HIGH SPECIFICITY HOMOCYSTEINE ASSAYS FOR BIOLOGICAL SAMPLES

Abstract: The novel methods of the invention involve use of particular homocysteinase enzymes that permit the determination of homocysteine concentrations in biological samples without interference from the concentrations of cysteine and/or of methionine that are routinely present in such samples. There is also provided a diagnostic kit for use in determining the amount of homocysteine in a biological sample comprising (a) a homocysteinase having the aformentioned characteristics, and (b) at least one reagent capable of being used to determine the amount of product formed in the homocysteinase reaction. In a further aspect, the homocysteinase is provided as a chimeric molecule that comprises amino acid subsequences derived from, or patterned on, more than one homocysteinase, and which is typically produced from a chimeric polynucleotide that encodes therefor. Additional enhancements in homocysteine assay methodology include use of the enzyme gamma -glutamylcysteine synthetase to further limit any interference from cysteine present in the biological samples.

Description: High Specificity Homocysteine Assays for Biological Samples Field of the Invention The present invention relates to the assay of homocysteine in biological fluids such as urine, whole blood, and blood plasma. More particularly, the invention relates to the use of enzyme preparations that comprise one or more homocysteinase enzymes and diagnostic kits containing these enzymes. In the preferred practice of the invention, homocysteine concentration is determined by measurement of hydrogen sulfide produced therefrom. K.S. McCully et al. (American Journal of Pathology, 56, pp. 111- 128, 1969) first reported the association of blood plasma homocysteine levels with risk from cardiovascular disease. McCully et al. identified a correlation between elevated plasma homocysteine concentrations and arteriosclerotic disease. More recent studies have determined that even moderately elevated plasma homocysteine levels are indicative of substantially increased risk to coronary heart disease, cerebrovascular disease, and peripheral vascular disorders. See, for example, J. Selhub et al., New England Journal of Medicine, 32, pp. 286-291, 1995, and S. Kang et al., Annual Review of Nutrition, 12, pp. 279-298, 1992. With respect to assessing risk to cardiovascular disease, there is substantial evidence that elevated homocysteine levels may be a better predictor of risk than elevated cholesterol levels. For example, in one study, plasma homocysteine concentrations only 12% above the upper limit of normal were associated with a 3.4-fold increase in risk for myocardial infarction (M. Stampfer et al., Journal of the American Medical Association, 268, pp. 877- 881, 1992). In another case study where blood was drawn before cardiovascular disease was diagnosed, a group of 271 men (who later suffered myocardial infarctions) presented significantly higher levels of homocysteine than control patients who did not later suffer infarctions (see P. Ueland et al., Cardiovascular Disease, Hemostasis and Endothelial Function, 1992, at pages 183-236, Marcel Dekker, New York). O. Nygard et al, New England Journal of Medicine, 337(4), pp. 230-236 (1997) report that total plasma homocysteine is a strong predictor of mortality in patients with confirmed coronary artery disease. A detailed understanding of the molecular mechanisms whereby homocysteine participates in cardiovascular disease processes has not yet been achieved. It has been suggested that homocysteine participates in reactions resulting, for example, in overproduction of oxygen free radicals, elastase activation, and calcium deposition (i.e. plaque formation). For a review of current theories, see K. McCully, Nature Medicine, 2(4), pp. 386-389 (1996) and K.S. McCully, Annals of Clinical and Laboratory Science, 23(6), pp. 477-493 (1993). Elevated levels of circulating homocysteine may also affect the coagulation process directly (see O. Nygard et al., above, and references cited therein). Accordingly, there is a need to develop accurate methods to determine homocysteine levels in patients, and to make such assays a recognized part of standard medical practice. As described below, there is also a critical need to make available widespread screening of infants for diseases involving abnormal homocysteine metabolism, such as homocystinuria. Reported Developments A. Araki et al, Journal of Chromatography, 422, pp. 43-52 (1987) describe a detection method for free and protein bound homocysteine in plasma that utilizes reaction with a thiol specific fluorogenic agent, followed by detection with high pressure liquid chromotogaphy (HPLC) to separate other thiol species. See also N.P.B. Dudman et al., Clinical Chemistry, 42(12), pp. 2028-2032 (1996). Disadvantages of such methods for clinical application may include the inherent limits on the number of samples that can be processed by HPLC, that use of HPLC may not permit high through-put screening, and that appropriate HPLC machines may not be present in clinical labs. Shipchandler et al. (see Clinical Chemistry, 41(7), pp. 991-994, 1995) have recently described a fluorescence polarization immunoassay for detection of homocysteine, and of homocystine (the disulfide dimer of homocysteine). The assay was stated to evidence little cross reactivity toward cysteine and methionine. The assay involves conversion of homocysteine to S-adenosylhomocysteine, and detection is based on use of a monoclonal antibody that recognizes S-adenosylhomocysteine, and also a fluorescein analog thereof. However, potential disadvantages of this detection technology may include cost associated with monoclonal antibody production and "nonspecificity" of the antibody, that is, cross reactivity toward, for example, S-adenosyl methionine. Additionally, it appears that this method can only be practiced with particular patented instrumentation available from only once source. K-W. Thong et al. (Experimental Parasitology, 63, pp. 143-151, 1987) describe enzymatic conversion of homocysteine to alpha-ketobutyrate, hydogen sulfide and ammonia followed by determination of the hydrogen sulfide with lead acetate (see also K-W. Thong et al., IRCS Medical Science, 13, pp. 493-494, and pp. 495-496, 1985) However, there is no suggestion of use of the involved enzyme (for example, from the parasite Trichomonas vaginalis) for clinical diagnosis. Additionally, in a clinical context, the disclosed procedure does not distinguish homocysteine from cysteine, nor does it account for that fraction of homocysteine molecules in biological samples that is disulfide bonded, or bonded to protein. An additional procedure for detection of homocysteine that involves use of a microbial enzyme from the bacterium Pseudomonas ovalis is disclosed in Yamaguchi et al., Annual Report of Sapporo City Institute of Public Health, No. 20, pp. 67-74, 1993. The authors focused on detection of the amino acid methionine as an indirect method to determine homocysteine levels (such as for homocystinuria patients), since homocysteine was considered unstable, for example being prone to disulfide bonding with proteins. As an indirect measure of homocysteine, it is possible that the method is less accurate, and determination of product ammonia instead of product hydrogen sulfide may be less sensitive given the high background levels of ammonia in biological fluids. It is noted also that ammonia assays are adversely affected by the presence of contaminating deaminases, a potentially pervasive problem. Similarly, assay of product alpha-keto butyrate is expected to be ineffective given the high background level in biological samples (for an assay procedure, see Y. Tan et al., Protein Expression and Purification, 9, pp. 233-245, 1997). An additional reference of relevance to the practice of the invention is U.S. Patent 5,631,127 to Sundrehagen, issued May 20, 1997. Given the increasing awareness of the role of homocysteine in cardiovascular disease, and the public health goal to screen patients for risk from cardiovascular disease, there is a critical need for novel analytical procedures that can be used to accurately determine homocysteine levels in patients. Such procedures are expected to provide considerable medical benefit both in those cases where elevated homocysteine concentrations are a cause of a particular disease state, and in cases where elevated homocysteine concentrations are a detectable byproduct of an exisiting disease state. Such analytical procedures would also provide great benefit by predicting a patient's susceptibility to cardiovascular disease before onset can be detected by other procedures. In this regard, great benefit would be achieved by adapting such procedures to the widespread screening of the general population, and in particular, to all infants. The present invention provides for such methods, including diagnostic kits for use in the clinical setting. In connection with the design of such improved analytical procedures, it would be of great value to develop enzymes useful to measure homocysteine in single step/enzyme assay, but which are substantially non-reactive toward the interfering concentrations of cysteine and methionine that are typically present in biological samples. As described below, the present invention provides for homocysteinase enzymes which have such specificity. Summary of the Invention The present invention provides for novel methods to determine the concentration of homocysteine in biological fluids. In a preferred embodiment of the invention, the biological fluid is a urine, tissue fluid, blood, blood serum, or blood plasma sample from a patient, and the methods of the invention are useful to assess risk for cardiovascular disease. In particular, the present invention provides novel methods to determine homocysteine concentrations in biological fluids while avoiding detection of related but interferring substances, most particularly cysteine and methionine. Accordingly, there is provided a method for determining the amount of homocysteine that is present in a biological sample containing, for example, homocysteine and cysteine, that comprises contacting said sample with an enzyme preparation capable of producing hydrogen sulfide from homocysteine and cysteine, and determining the amount of homocysteine in said sample by measuring the amount of hydrogen sulfide produced from homocysteine. In a most preferred embodiment, the enzyme preparation consists only of a single enzyme whose reactivity toward homocysteine is sufficient, compared to cysteine, that hydrogen sulfide produced from cysteine may be ignored. Said enzyme preparation comprises an enzyme referred to as a homocysteinase, and which may be referred to by other names as described below. Additionally, the term "enzyme preparation" should also be understood to include, unless otherwise noted, both a single and a plurality of aliquots, that is, either a single amount or multiple amounts of one or more enzymes may be added during the course of an assay procedure, and the use of the term is without limitation has to how, or how many, aliquots or steps are used to add all of the necessary enzymes during a reaction procedure. According to a preferred aspect of the invention, it is recognized that the total concentration of homocysteine present in biological samples, for example in body fluids, includes homocysteine molecules that are not present in free form, being instead covalently coupled to other molecules. The methods of the invention provide further steps for releasing this homocysteine prior to measuring of homocysteine-derived hydrogen sulfide. It should be noted, however, that since the methodology of the present invention provides for accurate measurement of free homocysteine levels absent interference from related substances (for example, cysteine), valuable information is provided to the clinician even if only free homocysteine is detected. Among many uses, it is expected that such information is very useful as a fast initial diagnostic tool, for example, in the testing of all newborn infants. Additional embodiments of the invention include methods to distinguish homocysteine from any cysteine that may be present in said biological sample. Representative of such methods are: (I) a procedure wherein hydrogen sulfide produced from homocysteine is distinguished from hydrogen sulfide produced from cysteine by including in said method the additional initial steps of: (a) contacting said biological sample with a reagent that protects homocysteine from said enzyme preparation; (b) permitting said enzyme preparation to produce hydrogen sulfide from cysteine present in said sample; (c) removing said hydrogen sulfide; and (d) deprotecting said homocysteine, and adding a further amount of enzyme preparation to produce hydrogen sulfide therefrom; and (II) a procedure that includes the additional initial steps of: (a) contacting homocysteine that is present in said biological sample with a further enzyme preparation capable of converting it to a compound that can be isolated; (b) isolating said compound; (c) contacting said compound with an enzyme preparation or other reagent under conditions where said compound is converted back to homocysteine; and (d) contacting the homocysteine so produced with said enzyme preparation capable of producing hydrogen sulfide therefrom; (III) a procedure wherein said biological sample is pretreated with an agent that renders cysteine non-reactive to said enzyme preparation; and (IV) a procedure wherein hydrogen sulfide produced from homocysteine is distinguished from hydrogen sulfide produced from cysteine, that includes the steps of: (a) permitting an enzyme preparation to produce hydrogen sulfide from both homocysteine and cysteine present in said sample, wherein said enzyme also produces pyruvate from cysteine in an amount equal to the amount of hydrogen sulfide produced therefrom; (b) determine the amount of pyruvate so produced; and (c) calculate the amount of homocysteine from the amount of hydrogen sulfide attributed thereto, wherein said sample is, or optionally is not, divided into a first part and a second part in order to conduct said assays. In a particularly preferred embodiment of the invention that is readily adapted to the rapid testing of samples from a large number of patient samples, there is provided a method for determining the amount of homocysteine that is present in a biological sample containing homocysteine and cysteine, comprising the steps of: (a) dividing said sample into 2 parts, part 1 and part 2; (b) contacting part 1 with a first enzyme preparation that is capable of converting homocysteine to a substance that is not a substrate for homocysteinase, wherein said first enzyme preparation does not recognize cysteine as a substrate; (c) independently contacting part 1 and part 2 with a second enzyme preparation that comprises a homocysteinase capable of producing hydrogen sulfide from both homocysteine and cysteine; (d) measuring the amount of hydrogen sulfide produced in part 1 and part 2; and (e) calculating the amount of homocysteine in said biological sample by subtracting the hydrogen sulfide measurement of part 1 from that of part 2, and doubling the result. For the clinical practice of the invention, there is provided a diagnostic kit for use in determining the amount of homocysteine in a biological sample, and wherein the homocysteinase included therewith is obtained from one of several sources. In a preferred embodiment, the diagnostic kit comprises: (a) a homocysteinase from Trichomonas vaginalis , or Pseudomonas (such as species ovalis or putida), and (b) at least one reagent capable of being used to determine the amount of hydrogen sulfide formed in the homocysteinase reaction. In a further aspect of the invention, there is provided a chimeric nucleotide sequence (whether of DNA or RNA), derived from more than one gene, or other polynucleotide, that codes for a homocysteinase, wherein expression of such sequence leads to the production of a chimeric homocysteinase. Such homocysteinases have valuable properties with respect to the practice of the invention, and a preferred example thereof includes a chimeric enzyme that comprises amino acid sequences derived from both Trichomonas vaginalis and Pseudomonas putida homocysteinases. Thus the present invention provides two basic approaches in the design of clinical assays for homocysteine: (1) to enhance detection of homocysteine in biological samples using novel assay techniques to detect homocysteinase reaction products, and (2) to further enhance the sensitivity of such assays by designing homocysteinase enyzmes that have substantially enhanced reactivity toward homocysteine, in comparison with other sulfur- containing amino acids. With respect to approach (2) above, use of the protein encoded by SEQ ID NO: 10 is representative. Such a homocysteinase enzyme is sufficiently non-reactive toward cysteine or methionine that the concentration of homocysteine that is present, for example, in a sample of tissue fluid, urine, blood, blood serum, or blood plasma of a subject may be determined in a single step assay, wherein is measured the amount of one or more products resulting from reaction of said homocysteinease on homocysteine in said sample, and wherein said measurement is substantially unaffected by the concentration of cysteine or methionine therein. In a preferred aspect, such a homocysteinase is patterned on a homocysteinase from Pseudomonas, Clostridium, Aeromonas or Trichomonas, and in an example thereof, one or more peptide sequences of such an enzyme are correspondingly replaced by one or more homologous peptide sequences of SEQ ID NO: 10, for example, those selected from the group consisting of: (a) Gly-Gly-Asn-Arg-Leu-Ala-Gly-Gln-Glu, (see residues 43-51 of SEQ ID NO: 10); (b) a subset of (a) that comprises Leu; (c) Arg-Val-Cys-Lys-Glu-Ala-His-Ser-Gln, (see residues 168-176 of SEQ ID NO: 10); (d) a subset of (c) that comprises Glu; (e) Gin-Met- Arg-Met-Tyr-Gly-Ser-Met-Ile, (see residues 304-312 of SEQ ID NO: 10); and (f) a subset of (e) that comprises Tyr. In a preferred aspect, such a homocysteinase is a substitution variant, addition variant, deletion variant, or derivative of SEQ ID NO: 10, wherein said variant or derivative has one or both of the following properties: (a) at least about 10% of the activity of SEQ ID NO: 10 toward homocysteine in a suitable assay; and/or (b) no more than about 1000 % of the activity of SEQ ID NO: 10 toward cysteine (or methionine) in a suitable assay. Additional aspects of the invention therefore include: (1) a purified and isolated DNA molecule that comprises a nucleotide sequence coding for a homocysteinase having such specificity, and wherein said coding sequence is operably linked to control sequences capable of directing expression therefrom in a host cell; and (2) appropriately transfected host cells. An additional preferred embodiment of the invention is a method for determining the amount of homocysteine that is present in a biological sample containing homocysteine and cysteine, which method comprises measuring the concentration of a product that is produced from homocysteine in a reaction catalyzed by a homocysteinase, which product may also be produced by reaction of said homocysteinase upon cysteine in said sample, and wherein said sample contains an otherwise interfering concentration of cysteine, said method comprising the steps of: (a) incubating said sample with γ-glutamylcysteine synthetase in the presence of substrates that permit conversion of cysteine to γ-glutamylcysteine; (b) incubating the mixture resultant from step (a) with homocysteinase; and (c) measuring the concentration of a product that is capable of being produced from both homocysteine and cysteine in a reaction catalyzed by a homocysteinase, and wherein it is preferred (1) to use as homocysteinase the Trichomonas vaginalis enzyme corresponding to SEQ ID NO: 10, or to residues Met 1 through Leu 396 thereof, and (2) to use E. coli γ- glutamylcysteine synthetase. Diagnostic kits are also provided, as is an analogous procedure based upon the use of cysteine tRNA synthetase. Additional important features of the invention include a diagnostic kit for use in a single enzyme assay of the homocysteine concentration in a biological fluid of a subject, said kit comprising: (a) a homocysteinase enzyme; and (b) at least one reagent capable of being used to determine the amount of product hydrogen sulfide formed by reaction of homocysteinase on homocysteine; wherein said homocysteinase is sufficiently non-reactive toward cysteine that any hydrogen sulfide produced by reaction of said homocysteinase on cysteine does not substantially interfere with the use of said kit to assess risk for cardiovascular disease. According to this aspect of the invention, a diagnostic kit is typically provided wherein the homocysteinease included therein has the property that at least about 90% of the hydrogen sulfide produced by action of said homocysteinase upon contacting a biological sample in an assay for homocysteine is contributed by said homocysteine when, for example, the concentrations of homocysteine and cysteine in said fluid are, respectively, about 20 μmolar and about 300 μmolar respectively. Preferably, the included homocysteinase has the property that at least about 99% of the hydrogen sulfide produced by action of said homocysteinase upon contacting said biological fluid is contributed from homocysteine. Such diagnostic kits can be used, for example, to accurately determine the homocysteine concentration in biological fluids in the range of about 1-500 μmolar, or higher as necessary, wherein said fluids also contains from 0 to about 1000 μmolar of cysteine, in a typical example. With respect to the use of such kits, as with other assay procedures of the invention, it is generally preferred to include use of a disulfide reducing agent, such as dithiothreitol ("DTT"), so that homocysteine molecules in biological samples that are disulfide-bonded to other molecules (such as free cysteine, other homocysteine molecules or protein SH groups) can also be detected. Further representative and additional aspects of the invention are described according to the Detailed Description of the Invention which follows directly. It should be recognized also that the practice of the present invention should not be seen as dependent on, or limited by, any particular theory concerning the role of homocysteine in pathological processes. Brief Description of the Drawings Figure 1 (panels A and B) provides a comparison of the amino acid sequence of Trichomonas vaginalis homocysteinase encoded by the mgll gene with that encoded by the Trichomonas vaginalis pAC2-l clone. Figure 2 shows the comparative specific activities (units/mg crude E. coli extract) corresponding to homocysteinase of the pAC2-l clone for cysteine, methionine, and homocysteine. Figure 3 shows the comparative specific activities of homocysteinase (pAC2-l clone, purified using a DEAE-Fast Flow procedure) for cysteine, methionine, and homocysteine. Figure 4 shows the comparative specific activities of homocysteinase (pAC2-l clone, purified using a nickel cation affinity reagent) for cysteine, methionine, and homocysteine. Figure 5 shows determined κ cat values of homocysteinase (pAC2-l clone) for cysteine, methionine, and homocysteine. Detailed Description of the Invention Elevated blood plasma homocysteine levels are recognized as a risk factor for vascular disease. The atherogenic properties of homocysteine were first noted in association with a rare group of genetic diseases referred to collectively as homocystinuria. The disease state is characterized by a level of circulating homocysteine that is typically 10-fold (or greater) above that in normal blood. Under these circumstances, homocysteine is also detected in the urine. Premature vascular disease is strongly correlated with this condition. For example, if homocystinuria is left untreated, about 50% of patients suffer thromboembolic events, and mortality by the age of 30 is approximately 20% (see, for example, S.H. Mudd et al., American Journal of Human Genetics , 37, pp. 1-31, 1985). Given that more than 75 epidemiologic or clinical studies have now shown a relation between total homocysteine levels and coronary and peripheral arterial diseases, stroke, and venous thrombosis, there is a clear medical need for diagnostic procedures that can accurately determine homocysteine levels in, for example, blood plasma. It would be highly advantageous also if such methods could be adapted to mass screenings of large populations of patients, or of the general population or newborn infants. The development of such assay procedures has been hindered principally by two interrelated difficulties: (1) the chemical and biochemical properties of homocysteine are intimately related to those of other sulfhydryl-containing biomolecules, including the amino acids methionine and cysteine, and thus improved assay procedures for homocysteine must be uniquely responsive to homocysteine. (2) homocysteine is highly reactive and exists under physiological conditions in many forms besides the free homocysteine molecule, for example, as a disulfide-linked dimer, as a disulfide-linked heterodimer with free cysteine, as a disulfide-linked conjugate with cysteine residues of blood plasma proteins, and bonded (often as an N-Acyl derivative) to the R groups of other protein amino acids (for example, to the epsilon amino group of protein lysine). These conjugated forms contribute very substantially to the total amount of homocysteine that may participate in pathological processes. Given also that only minor increases, beyond the normal range, in plasma homocysteine concentrations lead to satistically significant disease risk (see, O. Nygard et al. above), it is useful that clinical assays provide the capability to detect the total homocysteine content of biological fluids. This said, the reproducible detection of free homocysteine (or of any of the other forms in which homocysteine is found), but absent interference from such other substances as methionine and cysteine, also provides valuable information to the clinician. The present invention is thus directed to the development of homocysteine assay procedures, and the selection of reagents therefor, that overcome the above complexities under conditions where large scale screening of patient samples is practicable. Homocysteinase enzymes useful in the practice of the invention According to the practice of the invention, homocysteine concentrations in biological samples are determined enzymatically with enzymes hereinafter referrred to as "homocysteinases", which are defined as enzymes capable of catalyzing reactions whereby hydrogen sulfide is produced from homocysteine. In the typical case, ammonia and alpha- ketobutyrate are also produced. As described herein, it is preferred to detect product hydrogen sulfide, although product ammonia and/or alpha-ketobutyrate or other products may also be detected. Homocysteinases, as defined herein, may catalzye other reactions involving other sulfhydryl compounds and have acquired multiple names in the literature; however, they are generally useful in the practice of the present invention if they possess the property of catalyzing production of hydrogen sulfide from homocysteine. A first homocysteinase that is preferred in the practice of the invention is L- methionine-alpha-deamino-gamma-mercaptomethane lyase (methionine lyase) derived from the bacterial source, Pseudomonas putida. The enzyme has been purified by S. Ito et al., Journal of Biochemistry, 79, pp.1263-1272 (1976), and determined to have a molecular weight of about 170kDa. In the context studied by S. Ito, the enzyme carried out the alpha- gamma elimination of methionine to alpha-keto butyrate, methanethiol, and ammonia. If homocysteine is the substrate, then alpha-keto butyrate, hydrogen sulfide, and ammonia would be the resultant products. The homologous enzyme has been isolated from Pseudomonas ovalis , H. Tanaka et al., Biochemistry ,16, pp.100-106 (1977). Methods for the recombinant production of this Pseudomonas enzyme have also been developed (see Y. Tan et al., Protein Expression and Purification, 9, pp. 233-245, 1997), and use of recombinant enzyme in the clinical practice of the present invention is expected to provide advantages in terms of diagnostic kit cost and assay reproducibility. The Tan et al. reference describes construction of a clone designated p AC 1-1 containg a single copy of enzyme-encoding sequence. An additional clone, designated pACl-11, has been constructed which contains two copies of the encoding sequence, in tandem. As with the p AC 1-1 structure, the pT7-7 plasmid was used to construct pACl-11. The two encoding sequences are linked together at a BamHI site, with the first gene linked Ndel to BamHI, and the second linked BamHI to a further BamHI site. The substrate specificity of the P. putida enzyme has also been determined. For example, N. Esaki et al. Methods in Enzymology, 143, pp. 459-465 (1987) report that on a relative activity scale where activity toward methionine is assigned 100, cysteine is 10, and homocysteine is 180. That the enzyme is reactive to all three sulfhydryl-containing amino acids underscores the need to define clinical assays for which the source of hydrogen sulfide can be properly determined. It should be noted that the apparent 10- fold preference of the enzyme for homocysteine over cysteine does not take into account that the concentration of cysteine in a biological sample may be high ~ in fact much higher than the concentration of homocysteine. Homocysteinase enzymes of suitable catalytic activity can be derived from other Pseudomonas species, or from other bacteria, using routine screening procedures and assays that are recognized in the art. An additional group of organisms that are a source of homocysteinase useful in the practice of the invention are species of the Trichomonad parasites and related protozoans. Trichomonads are important parasites of the urogenital tract and are aero tolerant (but nonetheless anaerobic) flagellate protozoa. Use of homocysteinase from Trichomonas vaginalis is preferred according to the practice of the invention. Trichomonas species are believed to use their capabilities for thiol metabolism in order to counter oxygen toxicity in host tissues, see K-W Thong et al., Experimental Parasitology, 63, pp.143-151 (1987), and papers cited therein. Although considerable variation in homocysteinase activity (termed homocysteine desulphurase activity therein) was found between Trichomonas species, it is routine to screen available species for acceptable levels of enzyme activity. Generally speaking, it is preferred that a homocysteinase should have a specific activity of at least about 1 unit/mg purified protein for use in the below- described assays, although it is well within the skill of those familiar with the relevant art to design variations on these assays that use greater or lesser amounts of enzyme or enzyme preparations with differing enzyme activity. It is noted that highly purified and active P. putida enzyme has a specific activity of about 20 units/mg (a unit of enzyme activity may be described as 1 micromole of substrate converted per minute under standard conditions (see Y. Tan et al. above). The "homocysteine desulphurase activity" reported by K-W. Thong et al. (1987) above appears to result from the same enzyme responsible for methionine catabolizing activity in Trichomonas , and later termed methionine-gamma-lyase by B.C. Lockwood and G.H. Coombs (Biochemical Journal, 279, pp. 675-682, 1991) wherein is also described purification of this enzyme. As aforementioned, use of methionine-gamma-lyase (a homocysteinase) from Trichomonas vaginalis is preferred in the practice of the present invention. Use of a recombinant version of the T. vaginalis enzyme is also preferred. One potential cloning strategy follows the observations by A. Marcos et al., FEMS Microbiology Letters, 135, pp. 259-264 (1996), that T. vaginalis genes may have few introns. Accordingly, a genomic library would be constructed (see D. E. Riley et al. Molecular and Biochemical Parasitology, 51, pp. 161-164, 1992) and screened with DNA fragments corresponding to the Psuedomonas putida enzyme, and which are expected to reflect some partially conserved sequence. Lockwood et al. also list other reports of bacteria having methionine-gamma-lyase activity involving species of Pseudomonas, Clostridium, and Aeromonas. It is expected that such species are sources of homocysteinase activity useful in the practice of the present invention. Additional organisms that are expected to provide useful homocysteinase enzymes are certain marine algae (see, for example, D.A. Gage et al. Nature, 387, pp. 891-897, (1997) describing species with extensive methionine-related metabolism); sulfur bacteria; and geomicrobes or other microbes living under extreme environmental conditions (see, for example, R.A. Kerr, Science, 276, pp.703-704, 1997 and E. Pennist, Science, 276, pp.705-706, 1997). Additional examples of microorganisms that may be useful sources for homocysteinase-type enzymes include bacteria involved in periodontal disease, such as are capable of forming volatile sulfur compounds from cysteine. In this regard, see S. Persson et al, Oral Microbiology and Immunology, 5(4), pp. 195-201, 1990. Finally, with respect to the definition of "homocysteinases" that are useful in the practice of the invention, it should not be viewed as a limitation herein that hydrogen sulfide necessarily be a product of such enzymes. Broadly speaking, the present invention provides for high through-put diagnostic procedures, permitting the cost effective analysis of a very large number of homocysteine-containing samples while avoiding detection of interfering substances. Accordingly, it is expected that other enzymes that metabolize homocysteine are useful in the practice of the invention if methods exists to detect such metabolites under conditions where interference from substances similar to homocysteine (such as cysteine) can be avoided. The following examples describe a considerable spectrum of techniques to accurately measure homocysteine while avoiding detection of interfering substances. Accordingly, it will be appreciated that the techniques disclosed below can be adapted by those skilled in the art to detection methods using many other enzymes that act on homocysteine. Chimeric Homocysteinases As aforementioned, in a preferred aspect of the invention there is provided a chimeric nucleotide sequence (whether of DNA or RNA), derived from more than one gene (or other polynucleotide such as a cDNA or other intron-less sequence), that codes for a chimeric homocysteinase enzyme. Such homocysteinases have properties that are very useful with respect to the practice of the invention, and a preferred example thereof includes a chimeric enzyme that comprises amino acid sequences corresponding to both Trichomonas vaginalis and Pseudomonas putida homocysteinases. It is believed that P. putida homocysteinase is more stable than the T. vaginalis enzyme under a variety of conditions. As one probable consequence thereof (see B. Lockwood et al., Biochemical Journal, 279, pp. 675-682 , (1991), recovery of the T. vaginalis enzyme during purification was very low (see p. 679 at Table 2, referring to "methionine γ lyase"). Thus in the practice of the present invention, it is preferred to include P .putida sequences in chimeric enzymes in order to take advantage of this enhanced stability. As discussed above, different homocysteinases have different respective reactivities toward their various sulfur-containing substrates, for example, cysteine, methionine, and homocysteine. Although the selection of particular reaction conditions may affect apparent reactivities under particular circumstances, evidence indicates that the T. vaginalis enzyme has a higher reactivity toward homocysteine as substrate than does the P. putida enzyme, which may show greater reactivity toward cysteine, or methionine as substrate. In this regard, the results reported by Lockwood et al., 1991 at Table 4 thereof) are of note in that relative activity data for the T. vaginalis enzyme evidence a very pronounced "preference" for homocysteine as substrate (see also the K,,, data reported on page 678 thereof). This is in contrast to the data reported by N. Esaki et al. above with respect to the P. putida enzyme. Thus, the properties of homocysteinease for use in the clinical diagnostic applications of the present invention can be improved by providing the enzyme as a chimeric molecule that draws upon functional features contributed to the chimeric protein from both of its component species. In particular, it is preferred to select for inclusion in the chimeric homocysteinases of the invention regions of the P. putida enzyme that contribute substantially to the stability thereof, while including also regions of the T. vaginalis enzyme that preferentially enhance reactivity toward homocysteine as substrate. In connection with the design of such enzymes, a recent report that T. vaginalis homocysteinase actually represents two protein species derived from two genes is of note (see J.C. Mottram, direct submission of sequences to Gene Bank, submitted July 17, 1997, as T. vaginalis mgll gene, accession number AJ000486, NID g2330884, and T. vaginalis mgl2 gene, accession number AJ000487, NID g2330886), the complete sequences of which are fully incorporated by reference herein, as if directly set forth. See also International Patent application WO 98/07872 published February 26, 1998, and A. McKie et al., Journal of Biological Chemisty, 278, pp.5549-5556, 1998. The present invention therefore provides for a purified and isolated DNA molecule comprising a chimeric nucleotide sequence that encodes amino acid sequence of Pseudomonas putida homocysteinase, and amino acid sequence of Trichomonas vaginalis homocysteinase (derived from either mgll, or mgl2, or both) from which can be expressed a functional protein having homocysteinase activity. In a preferred aspect, the nucleotide construct (or corresponding amino acid construct) corresponds predominantly to that of putida, and thus there is provided a DNA molecule that comprises an encoding nucleotide sequence for Pseudomonas putida homocysteinase, wherein one or more subsequences thereof that encode one or more amino acids of said enzyme are correspondingly replaced by one or more nucleotide subsequences that encode the corresponding amino acids of a Trichomonas vaginalis homocysteinase. In the practice of the present invention, the T. vaginalis enzyme encoded by the mgll gene is hereinafter referred to as the Tl protein, and that encoded by the mgl2 gene is referred to as the T2 protein. In connection with the selection of chimeric polypeptides and encoding polynucleotides, the following considerations are of note. The Tl and T2 proteins are encoded by remarkably similar DNA sequences. Accordingly, it is generally expected to be the case that substitution of an mgll -encoding subsequence into a P .putida backbone will have substantially the same effect, and generate improvements substantially similar to those resultant from an equivalent mgl2 subsequence substitution. However, it is noted that there are a limited number of subregions where the Tl and T2 sequences differ substantially, while at the same time, the Tl sequence (less so the T2 sequence) shows substantial homology with the published P. putida sequence. Indeed, the published P. putida sequence shows considerable homology with the Tl sequence. Thus, in a further preferred aspect of the invention, there are identified regions of the encoding sequence for T. vaginalis enzyme where the T2 sequence and corresponding Tl sequence are very different from each other, and yet the Tl sequence is substantially similar to that of P .putida. Without being limited as to theory, it is believed that these encoding subregions of T2 present unique opportunities to insert into the a P. putida encoding polynucleotide, amino acid sequence domains that provide enhanced reactivity of the chimeric species toward homocysteine, thereby facilitating the diagnostic procedures of the invention. The invention therefore provides for a DNA molecule that comprises an encoding sequence for Pseudomonas putida homocysteinase, wherein one or more subsequences thereof, that each encode one or more amino acids of said enzyme, are correspondingly replaced by one or more nucleotide subsequences than each encode the one or more corresponding amino acids of a Trichomonas vaginalis homocysteinase, and wherein said resultant amino acid replacements are selected from the group consisiting of: (a) Cys-Ser-Arg-Ala-Asp-Ile-Ile-Ala-Lys-Val-Lys-Ser (SEQ ID NO: 1), or any subset thereof, from Trichomonas vaginalis (T2) for Val-Gly-Ser-Gln- Ala-Leu- Val-Asp-Arg-Ile- Arg-Leu (SEQ ID NO: 2), or any subset thereof, from Pseudomonas putida; (b) Asp-Val-Asp (SEQ ID NO: 3), or any subset thereof, from Trichomonas vaginalis (T2) for Glu-Leu-Lys (SEQ ID NO: 4), or any subset thereof, from Pseudomonas putida ; (c) Cys-His-Val-Val (SEQ ID NO: 5), or any subset thereof, from Trichomonas vaginalis (T2) for Ala-Leu-Gin-Leu (SEQ ID NO: 6), or any subset thereof, from Pseudomonas putida ; (d) Cys-Glu-Asn-Val-Gln-Asp-Ile-Ile-Asp-Asp (SEQ ID NO: 7), or any subset thereof, from Trichomonas vaginalis (T2) for Gly-Leu-Glu-Asp-Ile- Asp- Asp-Leu-Leu- Ala (SEQ ID NO: 8), or any subset thereof, from Pseudomonas putida; and (e) any combination involving one, two, three or four of the replacements provided for by groups (a), (b), (c), and (d) above. With respect to the above-listed Trichomonas vaginalis mgl2 ("T2") amino acid subsequences it should be noted they correspond approximately to the following amino acid sequence positions: Cys-Ser-Arg-Ala-Asp-Ile-Ile-Ala-Lys-Val-Lys-Ser (SEQ ID NO:l), approximately 226-237; Asp-Val-Asp (SEQ ID NO:3), approximately 318-320; Cys-His-Val-Val (SEQ ID NO:5), approximately 331-335; and Cys-Glu-Asn-Val-Gln-Asp-Ile-Ile-Asp-Asp (SEQ ID NO:7), approximately 381-390. In connection with the practice of this aspect of the invention, the above-listed T. vaginalis subsequences can be additionally modified to accept conservative amino acid substitutions. Additionally, the T. vaginalis subsequences need not correspondingly replace the above listed P. putida sequences so that, for example, the T. vaginalis subsequences may be inserted nearby (i.e. up to about 10 amino acid positions distant therefrom), while the targeted P. putida sequences remaining in place, in whole or in part. Additionally, the T\ vaginalis inserts may comprise transfer of additional amino acids found on the N-termnal or C-terminal side of the above-depicted sequences thereof, as long as the benefit of increased reactivity toward homocysteine is not compromised owing to decreased enzyme stability. The construction of appropriate chimeric encoding polynucleotides is straightforward for those familiar with the art, and may involve techniques of site directed or loop out mutagenesis, or use of PCR technology. Appropriate host cells for the expression of such encoding polynucleotides are described, for example, in Y. Tan et al., Protein Expression and Purification, 9, pp. 233-245, 1997. Homocysteinases for One Step Assays that Lack Reactivity Toward Cysteine According to the practice of the invention, there are also provided novel homocysteinase enzymes that are essentially non-reactive toward cysteine, that is, any such reactivity is sufficiently low that the amount of homocysteine present in a sample of tissue fluid, urine, blood, blood serum, or blood plasma, or other biological fluids or samples from human subjects, may be determined in a single enzyme assay — absent the need to correct for the concentrations of cysteine and/or of methionine that are also present in the sample. The significance of these discoveries will be immediately apparent to the clinician, given the widespread public health need for simple and fast homocysteine assays, when it is recognized that the concentration of cysteine and/or of methionine in such biological samples is generally much higher than that of homocysteine, and particularly so in certain disease states. Although such enzymes may react with methionine, this is generally irrelevant since a different detectable reaction product (hydrogen sulfide) results. In this regard, the activity of the novel Trichomonas vaginalis homocysteinase represented by SEQ ID NO: 10 is of particular note (see Example 8, and Figures 2 through 4, as discussed further below). The relative activity of this enzyme toward homocysteine, compared to cysteine, will readily permit homocysteine measurements without the need to correct for any cysteine present in the biological test sample. In a preferred example of the invention, such novel enzymes are patterned on homocysteinases found in various microorganisms including Pseudomonas, Clostridium, Aeromonas or Trichomonas, and particularly preferred homocysteinases are derived from T. vaginalis, including that expressed from the mgll gene (SEQ ID NO:l 1), or the mgl2 gene thereof. In Figure 1 (panels A and B) are depicted the amino acid sequences of two Trichomonas vaginalis homocysteinases, the first derived from a novel clone (pAC2-l) of the present invention, and the second, the enzyme derived from the known mgll gene (see also SEQ ID NOS:10 and 12 for comparison of amino acids; SEQ ID NOS: 9 and 11 for comparison of encoding DNAs). It will be seen that pAC2-l is characterized by the existence of three differences (mutations), compared to mgll : Phe to Leu at amino acid position 47, Asp to Glu at amino acid position 172, and Ser to Tyr at amino acid position 308. Modified homocysteinase amino acid sequences, made by incorporating amino acid substitutions that include one or more of these differences (mutations), are particularly useful in the practice of the invention. Accordingly, a preferred example of the invention involves providing a homocysteinase to contain one or more peptide (sub)sequences of SEQ ID NO: 10 that are selected from the group consisting of: (a) Gly-Gly-Asn-Arg-Leu-Ala-Gly-Gln-Glu, (see residues 43-51 of SEQ ID NO: 10); (b) a subset of (a) that comprises Leu; (c) Arg-Val-Cys-Lys-Glu-Ala-His-Ser-Gln, (see residues 168-176 of SEQ ID NO: 10); (d) a subset of (c) that comprises Glu; (e) Gln-Met-Arg-Met-Tyr-Gly-Ser-Met-Ile; (see residues 304-312 of SEQ ID NO: 10); and (f) a subset of (e) that comprises Tyr. It should also be noted that SEQ ID NO: 10, a preferred homocysteinase of the invention, is expressed from a DNA sequence (pAC2-l, see SEQ ID NO:9) isolated as a gene from Trichomonas vaginalis genomic DNA. In contrast, the reported isolation of mgll and mgl2 appears to have involved a cDNA procedure. pAC2-l may thus represent a new homocysteinase Trichomonas gene. In a representative example, the homocysteinase is patterned on an enzyme from Pseudomonas, Clostridium, Aeromonas or Trichomonas wherein one or more peptide (sub)sequences of the original polypeptide sequence(s) thereof are correspondingly replaced by one or more homologous peptide sequences of SEQ ID NO: 10 that are selected from the group consisting of: (a) Gly-Gly-Asn-Arg-Leu-Ala-Gly-Gln-Glu, (see residues 43-51 of SEQ ID NO: 10); (b) a subset of (a) that comprises Leu; (c) Arg-Val-Cys-Lys-Glu-Ala-His-Ser-Gln, (see residues 168-176 of SEQ ID NO:10); (d) a subset of (c) that comprises Glu; (e) Gln-Met-Arg-Met-Tyr-Gly-Ser-Met-Ile, (see residues 304-312 of SEQ ID NO: 10); and (f) a subset of (e) that comprises Tyr. In this regard, use of the term "homologous" is not intented to suggest that such homology is exact, but rather, that a comparison of such sequences using generally recognized models would suggest that such sequences, even if now signficantly different, may have evolved from a common ancestor gene or subset thereof. Similarly, the term "mutation" as used herein, should be broadly understood to include modifications that arise by whatever means, whether natural, or experimental and the like. Accordingly, a generally preferred example includes a chimeric homocysteinase patterned on a first Trichomonas homocysteinase wherein one or more amino acids thereof, that correspond to the Leu 47 , Glu 172 , and Tyr 308 residues of a second Trichomonas homocysteinase (that from pAC2-l, as depicted in SEQ ID NO: 10), are correspondingly replaced by one or more of said Leu 47 , Glu 172 , and Tyr 308 . An additional example of the invention is defined by a homocysteinase that is a substitution variant, addition variant, deletion variant, or other derivative of SEQ ID NO: 10, wherein said variant or derivative has one or both of the following properties: (a) at least about 10%, preferably about 25%, more preferably at least 50%> of the activity of SEQ ID NO: 10 toward homocysteine in a suitable assay (such as described in Example 8); and/or (b) no more than about 1000%, preferably less than 500%, more preferably less than 200%> of the activity of SEQ ID NO: 10 toward cysteine or methionine in a suitable assay (such as described in Example 8). Such a range of properties is generally believed to maintain sufficiently enhanced activity of the enzyme with respect to homocysteine, in comparison with cysteine or methionine, that single step methodology is still practical, although it will be appreciated that the exact requirements for such relative sensitivity may be readily determined in each clinical application. For example, cysteine would generally be expected to be at low concentration in a urine sample, at least with respect to a non-diseased patient. With respect to the provision of mutant homocysteinases from other organisms, as produced for example by appropriate recombinant methods, the above relative guidelines are expected to prove useful. The enzyme represented by SEQ ID NO: 10 is a particularly useful example given that the apparent activity of this homocysteinase for homocysteine is at least about 100-fold greater than for cysteine and methionine. In a representative example whereby a homocysteinase is modified to improve its usefulness for single-step assay methodology, the wild type T. vaginalis amino acid sequence (from mgll, see SEQ ID NOS: 11 and 12) is altered (generally of course by modification of an appropriate encoding DNA) as follows: one or more of Phe 47 , Asp 172 , and Ser 308 thereof is deleted, or is replaced according to the following formula: (1) for Phe 47 , replace with Leu, He, Val, Ala, Gly, Met, and Trp; (2) for Asp 172 , replace with Glu, Gin, or Asn; (3) for Ser 308 , replace with Tyr, Phe, Met, Trp, Gin, Thr, or Asn, wherein are suggested relatively conservative amino acid substitutions, as is recognized in the art, and whose efficacy in this regard can be determined by routine experimentation. As further described below (see Example 7), nickel-NTA affinity purification procedures (available from the Qiagen Company, Germany) can be used to facilitate purification of homocysteinase from E. coli or other host cells transfected with an appropriate homocysteinase-encoding DNA sequence. These procedures take advantage of the selective binding of protein imidazole groups to nickel cations immobilized in the NTA resin matrix. In a preferred example of this aspect of the invention, by modification of an appropriate encoding DNA (see SEQ ID NOS: 9 and 10), an amino terminal tag that contains, for example, 6 consecutive histidine residues is introduced into the homocysteinase, thereby facilitating its purification from host cell materials. Homocysteinease SEQ ID NO: 10 (see also Figure 1AB), is representative of modified homocysteinase enzymes that comprise one or more histidine residues positioned on the N- terminal side of the natural Met 1 of said proteins, said modified enzymes being sufficiently non reactive toward cysteine or methionine that the concentration of homocysteine that is present in a sample of tissue fluid, urine, blood, blood serum, or blood plasma of a subject may be determined by use of such enzymes in a single step assay. In such an assay, the amount of one or more products resulting from reaction of said modified homocysteinease on homocysteine are measured, and such measurements are substantially unaffected by the concentration of cysteine or methionine in the sample. According to this aspect of the disclosure, a homocysteinase that is useful in the practice of the invention comprises one or more, preferably 2-15, and most preferably 5-8 histidine residues consecutively positioned on the N-terminal side of the natural homocysteinase Met 1 . Examples Example 1 Detection of Homocysteine by Double Enzymatic Subtraction in a Sample Containing Cysteine The concentration of free homocysteine in a blood plasma sample is determined as follows. Blood samples (0.1 to 5.0 mL) are collected with a standard vacutainer using EDTA as anticoagulant, and plasma is then prepared by centrifugation. While it is expected that measurement of free homocysteine in biological samples provides the clinician with useful predictive information, it is recognized that a large fraction of homocysteine exists disulfide bonded to other molecules (for example, free cysteine, other homocysteine molecules, and available protein sulfhydryl groups). According to the practice of the invention, it is generally preferred to use a disulfide reducing agent (such as dithiothreitol, "DTT") in a protocol to permit detection of the broader pool of homocysteine molecules. A stock homocysteinase solution of 150 units/ml Trichomonas vaginalis enzyme is also prepared. Stock solutions of enzyme generally range from 1-1000 units/ml, and preferably 100-200 units/ml, although the exact activity is subject to selection by the practitioner, depending on various factors, including the concentration of homocysteine in the samples. As described above, use of P. putida enzyme is also preferred, at similar activity levels. The plasma sample, at an appropriate dilution in buffer, is divided into two equal volumes, Parts 1 and 2. To Part 1 is added a suitable amount of S-adenosylhomocysteine hydrolase ("SAHH") and also a quantity of adenosine (SAHH is available from both eukaryotic and prokaryotic sources, and a procedure for cloning it in T. vaginalis has been presented, Bagnara et al. Molecular and Biochemical Parasitology, 81, pp.1-11, 1996). In the presence of sufficient adenosine, the plasma homocysteine is converted to S- adenosylhomocysteine. The Part 2 volume is not reacted with SAHH. Trichomonas vaginalis homocysteinase is then added to the Part 1 and Part 2 samples and reacted for about 10-30 minutes. A final concentration of homocysteinase in the range of 0.5-1 unit/mL is preferred, although the exact amount may be adjusted as needed. It is noted that the adenosine remaining in the Part 1 sample prevents the hydrolyase reaction from running in reverse, and so its initial concentration should be chosen with this in mind. The amount needed may depend on the SAHH selected, but a useful starting point is a concentration not less than that of the homocysteine present. Hydrogen sulfide is then measured in both Part 1 and Part 2, and the amount of homocysteine present in the original sample is calculated by subtracting the hydrogen sulfide measurement of part 1 from that of part 2, and doubling the result. The hydrogen sulfide measurement is performed by adding lead acetate to the samples in the range of about 0.1 to 1.0 Molar, thus precipitating the sulfide. The precipitate is measured in a standard spectrophotometer at 360 run, or other suitable wavelength (such as in the visible range) at which the insoluble precipitate can be determined. This procedure is readily automated. An additional enzyme useful in the practice of the double enzymatic subtraction approach is cystathionine β-synthase, which catalyzes production in the body of cystathionine from homocysteine and serine. For blood serum or plasma samples, the reaction may be driven in the direction of cystathionine by taking advantage of the relatively high concentration of serine in the blood. Suitable sources of this enzyme include material isolated from Trichomonas, from Pseudomonas, or from mammalian tissues including, for example, liver. The enzyme may be used in an amount that provides a units/ml equivalent to the range described above for S-adenosylhomocysteine hydrolase. As mentioned above in connection with the use of SAHH, it is preferable that steps be taken to prevent this enzymatic reaction from running backwards, which may lead to inaccurate data. The reaction may be driven, and then maintained, in the direction of cystathionine by adding exogenous serine to the reaction samples, or by irreversibly inhibiting the enzyme once conversion to cystathionine has been accomplished. Suitable inhibitors include serine analogs of high affinity for the active site. A further additional procedure based on the double enzymatic subtraction approach may be available. Recent published work may indicate that the enzyme methionine tRNA synthetase (methionine tRNA synthase) is capable of accepting homocysteine as a reactant (see H. Jakubowsky et al., FEBS Letters, 317, pp. 237-246, 1993). Thus it may be possible to reproduce the above result using this enzyme. Example 2 Alternate S-adenosylhomocysteine hydrolase procedure S-adenosylhomocysteine produced according to Example 1 is subjected to the following manipulations: following its production it is isolated, and contacted with a further amount of S-adenosylhomocysteine hydrolase under conditions where S- adenosylhomocysteine is converted back to homocysteine; after which the homocysteine so produced with said enzyme preparation is contacted with homocysteinase to produce hydrogen sulfide therefrom. Example 3 Detection of Homocysteine by a Direct Enzymatic Subtraction in a Sample Containing Cysteine Following the initial procedures of Example 1, a blood plasma sample containing both homocysteine and cysteine is prepared. The sample is not divided into parts; instead, a sample of the enzyme cysteine tRNA synthetase is added under conditions that cysteine present is combined with tRNA. The enzyme selected may be of eukaryotic or prokaryotic origin, and conditions for its reaction are well known in the art. The cysteine-depleted sample is then reacted with homocysteinase as aforementioned, and the homocysteine concentration is determined directly from the hydrogen sulfide measurement. An additional example of the direct enzymatic subtraction method is provided below in Example 10, and is based on the enzyme γ-glutamylcysteine synthetase which may of eukaryotic or prokaryotic origin. Assay systems that incorporate use of this enzyme are described. Additional enzymes useful to deplete cysteine include cysteine oxidase and cystathionine β-synthetase. Example 4 Methods to Detect Homocysteine Bonded to Other Biomolecules As aforementioned, homocysteine exists in biological fluids bonded to other biomolecules, and homocysteine "stored" in such forms is believed to contribute to pathological processes. Thus it is desireable to determine total homocysteine, and this includes (1) homocysteine dimers, (2) homocysteine/cysteine heterodimers, (3) conjugates of homocysteine and protein wherein homocysteine is attached by way of disulfide bonds to protein cysteine, and also (4) conjugates of homocysteine and protein wherein homocysteine (including N-Acylated forms thereof) is bonded to protein amino acid R groups, most notably to the epsilon amino group of protein lysine. With respect to forms (1) through (3), reducing solutions are used to release the covalently bound homocysteine. In one example, a stock sodium borohydride or sodium cyanoborohydride solution is prepared at about 0.1 M or less. The reaction is carried out by adding the borohydride to the plasma sample (prepared following the intitial procedure of Example 1) at pH 6 to 8 for 5-10 minutes at room temperature. The excess borohydride is destroyed, and the sample may then be subjected to the enzymatic procedures of Examples 1 or 3. Alternatively, the borohydride may be neutralized with HC1 if the sample is further manipulated according to the following procedure. It has been found, however, that sodium borohydride can denature homocysteinases under certain circumstances, so use of DTT is preferred. Example 5 Manipulations under Acid Conditions With respect to form (4) above, 6 N HC1 can be used by dilution at about 50/50 with the protein sample. Following reflux at boiling temperature for 1 -2 hours under nitrogen, water is removed from the sample under vacuum to complete dryness. This liberates homocysteine if bonded to other amino acids, and also completes the homocysteine lactonization process. At the point of dryness, sodium acetate (1M) and acetic anhydride (10M) are added. This is refluxed for one further hour at boiling temperature to create acetylated homocysteine thiolactone. The anhydride is removed by evaporation to dryness. The N-acetylated homocysteine thiolactone is selectively extracted with ether. The ether is then evaporated at 35 degrees C under vacuum. Physiologically buffered water (phosphate buffered saline, pH 7.5 ) is then added which then opens the thiolactone ring. At this point , either of two reactions is carried out. Homocysteinase may then be added and reacted for 10-30 minutes followed by detection of hydrogen sulfide as above, or preferrably a hog-kidney deacetylase is added to deacylate the homocysteine with subsequent addition of the homocysteinase. Example 6 Additional Methods for Detection of Homocysteine by a Direct Enzymatic Subtraction in a Sample that also Contains Cysteine Generally speaking, this additional method takes advantage of the fact that pyruvate produced from cysteine by the action of homocysteinases may be detected independently of the hydrogen sulfide that is generated by the action of homocysteinases on both homocysteine and cysteine present in biological samples, such as blood plasma. Thus determination of homocysteine may be made by simple subtraction. According to this method, pyruvate can be determined enzymatically by any number of enzymes that generate a detectable product therefrom. Similarly, α-ketobutyrate (produced from homocysteine but not cysteine) can be determined in relation to hydrogen sulfide using leucine dehydrogenase. The two principal processes of the method, detection of pyruvate and detection of hydrogen sulfide, may be conducted simultaneously in a single sample, or they may be conducted separately in parallel samples, in either case with appropriate controls. In a preferred example, the enzyme used to detect pyruvate is a mammalian lactate dehydrogenase, and determinations of both pyruvate and total hydrogen sulfide are performed in a single cuvette, using a standard spectrophotometer. Following the initial procedures of Example 1 , a blood plasma sample containing both homocysteine and cysteine is prepared. The sample is not divided into parts, instead a sample of a homocysteinase (from Trichomonas vaginalis or P. putida ) is added under conditions such that hydrogen sulfide, ammonia, and α-keto butyrate are produced from homocysteine, and additionally, hydrogen sulfide, ammonia and pyruvate are produced from cysteine. A sample of the enzyme lactate dehydrogenase (at an amount of units/ml comparable to that used in previous examples) is also added under conditions such that it will react with the pyruvate produced from cysteine, which reaction is then monitored at or in the vicinity of 340 nm (for NADH), per well known procedures. The total concentration of both homocysteine and cysteine in a sample may be determined colorimetrically, as before (see Example 1), by measuring the total hydrogen sulfide produced from homocysteinase, and at any suitable wavelength at which the precipitating colored lead salt can be detected, such as in the visible range, or also at or near, for example, 360 nm. Hydrogen sulfide may also be detected based on formation of other chromophoric metal salts, as is known in the art. The homocysteine concentration in the original sample is determined by comparison of the total hydrogen sulfide produced (from cysteine and homocysteine) with the total pyruvate produced (from cysteine only). This procedure is also readily automated, and both detection wavelengths can be monitored simultaneously. Although use of lactate dehydrogenase to detect pyruvate in blood, or in solutions derived from blood, may follow one of several art-recognized procedures, certain precautions should be repeated. Pyruvate is very unstable in blood, and in materials derived therefrom, and it may even polymerize. Accordingly, it may be desirable to conduct pyruvate determinations on protein- free filtrates of recovered samples, and on such samples only when processed with metaphosphoric acid, as is known in the art. Example 7 Production of the E. coli BL21 (DE3) pAC2-l clone The cloning of the pAC2-l homocysteinase gene of Trichomonas vaginalis was accomplished as described below. Generally speaking, recombinant methodology recognized as applicable in other microorganisms is useful with respect to manipulation of Trichomonas DNA, particularly so since many Trichomonas genes, as aforementioned, lack introns. Useful reference may therefore be made to Y. Tan et al., Protein Expression and Purification, 9, pp. 233-245, (1997) and International Patent publication number WO 96/40284 of Y. Tan et al., published December 19, 1996. Genomic DNA from Trichomonas vaginalis was isolated by standard procedures (Wizard Minipreps, Promega, Madison, WT), and used as a template for a PCR reaction conducted according to the method provided with a PCR reagent kit (Roche, Branchburg, NJ). Oligonucleotide primers were developed based on the nucleotide sequence of the mgll gene (J.C. Mottram et al, Gene Bank, accession number AJ000486, NID g2330884, submitted July 17, 1997). The specific primers used were: (sense) 5 '-GGATTACATATGCATCATCATCATCATCACATGAGTGGCCACGCTATCGAC-3 ' (SEQ ID NO: 13), which includes a CAT ATG Ndel site; and (antisense) 5'-GGATTAGGATCCTTAGAGGACTAAGTCGAGAGCC-3' (SEQ ID NO: 14), which includes a GGATCC BamHI site. Additional reagents used included restriction endonucleases, T4 DNA ligase, and BL21(DE3) competent cells, all purchased from Stratagene (San Diego, CA). The GeneAmp PCR reagent kit was purchased from Roche (Branchburg, NJ), and the DNA purification kit was purchased from Promega (Madison, WI). The oligonucleotide probes for PCR amplification were synthesized by IDT Inc. (Coralville, IA). All other reagents were purchased from Sigma (St. Louis, MO). Wild type Trichomonas vaginalis was purchased from the American Type Culture Collection (Rockville, MD). The PCR reaction conditions were as follows: 35 cycles of denaturation at 94°C for 1 minute; annealing at 50°C for 2 minutes; and extension at 72°C for 2 minutes. This was followed by a final extension step at 72°C for 10 minutes. The PCR-amplified product (which appeared as one band of 1.2K bp identified by Kb-ladder markers) was collected, digested with the Ndel and BamHI restriction enzymes, and then ligated into the pT7-7 vector at the Ndel and BamHI cloning sites thereof, using standard protocols (the pT7-7 vector was provided by Dr. Stan Tabor, Harvard Medical School, Boston , MA, see Tabor, S. "Expression using the T7 RNA polymerase/promoter system," in Current Protocols in Molecular Biology , F.A. Ausubel, et al., eds., 1990, pp. 16.2.1-16.2.11, Greene Publishing and Wiley- Interscience, New York). The resulting plasmid was then transformed into E. coli BL21 (DE3) cells by electrotransformation. After incubation at 37°C overnight, ampicillin-resistance clones were selected from ampicillin-containing plates. The cells from the selected colonies were grown in 5 ml LB medium (Fisher) at 37°C overnight. Suitable clones were selected based on enzyme activity of crude culture extracts in the α-ketobutyrate assay (using a modification of the method of K. Soda et al., Methods in Enzymology, 143, 459-465, 1981 - based on reaction of methyl-2- benzothiazolinone hydrazone ), and/or a dethiomethylation assay in which H 2 S is produced and quantified (see A.E. Braunstein et al., Biochimica et Biophysica Acta, 242, pp. 247-260, 1971), which are both recited directly below. A variant of the E. coli BL21 (DE3) pAC2-l clone was also constructed which differs only in that the included plasmid (expression vector pAC2-l 1, again based on pT7-7) contains two copies of the homocysteinase-encoding sequence, which are present in tandem. This particular clone, designated E. coli BL21 (DE3) pAC2-l 1 provides high level expression of recombinant homocysteinase for use in the diagnosis procedures of the invention. Briefly the 2 copies of the encoding sequence were placed between the Ndel and Bspl06I sites of pT7-7. The first copy was linked Ndel to BamHI, and the second was linked BamHI to BsplOόl. Recombinant homocysteinase may be purified from either the pAC2-l or pAC2-l 1 clones by the same procedures. The α-ketobutryate/pyruvate assay In the first step of this assay, 1 ml volumes of 100 mM phosphate buffer pH 8.0, containing also 10 μM pyridoxal phosphate, and different concentrations (25 uM-25 mM) of DL-homocysteine, or L-methionine, or L-cysteine, respectively, were incubated, for 10 min at 37°C, with a sufficient sample (typically 50μl) of crude cell extract (cells were sonicated, and the supernatant was recovered following centrifugation) to provide about 1-100 units of homocysteinase ("HCYase") enzyme. The reactions were stopped by adding 0.1 ml of 50% TCA. The suspensions were then centrifuged using an Eppendorf centrifuge at 13k rpm for 2 minutes. 0.5 ml samples of the supernatants were then added to 0.5 ml of 0.05% 3-methyl-2- benzothiazolinone hydrazone ("MBTH") in 0.8 ml of 1M sodium acetate, pH 5.2 and incubated at 50°C for 30 min. The amount of reaction products was determined for each sample, by spectrophotometry at OD 320 . The amount of protein was determined with a Bio- Rad 500-0006 kit (Bio-Rad, Richmond, CA) with bovine serum albumin as a standard. The enzyme specific activity was calculated as units/mg protein, with one unit of enzyme defined as the amount that catalyzes the formation of lμmol of α-ketobutyrate from homocysteine per minute. The assay procedure can, of course, also be used with purified homocysteinase samples that provide, for example, 1-100 units of enzyme per assay test. The dethiomethylation screening assay As aforementioned, the assay used was a modification of the method of A.E. Braunstein et al. above. The standard reaction mixture consisted of potassium phosphate buffer (pH 7.5, lOOmM), lead acetate (0.33 mM), and sufficient crude cell extract to provide 1-100 units of homocysteinase, to which mixture different concentrations (5μM-100 μM) of substrate DL-homocysteine, or L-cysteine or L-methionine were added, such that the total reaction volume was 1.5 ml. After incubation at 37°C for 10 min, the determination of lead sulfide was obtained on a spectrophotometer at OD 360 . The assay procedure can also be used with purified homocysteinase samples, for example, those providing 1-100 units per assay test. Purification of product homocysteinase Following use of the above initial assay screens, homocysteinases ("HCYases") from promising clones were purified using protocols that included DEAE Sepharose Fast Flow chromatography (for resultant enzyme activity profiles from the pAC2-l clone, see Figure 3), or included nickel-NTA affinity chromatography (for resultant enzyme activity profiles from the pAC2-l clone, see Figure 4). Cell Growth Conditions 10 μl of frozen E. coli stock corresponding to a particular clone was seeded into 5 ml LB medium (Fisher) containing also lOμg/ml ampicillin, and cultured overnight at 30°C with moderate shaking (300 rpm). The culture was then divided into 2 fresh 100 ml volumes of LB medium in 500 ml flasks, and further cultured at 30°C for 6 hours, also at 300 rpm. The entire culture was then divided into 18 flasks (each of 6 liters) containing 800 ml of LB medium. Growth was continued, overnight, at 37°C at 300 rpm and an OD 600 of approximately 10 was achieved. The bacteria were then spun down at 4000g permitting replacement of the growth medium with fresh LB, and the incubation was continued for a further 6 hours, again at 37°C and 300 rpm. When the OD 600 reached 20, the bacteria were harvested by centrifugation for 10 minutes at 4000g, 4°C . It will be recognized that a wide variety of variations on the above procedure are also effective. Pre-treatment prior to chromatography The bacterial pellet was first suspended in extraction solution (20mM potassium phosphate, 10 μM pyridoxal phosphate and 0.01 %> β-mercaptoethanol, pH 9.0), and the cells were then disrupted with a cavitator-type homogenizer (model HC 8000, Microfluidics Corporation, Newton, MA). The suspension was centrifuged with a refrigerated centrifuge (Sorvall, superspeed RC2-B) at 8000g, 4°C for 30 minutes. The supernatant was then collected, heated at 50°C for 1 minute, and then subject to ultrafiltration using a preparative scale device (model TFF PLHK 100k, Millipore, Bedford, MA) using a 2.5ft 2 pressure cartridge containing lOmM potassium phosphate buffer, pH 8.3). The pH was adjusted to 7.2 during the ultrafiltration. Chromatographic conditions-first column The above-derived concentrate (at pH 7.2) was applied to a first column (100 mm diameter x 30 cm) containing a 2400 ml packed volume of DEAE Sepharose® FF (Pharmacia, Uppsala, Sweden) in 40 mM KC1-PPM buffer (40 mM potassium chloride; and lOmM potassium phosphate, lOμM pyridoxal phosphate, 0.01% β-mercaptoethanol, pH 7.2). After loading the protein sample, the column was pre-washed with about 10 volumes of 40 mM KC1-PPM buffer until the OD 280 dropped below 0.1. The protein was then eluted with a linear gradient of 40-300 mM KC1 in PPM buffer, and 500 ml fractions were collected. The fractions containing homocysteinase were identified by their yellow color, and an activity assay. Chromatographic conditions-second column Following a 24 hour dialysis period against a solution of 80 mM KC1, lOmM potassium phosphate, pH 8.3, the recovered eluate (about 2-5 mg/ml representing a recovery of 5-10 total grams protein) was applied to a second column. After loading, the second column (Pharmacia XK 50/30 filled with DEAE Sepharose® FF - 500 ml volume, 50mm diameter x 25 cm) was pre-washed with 4 volumes of 80 mM KC1, 10 mM potassium phosphate, pH 8.3 (at a flow rate of about 6-8 ml/minute) until the OD 280 dropped below 0.1. The homocysteinase was then eluted with a linear gradient of 80 to 300 mM potassium chloride in 10 mM potassium phosphate buffer (pH 8.3). Eluate was collected in 300 ml fractions, and active fractions were identifiable by yellow color and homocysteinase enzyme activity. Enzyme activity results (toward homocysteine, cysteine, and methionine) for homocysteinase purified in this manner are discussed in Example 8 below (see also Figure 3). Alternatively, an additional two stage strategy may be used. In the first stage, the partitioning material was DEAE Sepharose® FF, and loading and pre-washing were conducted with 20 mM sodium phosphate buffer, pH 7.2. Elution of the protein was accomplished using a linear gradient (8 ml min) from 20 mM sodium phosphate buffer, pH 7.2 (solution A) to 20 mM sodium phosphate buffer, pH 7.2, 500 mM NaCl (solution B). In the second stage, the partitioning material was phenyl Sepharose® 6-FF, and loading and pre- washing were conducted with 0.6 M NH 4 SO 4 in 20 mM sodium phosphate buffer, pH 7.2. Elution of the protein was accomplished using a linear gradient (5 ml/min) from 0.6 M NH 4 SO 4 , 20 mM sodium phosphate buffer, pH 7.2 (solution A) to 20 mM sodium phosphate buffer, pH 7.2, (solution B). The following purification results were obtained for this alternate procedure. The above-described cell-lysate contained 8400 units of homocysteinase at a specific activity (units/mg) of 5.2. Following completion of pre-column procedures, 6,300 units were recovered at a specific activity of 64 (about 75% yield). Upon completion of the DEAE Sepharose® -FF step (first column), 5,040 units were recovered at a specific activity of 172 (60% yield), while after the phenyl Sepharose® 6-FF step (second column), 4,200 units remained at a specific activity of 300 (50% yield). Analysis of resultant homocysteinase For HPLC analysis, an L-6200A Intelligent pump (Hitachi, Ltd., Tokyo, Japan) with a Supelco Progel™ TSK column (G3000 SW XL , 30 cm x 7.8 mm, Supelco, Bellefonte, PA) was used. Typically samples of a 20μl size (containing about 0.1 to 0.5 mg/ml protein were loaded, and eluted with a solution of 0.12 M NaCl, lOmM sodium phosphate buffer, pH 7.2, at a flow rate of about lml/min. The protein containing fractions were identified with a spectrophotometer (Hitachi U2000) at a wavelength of 280 nm. Bovine serum albumin (MW 66,000) and β-amylase from sweet potato (MW 200,000) (Sigma, St. Louis, MO) were used as MW standards. Resultant retetion times were: for BSA, 8.88 min; for β-amylase, 7.82 min; and, for the product homocysteinase, 8.28 minutes. Electrophoresis of resultant proteins was carried out on (non-reducing) 7.5% or 10%> polyacrylamide-precasted plates in 0.2 M Tris-glycine buffer, pH 8.3, with and without 0.1%> SDS. The molecular weight standards used were the Kaleidoscope Prestained Standards (Bio-Rad, Richmond, CA). The product homocysteinase resolved as a single band of about 43 kD in the presence of 0.1 % SDS, and as a single band at about 172 kD absent 0.1% SDS. DNA sequencing of suitable clones was then performed by ACGT Inc. (Northbrook, IL) using T7 DNA polymerase and the dideoxy nucleotide termination reaction. The primer walking method was used, and the sequences were analyzed on a DNA analyzer. Purification of homocysteinase as a 6x histidine-tagged protein using Ni-NTA methodology An alternate method to purify recombinant homocysteinase from E. coli involves use of nickel-NTA affinity chromatography. This technology takes advantage of the affinity of protein histidine imidazoles for nickel cations immobilized in the NTA resin matrix. In order to take full advantage of this technology (Qiagen Company, Germany), a sequence of 6 additional histidine residues is added (preferably by modification of an encoding DNA which is then expressed) to the protein on the N-terminal side of the natural Met 1 of the homocysteinase (see Example 7 above, and SEQ ID NOS 9 and 10). According to this purification procedure (for additional information, see also "The QIAexpressionist, A handbook for high level expression and purification of 6xHis-tagged proteins" March 1997, 3rd edition, available from Qiagen) 100 ml of densely grown culture of E. coli BL21 (DE3), clone pAC2-l were harvested by centrifugation, and the resultant pellet was resuspended in 4 ml lysis buffer (300mM NaCl, 10 mM imidazole, 50mM NaH 2 PO 4 , pH 8.0). The resultant cell suspension was then sonicated on ice for one minute. Cellular debris was removed using a benchtop centrifuge set at maximum speed for 20 minutes. The clear supernatant was then mixed with 1 ml of Ni-NTA slurry (Qiagen), and gently shaken on ice for 60 minutes to allow adsorption. The mixture was then transfered to a disposable polypropylene column and the flowthrough fraction was discarded. The Ni-NTA resin beads were then washed with 8 ml of wash buffer (300mM NaCl, 20 mM imidazole, 50mM NaH 2 PO 4 , pH 8.0), after which the recombinant protein was finally collected by elution using 2 ml of elution buffer (300mM NaCl, 250 mM imidazole, 50mM NaH 2 PO 4 , pH 8.0). The purified protein was then characterized as above. Enzyme activity results (toward homocysteine, cysteine, and methionine) for homocysteinase purified in this fashion are described in Example 8 below (see also Figure 4). Example 8 Representative Catalytic Properties of the Homocysteinases of the Invention Figures 2 through 4 evidence the significantly enhanced usefulness of the homocysteinases of the invention for single step assays. The results depicted in Figure 2 reflect use of a crude lysate of the host E. coli cells containing the pAC2-l clone, and show specific activity (u/mg) with respect to the 3 substrates (homocysteine, methionine, and cysteine) when each is present in a separate assay at 25mM. The results depicted in Figures 3 and 4 (again for E. coli containing the pAC2-l clone) reflect relative activity of purified enzyme preparations for homocysteine, methionine, and cysteine.. For the assays shown in Figure 3, the enzyme was purified using a DEAE Sepharose Fast Flow procedure as described in Example 7; for Figure 4, the purification procedure of Example 7 involving nickel-NTA agarose affinity reagent, was used (see above under the heading "The α-ketobutyrate assay/pyruvate assay" for methodology). It is well recognized that numerous procedures are available to purify recombinant homocysteinases from host E. coli cells. As exemplified, the novel homocysteinases of the invention exhibit activities toward homocysteine (using the assays of the invention, see for example, Example 1) that are typically 100-fold, and even up to about 1000-fold, higher than their activities exhibited toward cysteine or methionine. Example 9 Single Step/Single Enzyme Clinical Assays It will be immediately recognized that procedures for multi-step/multi-enzyme assays of homocysteine in biological fluids are readily adapted for use in simplified single step/single enzyme assays in order to take advantage of the substrate specificity of novel species of homocysteinase (the terms "single step assay" and "single enzyme assay" are used interchangeably herein to indicate that the biological sample need only be contacted with one enzyme in order to accurately measure homocysteine in the presence of cysteine, and/or methionine). The concentrations of homocysteinase appropriate for such procedures are broadly equivalent those described above in Example 1, or elsewhere in the Specification, or are readily determined. According to the single-enzyme diagnostic methods of the invention, the concentration of homocysteine in a biological sample such as, for example, whole blood, blood plasma, serum, urine, or tissue fluid may be determined with sufficient accuracy to provide valuable diagnostic information to physicians, absent interference from contaminating concentrations of cysteine and/or methionine. Such diagnostic assays are rapid, accurate, involve a minimum of reagents, and are therefore applicable to large scale screenings operations in clinical laboratories, and as public health measures. In a representative example, the concentration of cysteine in normal blood plasma may be about 30-120 μM (micromolar), and that of homocysteine only about 5-15 μM. As is recognized in the art, a subject's risk for cardiovascular disease may increase significantly as homocysteine concentrations in the blood plasma rise above the 15 μM level. High risk cardiovascular patients have been identified that have homocysteine levels up to several hundred micromolar. Similarly, patients affected by disorders of cysteine metabolism may have cysteine levels up to 500 or even 1000 μM. The diagnostic kits of the present invention are useful to determine homocysteine concentrations in biological fluids in the range of 1-500 μmolar, for example, wherein said fluids also contain from 0 μmolar (typically from 10 μmolar) to about 1000 μmolar of cysteine, or higher. According to the preferred practice of the invention, homocysteineases are provided wherein the relative activities of said enzymes toward homocysteine versus cysteine are such that, by detecting product hydrogen sulfide, the apparent concentration of homocysteine measured in a patient sample (which reflects also a contribution from cysteine) is less that about 150%, preferably less than about 110%, and most preferably less than about 102%, of the actual value thereof. In a further prefered example of the invention, for example using the novel Trichomonas homocysteinase expressed from pAC2-l, the apparent concentration of homocysteine measured in a single enzyme assay reflects no more than about a 1% false contribution from cysteine. According to the practice of the present invention, such single enzyme assay methodology (using for example, SEQ ID NO: 10 as homocysteinase) is applicable over a wide range of natural concentrations (and ratios of concentrations) of both homocysteine and cysteine (and also methionine where appropriate), and is thus accurately able to reflect the body fluid chemistry of both healthy subjects and those at risk for disease states, most particulary cardiovascular disease. Accordingly, in its most preferred aspect, the present invention provides a diagnostic kit for use in the single enzyme assay of the homocysteine concentration in a biological fluid of a subject, said kit comprising: (a) a homocysteinase enzyme; and (b) at least one reagent capable of being used to determine the amount of product hydrogen sulfide formed by reaction of homocysteinase on homocysteine; wherein said homocysteinase is sufficiently non-reactive toward cysteine that any hydrogen sulfide produced by reaction of said homocysteinase on cysteine does not substantially interfere with the use of said kit to assess risk for cardiovascular disease. Since the diagnostic kits of the present invention are useful over a wide spectrum of homocysteine and cysteine concentrations, and relative concentrations, it is to be expected that there are a variety of examples of the successful practice of the invention. Representative examples are as follows: (1) most generally, the fraction of measured hydrogen sulfide that is attributed to cysteine instead of homocysteine should be sufficiently small to prevent useful interpretation of the results by a qualified medicial practitioner. In general, useful diagnostic information can be obtained when the amount of produced hydrogen sulfide attributable to homocysteine is about one half of the total. Given the generally higher concentration of cysteine in patient samples, compared to homocysteine, the specificity of such enzymes is still evident. Preferably, however the fraction of total hydrogen sulfide detected in an assay that is attributable to homocysteine is at least 90%, preferably 95%, more preferably 98%, and most preferably 99% or greater. The 99% level is readily obtained according to the practice of the invention, for example using the homocysteinase represented by SEQ ID NO: 10. (2) in an assay of a patient sample which evidences risk for cardiovascular disease, wherein the concentration of homocysteine therein is about 20 μmolar or higher, concentrations of 300 μmolar, or even greater, of cysteine contribute less than 1% to the total amount of hydrogen sulfide detected. Obviously such enzymes remain very useful with respect to biological samples having even greater cysteine concentrations. (3) in an assay of a patient sample which evidences risk for cardiovascular disease, and wherein the ratio of the cysteine to homocysteine concentration is, for example, about 15:1, cysteine contributes less than about 1% to the total amount of hydrogen sulfide detected. The following further evidences the differences between the properties of known homocysteinases and the novel enzymes of the present invention. Homocysteinase isolated from Trichomonas vaginalis (which may reflect expression from more than one gene) was reported (see Lockwood et al., 1991 above) to have a ¥L ^ for homocysteine of about 0.5 mM, whereas K,,, values determined for the novel enzyme expressed from the pAC2-l clone are (all at 37°C, pH 8.0): 4.8 mM, 3.45 mM, and 3.1 mM for homocysteine, cysteine, and methionine, respectively. The K calculations were determined from assays carried out in one ml volumes of lOOmM phosphate buffer, pH 8.0 containing lOμM pyridoxal phosphate at different concentrations (lOμM to 1.6 mM) of, separately, DL-homocysteine, L-methionine, or L- cysteine, for 10 minutes at 37° C, pH 8.0 using 50 μl of enzyme (300 units/ml). The reaction was stopped by adding 0.1 ml of 50%> TCA. The resultant suspension was then centrifuged with an Eppendorf centrifuge at 13k rpm for 2 minutes. 0.5 ml samples of supernatant were then added to 0.5 ml volumes of 0.05% 3-methyl -2-benzothiazolinone hydrazone in 0.8 ml of 1 M sodium acetate, pH 5.2, and incubated at 50°C for 30 minutes. The amount of reaction product was then determined by spectrophotometry at OD 320 . The K cat (turnover number) values reported in Figure 5, were determined from the calculation of V max values using standard kinetic expressions and plots. It is apparent that the kinetics of the enzyme expressed from the pAC2-l clone are quite different from those of the wild type enzyme. An additional aspect of the present invention involves recognition that although certain changes have been described in the amino acid sequence of the T. vaginalis enzyme in order to provide for enhanced kinetic properties, those skilled in the art will recognize other potential mechanisms to accomplish this. An example thereof is covalent modification of the enzyme, whether or not at the active site. Such other methods of modifying a homocysteinase are within the practice of the invention if they lead to production of an enzyme having kinetic properties such as those first disclosed herein. Example 10 Assay Technology Based on γ-Glutamylcysteine Synthetase The enyzme γ-glutamylcysteine synthetase (EC 6.3.2.2, also referred to as glutamate- cysteine ligase) is another enyzme that may be used to enhance accuracy of assays for homocysteine in biological samples. The enzyme catalyzes the first (and rate-limiting) step in the synthesis of glutathione (γ-L glutamyl-L-cysteinylglycine), a tripeptide having numerous important functions in living cells. Given the important roles that glutathione plays in metabolic pathways, it is believed highly unlikely that γ-glutamylcysteine synthetase would accept homocysteine as a replacement for substrate cysteine. Accordingly, use of this enzyme contributes to highly specific determinations of homocysteine in biological samples (via a homocysteinase), by removing any interfering cysteine. In a preferred example, the assay is conducted using the direct enzymatic substraction method (see Example 3 above). part A - expression of encoding DNA for E. coli γ-glutamylcysteine synthetase A principal achievement of the invention is highly sensitive enzyme-based assay methodology that provides for accurate determinations of homocysteine concentrations in biological samples. As aforementioned, in many kinds of biological samples, naturally occurring concentrations of cysteine interfere with homocysteine determinations. As described below, a particularly preferred enzyme-based assay for homocysteine involves use of two enzymes, (1) a homocysteinase having high specificity for homocysteine and very low specificity for cysteine (such as the Trichomonas vaginalis enzyme expressed from pAC2-l, SEQ ID NO: 10), and (2) γ-glutamylcysteine synthetase such as from clone GSH-I His (see below) use of which further eliminates interference from cysteine. It will immediately be appreciated that once the value of this multi enzyme approach is recognized, the γ-glutamylcysteine synthetase enzyme utilized can be provided from a wide variety of sources, such as hog liver, wheat germ, bovine lens, rat liver, or bacteria including Proteus mirabilis or E coli. B. The γ-glutamylcysteine synthetase enzyme may be purified from such sources by recognized methods, or expressed recombinantly, for example, from host prokaryotic cells such as E. coli, or eukaryotic cells generally recognized as useful in this regard. Suitable E. coli strains include E.coli BL21 (DE3), HB 101, and JM 109. Given the broad utility of this approach, it will also be immediately apparent that a wide variety of particular assay conditions, enzyme concentrations, detection chromophores, and the like, are available to the clinician based upon the teachings herein. For example, the nucleotide sequence for the E. coli B γ-glutamylcysteine synthetase (gsh I)gene has been reported by K. Watanabe et al.(see Nucleic Acids Research, 14(11), pp. 4393-4400,1986, and references cited therein). An open reading frame of 518 amino acids was deduced, and the unusual codon TTG was determined as the translational start (Met 1 ). The enzyme and its function have also been characterized on a molecular level, and expression and purification schemes therefor described (Y. Inoue et al., Applied Microbiology and Biotechnology, 38, pp. 473-477, 1993). In the practice of the present invention, and in order to clone the E. coli γ- glutamylcysteine synthetase gene, genomic DNA for E.coli HB 101 was isolated and used as a template for PCR. Primers were selected to permit amplification of the encoding sequence in two overlapping fragments. The primers selected were GSH-1 and GSH-2, corresponding to the smaller resultant gene fragment, and GSH-3 and GSH-4 that were similarly selected for the larger of the fragments. The unusual start codon TTG (see GSH-1, SEQ ID NO: 15 below) was also replaced by an ATG. The nucleotide sequences for primers GSH-1 through GSH-4 were as follows (the nucleotide numbers are assigned to correspond with those used by K. Watanabe et al. at p. 4396, based on an isolated 2098 bp HincII-PstI restriction fragment reported therein that contained the open reading frame): GSH-1 (sense strand), 5'-ATCGCTAG [CAT ATG 1 ] ATC CCG GAC GTA TCA CAG-3', which includes encoding nucleotides No. 338-358 (K. Watanabe et al.), wherein [CAT ATG 1 ] encodes the Met 1 and provides an Ndel site (SEQ ID NO: 15); GSH-2 (antisense strand), 5'- ATCACCATCC ACTGGTGTAA TG -3', corresponding to nucleotides No. 541-562 (SEQ ID NO:16); GSH-3 (sense strand), 5'- CTA CCG ATT TTG CGG AAG -3', corresponding to encoding nucleotides No. 510-527 (SEQ ID NO: 17); and GSH-4 (antisense strand), 5'- CATGACGGAT CCTCAGGCGT GTTTTTCCAG -3', corresponding to nucleotides No. 1877-1894 (1894 is the stop codon), with 12 additional nucleotides attached (SEQ ID NO: 18). To add an oligo (6-mer) histidine tag at the N-terminus of the resulting protein (compare also Example 7 above), a series of consecutive histidine codons (CAT/CAC) were included at the 5' end of the encoding sequence for primer GSH-1H as follows (GSH-1H was then used in replacement of GSH-1 with GSH-2, and as before, the unusual start codon was replaced): GSH-1H 5'- ATCGCTAG CAT ATG CAT CAT CAT CAT CAT CAC ATG (original Met 1 ) ATC CCG GAC GTA TC- 3' (SEQ ID NO: 19) In order to assemble appropriate expression plasmids, the small fragment was digested with Ndel and EcoRI, and the large fragment was digested with EcoRI and BamHI. Both fragments were then used in a 3-way ligation with Ndel/BamHI predigested pT7-7 plasmid which includes Nde I and BamHI sites, to yield the plasmids pGSH-I and pGSH-I-HIS (which differ only in the encoding sequence for the N' end histidine tag). Correct cloning was comfirmed by RE analysis. The plasmids were then transformed into E. coli BL21 (DE3) cells and protein minipreps were prepared to confirm the expression of the recombinant protein. Sufficient γ-glutamylcysteine synthetase was purified from E. coli BL21 (DE3) by nickel-NTA affinity chromatography, using the procedure described in Example 7 for purification of pAC2-l T. vaginalis homocysteinase. part B - assay methodology using γ-glutamylcysteine synthetase and Zn(OH) ? trapping According to the practice of the invention, following use of γ-glutamylcysteine synthetase to deplete cysteine from a biological sample, determination of the homocysteine concentration therein may then be accomplished by any of several procedures, preferably those involving use of a homocysteinase, with detection of one or more resulting reaction products. A further example of an effective assay procedure is presented below, and involves colorimetric detection of methylene blue which is itself produced, in situ, in a reaction that efficiently consumes the hydrogen sulfide produced in the homocysteinase reaction. This procedure takes advantage of the efficient detection of the dye molecule methylene blue (methyl thionine chloride) in the visible range, such as at 740 nm. Specifically, methylene blue is produced by oxidation of N,N- dimethyl-p-phenylenediamine ("NDPD") with ferric chloride in the presence of hydrogen sulfide. In a variation on this reaction scheme, N,N- diethyl-p-phenylenediamine is substituted for N,N- dimethyl-p-phenylenediamine, with the result that chromophoric detection is enhanced, by up to about 5-fold. As will be recognized in the art, other sources of ferric iron are suitable. While in view of the present disclosure, those skilled in the art will recognize that numerous methods are available to detect sample homocysteine, this "methylene blue procedure" has been found to be particularly useful in a clinicial setting where rapid, accurate, and economical screening of a large number of samples is required. Procedures demonstrating the method were performed as follows. A sample of human serum was diluted (1:1) with phosphate buffered saline, and then deproteinized by ultrafilitration using an Amicon 30k Centriprep, with centrifugation at 4,000 rpm for 60 minutes at 4°C. The deproteinized and diluted serum was then collected and divided into 1ml aliquots in Eppendorf tubes for use in the assays. In the below-described procedures, free homocysteine was determined. That fraction of total serum homocysteine that is present as a disulfide dimer (homocystine or cysteine/homocysteine heterodimer), or is found disulfide- bonded to protein, may also be determined by including a reducing step (for example, addition of 5mM β-mercaptoethanol) at an appropriate point in the protocol. However, in view of the capacity of such agents to inactivate proteins (including γ-glutamylcysteine synthetase and homocysteinases), it is preferable to select as reducing agent (see below) a specialized compound capable of reducing the disulfide bond of homocystine, for example, without inactivating γ-glutamylcysteine synthetase or homocysteinase. In these protocols, validations were first performed to confirm assay reproducibility. Accordingly, in a first test, separate tubes of serum (1:1 diluted and deproteinized) were spiked with different amounts of homocysteine (to attain final concentrations of 1 μM, 5 μM, 10 μM, 15 μM, and 20 μM). A 50 μl aliquot of homocysteinase (either the Trichomonas vaginalis enzyme expressed from pAC2-l, SEQ ID NO: 10, or wild type Pseudomonas pudita enzyme), was then added to each tube, and each tube was incubated at 37°C for 20 minutes with the lid tightly closed (to retain H 2 S). The reaction was stopped by addition of 100 μl of 1M NaOH, with mixing, followed immediately by a further addition of 0.3 ml of trapping solution (solution A), rapid mixing, and then centrifugation at 13,000 rpm for 1 minute (solution A was made by dissolving 4g NaOH, 2.94 g trisodium citrate, and lOg zinc acetate in 1 liter of water). The supernatant in each tube was then discarded, and the pellets were dissolved in 0.15 ml of solution B (2.7% w/v FeCl 3 in 6N HCl) followed by addition of 0.75 ml solution C (0.2% w/v NDPD in IN HCl). The contents were mixed thoroughly, followed by incubation at room temperature for 30 minutes, again with the lids closed. The concentration of product methylene blue, which is reflective of H 2 S produced in the homocysteinase reaction, was read at 740 nm in a spectrophotometer. Linearity of the result was good for both the recombinant Pseudomonas putida and Trichomonas vaginalis enzymes. A similar set of assays was performed in which serum was spiked to achieve a cysteine concentration of between 1 and 20 micromolar. In comparing the outcome for a recombinantly produced Trichomonas vaginalis enzyme (SEQ ID NO: 10, from clone pAC2- 1) and a recombinantly produced wild type Pseudomonas pudita enzyme, the experiment demonstrated that in spite of the high activities both enzymes show toward homocysteine, the pAC2-l variant has much less activity toward cysteine and is therefore preferred. The following procedure demonstrates the use of the preferred combination of Trichomonas vaginalis enzyme from clone pAC2-l, and γ-glutamylcysteine synthetase from E. coli B. In an example of the assay, in a sealable Eppendorf tube an incubation mixture (1 ml) was prepared to contain 10 mM in L-glutamate, 2mM ATP, 25 mM MgCl 2 , buffered with Tris-Cl, 100 mM at pH. 8.0, and included 5 μg recombinant γ-glutamylcysteine synthetase from clone GSH-I His, and a sufficient amount of the biological sample to deliver, for example, about 50 μmoles of homocysteine. Note also that it is prefered to add the reagents to the tube in the following order, from the following stock solutions: (1) 700 μl of water; (2) 100 μl of lM Tris pH 8.0; (3) homocysteine/cysteine-containing sample (about 50 μl), or other amount as appropriate; (4) 100 μl of 100 mM L-glutamate; (5) 20 μl of lOO mM ATP; (6) lOμl of 2.5M MgCl 2 ; and (7) 10 μl of 0.5 mg/ml recombinant γ-glutamylcysteine synthetase from clone GSH-I His. The contents are incubated at 25 °C for 30 minutes, after which 10 μl of homocysteinase enzyme (4.5 mg/ml of Trichomonas vaginalis enzyme, SEQ ID NO: 10, from clone pAC2-l) is added, the vial sealed, followed by a further incubation at 37°C for 20 minutes. Production of H 2 S is stopped by addition of 100 μl of 1M NaOH, with mixing, followed by 300 μl of solution A, with mixing, after which the tube is spun at 13,000 rpm for 45 seconds as above. Following careful removal of all of the supernatant, the pellet is resuspended in 150 μl solution B, followed by addition of 750 μl of solution C and a further 20 minute incubation period at room temperature. Methlyene blue production is monitored within the visible range, most preferably from about 650 to 750 nm. It is noted that although literature reports indicate that γ-glutamylcysteine synthetase may function optimally around 45 degrees C, in the practice of the present invention it has been found to work quite well at 25 degrees C. As previously mentioned (see Example 4), a significant fraction of homocysteine that is present in biological samples may be bonded to other biological molecules, and such "stored" forms of homocysteine are believed to contribute to pathological processes. Thus, in determining homocysteine concentrations, it is desireable to include these other forms. Major contributors to the total homocysteine concentration include (1) homocysteine dimers, (2) homocysteine/cysteine heterodimers, and (3) conjugates of homocysteine and protein wherein homocystiene is attached by way of a disulfide bond to protein cysteine. According to the practice of the present invention, the total of such additional homocysteine concentrations may be accurately determined by appropriate use of a disulfide bond reducing agent. Not all disulfide reducing agents are particularly suitable for this function since they also present the capacity to reduce disulfide bonds in homocysteinases or γ-glutamylcysteine synthetases, participate in thiol exchange reactions, or react directly with enzyme thiol groups, thereby altering the activity of the enzymes. One such less preferred agent is β-mercaptoethanol. A disulfide reducing agent that is potentially useful in the practice of the invention is tris-(2-carboxyethyl)phosphine ("TCEP"), available from Molecular Probes Inc., Eugene ,OR which may be prepared fresh as a lOOrnM stock solution (for use at 10 μl/lml tube sample) and which may be preferably dispensed into the vial just prior to addition of the homocysteine-containing biological sample itself. Additional suitable reducing agents will be recognized by those familiar with the art. In fact, DTT has been found to be the most preferred reducing agent, and causes minimal effects on assay enzymes. This aspect of the invention is represented by a method for determining the amount of homocysteine that is present in a biological sample containing homocysteine and cysteine, which method comprises measuring the concentration of a product that is produced from homocysteine in a reaction catalyzed by a homocysteinase, which product may also be produced by reaction of said homocysteinase upon cysteine in said sample, and wherein said sample contains an otherwise interfering concentration of cysteine, said method comprising the steps of: (a) incubating said sample with γ-glutamylcysteine synthetase in the presence of substrates that permit conversion of cysteine to γ-glutamylcysteine; (b) incubating the mixture resultant from step (a) with homocysteinase; and (c) measuring the concentration of a product that is capable of being produced from both homocysteine and cysteine in a reaction catalyzed by a homocysteinase, wherein said step of making disulfide bonded homocysteine molecules available for reaction with homocysteinase is performed by treating said sample with tris-(2-carboxyethyl) phosphine at a point prior to step (a) thereof. alternate procedure involving detection of product ammonia As aforementioned, the ammonia product of the homocysteinase reaction may also be used to quantify homocysteine in biological samples. However, in addition to interference caused by concentrations of cysteine (which is addressed appropriately according to the above-described procedures), interference by methionine is possible if ammonia is being detected. In this case, correction may be accomplished generally by recognizing that methionine can also be detected by release of methanethiol during the homocysteinase reaction itself. Thus, its concentration may be measured and then substracted. The resultant methanethiol can be determined by production of methylene blue (including variants thereof) by measuring OD at about 510 nm, using as reactant N,N- dimethyl [or diethyl]-p- phenylenediamine. See Tanaka et al., Journal of Applied Biochemistry, 2, pp. 439-444, 1980. Ammonia, as is recognized in the art, may also be measured in assays that involve glutamate dehydrogenase, and such procedures may be adapted to the practice of the present invention. Example 11 An Enhanced Detection Procedure for Hydrogen Sulfide An enhanced methodology for the detection of hydrogen sulfide has been developed which is related to that presented in Example 10 above. Diagnostic kits according to this aspect of the invention are prepared as follows. The following reagents/stock solutions are prepared: (a) buffer solution - a stock solution of 50 mM borate buffer, pH 7.5, prepared from 309 mg H 3 BO 3 , f.w.61.83, added to 90 ml dd water, the pH is adjusted to 7.5 with NaOH, after which the volume is brought up to a total of 100 ml with dd water. (b) reducing agent - 15.4 mg DL-dithiothreitol ("DTT"),Sigma, St. Louis, MO, is dissolved in 1 ml of borate buffer(a) resulting in a lOOmM solution. (c) chromogenic reagent I - 33.25 mg of potassium ferricyanate, K 3 Fe(CN) 5 , Sigma, is dissolved in a solution of 10 ml of IN HCl, 1% (w/v) Triton X-100, Sigma, creating a 10 mM stock solution of ferric iron. This solution should be mixed well before use. (d) chromogenic reagent II - 52.5 mg of N,N-dipropyl-phenylene diamine ("DPPDA"), Wako Pure Chemical Industries, Ltd., Japan, is dissolved in dd water up to a final concentration of lOOmM. (e) recombinant homocysteinase - the enzyme represented by SEQ ID NO: 10 (derived from E. coli clone pAC2-l, ATCC 98549) may be purchased as product HYase™ from AntiCancer, Inc, San Diego, CA and has a molecular weight of 172,000 and a specific activity of 40 units/mg. The enzyme should be diluted to 4 units/ml with borate buffer before use, stored during assays at 0-4°C, and prepared fresh for each use. (f) DL-homocysteine - 2.7 mg of DL-homocysteine, Sigma, is added to 2 ml borate buffer (4 mM final concentration), and prepared fresh daily as a calibration standard, with storage during use at 0-4°C (Note: in general homocysteinases use only the natural L-optical isomer of homocysteine, but the D/L mixture provides an economical source of of the L-isomer, requiring of course use of a correction factor of 50% to provide the effective L- concentration). A representative assay protocol is as follows. O.l ml of blood plasma is added to 0.9 ml of borate buffer. 10 μl of the 100 mM DTT solution is then added, mixed well, followed by an incubation at 37°C for 30 minutes. 30 μl of HYase™ recombinant homocysteinase (prediluted to 4 units/ml in borate buffer) is then added with mixing, followed by a further incubation period at 37°C for 1 minute. 200 μl of chromogenic reagent I and 10 μl of chromogenic reagent II are then added simultaneously with mixing and the sample is incubated at 37°C for 20 minutes. The product methylene blue-type chromophore is then detected by measuring the OD at 677 nm in a spectrophotometer. The plasma concentration of L-homocysteine is then determined from a constructed calibration curve, which may use homocysteine and/or the disulfide bonded dimer thereof, homocystine, as standards. In providing this representative example, it is of course apparent that considerable variation in the selection of assay reagents or reagent concentrations is possible, and such modifications will be recognized immediately as within the skill of art. Example 12 Additional Methodology The following provides additional examples of procedures, and variations thereof, which the practitioner may find useful in the practice of the invention. Growth of recombinant bacteria Each fermentation run was started with one vial of recombinant E. coli containing recombinant homocysteinase (pAC2-l, see SEQ ID NOS: 9 and 10) from the master cell bank (MCB). Ten microliters of bacteria from the MCB were seeded into 5 ml LB medium and grown overnight at 37°C with shaking at 400 rpm . The culture was transferred to 800 ml LB medium in 6 L flasks and grown overnight at 37°C at 400 rpm at which time the OD 600 was approximately 10. The bacteria were collected by centrifugation at 4000 rpm for 20 min. The bacteria were then transferred into 800-ml LB-medium cultures in 6 L flasks and grown at 37°C at 400 rpm for 16 h. The bacteria were harvested by centrifugation at 4000 rpm at 4°C for 20 minutes. precolumn treatment The bacterial pellets, derived from 90x 800-ml cultures of recombinant E. coli, were combined and disrupted with a cavitation type homogenizer (Micro fluidics Corp., Newton, MA; model HC 8000). The homogenate was suspended in 2 vol. of 20 mM potassium phosphate pH 7.2 containing 10 μM pyridoxal phosphate, 0.01% β-mercaptoethanol and 20% ethanol. Heat treatment of the homogenate was carried out at 50°C for 1 minute. 1% (w/v) polyethyleneimine (PEI) was added to the suspension solution and mixed for 20 minutes. The suspension was centrifuged with an automatic refrigerated centrifuge (Sorvall, Superspeed RC 2-B) at 4°C at 12,000 rpm for 40 minutes to remove nucleic acids. The supernatant was then collected and mixed with polyethylene glycol 8000 (PEG 8000) to a final concentration of 10-12% (w/v) and stirred at 4°C for 60 minutes. The precipitate (containing contaminating proteins) was removed by centrifugation at 12,000 rpm for 40 minutes. 3.0 M sodium chloride was then added to the supernatant to give a final concentration of 0.12 M which precipitated recombinant homocysteinase. The pellet was collected by centrifugation at 12,000 rpm for 40 minutes and dissolved in 20 mM potassium phosphate buffer pH 7.6, containing 10 μM pyridoxal phosphate and 0.01% β- mercaptoethanol. DEAE-Sepharose FF Chromatography The enzyme sample from the precolumn treatment was applied to a column of DEAE- Sepharose FF (30 x 5 cm) (Pharmacia) that was previously equilibrated with 20 mM potassium phosphate pH 7.2. After loading the column, it was prewashed with 50 mM sodium chloride, 20 mM potassium phosphate pH 7.2 for approximately 3 volumes, until the OD 280 dropped below 0.1. Recombinant homocysteinase was then eluted with a linear sodium chloride concentration gradient of 0.05-0.5 M in the same buffer over 90 minutes. Phenyl-Sepharose 6 FF Chromatography Solid ammonium sulfate (79.3 mg/ml) was added to the active fractions of the DEAE- Sepharose FF chromatography to give a final concentration of 0.6 M. Before loading the supernatant on the Phenyl-Sepharose 6 FF column (20 x 2.6 cm), the column was equilibrated with Buffer A - consisting of 0.6 M ammonium sulfate in 20 mM potassium phosphate pH 7.6. Bound protein was eluted by linearly decreasing the ammonium sulfate gradient with Buffer B (which contained 20 mM potassium phosphate pH 7.6, 10 μM pyridoxal phosphate, 0.02%) β-mercaptoethanol, and 5.0%) ethylene glycol). The active fractions were concentrated by DEAE-Sepharose FF (20 x 1.6 cm) which removed ammonium sulfate and polyethylene glycol. Purified enzyme was stored at -80°C. Activity Assays for Recombinant Homocysteinase An activity assay for the keto product of the α, γ or α,β-elimination (alpha keto butryate for homocysteine) was carried out in in a 1.5 ml Eppindorf tube using 1-ml. of 50 mM phosphate buffer pH 8.0 (also containing 10 μM pyridoxal phosphate and 20 mM homocysteine) for 10 minutes at 37°C, with varying amounts of enzyme. The reaction was stopped by adding 0.5 ml of 4.5% TCA. 0.05 ml of the resultant solution was then combined with 0.45 ml of 0.05% 3-methyl-2-benzothiazothiazolinone hydrazone (MBTH) and added to a 1.0 ml sample of 1.0 M sodium acetate, pH 5.2, followed by incubation at 50°C for 30 minutes. The amount of reaction product was determined by spectrophotometry at OD 335 . The hydrazone product has an extinction coefficient of 7.6 x 10 3 1/mol x cm.. The amount of protein was determined with the Lowry Reagent Kit (Sigma) with bovine serum albumin as a standard. The specific activity was calculated as units/ mg protein, with one unit of enzyme defined as the amount that catalyzed the formation of 1 μmol of α-ketobutyrate/minute. Activity assays for the γ and β elimination reactions of homocysteine and cysteine, respectively, were carried out at 37°C for 30 seconds. The resulting H 2 S produced was measured using methylene blue formation at OD 671 . SDS-PAGE and PAGE Electrophoresis was performed according to the methods described in the NOVEX Kit (NOVEX Experimental Technology, San Diego). A 12% Tris-Gel was used for SDS-PAGE. After staining with Coomassie brilliant blue, the intensity of protein bands was estimated with molecular standards (NOVEX mark 12 Wide Range Protein Standard), including myosin, 200 kD; β-galactosidase, 116.3 kD; phosphorylase b, 97.4 kD; bovine serum albumin, 66.3 kD; carbonic anhydrase, 31.0 kD; trypsin inhibitor, 21.5 kD; lysozyme, 14.4 kD; aprotinin, 6.0 kD. Native gels were stained for recombinant homocysteinase activity by immersion in a reaction mixture containing 3.3 mM-DL-homocysteine, 0.33 mM lead acetate, 28.4 mM β-mercaptoethanol, and 100 mM Tris-HCl buffer, pH 7.5. Bands containing homocysteinase activity will convert homocysteine to α-ketobutyrate, ammonia and H 2 S. Hydrogen sulfide reacts with lead acetate to form a dark brown precipitate (Pb 2 S). The gels were then stained for protein with Coomassie Blue. In further support of the present disclosure, on September 26, 1997, a deposit of biological material of confirmed viability was made with the American Type Culture Collection, Rockville, MD, USA under the Budapest Treaty. The material is identified as E. coli BL21 (DE3), clone pAC2-l, and has been assigned ATCC number 98549. Upon the granting of a patent herein, all restrictions on the availability of this material to the public will be irrevocably removed. Additional Statements of the Invention Following disclosure of the above detailed description of the invention, the practitioner of the art will appreciate the following statements as further examples of the invention. 1. A method for determining the amount of homocysteine that is present in a biological sample containing homocysteine and cysteine, that comprises contacting said sample with an enzyme preparation capable of producing hydrogen sulfide from homocysteine and cysteine, and determining the amount of homocysteine in said sample by measuring the amount of hydrogen sulfide produced from homocysteine. 2. A method according to statement 1 wherein said biological sample comprises homocysteine molecules that are covalently bonded to other molecules, and said method includes the further step of dissociating said homocysteine molecules therefrom prior to measuring of hydrogen sulfide. 3. A method according to statement 2 wherein said biological sample comprises homocysteine molecules that are bonded to amino acid R groups in protein. 4. A method according to statement 2 wherein said biological sample comprises homocysteine molecules that are disulfide-bonded to protein cysteine residues, free cysteine molecules, or to other homocysteine molecules. 5. A method according to statement 1 wherein hydrogen sulfide is produced from both cysteine and homocysteine, and hydrogen sulfide produced from homocysteine is distinguished from hydrogen sulfide produced from cysteine by including in said method the additional initial steps of: (a) contacting said biological sample with a reagent that protects homocysteine from said enzyme preparation; (b) permitting said enzyme preparation to produce hydrogen sulfide from cysteine present in said sample; (c) removing said hydrogen sulfide; and (d) deprotecting said homocysteine, and adding a further amount of enzyme preparation to produce hydrogen sulfide therefrom. 6. A method according to statement 5 wherein homocysteine is protected by conversion to lactone form by acid treatment, followed by deprotection using bromide or mild base. 7. A method according to statement 1 that includes the additional initial steps of: (a) contacting homocysteine that is present in said biological sample with a further enzyme preparation capable of converting it to a compound that can be isolated; (b) isolating said compound; (c) contacting said compound with an enzyme preparation or other reagent under conditions where said compound is converted back to homocysteine; and (d) contacting the homocysteine so produced with said enzyme preparation capable of producing hydrogen sulfide therefrom. 8. A method according to statement 7 that includes the steps of: (a) contacting homocysteine that is present in said biological sample with a further enzyme preparation, itself comprising S-adenosylhomocysteine hydrolase, under conditions where homocysteine is converted to S-adenosylhomocysteine; (b) isolating said S-adenosylhomocysteine; (c) contacting said S-adenosylhomocysteine with a further amount of S- adenosylhomocysteine hydrolase under conditions where S-adenosylhomocysteine is converted back to homocysteine; and (d) contacting the homocysteine so produced with said enzyme preparation capable of producing hydrogen sulfide therefrom. 9. A method according to statement 1 wherein said biological sample is pretreated with an agent that renders cysteine non-reactive to said enzyme preparation. 10. A method according to statement 9 wherein said agent is cysteine tRNA synthetase. 11. A method according to statement 1 wherein said enzyme preparation includes a homocysteinase of prokaryotic origin. 12. A method according to statement 11 wherein said homocysteinease is from Pseudomonas putida. 13. A method according to statement 1 wherein said enzyme preparation includes a homocysteinase of eukaryotic origin. 14. A method according to statement 13 wherein said homocysteinease is from Trichomonas vaginalis. 15. A method according to statement 1 wherein said biological fluid is selected from the group consisting of urine, tissue fluid, blood, blood serum, and blood plasma. 16. A method for determining the amount of homocysteine that is present in a biological sample containing homocysteine and cysteine, comprising the steps of (A) dividing said sample into 2 parts, part 1 and part 2; (B): contacting part 1 with an first enzyme preparation that is capable of converting homocysteine to a substance that is not a substrate for homocysteinase, wherein said first enzyme preparation does not recognize cysteine as a substrate; (C) independently contacting part 1 and part 2 with a second enzyme preparation that comprises a homocysteinase capable of producing hydrogen sulfide from both homocysteine and cysteine; (D) measuring the amount of hydrogen sulfide produced in part 1 and part 2; and (D) calculating the amount of homocysteine in said biological sample by subtracting the hydrogen sulfide measurement of part 1 from that of part 2, and doubling the result. 17. A method according to statement 16 wherein said first enzyme preparation comprises S-adenosylhomocysteine hydrolyase, and homocysteine is converted to S- adenosylhomocysteine. 18. In a method for determining the amount of homocysteine present in a biological sample wherein said method comprises the step of contacting said sample with an enzyme preparation capable of catalyzing production from homocysteine of a detectable quantity of hydrogen sulfide, and wherein said sample further comprises an amount of cysteine from which a detectable quantity of hydrogen sulfide can be produced that interferes with said determination, the improvement comprising including one or more additional steps that permit the separate detection of hydrogen sulfide derived from homocysteine. 19. A diagnostic kit for use in determining the amount of homocysteine in a biological sample comprising: (a) Trichomonas vaginalis, Pseudomonas ovalis or Pseudomonas putida homocysteinase; and (b) at least one reagent capable of being used to determine the amount of hydrogen sulfide formed in the homocysteinase reaction. 20. A diagnostic kit according to statement 19 that includes a further reagent useful to dissociate homocysteine from other molecules to which it may be covalently bonded. 21. A diagnostic kit according to statement 19 that includes a further reagent useful to convert homocysteine to a form non reactive with said homocysteinase. 22. A diagnostic kit according to statement 19 that includes a further reagent useful to isolate homocysteine from said biological sample prior to reacting said homocysteine with homocysteinase. 23. A diagnostic kit according to statement 19 that includes a further reagent useful to convert cysteine to a form non-reactive with said homocysteinase. 24. A method for determining the amount of homocysteine that is present in a biological sample containing homocysteine and cysteine, that comprises: (a) contacting said sample with an enzyme preparation that is capable of producing hydrogen sulfide and α-ketobutyrate from homocysteine, and further capable of producing hydrogen sulfide and pyruvate from cysteine, and (b) contacting said sample, or a duplicate or a part thereof, with an enzyme preparation by whose action the amount of pyruvate in said sample, or in said duplicate or part thereof, can be determined, and then (c) calculating the amount of homocysteine in said sample by comparing the amount of pyruvate so determined with the amount of hydrogen sulfide produced therein. 25. A method according to statement 24 wherein said enzyme preparation by which pyruvate in said sample can be determined comprises lactate dehydrogenase. 26. A method according to statement 25 wherein said biological sample comprises homocysteine molecules that are bonded to amino acid R groups in protein, and said method further comprises the step of first making said bonded homocysteine molecules available for detection therein. 27. A method according to statement 25 wherein said biological sample comprises homocysteine molecules that are disulfide-bonded to protein cysteine residues, free cysteine molecules, or to other homocysteine molecules, and said method further comprises the step of first making said bonded homocysteine molecules available for detection therein. 28. A method for determining the amount of homocysteine that is present in a biological sample containing homocysteine and cysteine, comprising the steps of (a) dividing said sample into 2 parts, part 1 and part 2; (b) contacting part 1 with an first enzyme preparation that is capable of converting homocysteine to a substance that is not a substrate for homocysteinase, and wherein said first enzyme preparation does not recognize cysteine as a substrate, said preparation further comprising an enzyme selected from the group consisting of S-adenosylhomocysteine hydrolyase or cystathionine β-synthase; (c) independently contacting part 1 and part 2 with a second enzyme preparation that comprises a homocysteinase capable of producing hydrogen sulfide from both homocysteine and cysteine; (d) measuring the amount of hydrogen sulfide produced in part 1 and part 2; and (e) calculating the amount of homocysteine in said biological sample by subtracting the hydrogen sulfide measurement of part 1 from that of part 2, and doubling the result. 29. A method according to statement 24 wherein said homocysteinease is derived from (a) Pseudomonas putida, (b) Trichomonas vaginalis, or (c) is a chimeric enzyme produced by expression of a chimeric encoding polynucleotide that includes nucleotide sequence derived from both Pseudomonas putida and Trichomonas vaginalis . 30. A method according to statement 28 wherein said homocysteinease is derived from (a) Pseudomonas putida , (b) Trichomonas vaginalis, or (c) is a chimeric enzyme produced by expression of a chimeric encoding polynucleotide that includes nucleotide sequence derived from both Pseudomonas putida and Trichomonas vaginalis . 31. A method according to statement 24 wherein said biological fluid is selected from the group consisting of urine, tissue fluid, blood, blood serum, and blood plasma. 32. A method according to statement 28 wherein said biological fluid is selected from the group consisting of urine, tissue fluid, blood, blood serum, and blood plasma. 33. A diagnostic kit for use in determining the amount of homocysteine in a biological sample comprising: (a) Trichomonas vaginalis or Pseudomonas putida homocysteinase, or a chimeric enzyme produced by expression of a chimeric encoding polynucleotide that includes nucleotide sequence derived from both Pseudomonas putida and Trichomonas vaginalis; (b) at least one reagent capable of being used to determine the amount of hydrogen sulfide formed in the homocysteinase reaction; and (c) at least one reagent capable of being used to determine the amount of pyruvate formed in the homocysteinase reaction from cysteine present in said sample. 34. A diagnostic kit for use in determining the amount of homocysteine in a biological sample comprising: (a) Trichomonas vaginalis or Pseudomonas putida homocysteinase, or a chimeric enzyme produced by expression of a chimeric encoding polynucleotide that includes nucleotide sequence derived from both Pseudomonas putida and Trichomonas vaginalis; (b) at least one reagent capable of being used to determine the amount of hydrogen sulfide formed in the homocysteinase reaction; and (c) cystathionine β-synthase. 35. A purified and isolated DNA molecule comprising a chimeric nucleotide sequence that encodes amino acid sequence of Pseudomonas putida homocysteinase and amino acid sequence of Trichomonas vaginalis homocysteinase, from which can be expressed a functional protein having homocysteinase activity. 36. A DNA molecule according to statement 35 that comprises an encoding nucleotide sequence for Pseudomonas putida homocysteinase, wherein one or more subsequences thereof that encode one or more amino acids of said enzyme are correspondingly replaced by one or more nucleotide subsequences that encode the corresponding amino acids of a Trichomonas vaginalis homocysteinase. 37. A DNA molecule according to statement 36 wherein said Trichomonas vaginalis homocysteinase is that encoded by the mgll gene or the mgll gene. 38. A DNA molecule according to statement 37 wherein said Trichomonas vaginalis homocysteinase is that encoded by the mgl2 gene. 39. A DNA molecule according to statement 38 that comprises an encoding sequence for Pseudomonas putida homocysteinase, wherein one or more subsequences thereof, that each encode one or more amino acids of said enzyme, are correspondingly replaced by one or more nucleotide subsequences than each encode the one or more corresponding amino acids of said Trichomonas vaginalis homocysteinase, and wherein said resultant amino acid replacements are selected from the group consisting of: (a) Cys-Ser-Arg-Ala-Asp-Ile-Ile-Ala-Lys-Val-Lys-Ser (SEQ ID NO: l),or any subset thereof, from Trichomonas vaginalis for Val-Gly-Ser-Gln-Ala-Leu-Val-Asp-Arg-Ile-Arg-Leu (SEQ ID NO: 2), or any subset thereof, from Pseudomonas putida ; (b) Asp-Val-Asp (SEQ ID NO: 3),or any subset thereof, from Trichomonas vaginalis for Glu-Leu-Lys (SEQ ID NO: 4), or any subset thereof, from Pseudomonas putida ; (c) Cys-His-Val-Val (SEQ ID NO: 5),or any subset thereof, from Trichomonas vaginalis for, Ala-Leu-Gin-Leu (SEQ ID NO: 6),or any subset thereof, from Pseudomonas putida ; (d) Cys-Glu-Asn-Val-Gln-Asp-Ile-Ile-Asp-Asp (SEQ ID NO: 7),or any subset thereof, from Trichomonas vaginalis for, Gly-Leu-Glu-Asp-Ile-Asp-Asp-Leu-Leu-Ala (SEQ ID NO: 8) ,or any subset thereof, from Pseudomonas putida ; and (e) any combination involving one, two, three or four of the replacements provided for by groups (a), (b), (c), and (d) above; and (f) a nucleotide sequence complementary to a nucleotide sequence (a), (b), (c), (d) or (e). 40. A diagnostic kit for use in determining the amount of homocysteine in a biological sample comprising: (a) a homocysteinase according to statement 35; and (b) at least one reagent capable of being used to determine the amount of hydrogen sulfide formed in the homocysteinase reaction. 41. A diagnostic kit for use in determining the amount of homocysteine in a biological sample comprising: (a) a homocysteinase according to statement 39; and (b) at least one reagent capable of being used to determine the amount of hydrogen sulfide formed in the homocysteinase reaction. 42. A recombinant DNA molecule according to statement 39 in which the DNA sequence encoding said chimeric homocysteinase is operably linked to an expression control sequence. 43. A host cell modified to contain a recombinant DNA molecule according to statement 42. 44. A method for determining the concentration of homocysteine in a biological fluid of a patient that comprises use of a diagnostic kit according to statement 33, under conditions where the determination of hydrogen sulfide and of pyruvate in said sample that permit determination of said homocysteine concentration therein are both accomplished in a single cuvette. 45. A homocysteinase enzyme that is sufficiently non reactive toward cysteine or methionine that the concentration of homocysteine that is present in a sample of tissue fluid, urine, blood, blood serum, or blood plasma of a subject may be determined in a single enzyme assay wherein is measured one or more products resulting from reaction of said homocysteinease on homocysteine in said sample, and wherein said measurement is substantially unaffected by the concentration of cysteine or methionine therein. 46. A homocysteinase enzyme according to statement 45 patterned substantially on the amino acid sequence of a homocysteinase of Trichomonas vaginalis. 47. A homocysteinase according to statement 46 that is patterned on Trichomonas vaginalis enzyme encoded by the mgll gene (SEQ ID NO:l 1). 48. A homocysteinase according to statement 46 that is patterned on Trichomonas vaginalis enzyme encoded by the mgl2 gene. 49. A homocysteinase according to statement 46 having the amino acid sequence corresponding to SEQ ID NO: 10, or to residues Met 1 through Leu 396 thereof. 50. A homocysteinase containing one or more peptide sequences of SEQ ID NO: 10 that are selected from the group consisting of: (a) Gly-Gly-Asn-Arg-Leu-Ala-Gly-Gln-Glu; (b) a subset of (a) that comprises Leu; (c) Arg-Val-Cys-Lys-Glu-Ala-His-Ser-Gln (SEQ ID NO: 10, residues 168-176); (d) a subset of (c) that comprises Glu; (e) Gln-Met-Arg-Met-Tyr-Gly-Ser-Met-Ile (SEQ ID NO: 10, residues 304-312); and (f) a subset of (e) that comprises Tyr. 51. A homocysteinase according to statement 45 patterned on a homocysteinase from Pseudomonas, Clostridium, Aeromonas or Trichomonas. 52. A homocysteinase according to statement 45 derived from Pseudomonas, Clostridium, Aeromonas or Trichomonas wherein one or more peptide sequences thereof are correspondingly replaced by one or more peptide sequences of SEQ ID NO: 10 that are selected from the group consisting of: (a) Gly-Gly-Asn-Arg-Leu-Ala-Gly-Gln-Glu (SEQ ID NO: 10, residues 43-51); (b) a subset of (a) that comprises Leu; (c) Arg-Val-Cys-Lys-Glu-Ala-His-Ser-Gln (SEQ ID NO:10, residues 168-176); (d) a subset of (c) that comprises Glu; (e) Gln-Met-Arg-Met-Tyr-Gly-Ser-Met-Ile (SEQ ID NO: 10, residues 304-312); and (f) a subset of (e) that comprises Tyr. 53. A chimeric homocysteinase patterned on a Trichomonas homocysteinase wherein one or more amino acids thereof that correspond to Leu 47 , Glu 172 , and Tyr 308 of SEQ ID NO: 10 are correspondingly replaced thereby. 54. A homocysteinase that is a substitution variant, addition variant, deletion variant, or derivative of SEQ ID NO: 10, or of residues Met 1 through Leu 396 thereof, wherein said variant or derivative has one or both of the following properties: (a) at least about 25% of the activity of SEQ ID NO:10 toward homocysteine in an assay of Example 8; and/or (b) no more than about 500 %> of the activity of SEQ ID NO: 10 toward cysteine in an assay of Example 8. 55. A homocysteinase enzyme according to statement 45 that is: (a) sufficiently non reactive toward cysteine or methionine that the concentration of homocysteine that is present in a sample of tissue fluid, urine, blood, blood serum, or blood plasma of a subject may be determined in a single enzyme assay wherein is measured one or more products resulting from reaction of said homocysteinease on homocysteine in said sample, wherein said measurement is substantially unaffected by the concentration of cysteine or methionine therein; and is (b) patterned on SEQ ID NO: 12, but wherein one or more of Phe 47 , Asp 172 , and Ser 308 thereof is deleted, or is replaced according to the following formula: (1) for Phe 47 , replace with Leu, He, Val, Ala, Gly, Met, and Trp; (2) for Asp 172 , replace with Glu, Gin, or Asn; (3) for Ser 308 , replace with Tyr, Phe, Met, Trp, Gin, Thr, or Asn. 56. A homocysteinase according to statement 45 having a specific activity for homocysteine that is at least about 100-fold greater than for cysteine and methionine. 57. A purified and isolated DNA molecule comprising a nucleotide sequence coding for a homocysteinase according to statement 45 wherein said coding sequence is operably linked to control sequences capable of directing expression therefrom in a host cell. 58. A host cell that contains a DNA molecule according to statement 57. 59. A purified and isolated DNA molecule according to statement 57 comprising a nucleotide sequence that codes for a homocysteinase according to SEQ ID NO: 10, or for residues Met 1 through Leu 396 thereof. 60. A DNA molecule according to statement 59 that comprises a nucleotide sequence that is SEQ ID NO:9 or the nucleotide 39-1226 fragment of SEQ ID NO:9, or a complement thereof. 61. An RNA molecule that corresponds to SEQ ID NO:9, or a homocysteinase encoding fragment thereof. 62. A method for determining the amount of homocysteine that is present in a biological sample containing homocysteine and cysteine, which method comprises measuring the concentration of a product that is produced from homocysteine in a reaction catalyzed by a homocysteinase, which product may also be produced by reaction of said enzyme upon cysteine in said sample, and wherein said sample contains an otherwise interfering concentration of cysteine, said method including the step of providing said enzyme in a form that is sufficiently non-reactive toward cysteine that the concentration of homocysteine in said sample may be determined in a single enzyme assay. 63. A method according to statement 62 wherein said biological sample is tissue fluid, urine, blood, blood serum, or blood plasma from a human subject. 64. A method according to statement 63 wherein said biological sample comprises homocysteine molecules that are bonded to amino acid R groups in protein, and said method further comprises the step of first making said bonded homocysteine molecules available for detection therein. 65. A method according to statement 63 wherein said biological sample comprises homocysteine molecules that are disulfide-bonded to protein cysteine residues, free cysteine molecules, or to other homocysteine molecules, and said method further comprises the step of first making said bonded homocysteine molecules available for detection therein. 66. A method for determining the amount of homocysteine that is present in a biological sample containing homocysteine and methionine, which method comprises measuring the concentration of a product that is produced from homocysteine in a reaction catalyzed by a homocysteinase, which product may also be produced by reaction of said enzyme upon methionine in said sample, and wherein said sample contains an otherwise interfering concentration of methionine, said method including the step of providing said enzyme in a form that is sufficiently non-reactive toward methionine that the concentration of homocysteine in said sample may be determined in a single enzyme assay. 67. A method according to statement 65 wherein said biological sample is tissue fluid, urine, blood, blood serum, or blood plasma from a human subject. 68. A diagnostic kit for use in determining the amount of homocysteine in a biological sample comprising: (a) a homocysteinase according to statement 45; and (b) at least one reagent capable of being used to determine the amount of product formed in the homocysteinase reaction. 69. A diagnostic kit according to statement 68, wherein said reagent of part (b) thereof is used to determine the amount of hydrogen sulfide formed in the homocysteinase reaction. 70. A modified homocysteinase enzyme that comprises one or more histidine residues positioned on the N-terminal side of the normal Met 1 of said protein, said enzyme being sufficiently non reactive toward cysteine or methionine that the concentration of homocysteine that is present in a sample of tissue fluid, urine, blood, blood serum, or blood plasma of a subject may be determined in a single enzyme assay wherein is measured one or more products resulting from reaction of said modified homocysteinease on homocysteine in said sample, and wherein said measurement is substantially unaffected by the concentration of cysteine or methionine therein. 71. A modified homocysteinase according to statement 70 that comprises about 6 histidine residues positioned on the N-terminal side of the normal Met 1 of said protein. 72. A DNA molecule that encodes a homocysteinase according to statement 70. 73. A method for determining the amount of homocysteine that is present in a biological sample containing homocysteine and cysteine, which method comprises measuring the concentration of a product that is produced from homocysteine in a reaction catalyzed by a homocysteinase, which product may also be produced by reaction of said homocysteinase upon cysteine in said sample, and wherein said sample contains an otherwise interfering concentration of cysteine, said method comprising the steps of: (a) incubating said sample with γ-glutamylcysteine synthetase in the presence of substrates that permit conversion of cysteine to γ-glutamylcysteine; (b) incubating the mixture resultant from step (a) with homocysteinase; and (c) measuring the concentration of a product that is capable of being produced from both homocysteine and cysteine in a reaction catalyzed by said homocysteinase. 74. The method of statement 73 wherein the product measured is hydrogen sulfide or ammonia. 75. The method of statement 73 wherein said homocysteinase is from Trichomonas vaginalis. 76. The method of statement 75 wherein said homocysteinase is Trichomonas vaginalis enzyme encoded by the mgll gene (SEQ ID NO: 11). 77. The method of statement 75 wherein said homocysteinase is the Trichomonas vaginalis enzyme encoded by the mgl2 gene. 78. The method of statement 75 wherein said homocysteinase has an the amino acid sequence corresponding to SEQ ID NO: 10, or to residues Met 1 through Leu 396 thereof. 79. The method of statement 73 wherein said homocysteinase is from Pseudomonas putida. 80. The method of statement 73 wherein said biological sample is selected from the group consisting of tissue fluid, urine, blood, blood serum, or blood plasma. 81. The method of statement 73 wherein said homocysteinase is from Pseudomonas, Clostridium, Aeromonas or Trichomonas. 82. The method to statement 73 wherein said biological sample comprises homocysteine molecules that are disulfide-bonded to protein cysteine residues, free cysteine molecules, or to other homocysteine molecules, and said method further comprises the step of first making said bonded homocysteine molecules available for reaction with homocysteinase. 83. The method of statement 82 wherein said step of making disulfide bonded homocysteine molecules available for reaction with homocysteinase is performed by treating said sample with tris-(2-carboxyethyl) phosphine at a point prior to step (a) thereof. 84. The method of statement 73 wherein said γ-glutamylcysteine synthetase is from E. coli. 85. The method of statement 74 wherein detection of hydrogen sulfide is performed by colorimetric detection of methylene blue-type chromophore produced by oxidation of an N,N-dialkyl-p-phenylenediamine with ferric iron and said hydrogen sulfide. 86. The method of statement 85 wherein said N,N-dialkyl-p-phenylenediamine is N,N-dimethyl, N,N-diethyl, N,N-di-n-propyl, or N,N-di-n-butyl. . 87. A diagnostic kit for use in determining the amount of homocysteine in a biological sample comprising: (a) a γ-glutamylcysteine synthetase; (b) a homocysteinase; and (c) at least one reagent capable of being used to determine the amount of hydrogen sulfide, or ammonia, formed by reaction of homocysteinase on said sample. 88. A diagnostic kit according to statement 87, wherein said reagent of part (c) is used to determine hydrogen sulfide or ammonia. 89. A method for determining the amount of homocysteine that is present in a biological sample containing homocysteine and cysteine, which method comprises measuring the concentration of a product compound that is produced from homocysteine in a reaction catalyzed by a homocysteinase, which product compound may also be produced by reaction of said homocysteinase upon cysteine in said sample, and wherein said sample contains an otherwise interfering concentration of cysteine, said method comprising the steps of: (a) incubating said sample with an enzyme that, in a suitable environment therefor, is capable of converting cysteine to a compound that is not a substrate for homocysteinase; (b) incubating the mixture resultant from step (a) with homocysteinase in order to generate one or more product compounds; and (c) determining the amount of homocysteine in said sample by measuring the concentration of said product compound. 90. The method according to statement 89 wherein said product compound is hydrogen sulfide or ammonia. 91. The method according to statement 89 wherein the enzyme used in step (a) thereof is γ-glutamylcysteine synthetase, cysteine oxidase, cystathionine beta-synthetase, or cysteine tRNA synthetase. 92. The method according to statement 73 wherein said biological sample contains methionine.

Claims: Claims Claim 1. A method for determining the amount of homocysteine that is present in a biological sample comprising homocysteine and cysteine, which method comprises measuring the concentration of a product compound that is produced from homocysteine in a reaction catalyzed by a homocysteinase, which product compound may also be produced by reaction of said homocysteinase upon cysteine in said sample, and wherein said sample contains an otherwise interfering concentration of cysteine, said method comprising the steps of: (a) incubating said sample with a first enzyme that, in a suitable environment therefor, is capable of converting cysteine to a compound that is not a substrate for homocysteinase; (b) incubating the mixture resultant from step (a) with homocysteinase in order to generate a product compound; and (c) determining the amount of homocysteine in said sample by measuring the concentration of said product compound. Claim 2. The method according to claim 1 wherein the enzyme used in substep (a) thereof is selected from the group consisting of γ-glutamylcysteine synthetase, cysteine oxidase, cystathionine beta-synthetase, or cysteine tRNA synthetase. Claim 3. The method according to claim 1 wherein said biological sample contains methionine. Claim 4. The method according to claim 1 wherein said biological sample is selected from the group consisting of urine, tissue fluid, blood, blood serum, and blood plasma. Claim 5. The method according to claim 1 wherein said homocysteinase enzyme is derived from a prokaryotic or eukaryotic microorganism. Claim 6. The method according to claim 5 wherein said homocysteinease is from Clostridium sp., Aeromonas sp., Pseudomonas putida or Trichomonas vaginalis. Claim 7. The method according to claim 1 wherein said homocysteinase used in step (b) thereof is selected from the group consisting of: (a) Trichomonas vaginalis enzyme encoded by the mgll gene (SEQ ID NO: 11); (b) Trichomonas vaginalis enzyme encoded by the mgl2 gene; (c) SEQ ID NO: 10, or residues Met 1 through Leu 396 thereof; (d) a polypeptide patterned on SEQ ID NO: 12, but wherein one or more of Phe 47 , Asp 172 , and Ser 308 thereof is deleted, or is replaced according to the following formula: (1) for Phe 47 , replace with Leu, He, Val, Ala, Gly, Met, and Trp; (2) for Asp 172 , replace with Glu, Gin, or Asn; (3) for Ser 308 , replace with Tyr, Phe, Met, Trp, Gin, Thr, or Asn; (e) a homocysteinase patterned on an enzyme from Pseudomonas, Clostridium, Aeromonas or Trichomonas wherein one or more peptide subsequences of the original polypeptide sequence thereof are correspondingly replaced by one or more homologous peptide sequences of SEQ ID NO: 10 that are selected from the group consisting of: (1) Gly-Gly-Asn-Arg-Leu-Ala-Gly-Gln-Glu, (see residues 43-51 of SEQ ID NO:10); (2) a subset of (1) that comprises Leu; (3) Arg-Val-Cys-Lys-Glu-Ala-His-Ser-Gln, (see residues 168-176 of SEQ ID NO: 10); (4) a subset of (3) that comprises Glu; (5) Gin-Met- Arg-Met-Tyr-Gly-Ser-Met-Ile, (see residues 304-312 of SEQ ID NO: 10); and (6) a subset of (5) that comprises Tyr; (f) Pseudomonas putida homocysteinase, wherein one or more peptide sequences thereof are correspondingly replaced by one or more peptide sequences of a Trichomonas vaginalis homocysteinase, and wherein said resultant amino acid replacements are selected from the group consisiting of: (1) Cys-Ser-Arg-Ala-Asp-Ile-Ile-Ala-Lys-Val-Lys-Ser (SEQ ID NO: 1), or any subset thereof, from Trichomonas vaginalis (T2) for Val-Gly-Ser-Gln-Ala-Leu-Val-Asp-Arg- Ile-Arg-Leu (SEQ ID NO: 2), or any subset thereof, from Pseudomonas putida; (2) Asp-Val-Asp (SEQ ID NO: 3), or any subset thereof, from Trichomonas vaginalis (T2) for Glu-Leu-Lys (SEQ ID NO: 4), or any subset thereof, from Pseudomonas putida ; (3) Cys-His-Val-Val (SEQ ID NO: 5), or any subset thereof, from Trichomonas vaginalis (T2) for Ala-Leu-Gin-Leu (SEQ ID NO: 6), or any subset thereof, from Pseudomonas putida ; (4) Cys-Glu-Asn-Val-Gln-Asp-Ile-Ile-Asp-Asp (SEQ ID NO: 7), or any subset thereof, from Trichomonas vaginalis (T2) for Gly-Leu-Glu-Asp-Ile- Asp- Asp-Leu-Leu-Ala (SEQ ID NO: 8), or any subset thereof, from Pseudomonas putida; and (5) any combination involving one, two, three or four of the replacements provided for by groups (1), (2), (3), and (4) above; (g) a substitution variant, addition variant, deletion variant, or derivative of SEQ ID NO: 10, or of residues Met 1 through Leu 396 thereof, wherein said variant or derivative has one or both of the following properties: (1) at least about 25% of the activity of SEQ ID NO: 10 toward homocysteine in a suitable assay therefor, and/or (2) no more than about 500 % of the activity of SEQ ID NO: 10 toward cysteine in a suitable assay therefor; and (h) a chimeric homcysteinase enzyme produced by expression of a chimeric polynucleotide that includes homocysteinase-encoding nucleotide sequence derived from both Pseudomonas putida and Trichomonas vaginalis other than as described in (e) and (f) above. Claim 8. A method for determining the amount of homocysteine that is present in a biological sample comprising homocysteine and cysteine, which method comprises measuring the concentration of a product compound that is produced from homocysteine in a reaction catalyzed by a homocysteinase, which product compound may also be produced by reaction of said homocysteinase upon cysteine in said sample, and wherein said sample contains an otherwise interfering concentration of cysteine, said method comprising the steps of: (a) incubating said sample with a first enzyme that, in a suitable environment therefor, is capable of converting cysteine to a compound that is not a substrate for homocysteinase; (b) incubating the mixture resultant from step (a) with homocysteinase in order to generate a product compound; and (c) determining the amount of homocysteine in said sample by measuring the concentration of said product compound; wherein said biological sample further comprises homocysteine molecules that are disulfide-bonded to protein cysteine residues, free cysteine molecules, or to other homocysteine molecules, said method further comprising the step of making said bonded homocysteine molecules available for reaction with homocysteinase in step (b). Claim 9. The method of claim 8 wherein said step of making disulfide bonded homocysteine molecules available for reaction with homocysteinase is performed by treating said sample with tris-(2-carboxyethyl) phosphine, DL-dithiothreitol, β-mercaptoethanol, or other disulfide reducing agent, at a point prior to step (a) thereof. Claim 10. A purified and isolated DNA molecule that comprises an encoding sequence for a protein having homocysteinase activity, wherein said encoding sequence is selected from the group consisting of: (a) a nucleotide sequence that encodes SEQ ID NO: 10, or residues Met 1 through Leu 396 thereof; (b) a nucleotide sequence that encodes a polypeptide patterned on SEQ ID NO: 12, but wherein one or more of Phe 47 , Asp 172 , and Ser 308 of said polypeptide is deleted, or is replaced according to the following formula: (1) for Phe 47 , replace with Leu, He, Val, Ala, Gly, Met, and Trp; (2) for Asp 172 , replace with Glu, Gin, or Asn; (3) for Ser 308 , replace with Tyr, Phe, Met, Trp, Gin, Thr, or Asn. (c) a nucleotide sequence that encodes a homocysteinase patterned on an enzyme from Pseudomonas, Clostridium, Aeromonas or Trichomonas, wherein one or more peptide subsequences of the original amino acid sequences thereof are correspondingly replaced by one or more homologous peptide sequences of SEQ ID NO: 10 that are selected from the group consisting of: (1) Gly-Gly-Asn-Arg-Leu-Ala-Gly-Gln-Glu, (see residues 43-51 of SEQ ID NO: 10); (2) a subset of (1) that comprises Leu; (3) Arg-Val-Cys-Lys-Glu-Ala-His-Ser-Gln, (see residues 168-176 of SEQ ID NO: 10); (4) a subset of (3) that comprises Glu; (5) Gin-Met- Arg-Met-Tyr-Gly-Ser-Met-Ile, (see residues 304-312 of SEQ ID NO: 10); and (6) a subset of (5) that comprises Tyr. (d) a nucleotide sequence that encodes a Pseudomonas putida homocysteinase, wherein one or more peptide sequences of said homocysteinase are correspondingly replaced by one or more peptide sequences of a Trichomonas vaginalis homocysteinase, and wherein said resultant amino acid replacements are selected from the group consisiting of: (1) Cys-Ser-Arg-Ala-Asp-Ile-Ile-Ala-Lys-Val-Lys-Ser (SEQ ID NO: 1), or any subset thereof, from Trichomonas vaginalis (T2) for Val-Gly-Ser-Gln-Ala-Leu-Val-Asp-Arg- Ile-Arg-Leu (SEQ ID NO: 2), or any subset thereof, from Pseudomonas putida; (2) Asp-Val-Asp (SEQ ID NO: 3), or any subset thereof, from Trichomonas vaginalis (T2) for Glu-Leu-Lys (SEQ ID NO: 4), or any subset thereof, from Pseudomonas putida ; (3) Cys-His-Val-Val (SEQ ID NO: 5), or any subset thereof, from Trichomonas vaginalis (T2) for Ala-Leu-Gin-Leu (SEQ ID NO: 6), or any subset thereof, from Pseudomonas putida ; (4) Cys-Glu-Asn-Val-Gln-Asp-Ile-Ile-Asp-Asp (SEQ ID NO: 7), or any subset thereof, from Trichomonas vaginalis (T2) for Gly-Leu-Glu-Asp-Ile-Asp-Asp-Leu-Leu-Ala (SEQ ID NO: 8), or any subset thereof, from Pseudomonas putida; and (5) any combination involving one, two, three or four of the replacements provided for by groups (1), (2), (3), and (4) above; (e) a nucleotide sequence that encodes a substitution variant, addition variant, deletion variant, or derivative of SEQ ID NO: 10, or of residues Met 1 through Leu 396 thereof, wherein said variant or derivative has one or both of the following properties: (1) at least about 25% of the activity of SEQ ID NO: 10 toward homocysteine in a suitable assay therefor, and/or (2) no more than about 500 % of the activity of SEQ ID NO: 10 toward cysteine in a suitable assay therefor, and (f) a nucleotide sequence that encodes a chimeric homocysteinase enzyme produced by expression of a chimeric polynucleotide that includes homocysteinase-encoding nucleotide sequence derived from both Pseudomonas putida and Trichomonas vaginalis other than as described in (c) and (d) above. Claim 11. A purified and isolated DNA molecule according to claim 10 wherein said coding sequence is operably linked to control sequences capable of directing expression therefrom in a host cell. Claim 12. A host cell that contains a DNA molecule according to claim 11. Claim 13. A homocysteinase enzyme that is: (a) SEQ ID NO: 10, or residues Met 1 through Leu 396 thereof; (b) patterned on SEQ ID NO:12, but wherein one or more of Phe 47 , Asp 172 , and Ser 308 thereof is deleted, or is replaced according to the following formula: (1) for Phe 47 , replace with Leu, He, Val, Ala, Gly, Met, and Trp; (2) for Asp 172 , replace with Glu, Gin, or Asn; (3) for Ser 308 , replace with Tyr, Phe, Met, Trp, Gin, Thr, or Asn; (c) a homocysteinase patterned on an enzyme from Pseudomonas, Clostridium, Aeromonas or Trichomonas wherein one or more peptide subsequences of the original polypeptide sequences thereof are cooespondingly replaced by one or more homologous peptide sequences of SEQ ID NO: 10 that are selected from the group consisting of: (1) Gly-Gly-Asn-Arg-Leu-Ala-Gly-Gln-Glu, (see residues 43-51 of SEQ ID NO:10); (2) a subset of (1) that comprises Leu; (3) Arg-Val-Cys-Lys-Glu-Ala-His-Ser-Gln, (see residues 168-176 of SEQ ID NO: 10); (4) a subset of (3) that comprises Glu; (5) Gin-Met- Arg-Met-Tyr-Gly-Ser-Met-Ile, (see residues 304-312 of SEQ ID NO: 10); and (6) a subset of (5) that comprises Tyr. (d) Pseudomonas putida homocysteinase, wherein one or more peptide sequences thereof are correspondingly replaced by one or more peptide sequences of a Trichomonas vaginalis homocysteinase, and wherein said resultant amino acid replacements are selected from the group consisiting of: (1) Cys-Ser-Arg-Ala-Asp-Ile-Ile-Ala-Lys-Val-Lys-Ser (SEQ ID NO: 1), or any subset thereof, from Trichomonas vaginalis (T2) for Val-Gly-Ser-Gln- Ala-Leu- Val-Asp- Arg- Ile-Arg-Leu (SEQ ID NO: 2), or any subset thereof, from Pseudomonas putida; (2) Asp-Val-Asp (SEQ ID NO: 3), or any subset thereof, from Trichomonas vaginalis (T2) for Glu-Leu-Lys (SEQ ID NO: 4), or any subset thereof, from Pseudomonas putida ; (3) Cys-His-Val-Val (SEQ ID NO: 5), or any subset thereof, from Trichomonas vaginalis (T2) for Ala-Leu-Gin-Leu (SEQ ID NO: 6), or any subset thereof, from Pseudomonas putida ; (4) Cys-Glu-Asn-Val-Gln-Asp-Ile-Ile-Asp-Asp (SEQ ID NO: 7), or any subset thereof, from Trichomonas vaginalis (T2) for Gly-Leu-Glu-Asp-Ile-Asp-Asp-Leu-Leu-Ala (SEQ ID NO: 8), or any subset thereof, from Pseudomonas putida; and (5) any combination involving one, two, three or four of the replacements provided for by groups (1), (2), (3), and (4) above; (e) a substitution variant, addition variant, deletion variant, or derivative of SEQ ID NO: 10, or of residues Met 1 through Leu 396 thereof, wherein said variant or derivative has one or both of the following properties: (1) at least about 25% of the activity of SEQ ID NO:10 toward homocysteine in a suitable assay therefor, and/or (2) no more than about 500 % of the activity of SEQ ID NO: 10 toward cysteine in a suitable assay therefor, and (f) a chimeric homcysteinase enzyme produced by expression of a chimeric polynucleotide that includes homocysteinase-encoding nucleotide sequence derived from both Pseudomonas putida and Trichomonas vaginalis other than as described in (c) and (d) above. Claim 14. A homocysteinase enzyme that is sufficiently non reactive toward cysteine or methionine that the concentration of homocysteine that is present in a sample of tissue fluid, urine, blood, blood serum, or blood plasma of a subject may be determined in a single enzyme assay wherein is measured one or more products resulting from reaction of said homocysteinease on homocysteine in said sample, wherein said measurement is substantially unaffected by the concentration of cysteine or methionine therein. Claim 15. A diagnostic kit for use in determining the amount of homocysteine in a biological sample comprising: (a) an enzyme selected from the group consisiting of γ-glutamylcysteine synthetase, cysteine oxidase, cystathionine beta-synthetase, and cysteine tRNA synthetase. (b) a homocysteinase enzyme; and (c) at least one reagent capable of being used to determine the amount of hydrogen sulfide, or ammonia, formed by reaction of homocysteinase on said sample. Claim 16. The kit of claim 15 wherein said enzyme (a) is γ-glutamylcysteine synthetase from E. coli. Claim 17. A method for determining the amount of homocysteine that is present in a biological sample comprising homocysteine and cysteine, which method comprises: (a) contacting said sample with an enzyme preparation that is capable of producing hydrogen sulfide and α-ketobutyrate from homocysteine, and further capable of producing hydrogen sulfide and pyruvate from cysteine, and (b,) contacting said sample, or a duplicate or a part thereof, with an enzyme preparation by whose action the amount of pyruvate in said sample, or in said duplicate or part thereof, can be determined, or (b 2 ) contacting said sample, or a duplicate or a part thereof, with an enzyme preparation by whose action the amount of α-ketobutyrate in said sample, or in said duplicate or part thereof, can be determined; and then (c) calculating the amount of homocysteine in said sample by comparing the amount of pyruvate or α-ketobutyrate so determined with the amount of hydrogen sulfide produced therein. Claim 18. A method according to claim 17 wherein said enzyme preparation comprises lactate dehydrogenase by which pyruvate in said sample can be determined, or leucine dehydrogenase by which α-ketobutyrate in said sample can be determined. Claim 19. A method for determining the amount of homocysteine that is present in a biological sample comprising homocysteine and cysteine, which method comprises contacting said sample with an enzyme preparation capable of producing hydrogen sulfide from homocysteine and cysteine, and determining the amount of homocysteine in said sample by measuring the amount of hydrogen sulfide produced from homocysteine, and wherein, optionally, said biological sample comprises homocysteine molecules that are covalently bonded to other molecules, and said method includes the further step of dissociating said homocysteine molecules therefrom prior to measuring of hydrogen sulfide. Claim 20. The method according to claim 19 wherein hydrogen sulfide is produced from both cysteine and homocysteine, and hydrogen sulfide produced from homocysteine is distinguished from hydrogen sulfide produced from cysteine by including in said method the additional initial steps of: (a) contacting said biological sample with a reagent that protects homocysteine from said enzyme preparation; (b) permitting said enzyme preparation to produce hydrogen sulfide from cysteine present in said sample; (c) removing said hydrogen sulfide; and (d) deprotecting said homocysteine, and adding a further amount of enzyme preparation to produce hydrogen sulfide therefrom. Claim 21. A method according to Claim 19 for determining the amount of homocysteine that is present in a biological sample containing homocysteine and cysteine, comprising the steps of (a) dividing said sample into 2 parts, part 1 and part 2; (b) contacting part 1 with a first enzyme preparation that is capable of converting homocysteine to a substance that is not a substrate for homocysteinase, wherein said first enzyme preparation does not recognize cysteine as a substrate; (c) independently contacting part 1 and part 2 with a second enzyme preparation that comprises a homocysteinase capable of producing hydrogen sulfide from both homocysteine and cysteine; (d) measuring the amount of hydrogen sulfide produced in part 1 and part 2; and (e) calculating the amount of homocysteine in said biological sample by subtracting the hydrogen sulfide measurement of part 1 from that of part 2, and doubling the result. Claim 22. The method according to Claim 21 wherein said enzyme preparation used in step (b) thereof comprises an enzyme selected from the group consisting of S- adenosylhomocysteine hydrolyase and cystathionine β-synthase. Claim 23. A diagnostic kit for use in determining the amount of homocysteine in a biological sample comprising: (a) Trichomonas vaginalis homocysteinase or Pseudomonas putida homocysteinase, or a chimeric enzyme produced by expression of a chimeric encoding polynucleotide that includes nucleotide sequence derived from both Pseudomonas putida and Trichomonas vaginalis; (b) at least one reagent capable of being used to determine the amount of hydrogen sulfide formed in the homocysteinase reaction; and (c) at least one reagent capable of being used to determine the amount of pyruvate formed in the homocysteinase reaction from cysteine present in said sample. Claim 24. A diagnostic kit for use in a single enzyme assay of the homocysteine concentration in a biological fluid of a subject, said kit comprising: (a) a homocysteinase enzyme; and (b) at least one reagent capable of being used to determine the amount of product hydrogen sulfide formed by reaction of homocysteinase on homocysteine; wherein said homocysteinase is sufficiently non-reactive toward cysteine that any hydrogen sulfide produced by reaction of said homocysteinase on cysteine does not substantially interfere with the use of said kit to assess risk for cardiovascular disease. Claim 25. The diagnostic kit of claim 24 wherein the homocysteinease included therein has the property that at least about 90% of the hydrogen sulfide produced by action of said homocysteinase upon contacting a biological fluid in an assay for homocysteine is contributed by said homocysteine, when the concentrations of homocysteine and cysteine in said fluid are, respectively, about 20 μmolar and about 300 μmolar respectively. Claim 26. The diagnostic kit of claim 25 wherein said homocysteinase included therein has the property that at least about 99% of the hydrogen sulfide produced by action of said homocysteinase upon contacting said biological fluid in an assay for homocysteine is contributed by said homocysteine. Claim 27. The diagnostic kit of claim 24 capable of being used to determine a homocysteine concentration in said biological fluid in the range of 1-500 μmolar, wherein said fluid also contains about 10 to about 1000 μmolar of cysteine. Claim 28. The diagnositic kit of claim 24 capable of being used to determine the homocysteine concentration in tissue fluid, blood, blood plasma, blood serum or urine. Claim 29. The diagnostic kit of claim 24 further comprising one or more reagents selected from the group consisting of: (a) an agent capable of reducing disulfide bonds; (b) a source of ferric iron; (c) an N,N-dialkyl-p-phenylenediamine; and (d) homocysteine as a calibration standard. Claim 30. The diagnostic kit of claim 24 comprising: (a) SEQ ID NO: 10, or residues Met 1 through Leu 396 thereof, as homocysteinase; (b) the reagents 50 mM borate buffer, pH 7.5; lOOmM DL-dithiothreitol; 10 mM potassium ferricyanate, 1%> w/v Triton® X-100; a 100 mM solution of N,N-dialkyl- phenylene diamine selected from the group consisting of dimethyl, diethyl, dipropyl or dibutyl; and (c) homocysteine and/or homocystine as a calibration standard. Claim 31. A host bacterial cell that contains one or more copies of an encoding sequence for SEQ ID NO: 10, or residues Met 1 through Leu 396 thereof. Claim 32. A host cell according to claim 31 that is E. coli pAC2-l or pAC2-l 1. Claim 33. A host cell according to claim 31 that is ATCC 98549. Claim 34. In a method for determining the amount of homocysteine present in a biological fluid of a subject wherein said method comprises the step of contacting said sample with an enzyme capable of catalyzing production from homocysteine of a detectable quantity of hydrogen sulfide, and wherein said sample further comprises an amount of cysteine from which a detectable quantity of hydrogen sulfide can be produced that interferes with said determination, the improvement comprising selecting, for use therein, a homocysteinase having the property that at least about 90% of the hydrogen sulfide produced by action of said homocysteinase upon contacting said biological fluid therewith, in an assay for homocysteine, is contributed by said homocysteine, when the concentrations of homocysteine and cysteine in said fluid are, respectively, about 20 μmolar and about 300 μmolar respectively. Claim 35. A single enzyme method for determining the concentration of homocysteine in a biological fluid of a subject that comprises use of a diagnostic kit according to claim 24. Claim 36. A diagnostic method to assess risk for cardiovascular disease in a subject, wherein the concentration of homocysteine in a biological fluid of said subject is determined, absent interference from concentrations of cysteine and methionine in said fluid, said method comprising the steps of: (a) contacting the biological fluid sample with an agent capable of reducing disulfide bonds in order to liberate as free homocysteine that fraction of total homocysteine present therein in disulfide-bonded form; (b) contacting said sample with a homocysteinase that is substantially non-reactive to cysteine compared to homocysteine, such that at least about 90% of the hydrogen sulfide produced by said contacting is produced from homocysteine, under conditions where said hydrogen sulfide does not escape; (c) adding ferric iron and an N,N-dialkyl-phenylene diamine to said hydrogen sulfide- containing sample in order to produce, in situ, a methylene blue-type chromophore; (d) determining the concentration of homocysteine that was present in said sample based on the concentration of chromophore produced; and (e) comparing said result to levels of homocysteine in biological fluids of subjects that are known to correlate with increased risk for cardiovascular disease. Claim 37. The method of claim 36 wherein said homocysteinase is SEQ ID NO: 10, or residues Met 1 through Leu 396 thereof. Claim 38. The method according to any of claims 1, 8, 17 and 19 wherein said biological sample or fluid also comprises methionine. Claim 39. The method of claim 36 wherein said biological fluid sample is selected from the group consisting of tissue fluid, urine, blood, blood serum, and blood plasma. Claim 40. The diagnostic kit of claim 24 comprising: (a) a homocysteinase that is substantially non-reactive to cysteine compared to homocysteine; (b) a buffer; a reducing agent, an agent useful to detect hydrogen sulfide; and optionally, (c) homocysteine and/or homocystine as a calibration standard.

Inventors: HOFFMAN ROBERT M, LENZ MARTIN, PERRY ANDREW W, TAN YUYING, HOFFMAN, ROBERT, M., LENZ, MARTIN, PERRY, ANDREW, W., TAN, YUYING
Applicants: ANTICANCER INC, HOFFMAN ROBERT M, LENZ MARTIN, PERRY ANDREW W, TAN YUYING, ANTICANCER, INC., HOFFMAN, ROBERT, M., LENZ, MARTIN, PERRY, ANDREW, W., TAN, YUYING
Application year: 1998
Main classification: C12Q1
IPC codes: C12N1, C12N9, C12N15, C12Q1, C12R1, G01N33