IMMUNOASSAY FOR HOMOCYSTEINE

Abstract: This invention pertains to methods to detect a compound in the presence of a homolog that is immunologically related to the analyte. The invention is particularly suited for the detection of homocysteine in the presence of cysteine. The methods of this invention involve chemically modifying both the analyte and the homolog to increase their immunogenicity and facilitate antibody recognition. More importantly, this modification is done to make these compounds immunologically distinct. Antibodies to the immunologically distinct compounds are then prepared. An assay protocol comprises chemically modifying the analyte and homolog and then immunochemically detecting the modified analyte by means of the aforementioned antibodies.

Description: IM UNOASSAY FOR HOMOCYSTEINE Background of the Invention Field of the Invention Homocysteine (Hey) is one in a series of intermediates produced along the transsulfuration pathway in which methionine is eventually converted to cysteine (Cyε) . The exclusive source of Hey in mammals derives from the product of the enzyme catalyzed hydrolysis of S-adenosyl-homocysteine. Once formed, Hey may reenter the cycle through remethylation and conversion to methionine or combine with serine to form cystathionine, which is ultimately converted to Cys. The major metabolic pathway for the methylation of Hey by methionine synthase requires vitamin B 12 (cobalamine) as a methyl-transfer cofactor and 5-methyl-tetrahydrofolate as the ultimate methyl source. Not surprisingly, elevated levels of serum Hey have been associated with insufficient intake of vitamin B ]2 or folate, or a deficiency in the ability to properly utilize these two vitamins. Moderately elevated levels of Hey usually can be brought into balance by administering folate, a treatment for which there are few adverse side- effectε. In addition to anemia, individuals who are heterozygous for a defective cystathionine synthase gene, which results in a 50% diminution of normal enzymatic activity, are particularly susceptible to elevated Hey levels following methionine loading. This genetic predisposition is the most common cause of moderate homocysteinuria (build-up of Hey in urine) in otherwise healthy patients. Lastly, there appears to be a correlation between moderately elevated levels of Hey and cardiovascular disease. For these reasons, there has been great interest in developing an accurate Hey assay. Description of the Related Art There are several techniques to quantitate total homocysteine (Hey) as well as distinguish between the free (reduced and disulfide) and protein-bound (primarily albumin) forms. An excellent overview of the causes of homocysteinuria as well as an update on the current methods of clinical analysis can be found in Ueland, et al., Clin. Chem. 39 (9) :1764-1779 (1993) . An enzymatic method for a Hey assay is described by Sundrehagen, et al. , PCT/GB93/00138, where Hey is assayed indirectly by measuring the product concentration following the enzyme catalyzed conversion of Hey to S-adenosyl homocysteine. High performance liquid chromatographic ("HPLC") methods for Hey and Cys are known in the art. This analytical method discriminates between Hcy and Cyε by differential adsorption and elution of the compounds on a chromatographic support. Andersson, et al . , Clin. Chem. 39 (8) :1590-1597 (1993) describes the determination of total, free and reduced Hey and Cys. Hey and Cys analysis by means of a gas chromatograph-mass spectrometer is described in Allen, et al . , U.S. Patent No. 4,940,658. Allen, et al . , PCT/US92/05727 describes a chromatographic assay for cystathionine, the intermediary amino acid between Hey and Cys produced in the metabolism of methionine. Fiskerstrand, et al., Clin. Chem. 39 (2) :263-271 (1993) describes a fully automated analysis of total Hey involving fluorescent labeling of serum thiolε, followed by chromatographic separation of the Hey derivative from the other sulfur-containing compounds. Identification of Hey by HPLC methods often involves derivatization with fluorescent reagents such as is described in Fiskerstrand, supra or a radioenzymatic technique such as is described in Refsum, et al . , Clin. Chem. 31(4) 624-628 (1985). In addition, identification of Cys by protein sequence analysis involves derivatization with alkylating reagents. See, for example, Jue, et al . , Analytical Biochemistry 210:39-44 (1993) . Unfortunately, chromatographic methods have the disadvantage of being slow and labor intensive. Furthermore, current methods of Hey analysis require prior derivatization with fluorescent labels, such as bromobimane, in which the bromomethyl group reacts with the free thiol of Hey, thus forming a thioether and releasing free bromide ion. The bromobimane reagent also reacts with all other free thiols in solution, therefore chromatographic separation of the various derivatized sulfur-containing species is necessary. As many of the current methods of Hey analysis rely on cumbersome chromatographic techniques, there is a need for a faster and simpler antibody-based assay for Hey. To date, an immunoassay for Hey has not been employed, based on the expectation that it would be problematic because the Hey molecule is likely to be too small and contain too few antibody recognition features to serve as an effective hapten toward antibody development. Furthermore, antibodies may not attain the degree of antigenic specificity required to distinguish between Hey and Cys, which differ in structure by a single methylene group. Accordingly, there is a present need for the development of an immunoassay method that would permit highly specific quantitation and differentiation of these compounds. There are numerous techniques for handling undesirable cross- reactants. Brynes, et al . , U.S. Patent No. 4,952,336, describes a method of pretreating a sample with an aqueous periodate solution to eliminate cross-reactants in an amphetamine-methamphetamine immunoassay. Stevenson, PCT/GB90/01649, pertains to an improved immunoassay where the level of interference from rheumatoid factor is reduced by pretreating the sample with a reducing agent. Mach, et al . , U.S. Patent No. 4,978,632 pertains to an improved immunoassay where the level of interference from blood and blood products is eliminated by pretreating the sample with an oxidizing agent. These pretreatment methods only affect the cross-reactants; none of the methods affect the analyte. SUMMARY OF THE INVENTION The present invention pertains to a method of determining the presence or amount of a first compound in the presence of a second compound in a sample suspected of containing the compounds, wherein the compounds are immunologically related, comprising the steps of (a) bringing together in an aqueous medium: the sample, a modifying reagent capable of chemically modifying the first compound and the second compound to form a modified first compound and a modified second compound, wherein the modified compounds are immunologically distinct, and an antibody capable of specifically binding to the modified first compound but not to the modified second compound to form an immunecomplex; and (b) measuring the extent of binding of the antibody to the modified first compound, the extent thereof being related to the presence or amount of the first compound in the sample. This method is particularly well suited to an assay for Hey in the presence of immunologically related Cys. Another embodiment of the invention pertains to a method for determining the amount of Hey in the presence of Cys in a sample suspected of containing Hey, comprising the steps of (a) bringing together in an aqueous medium: the sample, a modifying reagent capable of chemically modifying Hey and Cys to form modified Hey and modified Cys, and an antibody capable of specifically binding to the modified Hey but not to the modified Cys to form an immunecomplex; and (b) measuring the amount of the immunecomplex, the amount thereof being related to the amount of Hey in the sample. One embodiment of the invention relates to a method of determining the amount of Hey in the presence of Cys in a sample suspected of containing Hey, comprising the steps of (a) bringing together in an aqueous medium: the sample, a modifying reagent capable of chemically modifying Hey and Cys to form modified Hey and modified Cys, an analog of the modified Hey bound to a detectable label, and an antibody capable of specifically binding to the modified Hey and to the analog, but not to the modified Cys and (b) measuring the extent of binding of the analog to the antibody, the extent thereof being related to the amount of Hey in the sample. Another embodiment of the invention pertains to a method of determining the amount of Hey in a sample suspected of containing Hey and Cys, comprising the steps of (a) bringing together in an aqueous medium: the sample, a modifying reagent capable of chemically modifying Hey and Cys to form modified Hey and modified Cys, an antibody capable of specifically binding to the modified Hey but not to the modified Cys to form an immunecomplex, a surface capable of binding the antibody, and a labeled analog of the modified Hey; and (b) measuring the extent of binding of the labeled analog to the antibody, the extent thereof being related to the amount of Hey in the sample. Still yet another embodiment concerns a method of determining the amount of Hey in a sample suspected of containing Hey and Cys comprising the steps of (a) bringing together in an aqueous medium: the sample, a modifying reagent capable of chemically modifying Hey and Cys to form modified Hey and modified Cys, an antibody capable of specifically binding to the modified Hey but not to the modified Cys to form an immunecomplex, and a receptor capable of binding to a portion of the modifying reagent, wherein the portion is present on the modified Hey; and (b) measuring the extent of binding of the antibody to the receptor, the extent thereof being related to the amount of Hey in the sample. Yet another embodiment pertains to method of determining the amount of Hey in the presence of Cys in a serum sample suspected of containing Hey wherein at least a portion of Hey is in the disulfide form (protein-bound and free-disulfide) , comprising the steps of (a) bringing together in an aqueous medium: the sample, a releasing agent to release Hey from the disulfide form, a modifying reagent capable of chemically modifying the sulfhydryl groups of Hey and Cys to form modified Hey and modified Cys, and an antibody capable of specifically binding to the modified Hey but not to the modified Cys to form an immunecomplex; and (b) examining the medium for the amount of the immunecomplex, the amount thereof being related to the amount of Hey in the sample. Another embodiment of the invention relates to a kit for use in a method for detecting Hey, comprising in a packaged combination: a modifying reagent capable of chemically modifying Hey and Cys to form modified Hey and modified Cys, and an antibody capable of specifically binding to the modified Hey but not to the modified Cys. The kit may also comprise a labeled analog of the modified Hey and a releasing agent. DESCRIPTION OF THE SPECIFIC EMBODIMENTS This invention pertains to methods to detect a compound in the presence of a homolog that is immunologically related to the analyte. The methods of this invention involve chemically modifying both the analyte and the homolog to increase their immunogenicity and facilitate antibody recognition. More importantly, this modification is done to make these compounds immunologically distinct. The chemical modification preferably comprises more than replacement of a hydrogen by a chemical group. Antibodies to the modified and immunologically distinct compounds are then prepared. An assay protocol comprises chemically modifying the analyte and homolog and then immunochemically detecting the modified analyte by means of the aforementioned antibodies. In particular, this invention relates to quantitative immunochemical methods that discriminate between the sulfhydryl amino acids homocysteine (Hey) and cysteine (Cys) . Hey and Cys are homologs: HOOC-CH(NH-) -CH 2 -CH 2 -SH Homocysteine HOOC-CH(NH 2 ) -CH 2 -SH Cysteine Furthermore, Hey and Cys are immunologically related because an antibody generated against Hey would be expected to crossreaet with Cys, as there is only a single carbon difference between the two compounds in the length of their amino acid side chains. Before proceeding further with the description of the specific embodiments of the present invention, a number of terms will be defined. Analyte: The compound or composition to be detected. The analyte can be a member of a specific binding pair ("sbp") and may be a ligand, which is monovalent (monoepitopic) or polyvalent (polyepitopic) , usually antigenic or haptenic, and is a single compound or plurality of compounds which share at least one common epitopic or determinant site. This invention provides a method to detect an analyte in a sample that also contains a homolog of the analyte, by chemically modifying the analyte and the homolog, then detecting the modified analyte. Therefore, the analyte is determined by detecting a reaction product whose presence will be detected only when the analyte of interest is present in the sample. Thus, the "modified" analyte is actually detected in the assay. The analyte can be found directly in a sample such as a body fluid from a host, for example, urine, blood, plasma, serum, saliva, semen, stool, sputum, cerebral spinal fluid, tears, mucus, or the like, but preferably is serum or plasma. The sample may be pretreated, as discussed below. Member of a specific binding pair ("sbp" member) : one of two different molecules, having an area on the surface or in a cavity that specifically binds to and is therefore defined as complementary with a particular spatial and polar organization of the other molecule. The members of the sbp can be referred to as ligand and receptor such as members of an immunological pair, e.g., antigen-antibody. As used herein, the term "ligand" refers to any compound for which a receptor naturally exists or can be prepared and the term "receptor" refers to any compound or composition capable of recognizing a particular spatial and polar organization of a molecule, i.e., epitopic or determinant site. Complementary sbp members bind to one another, as for example, a ligand and its complementary receptor. Sbp members can be immunological pairs such as antigen and antibody, or non-immunological such as avidin and biotin or the complementary strands of an oligonucleotide. Sbp members can also be small molecules or residues of small molecules and their receptors. Small molecules have a molecular weight of from 100 to 2000, preferably 150 to 1000, and either a receptor exists or can be prepared. Examples of small molecules include derivatives of biotin, lysergic acid, fluorescein or a fluorescein derivative, and vitamin B l2 , with the corresponding receptors being avidin or εtreptavidin, anti-lysergic acid, anti-fluorescein and intrinsic factor, respectively. Antibodies to small molecules can be prepared by linking the small molecule to an immunogenic carrier. Support or surface: The solid phase is typically a support or surface, which is a porous or non-porous water insoluble material that can have any one of a number of shapes, such as strip, rod, particle, including beads and the like. Suitable materials are well known in the art and are described in, for example, Ullman, et al . U.S. Patent No. 5,185,243, columns 10-11, which is incorporated herein by reference. Binding of sbp members to the support or surface may be accomplished by well-known techniques, commonly available in the literature. See, for example, "Immobilized Enzymes," Ichiro Chibata, Halsted Press, New York (1978) and Cuatrecasaε, J. Biol. Chem. 245:3059 (1970) . Whatever type of solid support is used, it must be treated εo as to have antibodies that specifically bind the modified analyte bound to its εurface or a receptor bound to itε surface, which receptor will specifically bind to such antibodies or to a small molecule conjugated to such antibodies. For example, avidin or streptavidin can be covalently bound to spherical glass beads of 0.5-1.5 mm and used to capture a biotinylated antibody. Signal producing system ("spε") : one or more components, at least one component being a label, which generate a detectable signal that relateε to the amount of bound and/or unbound label. Preferably, the label is a fluorescer, radiolabel, enzyme, chemi-luminescer or photoεensitizer, which are detected by obεerving enzyme activity, lumineεcence, light absorbance or radioactivity. Suitable labels include, by way of illustration and not limitation, enzymes such as alkaline phosphatase and horseradish peroxidase; a substrate for a replicase such as QB replicase; promoterε; dyes; fluoreεcerε, such as fluorescein, iεothiocyanate, rhodamine compoundε, phycoerythrin, phycocyanin, allophycocyanin, o-phthaldehyde, and fluoreεcamine; chemiluminescers such as isoluminol; sensitizers; coenzymes; enzyme εubεtrates; radiolabels such as l25 I, 13I I, l4 C, 3 H, 32 P and 35 S; particleε such as latex or carbon particles; metal sol; crystallite; liposomes; cells, etc., which may or may not be further labeled with a dye, catalyst or other detectable group and the like. Suitable enzymes and coenzymes are discloεed in Litman, et al . , U.S. Patent No. 4,275,149, columns 19 to 28, and Boguslaski, et al . , U.S. Patent No. 4,318,980, columnε 10 to 14; suitable fluorescers and chemiluminescers are disclosed in Litman, et al., U.S. Patent No. 4,275,149, at columns 30 and 31; which discloεures are all incorporated herein by reference. There are numerous methods by which the label can produce a signal detectable by external means, εuch as viεual examination, electromagnetic radiation, electrochemical detection, phonaccoustic spectroscopy, and the like. The label or other sps members can also be bound to an sbp member, anσther molecule or to a support. The label can directly produce a signal, and therefore, additional components are not required to produce a εignal. Numerous organic molecules, for example fluorescers, are able to absorb ultraviolet and visible light, where the light absorption transfers energy to these molecules and elevates them to an excited energy state. This absorbed energy is then disεipated by emission of light at a second wavelength. Other labels that directly produce a signal include radioactive isotopeε and dyes. Alternately, the label may need other components to produce a signal, and the sps would then include all the components required to produce a measurable εignal, which may include substrates, coenzymes, enhancers, additional enzymes, subεtanceε that react with enzymic products, catalysts, activators, cofactors, inhibitorε, scavengers, metal ions, specific binding substance required for binding of signal generating substances, and the like. A detailed discussion of suitable signal producing systems can be found in Ullman, et al. U.S. Patent No. 5,185,243, columns 11-13, which is incorporated herein by reference. The label can be bound covalently to numerous sbp members: an antibody that binds the modified analyte; a receptor for an antibody that binds the modified analyte; a receptor that is capable of binding to a small molecule conjugated to an antibody that binds the modified analyte,* or a ligand, particularly an analog of the modified analyte. Bonding of the label to the sbp member may be accomplished by chemical reactions which result in replacing a hydrogen atom of the label with a bond to the sbp member or may include a linking group between the label and the sbp member. Other sps members may also be bound covalently to εbp memberε. For example, two sps members such as a fluorescer and quencher can be bound respectively to an analog of the modified analyte and an antibody to the analyte that forms a complex with the analog. Formation of the complex brings the fluorescer and quencher in close proximity, thus permitting the quencher to interact with the fluorescer to produce a εignal. Methods of conjugation are well known in the art. See for example, Rubenstein, et al . , U.S. Patent No. 3,817,837, which is incorporated herein by reference. This invention also contemplates having an antibody bound to a first member of a signal producing system and a detectable label as the second member of a signal producing system. For example, when the detectable label is bound to an analyte analog, the extent of binding of the antibody to the analog can be measured by detecting the signal produced by the interaction of the signal producing system members. Ancillary Materialε: Various ancillary materialε will frequently be employed in the methods in accordance with the present invention. For example, buffers will normally be present in the assay medium, as well as stabilizerε for the aεεay medium and the aεsay components. Frequently, in addition to these additives, proteins may be included, such as albumins, organic solvents such as formamide, quaternary ammonium salts, polycations such aε dextran sulfate, or surfactants, particularly non-ionic surfactants, binding enhancers, e.g., polyalkylene glycols, or the like. As mentioned above, this invention pertains to methods to detect a compound in the preεence of a homolog that iε immunologically related to the analyte. One method of the present invention pertains to the determination of the presence or amount of a first compound in the presence of a second compound in a εample suspected of containing the compounds, wherein the compounds are immunologically related. The method comprises the εtepε of (a) bringing together in an aqueous medium: the sample, a modifying reagent capable of chemically modifying the first compound and the second compound to form a modified first compound and a modified εecond compound, and an antibody capable of εpecifically binding to the modified first compound but not to the modified second compound to form an immunecomplex; and (b) meaεuring the extent of binding of the antibody to the modified firεt compound, the extent thereof being related to the preεence or amount of the firεt compound in the εample. This method is particularly useful when the compounds are homologous. As used herein the term "immunologically related" is used to refer to two compounds, one of which is the analyte of interest, for which an antibody existε that exhibitε croεε-reactivity. Calculation of the cross- reactivity iε based upon the ratio of the concentrations of the two compounds that result in equal binding to the antibody. Antibodieε to different homologs often display high croεε-reactivity. Frequently, therefore, a compound can not be accurately measured in an immunoassay if a homolog is present in the medium. In that case, using an antibody reagent to detect the analyte, may often result in some of the antibodieε binding to the homolog and giving a falεe positive or an inaccurate quantitative reading. The extent of cross-reactivity that is acceptable in an immunoasεay dependε upon the highest concentration expected of the croεε- reactant, the εensitivity required for the assay and the accuracy needed. For example, if an antibody is 10% cross-reactive with Cys and Cys iε present in a sample in an amount five times greater than the lowest level of Hey to be detected, then the measured level of Hey will be 50% too high when Hey is at its lowest level. If only a 5% error is acceptable, then the cross-reactivity would have to be less than 1%. In the present invention, antibodies directed against the modified analyte must exhibit a low degree of cross-reactivity with the modified homolog. As exemplified above, quantification of a "high" or "low" degree of cross-reactivity will depend upon the maximum expected concentration of the modified homolog and the sensitivity and accuracy desired. As used herein, the term "homologous" describeε a compound in a εeries of organic compounds that have the same chemical functional groups, and which differ from the next by an insertion of a -CH 2 - group in the molecule. For example, CH 3 OH (methanol) , C 2 H 5 OH (ethanol) and C 3 H 7 OH (propanol) , etc., are all homologs and form a homologous serieε. Exampleε of homologouε compoundε within the εcope of thiε invention are homocysteine and cysteine, amphetamine and methamphetamine, serine and homoserine, citrate and homocitrate, aεpartate and glutamate, alanine and β-amino- isobutyrate, tetrahydrofolate and N 5 -methyltetrahydrofolate, oxaloacetate and α-ketoglutarate, gentisic acid and homogentisic acid, morphine and normorphine, and the like. As used herein the term "modifying reagent" is used to mean a reagent that reacts with two homologs to form two new and immunologically distinct compounds. The term "immunologically distinct" means that antibodies to the modified analyte are available that exhibit low cross-reactivity with the modified homolog. Homologs have the same functional groups, and antibodies to different homologs often display a high degree of cross- reactivity. Therefore a compound can not be accurately measured in an immunoaεεay if a homolog is present in the medium. The modifying reagent is capable of chemically modifying the compound being assayed (the "analyte") and its homolog so that the two compounds become immunologically distinct. As a result, antibodies to the modified analyte exhibit lower cross-reactivity with the modified homolog. The modifying reagent will usually be soluble in aqueous solventε. The modifying reagent is selected so that it will modify both compounds, within a short time, usually less than 4 hours, preferably less than 20 minuteε, and moεt preferably leεε than 5 minutes. It must be used in εufficient quantitieε that if it reactε with other componentε in the sample, it will be available to react with the analyte. Examples of suitable modifying reagents include alkylating agents, acylating agents, metals, nucleophiles and electrophileε. Thiε invention alεo contemplateε having a modifying reagent with a detectable label bound to it, εuch as a radiolabel. In that instance, the amount of modified analyte can then be measured by detecting the label. In the above described method for detecting a first compound in the presence of a second, immunologically related compound, the antibody can be detectably labeled or be capable of being detectably labeled. Alternatively, when the modifying reagent is labeled, the antibody can be bound to a support or can be capable of being bound to a support. As used herein, the termε "capable of being detectably labeled" and "capable of being bound to a εupport" means that a reagent, such as the antibody in this instance, is bound to a first εbp member or a εmall molecule and a complementary second sbp member or receptor for the small molecule, iε in turn bound to a label or a support. Therefore, the antibody is not actually labeled or bound to a support, but will become labeled or bound, when the complementary sbp member or receptor is added. The method deεcribed above can further comprise bringing together an analog of the modified first compound bound or capable of being bound to a detectable label and an antibody that can specifically bind both the analog and the modified firεt compound. The extent of binding of the antibody to the modified first compound iε meaεured by measuring the extent of binding of the analog to the antibody. The antibody can be bound to a εupport or capable of being bound to a εupport. More particularly, thiε invention involves chemically modifying Hey and Cyε in a sample in a manner that causes them to be immunologically distinct, and then detecting Hey. One method of determining the amount of Hey in the presence of Cyε in a sample suspected of containing Hey, comprises the steps of (a) bringing together in an aqueous medium: the sample, a modifying reagent capable of chemically modifying Hey and Cys to form modified Hey and modified Cyε, and an antibody capable of specifically binding to the modified Hey but not to the modified Cyε to form an immunecomplex; and (b) meaεuring the amount of the immunecomplex, the amount thereof being related to the amount of Hey in the εample. As used herein the term "immunecomplex" means the complex formed by the immunological binding of an antigen to an antibody. The same labeled reagents are suitable for use in thiε method aε thoεe deεcribed above with reference to a method of detecting one compound in the preεence of an immunologically related compound, i.e., labeled modifying reagent, labeled antibody, and labeled analog. In one embodiment of the invention, the modifying reagent is deεigned to form a bond to at leaεt one of the three functionalitieε of Hey and Cyε, the amino, εulfhydryl and carboxyl groups. Usually in at least one of the products, a εecond bond will form to one of these groups after formation of the first bond. This will frequently produce a ring. For example, the first bond can be to the sulfhydryl group and the εecond bond can involve the amine. Modifying reagents capable of reacting with the sulfhydryl group of Hey and Cys usually have a group that is suitably positioned to react with the Hey or Cys amino or carboxyl groups. This group may be, by way of illustration and not limitation, an alkylating or acylating group, a metal, or an aryl group activated for nucleophilic substitution. Particularly suitable are ketones εubεtituted at the alpha position by a leaving group selected from the group consiεting of Cl, Br, I, sulfonates, and εulfonium salts. As indicated above, alkylating agents, acylating agents and metals are particularly suited as modifying reagents for use in an Hey assay. As uεed herein the term "alkylating agent" meanε a modifying reagent that reacts with an organic molecule to introduce an alkyl or aryl group into the molecule. An alkyl group is a group derived from an alkane by dropping one hydrogen from the formula. For example, methyl, ethyl, propyl, etc. An aryl group is a group whose molecules have the ring structure characteristic of benzene, naphthalene, phenanthrene, anthracene, etc., i.e., either the six-carbon ring of benzene or the condensed six-carbon ringε of the other aromatic derivatives. Preferably, the alkylating agent has a group εuch aε an alkyl halide or an aryl halide or electronegatively εubstituted olefins such as a maleimide or acrylonitrile. When an alkyl halide iε uεed, a ketone, aldehyde, eεter, olefin, acetylene, nitrile, aryl, or the like will usually be adjacent to the carbon bearing the halogen. Suitable alkylating agents include, by way of illustration and not limitation, Modifying Reagents I-VIII, shown in the table below. A particularly useful alkylating agent is p-bromoacetylbenzoic acid (BABA) , Modifying Reagent II. As used herein the term "acylating agent" means a modifying reagent, usually a sulfonie acid or carboxylie acid derivative such as its anhydride or chloride or active ester, that reacts with an organic molecule to introduce a sulfonyl or acyl group into the molecule. An acyl group is an organic acid group in which the -OH of the carboxyl group is replaced by some other substituent. For example, acetyl (CH 3 CO-) and benzoyl (C^CO-) . Suitable acylating agents include, by way of illustration and not limitation, Modifying Reagents III-V and VIII, shown in the table below. As used herein the term "metal" means heavy metal chelateε, including organometallic reagents hat are capable of forming chelates with bidentate ligandε. Suitable metalε include, by way of illuεtration and not limitation, Modifying Reagentε IX and X, εhown in the table below. Particularly useful modifying reagents have an alkylating group εuch aε an alkyl halide or an aryl halide. When an alkyl halide iε used, a ketone, aldehyde, ester, olefin, acetylene, nitrile, aryl, or the like will usually be adjacent to the carbon bearing the halogen. Typical reagentε and their reaction productε with Hey and Cys, i.e., the "modified" Hey and Cyε, are given in the table below: -12* MODIFYING MODIFIED MODIFIED HOMOCYSTEINE REAGENT CYSTEINE (VIII ) (Villa) (VHIb) (IX) (IXa) (IXb) (X) (Xa) (Xb) The underlined group in each reagent is exemplary of a solubilizing group that iε not necessary for the derivatization reaction: Reaqent Solubilizinq Grouo I -COOH II -COOH III -COOH IV -SO 3 H V -SO 3 H VI -COOH VII -COOH VII -COOH IX -COOH X -COOH When a label is present in the modifying reagent, it may aε a matter of convenience, be attached through the solubilizing group by means of an amide or ester bond. Preferably, the modifying reagent modifies the sulfhydryl groups of the Hey and the Cys to form a modified Hey and Cys where the εulfur of at leaεt one of the modified compoundε iε part of a ring. Exemplary of modifying reagents where one of the modified compounds forms a ring are Modifying Reagents I-X, shown above. More preferably, a modifying reagent is selected so that both the resulting modified Hey and Cys have a ring, as is the case with Modifying Reagentε VI and X. Of the aforementioned modifying reagentε, a preferred reagent iε BABA (Modifying Reagent II) , which reactε rapidly with both Hey and Cyε at room temperature and neutral pH. BABA haε the following advantageε: the phenyl ring provideε a highly immunogenic moiety when preεented to an immune εystem; the carboxylate provides some degree of water solubility aε well as a convenient point of attachment to a carrier protein; and the earbonyl of the bromoacetyl group provides a convenient means of differentiating between modified Hey and Cys. As mentioned above, antibodies to Hey potentially exhibit a high degree of crosε-reactivity with Cyε, becauεe Hey and Cys are homologs and are immunologically related. There are numerous ways to modify Hey and Cys to make them immunologically distinct. For example, a six-membered ring iε εtereochemically quite different from a chain and therefore antibody recognition can be improved and cross-reactivity reduced when Cys is modified to form a six-membered ring and the modified Hey is acyclic. This iε illuεtrated with Modifying Reagent (VIII) : (VIII) which modifieε Cyε and Hey to form Modified Cyε (Villa) , where the εulfur iε part of a six-membered ring: (Villa) and Modified Hey (Vlllb) , where the sulfur is not part of a ring: (VIb) Another embodiment of this invention involves an asεay for determining the amount of Hey in the presence of Cys in a sample εuεpected of containing Hey, uεing the following method: (a) bringing together in an aqueouε medium: the εample, a modifying reagent capable of chemically modifying Hey and Cys to form modified Hey and modified Cys, an analog of the modified Hey bound to a detectable label, and an antibody capable of specifically binding to the modified Hey and to the analog, but not to the modified Cys; and (b) measuring the extent of binding of the analog to the antibody, the extent thereof being related to the amount of Hey in the sample. In this embodiment, the signal measured is indicative of the extent of binding of the analog to the antibody. Hence the signal is inversely related to the amount of Hey in the sample. Still another embodiment of this invention involves an assay for the amount of Hey in a sample suspected of containing Hey and Cys, using the following method: (a) bringing together in an aqueous medium: the sample, a modifying reagent capable of chemically modifying Hey and Cys to form modified Hey and modified Cys, an antibody capable of specifically binding to the modified Hey but not to the modified Cys to form an immunecomplex, a surface capable of binding the antibody, and a labeled analog of the modified Hey; and (b) measuring the extent of binding of the labeled analog to the antibody, the extent thereof being related to the amount of Hey in the sample. The method can further comprise bringing together a labeled first specific binding pair member, wherein the label of the labeled analog iε a complementary εecond specific binding pair member, for example an analog bound to fluorescein and an enzyme-labeled anti-fluorescein antibody. In this embodiment, the signal measured is indicative of the extent of binding of the analog to the antibody. Hence the signal is inversely related to the amount of Hey in the sample. Yet another embodiment of the invention relates to an asεay for determining the amount of Hey in a εample εuεpected of containing Hey and Cys, comprising the steps of (a) bringing together in an aqueous medium: the sample, a modifying reagent capable of chemically modifying Hey and Cys to form modified Hey and modified Cys, an antibody capable of specifically binding to the modified Hey but not to the modified Cys to form an immunecomplex, and a receptor capable of binding to a portion of the modifying reagent, wherein the portion is present on the modified Hey; and measuring the extent of binding of the antibody to the receptor, the extent thereof being related to the amount of Hey in the sample. The modifying reagent may be comprised of a hapten, a ligand, or a Hey or Cys receptor, wherein the receptor that binds a portion of the modifying reagent may be an anti-hapten antibody, a receptor for the ligand, or an antibody to the Hey or Cys receptor, respectively. Either or both the receptor and the specific antibody can be labeled, and one can be bound to a support. The following are merely illustrative of the embodiment encompassed by this method. For example, a modifying reagent bound to biotin forms a modified Hey bound to biotin. A complex iε formed that compriεes the modified Hey, a labeled antibody capable of specifically binding to the modified Hey and avidin bound to a εupport. Alternatively, a modifying reagent bound to a hapten forms a modified Hey bound to the hapten. A complex is formed that comprises the modified Hey, an antibody capable of specifically binding to the modified Hey bound to a first label and an anti-hapten antibody bound to a εecond label. Hey exists in human plasma as various mixed diεulfides. Normally, a major fraction of Hey (approximately 70%) is protein bound, via a disulfide bond to circulating proteins εuch aε albumin. The remaining "free" Hey (approximately 30%) iε in the form of Hey (reduced) or aε mixed diεulfides with other thiols εuch aε Cys. The sum of these Hey species present in plasma (protein-bound, free-disulfide and free-reduced) is referred to as the "total Hey". Measurement of total Hey in serum, preferably involves a pretreatment step to releaεe protein-bound Hey and reduce the mixed diεulfide form of Hey, i.e., the free-disulfide form. The preferred method of this invention measures the total Hey since the ratio of the three forms of Hey (protein-bound, free-disulfide or free-reduced) is dependent on factors such as Hey concentration, sample preparation, storage method, etc. However, measurement of free Hey may be done without pretreatment, if desired. To measure the total amount of Hey in a sample, the εample iε first combined with a releasing agent to release the Hey from the circulating proteinε aε well as from Hey dimers and other mixed disulfides. Particularly εuitable iε a reducing agent, which reduceε organic diεulfides. One such method of determining the amount of Hey in the presence of Cyε in a serum εample suspected of containing the Hey, wherein at least a portion of the Hey iε in the diεulfide form (protein-bound and free-diεulfide) , comprises the εtepε of (a) bringing together in an aqueouε medium: the εample, a releasing agent to release the Hey from the diεulfide form, a modifying reagent capable of chemically modifying the sulfhydryl groups of Hey and Cyε to form modified Hey and modified Cyε, and an antibody capable of εpecifically binding to the modified Hey but not to the modified Cyε to form an immunecomplex; and (b) examining the medium for the amount of the immunecomplex, the amount thereof being related to the amount of Hey in the sample. The releasing agent can be a reducing agent, εuch as described above, or it could react with diεulfides to form modified Hey and modified Cyε and thereby act aε both a releasing agent and the modifying reagent. Suitable releasing agents capable of liberating the Hey and Cyε sulfur compounds from εerum proteinε are well known in the art and include, without limitation, reducing agentε εuch aε sodium and potassium borohydride; thiols such as dithiothreitol, dithioerythritol, 2- mercaptoethanol, thioglycolic acid and glutathione; and phoεphineε and trialkylphoεphineε such aε tributylphoεphine and triε(2- carboxyethyDphoεphine. Particularly εuitable reducing agentε are dithiothreitol and triε (2-carboxyethyl)ρhoεphine (TCEP) , the latter of which iε deεcribed in Burnε, et al., J. Org. Chem. * 56 (8) :2648-2650 (1991). The reduction reaction iε illustrated below with TCEP, where protein-bound Hey and protein-bound Cys are liberated from the serum protein: When the releasing agent also acts as a modifying reagent, sulfiteε, phosphites, phosphorimidateε, and the like, including salts of their mono- and di-esters can be used. These reagents react with Hey and Cys disulfideε to give modified Hey and Cys that have thioεulfate or thiophosphate groups. On the other hand, when the releasing agent does not act as a modifying reagent, the liberated thiols are modified with a thiol-specific modifying reagent. Selection of the releasing agent may often be determined by the selection of the modifying reagent, as some combinations of reagents are more preferred than others. Many combinations of releasing agents and modifying reagents will react with each other, thus necessitating that the sample and the releasing agent firεt be combined and then εufficient modifying reagent added to react with the releasing agent and the liberated Hey and Cys. Alternatively, a releasing agent such as a phosphine can be used with an unsaturated modifying reagent εuch aε a maleimide with which it will not react. While this provides more flexibility in the protocol, the use of a less reactive modifying reagent can require a longer incubation period to asεure complete reaction with the Hey and Cyε. Thuε, selection of the releasing agent and modifying reagent will be based upon the desired protocol and timing of the assay. Appropriate reaction conditions are chosen for carrying out the methods in accordance with the present invention. The following description sets forth suitable conditions, which are subject to modification by those skilled in the art depending on the specific reagents and assay protocol chosen for any particular application. For example, the methods of this invention can be applied to numerous types of assays such as heterogeneous or homogeneous, competitive or direct, and the conditions and reagents used will be selected accordingly. The sample, preferably in a suitable medium, can be examined directly or may be pretreated before the sample is added to the aεsay medium. Pretreatment can render the analyte more readily available to one or more of the assay reagents or more readily detectible by reducing interference in the assay by removing any unwanted materials. The sample may be pretreated to separate or lyse cells; precipitate, hydrolyse or denature proteins; hydrolyze lipids; εolubilize the analyte; or the like. Such pretreatment may include, without limitation: centrifugation; treatment of the sample with an organic solvent, for example, an alcohol, preferably an alcohol having less than about 7 carbon atoms εuch aε methanol; treatment with detergents; and treatment with a releasing agent. The concentration of the compound to be assayed will generally vary from about 10 "4 to 10 "15 M, more usually from about 10" 5 to 10 *12 M. More specifically, for an Hey analyte, the concentration will generally vary from about 10" 4 to 10" 8 M, more usually from about 10 "5 to 10 "7 M. Considerationε such aε whether the aεεay iε qualitative, εemi-quantitative or quantitative, the particular detection technique, and the concentration of the analyte will normally determine the concentration of the other reagentε. In addition, the final concentration of each of the reagents will normally be determined empirically to optimize the senεitivity of the aεεay over the range of intereεt. The relative amounts of the various reagents used in the asεay and packaged in the kitε described below, can be varied widely to provide for concentrations of the reagents that substantially optimize the reactions that need to occur during the present method and to further εubεtantially optimize the εensitivity of any assay performed. The concentration of the antibodies in the assay medium is dependent on the type of assay, heterogeneous or homogeneous, competitive or direct, etc. Normally, the anti-modified analyte antibody will be present in the assay medium in a concentration of about half to 10 7 times the concentration of the modified analyte, more usually from about equal to about 10 3 times the concentration of the modified analyte. In carrying out the method of this invention, preferably an aqueous medium will be employed. Other polar cosolventε may alεo be employed in the medium, usually oxygenated organic solvents of from 1-6, more usually from 1-4 carbon atoms, including alcohols, ethers and the like. Usually theεe cosolvents will be present in less than about 70 weight percent, more usually, in lesε than about 30 weight percent. In aεsays in accordance with the present invention, the pH for the medium will usually be in the range of about 5-10, preferably, in the range of about 7-9. The pH is chosen so as to maintain a significant level of binding between sbp members, while optimizing signal producing proficiency. In some instances, a compromise will be made between these two considerations. In addition, the pH may also be selected so as to maintain a particular structural conformation such as cyclization. Various buffers may be uεed to achieve the deεired pH and maintain the pH during the determination. Illustrative buffers include borate, phosphate, carbonate, Triε, barbital and the like. The particular buffer employed iε not critical to this invention but in individual aεεayε, one buffer may be preferred over another. Moderate temperatures are normally employed for carrying out the method. The temperature may vary with the step being undertaken. More typically, however, a constant temperature is maintained during the period of the method. The temperatures will generally range from 10 to 50°C, usually from about 15° to 40°C. While the order of addition of the various reagents may be varied widely, there will be certain preferences depending on the nature of the particular assay format being used. The reagents and sample can be combined simultaneously or wholly or partially sequentially with each other and the εupport. Aε uεed herein, the term "wholly or partially εequentially" eanε that, when the εample and variouε reagents utilized in the present invention are combined other than concomitantly (simultaneously) , one or more of the reagents may subεequently be added alone, or together with other reagents. For example, the sample suspected of containing the analyte and the releasing agent may be combined first to allow the releasing agent to reduce any disulfide bonds, followed by the simultaneous or step-wise addition of the remaining reagents. Except when the releasing agent and the modifying reagent do not react together, addition of the releasing agent prior to the modifying reagent iε preferred. Optionally, one or more incubation steps may be involved after each reagent addition, generally ranging from about 30 secondε to 6 hourε, more uεually from about 2 inuteε to 1 hour. It is understood that there are certain sequences of steps that are more convenient and the choice of the particular sequence to be employed depends upon the selection of reagents and assay format and is not critical to the invention. The final step of an immunoassay is to measure the extent of binding between the modified analyte and the antibody, which is related to the presence or amount of the analyte in the sample. There are numerous ways to meaεure the extent of binding which are well known in the art. Theεe methods differ depending on, for example, whether the assay iε competitive, homogeneous, heterogenous, etc. One meaεurement of the extent of binding between the modified analyte and the antibody can be accompliεhed by adding detectably labeled modified analyte, i.e., a labeled analyte analog, and detecting the signal produced by binding of the labeled modified analyte with the antibody in competition with binding of the modified analyte. For example, in a homogeneous aεsay, the detectable label can be an enzyme and the final step might involve activating the enzyme label by addition of subεtrate. The signal produced is usually inversely related to the extent of binding between the labeled modified analyte and the antibody. The presence or amount of signal is then related to the presence or amount of analyte in the εample. In another meaεurement in a homogeneous assay which is competitive, the extent of binding can be determined by use of a fluorescer-labeled modified analyte and a quencher-labeled antibody. The signal is again inversely related to the extent of binding and directly related to the amount of - 20 - analyte present in the εample. Alternatively, a homogeneouε non- competitive assay may be used in which the fluorescer-labeled modified analyte iε not added as a separate reagent but is formed by reaction of the analyte with a fluorescer-labeled modifying reagent. In thiε caεe, the εignal is again inversely related to the extent of binding, but is inversely related to the amount of analyte present in the sample. In a heterogeneous assay, unlabeled anti-modified analyte antibodies may be bound to a support. Following incubation of the εupport with the modified analyte and labeled modified analyte, the support is separated from the liquid phase the amount of signal from the solid phase or the liquid phase is then determined, which indicates the extent of binding between the modified analyte and the antibody, and thus the amount of analyte in the εample. For a more detailed discusεion of the above immunoaεεay techniques, see "Enzyme-Immunoassay, " by Edward T. Maggio, CRC Press, Inc., Boca Raton, Florida, 1980. For illustrative purpoεeε, the following asεay protocols can be utilized. These illustrations should not be construed aε limiting the scope of the invention, but are merely illustrative of the qualitative, semi-quantitative and quantitative asεay protocols in which the method of thiε invention can be uεed for determining Hey or Cyε in a εample. The εignal detected in theεe methods is compared to a standard or control, having a known concentration of Hey. (A) In an asεay for Hey uεing Modifying Reagent (I) , a monoclonal antibody that specifically binds to modified Hey (lb) and not to modified Cys (la) , iε utilized. A εample εuεpected of containing Hey and Cyε iε combined with Modifying Reagent (I) in a εuitable medium. The reactions will occur as follows.* Reagent (I) + Hey → modified Hey (lb) and Reagent (I) + Cys -» modified Cys (la) , if Hey and Cyε are preεent. An analog of modified Hey (lb) , conjugated to glucose-6-phosphate dehydrogenase, is also added to the medium. After enough time haε elapsed for the reactions and immunological binding to occur, the anti-Hey antibody is added to the medium. After incubation for 30 minutes, glucose-6-phosphate is added. The εignal produced iε directly related to the amount of Hey in the εample. (B) In an aεεay for total Hey in serum, a monoclonal antibody that specifically binds to modified Hey (Vb) and not to modified Cys (Va) , iε utilized. The antibody iε bound to glaεs beads. The serum sample is firεt treated with dithiothreitol (DTT) to reductively liberate any Hey tied up aε diεulfides. Excess DTT is sequestered by treatment with sodium arsenite. The pretreated εample iε then combined with Modifying Reagent (V) , which haε been labeled with 3 H, and then combined with the glaεs bead- bound antibody. The medium is incubated for 10 minutes and the beads separated and washed. The radioactivity of the beads iε then measured. The signal produced is directly related to the amount of Hey in the sample. C) In an assay for Hey using Modifying Reagent (IX) , a monoclonal antibody that specifically binds to modified Hey (IXb) and not to modified Cys (IXa) , is bound to a support. An analog of modified Hey (IXb) , conjugated to horseradish peroxidase (HRP) , is also utilized. A sample suspected of containing Hey and Cys is combined with Modifying Reagent (IX) in a εuitable medium. After enough time has elapsed for the reactions to occur, the anti-Hey-support conjugate and the modified Hcy-HRP conjugate are then added to the medium. After incubation of the suεpenεion for 30 minuteε, the liquid and εolid phaεeε are separated, and the solid phase washed. An aqueous medium, to which substrate has been added is then added to the solid phase and the signal produced is inversely related to the amount of Hey in the sample. The method of this invention further involveε the preparation of polyclonal and monoclonal antibodieε to immunogenε homologouε to the immunologically diεtinct analyte, i.e., the "modified" analyte. The immunogen used to obtain the antibodies uεeful in the methodε of the present invention is the modified analyte, which iε obtained by reacting the compound being assayed with the selected modifying reagent. Typically, for preparation of the immunogen, first the solubilizing groups of the modifying reagents are used for attaching the reagent to an immunogenic carrier protein. Usually these attaching groups are first activated and the reagent is then coupled to the carrier protein. The resulting reagent- protein conjugate is then allowed to react with the analyte, purified, and uεed as an immunogen. This invention pertains to a method of detecting a firεt compound in the preεence of a εecond compound, which iε a homolog of the firεt. Therefore, the antibodieε described above are screened for their ability to react specifically with the reaction product of the modifying reagent and either the first compound (analyte) or the second compound; i.e., bind to either the modified analyte or to the modified second compound. For example, in an assay for Hey, the immunogen would be the derivatized Hey linked to an immunogenic carrier protein. The resulting antibodies would then be capable of specifically recognizing the modified Hey analyte and would not recognize the modified Cys. Other groups for attaching the modifying reagent to the protein carrier can be used, including, for example, aldehydes, ketones, diazonium salts, alkylating agents, etc. Conveniently, carboxylie acids or εulfonic acidε are converted to active esters or acid halides for conjugation. The carrier protein may be any protein, preferably one that iε foreign to the animal being immunized such as keyhole limpet hemocyanin or bovine serum albumin, if the animal is, for example, a mouse, rabbit, εheep or goat. Preparation of polyclonal antibodieε useful in the methods of this invention, is by methods εuch as are well known in the art. Monoclonal antibodieε are essentially obtained by a process εimilar to that deεcribed in Milεtein and Kohler, Nature 256:495-497 (1975) . The details of the Milstein and Kohler proceεs are well establiεhed. Generally, the proceεε involves injecting a host, usually a mouse or other suitable animal, with an immunogen. Cellε are then taken from the εpleen of the animal. Alternatively, the host may be unsensitized εpleen cells, which are sensitized to the immunogen in vitro. The resulting cellε are fuεed with myeloma cellε. The result is a hybrid cell, referred to as a "hybridoma" that can be cultured in vi tro . The population of hybridomas iε screened and manipulated so aε to iεolate individual clones, each of which secretes a single antibody to the antigen. When the immunogen is introduced into the host, the host's immune syεtem reεponds by producing a variety of antibodies that are able to recognize various sites on the immunogen. These numerous antibodieε have different affinities and specificities. To obtain those antibodies that have desirable affinity and specificity traits for the particular asεay method being used, the different hybridoma cell lines are screened. As a matter of convenience, the reagents for use in the present invention can be provided in a kit for use in an asεay method. A typical kit for uεe in a method for detecting Hey, compriεes in a packaged combination: a modifying reagent capable of chemically modifying Hey and Cys to form modified Hey and modified Cys, and an antibody capable of specifically binding to the modified Hey but not to the modified Cys. The kit can further comprise a labeled analog of the modified Hey. The kit can also comprise a releasing agent suitable for releasing any Hey in the diεulfide form. Under appropriate circumstances one or more of the reagents in the kit can be provided in solution or as a dry powder, usually lyophilized, including excipientε, which on diεεolution will provide for a reagent solution having the appropriate concentrations for performing a method or asεay in accordance with the preεent invention. To enhance the verεatility of the εubject invention, the reagentε can be provided in packaged combination, in the same or separate containers, so that the ratio of the reagents provides for substantial optimization of the method and assay. The reagents may each be in separate containers or various reagents can be combined in one or more containerε depending on the cross-reactivity and stability of the reagents. As a matter of convenience, the reagents employed in the present invention can be provided in predetermined .amounts. The reagents can include a first antibody and a second antibody, which are capable of specifically binding the modified analyte and a reagent to chemically modify the analyte. The kit can also contain written inεtructionε on how to use the reagents and/or how to perform a particular aεεay, for example in the form of a package inεert. The invention iε demonstrated further by the following illustrative example. ABBREVIATIONS AP Alkaline Phosphataεe (calf inteεtinal) BABA p-Bromoacetylbenzoic Acid BSA Bovine Serum Albumin Cyε-ABA Cyεteine Acetylbenzoic Acid DCC Dicyclohexylcarbodiimide DPBS Dulbecco' ε Phosphate Buffered Saline DTT Dithiothreitol DMEM Dulbecco'ε Modified Eagle Medium DMSO Dimethylεulfoxide EDTA Ethylenediaminetetraacetic Acid FBS Fetal Bovine Serum Hcy-ABA Homocyεteine Acetylbenzoic Acid Hcy-ABA-BSA Homocyεteine Acetylbenzoic Acid-Bovine Serum Albumin Hcy-ABA-AP Homocysteine Acetylbenzoic Acid-Alkaline Phosphatase Conjugate NHS N-Hydroxyεuccinimide PBS Phoεphate Buffered Saline PEG Polyethylene Glycol PNPP Phenylphoεphate S-DMEM Super Dulbecco'ε Modified Eagle Medium RAM Rabbit Anti-Mouεe TCEP Triε (2-carboxyethy1)phoεphine THF Tetrahydrofuran PREPARATION OF MATERIALS BSA Cohn crystallized was from ICN. AP was from Pierce. All other chemicals were reagent grade and commercially available from sources such as Gibco and Sigma. All solutions were prepared in H 2 0 and all reactions were performed under ambient conditions unless otherwise stated. A) Preparation of p-Bromoacetylbenzoic Acid (mw 243) 2.0 g (12.2 mmole) of 4-acetyl benzoic acid was added to 90 ml of acetic acid in a 500 ml erlenmeyer flask and heated at 45°C until dissolved. Maintaining the temperature at 45°C, 1.95 g (12.2 mmole, 0.61 ml) of bromine dissolved in "2 ml acetic acid was added εlowly over 1 hour with vigorous stirring. The reaction required an induction period of several minutes before proceeding, which became evident by the loεs of the red bromine color and the evolution of HBr. The product, BABA (Modifying Reagent II) , waε precipitated on ice, collected by filtration and recryεtallized 3 timeε from hot (75°C) ethanol. B) Preparation of Bromoacetylbenzoic Acid N-hvdroxysuccinamide Ester (mw 340) BABA was prepared as described above and recrystallized once. To 0.6 g BABA (2.5 mmole) and 0.28 g NHS (2.5 mmole) dissolved in 8 ml dry THF at 0°C was added 0.51 g (2.5 mmole) DCC in 1 ml THF. A precipitate was observed after several minutes and the solution stirred overnight. The solution was filtered through a glass frit and the volume reduced to ~2 ml under vacuum. Some precipitation was observed following volume reduction. An excesε of petroleum ether was added to precipitate the remaining product and the solid was recrystallized by dissolving in a limiting amount of THF and adding back petroleum ether until turbid, then cooling to -20°C and collecting the solid by filtration. Yield of BABA-NHS, 0.2 g. C) Preparation of Cvsteine Acetylbenzoic Acid (mw 283) 60 mg of BABA and 60 mg of 1-cysteine were added to 5.0 ml of 200 mM Na phoεphate, pH 7.5, and εtirred for 10 minuteε, after which 0.5 ml of 1.0 M HC1 was added to precipitate the product. The solid was collected by filtration, washed with 0.1 M HC1 and then with water and finally dried under vacuum to yield Cys-ABA (modified Cys Ila) . TLC analysis (cellulose, 0.1 M ammonium acetate,pH 6.8) and NMR (spectra recorded in D 2 0 containing 100 mM Na-DP0 4 , pH 7.5; 100 mM NaDC0 3 , pH 8.4; and 100 mM Na 2 B 3 0 7 , pH 9.3) εhowed the preεence of at leaεt three different pH dependent products. The mass spectral analysiε (Cl) produced a parent peak at mw 221 indicative of decarboxylation. D) Preparation of Homocvεteine Acetylbenzoic Acid (mw 297) 100 mg of 1-homocyεteine thiolactone waε added to 5.0 ml of degaεεed 0.5 M NaOH, heated to 45°C for 10 minuteε and neutralized to pH 8.0 with a combination of 1.0 M and concentrated HC1. 80 mg of BABA waε added to the Hey εolution and stirred for 15 minutes. The solution waε then filtered through a 0.22 micron εyringe filter and the εolution pH lowered to ~2 by the addition of 1.0 M HC1 in order to precipitate the product. The εolid Hcy-ABA (modified Hey lib) was washed several times with 0.1 M HC1, collected by centrifugation and dried under vacuum. E) Preparation of Homocysteine Acetylbenzoic Acid-Bovine Serum Albumin Conjugate 25 mg of BSA was disεolved in 5.0 ml 100 mM Na phosphate, pH 7.0, and 2.2 ml of thiε εolution waε added to 200 μl of 100 mM BABA-NHS in DMSO (+BABA-NHS rxn) or to 200 μl of DMSO alone (-BABA-NHS rxn) and mixed for 3 hourε. The two samples were then centrifuged to precipitate the excess solid BABA-NHS, filtered through a 0.45 micron syringe filter and concentrated 5 times on a Centricon 30. The concentrated εolutionε were diluted to 1.0 ml with 100 mM Na phoεphate, pH 7.0. A 20 μl aliquot waε removed from each solution, diluted to 1.0 ml and the absorbance meaεured to determine the reaction efficiency. The protein concentration waε determined from the -BABA-NHS reaction (BSA, e m = 43,600) and the protein- bound BABA concentration from the +BABA-NHS reaction (BABA, 6 2 * 0 = 13,600) . Hey waε further reacted with the BABA-activated BSA solutions prepared above. 20 mM 1-homocysteine was prepared by diεεolving 1.5 mg of 1-homocyεteine thiolactone in 0.5 ml l M NaOH, incubating 10 minuteε at 37°C and finally diluting with 2.0 ml Na phosphate, pH 7.0, and 2.5 ml Na phosphate, pH 6.0 (final pH 7) . Both BSA solutionε prepared above were diluted to 4.0 ml with H 2 0 and 1.0 ml of the Hey εolution waε added to each and incubated overnight. The reactions were concentrated on a Centrieon 30 and the reaction efficiency was eεtimated from the Br " concentration of the retained filtrateε. Bromide εtandards were prepared by adding 2 μl of the appropriate KBr stock solutions to 98 μl of the -BABA-NHS reaction filtrate, producing standards with Br " concentrations of 0.0, 0.2, 0.4, 0.8, and 1.0 mM. 20 μl of the prepared Br " standardε or 20 μl of the filtrate from the +BABA-NHS reaction were added to 0.9 ml of 0.1 M Na citrate pH 5.5, followed by 20 μl of 0.8 mM fluorescein and 10 μl of 0.1 M chloramine T. After 10 minutes, 20 μl of 25 mM thiosulfate in 0.2 M Na_C0 3 waε added to εtop the reaction and the abεorbance recorded for each εample at 515 nm in a 1 cm cell. The abεorbance values of the standardε were plotted against the Br ' concentrations to produce a standard curve, from which the concentration of Br " contained in the +BABA-NHS reaction was determined. From this value an estimate was obtained of the coupling efficiency of the BABA groups on BSA with Hey. The +BABA-NHS reaction solution was concentrated 4 times on a Centrieon 30, diluted with 1 ml 1 x PBS (final cone. "10 mg/ml) and set aside for subεequent antibody development. F) Preparation of Homocysteine Acetylbenzoic Acid-Alkaline Phosphatase Conjugate The Hcy-ABA-AP conjugate was prepared in a εimilar faεhion to the BSA conjugate deεcribed above. 10 mg of AP was diluted with 15 ml 100 mM Na phosphate, pH 7.0, and concentrated 3 times on a Centrieon 30, after which 200 mM Na phosphate, pH 7.0, was added to a final volume of 2.0 ml. Two reactions were prepared: 0.48 ml of the AP solution was added to 45 μl of 100 mM BABA-NHS in DMSO (+BABA-NHS rxn) or 45 μl of DMSO alone (-BABA-NHS rxn) and the reactions mixed for 2.5 hours. The reactions were then diluted with 2.0 ml H 2 0, filtered through a 0.45 μ syringe filter, concentrated 5 times on a Centrieon 30 and finally diluted to 1.0 ml with 100 mM Na phosphate, pH 7.0. 20 μl of each sample were diluted to 1.0 ml, the absorbance recorded and the ratio of protein-bound BABA to AP determined as before (AP, ε^, = 110,000; BABA, e M = 13,600) . The BABA-activated AP was reacted with Hey as follows. 9.3 mg of 1- homocysteine thiolactone was disεolved in 0.5 ml 1.0 M NaOH and incubated for 15 minutes at 37°C. The Hey was neutralized with 0.5 M Na_HP0 4 and H 2 0 to a final volume of 5 ml and pH 7.0. 200 μl of the Hey solution waε added to both AP εolutionε prepared above and incubated for 4 hours at 37°C. The reactions were then concentrated by Centrieon 30 and the filtrateε retained. The extent of the coupling reaction of BABA-activated AP with Hey waε eεtimated from the concentration of Br " released. Standard solutionε were prepared by mixing 2 μl of stock KBr solutions with 98 μl of a buffer resembling the filtrate in composition (1.0 ml of 100 mM Na phosphate 7.0 added to 200 μl 0.5 M Hey) to give standardε of 0.0, 0.05, 0.1, 0.15, and 0.2 mM bromide ion. 80 μl of the Br " standards or the filtrate from the +BABA-NHS rxn was added to 0.9 ml of 0.1 M Na citrate, pH 5.5, followed by 40 μl 0.4 mM fluorescein and 30 μl 50 mM chloramine T and incubated for 10 minuteε. 50 μl of 15 mM thioεulfate in 0.1 M Na 2 C0 3 was added to stop the reaction and the abεorbance at 515 nm recorded. The Br' concentration contained in the +BABA-NHS rxn filtrate was determined from the standard curve and used to estimate the efficiency of the reaction with Hey. Both the -BABA-NHS and +BABA-NHS reactions were concentrated 5 times on a Centrieon 30 and 1 x PBS added back to a final volume of 400 μl. The activity of derivatized AP was evaluated by diluting 2 μl of each AP εolution in 100 μl H 2 0 and adding 2 μl of the diluted solution to 1.0 ml of 1 g/ml PNPP in 50 mM Tris Cl, pH 9.4, and comparing the change in absorbance recorded over 30 εecondε. G) Production and Iεolation of Hcv-ABA-BSA Monoclonal Antibodies 1) Immunogens, Immunizations, and Enzyme-Conjugates The Hcy-ABA-BSA immunogen was prepared as described above. Mice were immunized intraperitoneally once with 100 μg and twice with 50 μg of immunogen emulsified in RIBI Adjuvant System. Three days prior to the fusion, mice received a saline boost intraperitoneally containing 250 μg of immunogen. Hcy-ABA-AP waε uεed for εcreening. 2) Tiεεue Culture S-DMEM waε uεed for all tiεεue culture, which conεisted of DMEM supplemented with: 10% FBS 10% NCTC-135 (Gibco #440-1100EC) 4 mM glutamine 1 mM oxaloacetic acid 1 mM sodium pyruvate 0.148 nM L-cysteine 200 units insulin 37.9 mM sodium bicarbonate Conditioned media was prepared by growing P388D1 cellε (ATCC TIB 63) in S-DMEM and splitting 1:4 every four to five days. Spent media waε centrifuged at 1500 rpm for 15 minutes. The spent media was then filtered using a 0.2 micron sterile tissue culture filter unit to remove any remaining cells and debris. Glutamine (100 x stock = 58.5 g/L) was added to the spent media before using or freezing at -20°C for future use. Conditioned S-DMEM was prepared by εupplementing S-DMEM with 10% P38D1 spent media and then using it to support hybridoma growth after fuεion and during cloning. Mouse myeloma cell line P3X63 Ag8.653 (Ag8.653, ATCC CRL 1580) was maintained in culture by splitting 1:2 to 1:4 daily or by serial dilution in a 6-well plate for two days. 3) Fusions Fusions were done essentially according to the procedure of Milstein and Kohler, Nature 256:495-497 (1975) . Spleen were aεeptically removed from two immunized mice and placed in a 60 mm tissue culture dish containing 10 mL DMEM. They were minced a few times and then mashed between the ends of two frosted slideε. A εingle cell suspension of splenocytes was attained by pasεing the cell εuεpenεion through a monofilament screen cloth. The splenocytes, about 2xl0 8 cells, were combined with 40xl0 6 Ag8.653 cells, centrifuged at 800 rpm for 5 minutes, and washed 1 to 2 times with DMEM. Fusion was performed by addition of 4.0 mL PEG (50% solution in 75 mM Hepes), which was added over 3 minutes while gently stirring, and then 30 to 40 mL S-DMEM was added to inactivate the PEG. The cell suspension was centrifuged at 800 rpm for 5 minutes. Supernatant was poured off, and the cells were resuspended with 240 mL S-DMEM-HAT (stock HAT = 50 x Sigma #H0262; in media: 100 μM hypoxanthine, 0.4 μM aminopterin, and 16 μM thymidine) and plated at 200 μL/well into twelve 96-well culture plates. Cells were fed by removal of 100 μL/well of spent media and subsequent addition of 200 μL/well of conditioned S-DMEM-HAT 4 to 5 days after the fusion. Fusions were screened about 7 to 10 days after the fusion. Hybridoma colonies were usually then visible by eye while the media waε still pink or just turning yellow. 4) Cloning by Serial Dilution Hybridomas producing ELISA positive antibodies were then cloned several times by serial dilution to ensure single cell colonies. The procedure was as follows. Contents from a well of a 96-well plate were transferred to a well in a 24-well plate containing 1.5 mL/well of conditioned S-DMEM. Cellε were mixed by pipetting, and 100 μL/well were added to the firεt row of a 96-well plate containing 200 μL/well conditioned S-DMEM. One hundred μL/well were tranεferred to the εecond row uεing a Flow Multichannel pipettor, mixed by pipetting, and again 100 μL/well were transferred to the next row. Each "clone" was serially diluted 7 times, 1 to 4 clones per plate. Cells were recloned by limiting dilution 3 to 4 times or until stable. 5) Freezing and Thawing Cell Lines Cloned and stabilized cell lines that were ELISA+ were stored at -100°C. The chosen well (clone) from a 96-well plate was grown up by daily pasεaging of cells and sequentially expanding from a 24-well plate with 1.5 mL/well S-DMEM, next into a 6-well plate with 8 mL/well S-DMEM, and finally into a T-75 flask with 50 mL S-DMEM. Cells from a T-75 flask were centrifuged at 800 rpm for 5 minutes and resuspended in 3 mL of freezing medium, 10% DMSO (ATCC #76-68-5) and an additional 10% FBS in S-DMEM. One-mL (3 to 5xl0 6 cells) aliquots were pipetted into cryovialε, which were then placed in a freezing container. The container waε then εtored at -100°C for 1-2 dayε, and then the vialε were tranεferred to a -100°C freezer εtorage box for later use. Cells were thawed by warming the vials in a 37°C water bath. The cell suεpenεion waε tranεferred to a 15-mL centrifuge tube containing 5 mL of S-DMEM and then centrifuged at 800 rpm for 5 minuteε. The supernatant was decanted and the cells resuεpended in 8 mL of S-DMEM and pipetted into a 6-well plate for cell expansion. 6) Screening The hybridomas were initially εcreened by the competitive reverεe ELISA εcreen, then later εcreened by reverεe ELISA. All ELISA screens were performed at room temperature. The competitive reverεe ELISA protocol waε uεed for primary screens using only one concentration of Hcy-ABA, the modified form of Hey, to identify antibodies. To identify the best antibodieε, the competitive reverεe ELISA was performed using Hcy-ABA at two different levels to determine relative senεitivity of the antibodies. BABA and Cys-ABA were also used to determine if there was any undesirable crossreactivity. The reverεe ELISA protocol was used for subεequent screens of cloning plates. The reagents for the competitive reverse ELISA and the reverεe ELISA were: DPBS (10X) , pH 7.4, diluted 1:10 50mM Carbonate Buffer, pH 9.0 ELISA Waεh Buffer: 0.5% Tween 20 in DPBS Plate coat: Rabbit anti-Mouse IgG, A, M; heavy & light (Zymed) , reconstituted with 2mL water per manufacturer's instructionε, diluted 1:100 in DPBS (RAM) Plate block: 1% BSA in DPBS Diluent: 0.5% BSA in DPBS; diluted plate block 1:2 in DPBS ELISA enzyme-ligand conjugates: AP-Hcy-ABA, diluted 1:1500 in diluent; competitive reverse ELISA: for primary screen, Hcy-ABA was diluted to 0.8 μg/mL in diluted Hcy-ABA-AP; for characterization of antibodies, Hcy-ABA, Cys-ABA and BABA were diluted to 1 μg/mL and lng/mL of diluted AP-Hcy-ABA in carbonate buffer. Substrate buffer: 1M diethanolamine, 0.25 mM MgCl 2 , pH 9.8 Substrate: 0.25 mg p-nitrophenyl phosphate per mL of subεtrate buffer The protocol for competitive reverse ELISA and reverse ELISA waε: 1. Plateε were coated with RAM, 50 μL/well, incubated one hour or up to one week at 4°C, and then the contents of the wells were emptied. 2. Plates were blocked with 300 μL/well, incubated 15 minutes and then the cσntentε of the wellε were emptied. 3. Antibody (spent media) was added, 50 μL/well, incubated 45 minutes, and waεhed 3 timeε. 4. Competitive reverεe ELISA: 50 μL/well of AP-Hcy-ABA or AP-Hcy-ABA plus Hcy-ABA, AP-Hcy-ABA plus Cyε-ABA, or Hcy-ABA-AP pluε BABA were added to replicate plateε. Reverεe ELISA: 50 μL/well of AP-Hcy-ABA waε added. Plates were incubated 45 minutes, and washed 3 times. 5. Substrate was added, 100 μL/well, incubated on a shaker for 20 to 30 minutes and read at 405 nm. 6. Positiveε in the primary screen were antibodieε with ODs greater than 0.9 with AP-Hcy-ABA only and better than 50% competition with Hcy-ABA- AP + Hcy-ABA. Percent competition was computed as: % Competition = OD w/AP-Hcv-ABA - OD w/AP-Hcv-ABA and Hcy-ABA x 100 OD w/AP-Hcy-ABA Positives from screening cloning plates using the reverse ELISA protocol were the wells with a single colony or fewest colonies with an elevated OD. 7) Production and Purification of In Vi tro Antibody ELISA positive hybrido as that were cloned and stabilized were then expanded in static culture. Hybridomas were overgrown to two-T225 flasks (500 mL of spent media) or to four-T225 flasks (1-L of spent media) . Cells and debriε were separated by centrifugation and filtration. Sodium azide was added at 0.02% before purification. Antibodies produced in static culture were purified by Protein G column chromatography. Purification was performed at room temperature. Reagents used for Protein G purification were: Washing/binding buffer, PBS pH 7.0: 0.01 M εodium phosphate, 0.15 M sodium chloride, and 0.02% sodium azide Elution buffer: 0.5 M acetic acid adjusted to pH 3.0 with ammonium hydroxide Neutralizing solution: 1 M tris baεe Cleaning buffer: 1 M acetic acid The protocol used for Protein G purification was: 1. A column was packed with 10 mL of Protein G and washed with 25 mL of washing/binding buffer. 2. Antibody (εpent media), 0.5 to 1-L, was loaded onto the column. 3. The column was washed with washing/binding buffer until OD returned to baεeline or with about 50 mL. 4. Antibody waε eluted with elution buffer. Two mL were collected per fraction into a test tube already containing 1 mL of neutralizing εolution. 5. The antibody peak was pooled and dialyzed overnight against 4-L DPBS, pH 7.4. 6. The column was washed with about 25 mL of cleaning buffer and then re-equilibrated with the washing/binding buffer. 7. Antibody purity was checked by Paragon electrophoresiε for presence of a single band. Antibody concentration was determined by firεt getting an OD of the antibody εolution at 280 nm, then calculating antibody concentration uεing the extinction coefficient for IgG: A^ (1 mg/mL) = 1.35. Reεultε: Competitive reverse ELISA data showed thirteen antibodies that competed well with Hcy-ABA, and competed poorly, if at all, with Cys- ABA or BABA. Senεitivity at 1 μg/mL of Hcy-ABA waε very good. Antibodieε from the clone designated Hey I 5C12 were selected for uεe in the following experiment. H) Preparation of Biotin-Hcv I 5C12 400 μl of the 0.7 mg/ml antibody solution (Hey I 5C12) was concentrated 2 times on a Centrieon 30 diluting with 100 mM Na carbonate, pH 8.5. The εolution was filtered and buffer added to a volume of 300 μl (final cone. ~1 mg/ml) . To the antibody εolution waε added 36 μl of 1 mg/ml biotinamido-caproate-NHS eεter in DMSO and incubated for 2 hours. The reaction was concentrated 5 ti eε on a Centrieon 30, finally adding back 50 mM Na phoεphate, pH 7.5, to a final volume of 1 ml. The concentration of antibody (100 μg/ml) was determined from the 280 nm abεorbance ( 1.2 mg/ml → abs. of 1.0) . Successful biotinylation was verified by Paragon gel electrophoresis by comparing the mobility of the unmodified and biotinylated antibody in the absence and presence of streptavidin. The biotin-Hcy I 5C12 solution was finally concentrated to a volume of 50 μl on a Centrieon 30. EXAMPLE I HPLC HOMOCYSTEINE ASSAY WITH DTT-ARSENITE REDUCTION To 200 μl εampleε of whole blood, coagulation halted with EDTA, were added aliquotε of 1.0 mM Hey to give final concentrationε of 0, 10, 20, 30, 40 μM. The blood waε incubated for 2 hourε at room temperature, then stored for 3 days at 4°C to allow for equilibration between the free- reduced, free-disulfide and protein-bound forms of Hey. 20 μl of plasma obtained from each blood sample was reduced by adding 10 μl 100 mM DTT and 30 μl 1.0 M Na carbonate, pH 8.2, and incubating at 37°C for 30 minutes. The proteins in the εample were subsequently removed by filtration through a Millipore Ultrafree MC (10K MW cutoff) . Excesε DTT waε sequestered by adding 10 μl 1.0 M Na arsenite to 20 μl of the filtrate and incubating for 10 minuteε. 10 μl of 10 mM BABA waε added and the reaction incubated for an additional 10 minuteε. The derivatized plasma samples were then analyzed by HPLC and the Hcy-ABA peak quantitated, allowing for accurate determination of total plasma Hey. (20 μl injection, 1.0 ml/min, 10 cm Alltech Eeonosphere C18 column, gradient: 1-10 % CH 3 CN in 100 mM Et 3 N-HOAc, pH 5.5, over 15 minuteε) . The data fit well to a linear least squares analysiε. If extrapolated to zero area the plasma-alone sample registered ~28 mM Hey, a value larger than expected (~12 μM is normal) . However the Hey concentration in stored blood has been shown to increase substantially over time. EXAMPLE II HPLC HOMOCYSTEINE ANALYSIS OF PLASMA WITH AND WITHOUT TCEP REDUCTION TCEP at low mM concentrations was shown to quantita-tively reduce Hey in buffer. To show that TCEP was also effective at reducing disulfides in plasma, plasma-alone (no Hey) was reacted with BABA with or without prior reduction with TCEP. 20 μl of plasma was added to 30 μl of 1.0 M Na bicarbonate and 10 μl of H 2 0 or 10 μl of 20 mM TCEP and incubated at 37°C for 5 minutes: 60 μl of 10 mM BABA was then added and the reaction incubated for 5 minutes: HS NH, 2 BABA HO Proteinε were removed by filtration aε above and the reactionε analyzed for Hcy-ABA by HPLC (conditionε aε above except; gradient: 1-20 % CH 3 CN in 1% trifluoroacetic acid over 15 minuteε) . No product was observed unleεε TCEP waε present. HPLC analysis also indicated that the Hcy-ABA peak was not completely resolved from other compounds and the experiment muεt be viewed in qualitative terms aε evidence that reduction with TCEP will effect diεulfide reduction in plasma (or serum) . EXAMPLE III Hcy-ABA INHIBITION OF Hcy-ABA-AP BINDING TO BIOTIN-Hcv I 5C12 A P . C O O H Ab - b i o t i n T l t ι n 100 μl of 1 x PBS, 0.05 % Tween containing 5 ng of Hcy-ABA-AP waε added to 100 μl of H 2 0 containing Hcy-ABA at a concentration of 0.125, 0.25, 0.5, 1.0 or 4.0 μM. 100 μl of PBS-Tween containing 10 ng biotin-Hcy I 5C12 waε added and the reaction incubated for 20 minutes. Prewashed εtreptavidin-coated particleε (1 pellet) were εuεpended in 300 μl PBS-Tween and 20 μl was added to the reaction. The reaction tube was agitated for 15 minutes such that the particles remained suspended. The particles were then washed 5 times with PBS-Tween, the separation of the particles from εolution being facilitated by pulling the particleε to the εide with a tube holder containing a magnetic εtrip. 300 μl of 1 mg/ml PNPP substrate in 50 mM Tris Cl, pH 9.3, was added and mixed for 20 minuteε. 200 μl of εolution from each reaction tube was transferred to a microtiter plate and the absorbance recorded at 414 nm on a plate reader. As expected, an increase in the concentration of Hcy-ABA caused a diminution in the magnitude of the recorded absorbance. Signal modulation was >50 % over an effective solution concentration range of 0.04 to 0.67 mM (no zero concentration data point was recorded) , allowing for a εubstantial dilution of serum sample (typical Hey concentration "12 mM) , while still maintaining an adequate sensitivity level in the immunoassay. EXAMPLE IV ASSAY OF HOMOCYSTEINE-SPIKED SERUM Serum samples were spiked with 10, 20, 30 and 40 μM Hey (added aε Hey, i.e., the dimeric form of Hey) . Six reactions were prepared in which 5 μl of each Hey-spiked serum εample or serum alone (two reactions) was -34 - reduced by adding 10 μl of 1.0 M Na phosphate, pH 7.5, and 10 μl of 10 mM TCEP and incubating for 10 minuteε. The resulting free Hey in 5 samples was derivatized, i.e., "modified" by adding 40 μl of 8 mM BABA and incubating for 10 minuteε. To one of the "serum alone" εampleε, BABA waε replaced by adding bromoacetamide, creating a zero Hcy-ABA control. 40 μl of BABA (1 εample) or bromoacetamide (5 εampleε) was then added to balance the reagent mix, followed successively by 100 μl PBS-Tween containing 10 ng Hcy-ABA-AP and 100 μl PBS-Tween containing 20 ng biotin-Hcy I 5C12 and incubation for 15 minutes. 20 μl of a εtreptavidin-coated particle εuεpenεion was added and the reaction agitated for 15 minutes. After washing 5 times with PBS-Tween the reaction was developed with PNPP and analyzed as described above. The data points were fitted to a smooth curve in a plot of added Hey versus the recorded absorbance. The absorbance reading from the serum-alone, bromoacetamide derivatized sample was fitted to the curve and the endogenous Hey concentration of the serum-alone εample (0 mM added Hey) waε eεtimated by compariεon with the zero Hcy-ABA (•.bromoacetamide) control. From the graph the Hey concentration of the serum εample waε 15 μM, a value that iε within the expected range for a normal individual. Although the foregoing invention has been described in some detail by way of illustration and example for the purposes of clarity and understanding, it will be obvious that certain changes or modifications may be practiced within the scope of the appended claims.

Claims: - 35 - CLAIMS What is claimed is: 1. A method of determining the presence or amount of a first compound in the presence of a second compound in a sample suspected of containing said compounds, wherein said compounds are immunologically related, comprising the stepε of: (a) bringing together in an aqueous medium: (1) said sample, (2) a modifying reagent capable of chemically modifying said first compound and said second compound to form a modified first compound and a modified second compound, and (3) an antibody capable of specifically binding to said modified first compound but not to said modified second compound to form an immunecomplex; and (b) measuring the extent of binding of said antibody to said modified first compound, the extent thereof being related to the presence or amount of said first compound in the sample. 2. The method of Claim 1 wherein said compounds are homologous. 3. The method of Claim 1 wherein said first and second compounds are homocysteine and cysteine. 4. The method of Claim 1 wherein said modifying reagent is selected from the group consiεting of alkylating agents, acylating agents and metals . 5. The method of Claim 1 wherein said modifying reagent has a detectable label, and the extent of binding of the antibody to the modified first compound iε meaεured by detecting said label . 6. The method of Claim 5 wherein said antibody iε bound to a εupport or iε capable of being bound to a εupport. 7. The method of Claim 1 wherein εaid antibody iε detectably labeled or iε capable of being detectably labeled and the extent of binding of the antibody to the modified firεt compound iε meaεured by detecting εaid label. 8. The method of Claim 1 which further compriεes bringing together an analog of said modified first compound bound to a detectable label, wherein said antibody is also capable of specifically binding to εaid analog, and wherein the extent of binding of said antibody to said modified first compound iε meaεured by meaεuring the extent of binding of εaid analog to εaid antibody. 9. The method of Claim 8 wherein εaid antibody iε bound to a εupport or is capable of being bound to a support. 10. A method of determining the amount of homocysteine in the presence of cyεteine in a εample suspected of containing said homocysteine, comprising the stepε of: (a) bringing together in an aqueouε medium: (1) εaid sample, (2) a modifying reagent capable of chemically modifying homocysteine and cyεteine to form modified homocyεteine and modified cyεteine, and (3) an antibody capable of εpecifically binding to εaid modified homocyεteine but not to said modified cysteine to form an immunecomplex; and (b) meaεuring the amount of εaid immunecomplex, the amount thereof being related to the amount of homocyεteine in said sample. 11. The method of Claim 10 wherein εaid modifying reagent modifieε the εulfhydryl groups of εaid homocysteine and said cyεteine to form a modified homocyεteine where the εulfur iε not part of a ring, and a modified cyεteine where the εulfur is part of a ring. 12. The method of Claim 10 wherein said modifying reagent is selected from the group consisting of alkylating agents, acylating agentε and metalε. 13. The method of Claim 10 wherein εaid modifying reagent haε a detectable label, and εaid label iε detected to measure the amount of said immunecomplex. 14. The method of Claim 13 wherein said antibody is bound to a support or is capable of being bound to a support. 15. The method of Claim 10 wherein εaid antibody iε detectably labeled or is capable of being detectably labeled and said label iε detected to measure for the presence or amount of said immunecomplex. 16. The method of Claim 10 which further compriεeε bringing together an analog of εaid modified homocyεteine bound to a detectable label, wherein εaid antibody is also capable of specifically binding to said analog to form a εecond immunecomplex, wherein the preεence or amount of said first immunecomplex is measured by measuring the extent of binding of εaid analog to said antibody to form εaid εecond immunecomplex. 17. The method of Claim 16 wherein εaid antibody iε bound to a εupport or is capable of being bound to a support. 18. A method of determining the amount of homocysteine in the presence of cysteine in a sample suspected of containing εaid homocyεteine, compriεing the steps of: (a) bringing together in an aqueous medium: (1) said sample, (2) a modifying reagent capable of chemically modifying homocysteine and cysteine to form modified homocysteine and modified cysteine, (3) an analog of said modified homocysteine bound to a detectable label, and (4) an antibody capable of specifically binding to said modified homocysteine and to said analog, but not to said modified cysteine; and (b) measuring the extent of binding of said analog to said antibody, the extent thereof being related to the amount of homocysteine in εaid εample. 19. The method of Claim 18 wherein εaid modifying reagent iε selected from the group consisting of alkylating agents, acylating agentε and metals. 20. The method of Claim 18 wherein said antibody is bound to a εupport or iε capable of being bound to a εupport. 21. The method of Claim 20 which further compriεes bringing together a support bound to a first specific binding pair member and said antibody is bound to a complementary second specific binding pair member. 22. The method of Claim 18 wherein said label is selected from the group consiεting of fluoreεcers, radiolabelε, enzymeε, chemilumineεcerε and photosenεitizers. 23. The method of Claim 18 wherein said antibody is bound to a first member of a signal producing syεtem and εaid detectable label iε a εecond member of a εignal producing system, and the extent of binding of said antibody to said analog is measured by detecting the signal produced by the interaction of said signal producing syεtem memberε. 24. A method of determining the amount of homocyεteine in a εample suspected of containing homocysteine and cysteine, comprising the steps of: (a) bringing together in an aqueous medium: (1) εaid εample, (2) a modifying reagent capable of chemically modifying homocyεteine and cysteine to form modified homocysteine and modified cyεteine, (3) an antibody capable of εpecifically binding to said modified homocysteine but not to said modified cyεteine to form an immunecomplex, (4) a εurface capable of binding εaid antibody, and (5) a labeled analog of εaid modified homocyεteine; and (b) measuring the extent of binding of said labeled analog to said antibody, the extent thereof being related to the amount of homocysteine in said sample . 25. The method of Claim 24 wherein said modifying reagent is selected from the group consisting of alkylating agents, acylating agents and metals. 26. The method of Claim 24 wherein the label of said labeled analog is selected from the group consisting of fluorescerε, radiolabelε, enzymeε, chemilumineεcerε and photosensitizerε. 27. The method of Claim 24 which further compriεeε bringing together a labeled firεt εpecific binding pair member, wherein the label of εaid labeled analog iε a complementary εecond εpecific binding pair member. 28. A method of determining the amount of homocysteine in a sample εuεpected of containing homocysteine and cysteine, comprising the εtepε of: (a) bringing together in an aqueous medium: (1) εaid εample, (2) a modifying reagent capable of chemically modifying homocyεteine and cyεteine to form modified homocyεteine and modified cyεteine, (3) an antibody capable of εpecifically binding to said modified homocysteine but not to said modified cysteine to form an immunecomplex, and (4) a receptor capable of binding to a portion of said modifying reagent, wherein said portion is present on said modified homocyεteine; (b) meaεuring the extent of binding of said antibody to said receptor, the extent thereof being related to the amount of homocysteine in said εample. 29. The method of Claim 28 wherein εaid portion of εaid modifying reagent iε εelected from the group conεiεting of haptenε, biotin and fluorescein; and said receptor is selected from the group consisting of antibodieε, avidin and streptavidin. 30. A method of determining the amount of homocysteine in the presence of cyεteine in a serum sample suspected of containing said homocysteine, wherein at least a portion of said homocyεteine iε in the diεulfide form (protein-bound and free-diεulfide) , comprising the steps of: (a) bringing together in an aqueous medium: (1) said εample (2) a releaεing agent to releaεe εaid homocyεteine from the diεulfide form, (3) a modifying reagent capable of chemically modifying the εulfhydryl groupε of homocysteine and cysteine to form modified homocysteine and modified cysteine, and (4) an antibody capable of specifically binding to said modified homocysteine but not to said modified cysteine to form an immunecomplex; (b) examining said medium for the amount of said immunecomplex, the amount thereof being related to the amount of homocysteine in said sample. 31. The method of Claim 30 wherein chemically modifying εaid εulfhydryl group of cysteine comprises the formation of a εix-membered ring. 32. The method of Claim 31 wherein chemically modifying said sulfhydryl group of homocysteine does not comprise the formation of a six- membered ring. 33. The method of Claim 30 wherein the releasing agent is a reducing agent. 34. The method of Claim 30 wherein said modifying reagent is a ketone εubεtituted at the alpha poεition by a leaving group εelected from the group conεiεting of Cl, Br, I, εulfonates, and sulfonium εaltε. 35. The method of Claim 30 wherein the amount of εaid immunecomplex iε determined by detection of enzyme activity, luminescence, light absorbance or radioactivity. 36. A kit for use in a method for detecting a first compound in the presence of a second compound, wherein said compounds are immunologically related, comprising in a packaged combination: a modifying reagent capable of chemically modifying εaid first compound and said second compound to form a modified first compound and a modified εecond compound, and an antibody capable of εpecifically binding to εaid modified firεt compound but not to εaid modified εecond compound. 37. The kit of Claim 36 which further comprises a labeled analog of said modified firεt compound. 38. A kit for uεe in a method for detecting homocyεteine, compriεing in a packaged combination: a modifying reagent capable of chemically modifying homocyεteine and cyεteine to form modified homocysteine and modified cysteine, and an antibody capable of specifically binding to said modified homocysteine but not to said modified cyεteine. 39. The kit of Claim 38 which further comprises a labeled analog of said modified homocyεteine. 40. The kit of Claim 38 which further comprises a releasing agent to release any homocysteine present in a disulfide form.

Inventors: GOODMAN THOMAS C, ULLMAN EDWIN F, VAN ATTA REUEL B, GOODMAN, THOMAS, C., ULLMAN, EDWIN, F., VAN ATTA, REUEL, B.
Applicants: SYNTEX INC, SYNTEX (USA) INC.
Application year: 1995
Main classification: G01N33
IPC codes: G01N33