Method and composition for the determination of sodium ions in fluids

Abstract: The invention concerns a process and a composition for the determination of sodium ions in fluids, wherein the influence of these ions on the activity of an enzyme is measured. The enzymes which are used may be for example a transferase, a hydrolase or an oxidoreductase. An essential part of the invention is a method to exclude interferences by ions by masking the interfering ions with a binding agent.

Description: This invention is concerned with methods and reagents for the determination of ions, hereinafter also called analytes, in biological and non-biological fluids. The invention is based on the ability of many analytes to stimulate or inhibit the activity of a a sensitive enzyme. The analytes may be cations or anions, metallic or non-metallic, simple or compound. In practice it is frequently found that the analyte is present in the sample at a concentration that lies outside the range of sensitivity of the relevant analytical indicator enzyme, or that interference is caused by the presence of other ions to which the enzyme is also sensitive. This invention addresses and solves these problems in diverse ways. In the practice of Clinical Biochemistry the measurement of serum electrolytes are the most common analytical tests performed within hospitals. These measurements are requested not only for routine investigations, but frequently for emergency and life-threatening situations where speed of analysis is essential. Since a major source of delay in hospitals is the transport of specimens from the wards to the diagnostic laboratories, a method that is easily performed near the bedside would be of particular value in emergency situations. A common method of analysing potassium and sodium in clinical biochemistry practice is flame photometry. This process depends on the principle that certain atoms, when energized by heat, become excited and emit light of a characteristic wavelength when returning to ground state. The intensity of the characteristic wavelength of radiant energy produced by the atoms in the flame is directly proportional to the number of atoms excited in the flame, which is directly proportional to the concentration of the substance of interest in the sample. The apparatus required is complex and relatively expensive and requires the use of combustible gases. An alternative method especially for sodium, potassium and chloride makes use of ion-selective electrodes. Ideally, each electrode would possess a unique ion-selective property that would allow it to respond to only one ion. In practice this is not the case and interfering ions exist for all ion-selective electrodes. Moreover, ion-specific electrodes are not absolutely specific, although generally corrections are possible. The electrodes measure the potential developed in the presence of the specific ion. The instrumention is relatively expensive. Neither method can be performed spectrophotometrically and the clinical need for ion measurement, therefore, results in a substantial increase in the complexity of commercially available clinical analysers, most of which are designed primarily for spectrophotometric assays. Both methods require a considerable degree of skill and knowledge for their successful implementation. Similarily, the routine determination of chloride by coulometric methods requires special instrumentation. The endpoint of this titration procedure is detected by an increase in electrical flux completion of formation of insoluble silver chloride product. Alternatively, potentiometric determinations may be used which are also very time-consuming and involve additional instrumentation. For chloride, in addition, there are a number of photometric and titrimetric methods, which e. g. include: titrimetric determination of free Hg 2+ ions via diphenylcarbazone complex colorimetric determination of the rhodanide complex of iron, which is formed after dissociation of the mercury complexes upon precipitation of HgCl 2 [Skeggs, Clin. Chem. 10, 1964, 918f.; Schmidt, Zentralblatt Pharm. 124 (9), 1985, 527f] colorimetric determination of chloranilic acid from the respective mercury salt [Reuschler, Klin. Wochenschrift 40, 489 (1962)] determination of the coloured Cu 2+ complex of diethyldithiocarbaminic acid from the colourless mercury salt (German Offenlegungsschrift 2137146) the rather common TPTZ-method (tripyridile-s-triazine) (R. Fried, Zeitschr. Klin. Chem., Klin. Bioch. 10, 1972, 280f; the patent application DE 21 53 387) which is similarily based on the formation of a coloured metal complex upon dissociation of a mercury complex. A major drawback of these methods is the use of solutions containing highly toxic substances. Some of the methods are complicated and imprecise (e.g. the titration method). Many of the reagents are unstable and calibration curves are non-linear (e.g. the rhodanide method). Some of these methods in addition need a pretreatment in order to eliminate interferences by the protein content of the sample. An improved TPTZ-method is described in the PCT application WO 83/002670, the use of toxic mercury compounds, however, is still a disadvantage. The only colorimetric method without use of mercury ions is the determination of hexachlorocomplexes of Fe(III) in a perchloric acid solution [F. Hoppe, Ther. Ggw. 110 (4), 1971, 554f., H. Mahner, Zeitschr. Klin. Chem., Klin. Biochem. 11 (11), 1973, 451f.; W. T. Law, Clin. Chem. 26 (13) 1980, 1874f.; US-Patent 4 278 440]. A considerable limitation of this method is the use of strongly acid reagents which are corrosive and therefore not compatible with mechanical pipetting systems. A further disadvantage is the interference by bilirubin in the samples. Calcium is a further example of an electrolyte which is routinely determined in the clinical laboratory. The concentration of this metal ion in body fluids is regulated within a narrow range. Pathologically high or low concentrations can lead to life- threatening disorders such as renal insufficiency, pancreatitis, tetany and congestive heart failure. One of the earliest methods for the determination of calcium was that described by Tisdall (J. Biol. Chem. 63, 461 - 465, 1925) in which calcium is precipitated by oxalic acid which is in turn estimated colorimetrically. The method involves a centrifugation step and is therefore very time-consuming; it is not specific for calcium and depends on a careful handling of samples. The method has been succeeded in many laboratories by titrimetric and direct colorimetric procedures. The former also has the drawback of a complicated and cumbersome procedure and requires large sample volumes. In the latter procedure calcium affects the colour of a dye, for example orthocresolphthalein complexone, which can be measured in a photometer. Due to the simplicity of the method it lends itself to automation in the clinical laboratory. The method, however, involves the use of aggressive, highly alkaline solutions and toxic substances. It is particularly prone to interference by a number of serum components such as lipids, proteins, phosphate and bilirubin and as a result does not agree well with atomic absorption and flame photometric reference methods. A further disadvantage of the colorimetric procedure is that the calibration curves are non-linear and the colour is greatly dependent on temperature. In W. H. Outlaw and O. H. Lowry, Analytical Biochemistry 92, 370 - 374 (1979) an enzyme-mediated assay for measuring potassium ions in tissues is described. The method employs pyruvate kinase from rabbit muscle, which is activated by potassium ions and sodium ions, the former being about one forty-fold more effective. Because of this non-specificity the method may be suitable for plant material in which potassium ions are the predominant cations, but it is unsuitable for measurements in body liquids like serum which contains a thirty-fold excess of sodium ions. Therefore, sodium ions cause unacceptable interference when using the enzymatic photometric technique as described by Outlaw et al. to measure potassium in plasma or serum. A further problem is that ammonium ions give a similar activation to potassium ions. The above mentioned publication does neither address or solve these critical problems in regard to the analysis of potassium ions in biological fluids such as serum nor does it propose any method for the determination of sodium ions. The patent application EP 0 275 398 reaches a determination of chloride with a deactivated α-amylase. M. C. Wimmer et al., Clin. Chem., Vol. 32, No. 4, 1986, describe a procedure for the determination of magnesium. However, by this method the interference by other ions cannot be avoided. The European patent application EP 207 392 discloses an alternate method to determine the concentration of ions by means of a multilayer test assay. Neither the disclosed configuration of the multilayer test assay is used in the present inventation nor does EP 207 392 suggest to use enzymes for generating detectible signals. The patent application DE 36 14 470 lacks the disclosure to make it a method for the potassium determination in serum with no significant interference of other ions, e.g. sodium ions. I. F. Dalmonova and N. N. Ugarova, J. Anal. Chem. of USSR (1980), Vol. 35, No. 8, Part 2, 1042 - 1081, includes the general discussion of the effect of some ions like beryllium, zinc and mercury ions to some enzymes, but there is no evidence that any of these methods have been made to work without the ingredients according to the present invention. Therefore, it is an object of the present invention to provide a process and a reagent by which the above-mentioned problems are avoided. The invention solves the problems by a process for the determination ions (analytes) in fluids wherein the influence of these sodium ions on the activity of an enzyme is measured. A key feature of this invention is the use of selectively binding agents to bring the free concentration of the analyte within the optimal range for the analytical enzyme, particularly when dilution of the fluid is not practicable. An additional element of the invention is the use of competitive inhibitors of the relevant analytical enzyme in order to reduce its sensitivity to the analyte, thereby permitting measurement of the latter at a higher concentration. This is especially useful, for example, where selective binding agents are not readily available or are unacceptably expensive. Another feature of the invention is that selective binding agents are employed to reduce the free concentrations of interfering ions to levels where interference is no longer significant. This may be the same selective binding agents as for reducing the free concentration. Use is also made ofthe fact that a competitive inhibitor may compete more effectively with interfering ions than with the analyte, thereby increasing the sensitivity of the enzyme to the analyte with respect to the interfering ion. An important element is the choice of optimal reaction conditions, including the selection of an appropriate isoenzyme, such that the stimulatory or inhibitory effects of the analyte are substantially greater that those of the interfering ions. In addition, the action of the analyte and interfering ions on the activity of the analytical enzyme should be additive so that, if the concentration of interfering ions is known, the concentration of the analyte can readily be determined by difference. Where an interfering ion is known to occur at a relatively constant concentration in the fluid under analysis, allowance can be made for this by including an appropriate concentration ofthe interfering ion in standard (calibrating) solutions. Another method for assaying such analytes is the use of a competitive binding assay where the analyte displaces another ion from binding agent and the effects of the released ion on the activity of an appropriate enzyme are determined. Suitable Enzymes Enzymes which may be used can be for example (H. J. Evans et al., Ann. Rev. Plant Physiol. 17, 47; 1966); another list of suitable enzymes is given in Biochemistry (1975) 2nd ed.: Lehninger, Worth Publishers, New York, pg. 185: Transferases like phosphorous-containing group-transferring transferases. Such a transferase may be pyruvate kinase. In place of pyruvate kinase other kinases such as adenylate kinase or hexokinase, sensitive to magnesium ion or manganous ion may be employed. Another transferase is acetate kinase (from E. coli). Another example is pyridoxal kinase from brain which is sensitive to zinc ions. Hydrolases like glycosidases, for example α- or β-D-galactosidase (from Escherichia coli), carboxypeptidase A (from bovine pancreas), collagenase (from Clostridium hystolicum), amylase (from saliva or pancreas) or phosphoglycolate phosphatase. Also peptide hydrolases such as the cysteine or thioldependent proteinases, specific examples of which are Calpain I and II (also called calcium activated neutral protease) are described by Sasaki et al. in J. Biol. Chem. 259, 12489 - 12494, (1984). The latter enzymes can be isolated and purified from a variety of animal tissues such as: rat liver and kidney, human and porcine erythrocytes, bovine brain, and rabbit skeletal muscle according to the method of A. Kitahara et al., J. Biochem. 95, 1759 - 1766 (1984). A further example is dipeptidyl aminopeptidase I (E.C. 3.4.14.1, Cathepsin C), J. Ken McDonald, Bioch. Biophys. Res. Communication 24(5), 66, 771f. Oxidoreductases like glycerol dehydrogenase (from Enterobacter aerogenes), acetaldehyde dehydrogenase (from yeast) or tyrosinase (catechol oxidase). Other suitable enzymes are various enzymes from halophilic organisms. All enzymes do not have to be of natural source but can also be obtained by DNA recombination techniques. Selective Binding Agents: A wide variety of binding agents is available for the binding of analytes or interfering ions. Such binding or masking substances are cryptands, coronands, crown ethers, podands, spherands, hemispherands, calixarens and combinations thereof, naturally occurring ionophores, for example antibiotics, cyclic peptides like valinomycin, complexones and chelating agents, for example iminodiacetic acid, EDTA, nitrotriacetic acid and derivatives thereof. Such compounds are described in Kontakte (Merck), 1977, No. 1, p. 11 ff and p. 29 ff; Kontakte (Merck), 1977, No. 2, p. 16 ff; Kontakte (Merck), 1977, No. 3, p. 36 ff; Phase Transfer Catalysts, Properties and Applications (Merck-Schuchardt) 1987, Thermodynamic and Kinetic Data for Cation-Macrocycle Interaction; R. M. Izatt et al., Chemical Reviews 85, 271 - 339 (1985); Data for Biochemical Research, 1986, R. M. C. Dawson et al., Eds., 3rd Edition, 399 - 415 (Clarendon Press) Oxford; F. Vögtle et al., Chem. Macrocycles, Springer Verlag, New York, 1985; G. W. Gokel et al., Eds., Macrocyclic Polyether Synthesis, Springer Verlag, New York, 1982; M. Hiraoka, Ed., Crown Compounds, Elsevier, Amsterdam, Oxford, New York, 1982; J. M. Lehn et al., J. Am. Chem. Soc. 97, 6700 - 6707 (1975); G. Schwarzenbach et al., Helv. Chim. Acta 28, 828 (1945); S. F. A. Kettle, Koordinationsverbindungen, Taschentext 3, Verlag Chemie, Weinheim/Bergstr. 1972; A. E. Martell et al., Die Chemie der Metallchelatverbindungen, Verlag Chemie, Weinheim/Bergstr. 1958; M. Becke-Goehring et al., Komplexchemie, Springer-Verlag, 1970; F. Kober, Grundlagen der Komplexchemie, Otto-Salle-Verlag, Frankfurt/Main 1979; G. Schwarzenbach et al., Helv. Chim. Acta 31, 1029 (1948); R. G. Pearson et al., Science 151, 172 (1966). Examples of chelators capable of binding multivalent ions, in particular bivalent cations are ethyleneglycol-bis-(2-aminoethylether)-N,N,N',N'-tetraacetic acid (referred to as EGTA) and ethylenediamino-tetraacetic acid (EDTA). While many binding agents exist that can bind multivalent ions, e.g. EDTA and its derivatives, agents which bind monovalent ions are less common. Tetraphenylboron binds potassium ions. However, a group of compounds with wider possibilities are cryptands which are examples of reagents that can selectively bind monovalent cations in aqueous solutions (R. M. Izatt et al., Chem. Reviews 85, 271 - 339). Special examples for cryptands are the Kryptofix® compounds of Merck-Schuchardt, for example: 4,7,13,16,21-Pentaoxa-1,10-diazabicyclo[8.8.5]-tricosan, Kryptofix® 221, page 438, Merck-Schuchardt catalogue, dated 1987/88, no. 810646 (K 221). 4,7,13,16,21,24-Hexaoxa-1,10-diazabicyclol[8.8.8]hexacosan, Krypotfix® 222, page 438, Merck-Schuchardt catalogue, dated 1987/88, no. 810647 (K 222). As masking compounds for the elimination of interfering anions the following classes of substances may potentially be used: anion cryptands, heterocyclophanes, catapinands and inorganic metal complexes or insoluble salts. Special examples of anion complexing compounds are described in the literature, e.g. azamono- or azapolycycles, macrocyclic quaternary tetrahedron compounds, macrocyclic bis-metal complexes, macrocycles with covalently incorporated lewis acid centers, protonated or alkylated quaternary cryptands or catapinands [F. P. Schmidtchen, Nachrichten Chem. Techn. lab. 36 (1), 1988, S. 8f; E. Graf, J. Amer. Chem. Soc. 98 (20), 1976, 6403f; C. H. Park, J. Amer. Chem. Soc. 90 (9), 1968, 2431f] as well as e.g. the hexachlorocomplex of Fe(lll) or silver nitrate. The analytical use of cryptands (e.g. Kryptofix 221, 222) for the volumetric determination of Li-, Na- and K-ions is described in Chem. Abstr. 84:25 385 b. It is shown, that Li, K, Ca, Sr and Ba interfere Na detection. Function of Binding Agents: These binding agents are used for the following purposes: 1. The selective binding of interfering ions by means of a first selective binding agent. 2. To reduce the concentration of the analytes to optimal measuring levels by means of a second selective binding agent, if dilution of the sample is not feasible. The use of a selection binding agent to lower the concentration of the ion to be determined into the appropriate range for enzymatic analyses serves to double the sensitivity of the method. 3. An embodiment of the invention is a process, wherein the second binding agent is present and forms a complex with "indicator" ions, from which complex the indicator ions are displaced stoichiometrically by the analyte ions, and wherein the influence of the displaced indicator ions on the activity of an enzyme is assayed, thereby giving an indirect measure of the concentration of analyte ions. For example, in such a process the enzyme is pyruvate kinase, the indicator ions are potassium, the binding agent is Kryptofix® 221 and the ion to be determined is sodium; or the enzyme is a kinase, the indicator ion is Mg 2+ , the binding agent is a chelating agent, e.g. EDTA, and the analyte ion is a metal ion or the enzyme is pyridoxal kinase, the indicator ion is Zn 2+ , the binding agent is Kryptofix® 221, and the analyte ion is a heavy metal ion; or the enzyme is a-amylase, the binding agent is a metal of the first and second side group (subgroups I b and II b) ofthe periodic system according to the Mendelevian periodic table [Chemical Principles by W. J. Masterton and E. J. Slowinski (3rd Edition), Saunders Golden Series, W. B. Saunders Company 1973] for example Ag or Hg and the ion to be determined is chloride; or the enzyme is collagenase the binding agent is a chelating agent such as EDTA and the ion to be determined is calcium. 4. The first and second binding agent may be the same. Fluids for Analysis: The biological fluids in which the measurement of analytes is made are blood, serum, plasma, urine, sweat, cerebrospinal fluid, lymph or intestinal secretions, exudates or transudates for example. Nonbiological fluids are water or aqueous extracts or mixtures, like extracts of foodstuffs or fruits or fermented liquids such as wine. Application of General Principles to the Determination of Sodium Ions The measurement of sodium ions in serum or plasma best illustrates the application of these principles in regulating effective analyte concentration. One means of measuring sodium ions as embodied in this invention is to use an enzyme whose activity is sensitive to sodium ions. An example of such an enzyme is β-galactosidase (Kuby et al., J. Am. Chem. Soc. 75, 890, 1953; Lederberg, J. Bacteriology 60, 381, 1950). However, the range of sodium ion concentration to which this enzyme is most sensitive is much lower than can conveniently be obtained in a plasma sample, without a dilution step. In keeping with the principles embodied in this invention the following procedures are employed to lower the effective sodium ion concentration to optimal levels when dilution of the sample is not feasible. 1. Use of a sodium ion binding agent such as Kryptofix® 221. 2. Use of lithium ions as a competitive inhibitor of β-galactosidase, thereby decreasing the sensitivity of the enzyme to sodium ions. The combination of procedures I and 2 readily allows the determination of sodium ions in plasma or serum; using β-galactosidase, and the amount of binding agent can be manipulated to minimize the signal for sodium ion concentrations below 110 mmol/l while enhancing the signal in the usual analytical range (110 - 170 mmol/l) (Example A). Sodium ions may also be measured by means ofpyruvate kinase, provided that conditions are chosen whereby the stimulation of enzyme activity by potassium ions is reduced, and a potassium ion-binding agent. e.g. Kryptofix® 222, is included in the reaction mixture (Example B). In another method of determining plasma sodium ion concentration, the sodium ions are allowed to displace potassium ions from Kryptofix® 221, the released potassium ions stimulating the activity ofpyruvate kinase in proportion to the plasma sodium ion concentration (Example C). Other embodiments of the invention are compositions and reagents for the determination of ions in biological and non-biological fluids. The reagent according to the present invention can be present in dissolved or dry form. It can be present impregnated on an appropriate carrier. A diagnostic agent in the form of a test strip can be produced by impregnating a carrier material, preferably filter paper, cellulose or synthetic fibre fleece, with solutions of the necessary reagents conventionally used for the production of test strip in readily volatile solvents, such as acetone. This can take place in one or more impregnation steps. The finished test papers can be used as such or stuck in known manner on to handles or preferably sealed between synthetic resins and fine meshes. Determination of Sodium Ions In principle the measurement of sodium ions, using PK, is similar to that of potassium ions. However, certain key differences exist. In the first instance PK is some 40 - 100 times more sensitive to potassium ion than it is to sodium ion, depending on incubation conditions as described above. Hence even though sodium ions occur in the plasma at a concentration some 30 times that of potassium ions, the latter can interfere with sodium ion measurement. According to the general principles espoused in this invention, lithium ions are omitted, PK from rabbit muscle is preferred to the bacterial enzyme, and magnesium ions may be substituted for manganese. In one method the potassium ions are specifically bound with Kryptofix® 222 and the effects of sodium ions on PK activity measured directly (Example B). Typical concentration ranges of the main reagents for the enzymatic determination at 37 °C of sodium ions using pyruvate kinase are: PK (rabbit muscle) 50 U/l to 10,000U/l PEP (neutralized Tris salt) 0.3 mmol/l to 30 mmol/l Kryptofix® 222 0.4 mmol/l to 4 mmol/l NADH 0.01 mmol/l to 0.8 mmol/l Buffer, pH 7 - 9 50 mmol/l to 500 mmol/l Mg 2+ 1 mmol/l to 10 mmol/l ADP (free acid) 0.5 mmol/l to 10 mmol/l LDH (assays at 25 °C) 5,000 U/l to 100,000 U/l Serum albumin 0 g/l to 5 g/l GDH (assayed at 25 °C) 2,500 U/l to 20,000 U/l KG (free acid) 1 mmol/l to 10 mmol/l In another method the sodium ions are allowed to displace potassium ions from Kryptofix® 221, the released potassium ions stimulating the activity of PK to a degree dependent on the sodium ion concentration (Example C). Typical concentration ranges of the main reagents for the enzymatic determination at 37 °C of sodium ions using pyruvate kinase to measure the displacement of potassium ions from Kryptofix® 221 are: PK (rabbit muscle) 50 U/l to 10,000 U/l PEP (neutralized Tris salt) 0.3 mmol/l to 30 mmol/l Kryptofix® 221 1 mmol/l to 10 mmol/l NADH 0.01 mmol/l to 0.8 mmol/l Buffer, pH 9 - 10 50 mmol/l to 500 mmol/l Mg 2+ 1 mmol/l to 10 mmol/l ADP (free acid) 0.5 mmol/l to 10 mmol/l LDH (assays at 25 °C) 5,000 U/l to 100,000 U/l KCl 2 mmol/l to 10 mmol/l Serum albumin 0 g/l to 5 g/l GDH (assayed at 25 °C) 2,500 U/l to 20,000 U/l KG (free acid) 1 mmol/l to 10 mmol/l A more accurate and precise embodiment for determining uses a sodium ion-dependent enzyme such as β-galactosidase (Example D). Blood plasma is incubated with a buffered mixture containing 2-nitrophenyl-β-D-galactopyranoside (NPG) and the enzyme β-D-galactosidase. The reaction catalysed by β-D-galactosidase is dependent on the presence of sodium ions and the rate of activity is a measure of sodium ion concentration. A key feature of this method is the use of an appropriate amount of sodium ion-selective binding agent (e.g. Kryptofix® 221) for measurements in serum or other biological fluids where the sodium ion concentration may exceed 100 mmol/l, to reduce sodium ion concentration so that the enzyme is most sensitive to small changes in sodium ion concentration in the usual analytical range (110 - 170 mmol/l). Under the conditions chosen für analysis, which include the presence of moderately high concentrations of magnesium and lithium ions, the rate of 2-nitrophenol and galactose formation is virtually proportional to the concentration of sodium ions being measured. Magnesium ions are required for optimal β-galactosidase activity. Lithium ions are competitive with sodium ions and therefore raise the Michaelis constant K m of the enzyme for sodium ions. As substrate for β-D-galactosidase many other compounds are suitable. Quite generally, the galactosidase-containing sample is mixed with an appropriate β-D-galactosidase substrate, the substrate being split by the enzyme, one of the fission products then being detected in an appropriate manner. Either the glycone liberated by action of the enzyme or the aglycone can be measured. As a rule, the latter is determined. As substrate, the natural substrate lactose can be used, but especially advantageous is use of a chromogenic galactoside. Thus, in Biochem. Z., 333, 209 (1960), there are described phenylphenyl-β-D-galactoside, as well as some further derivatives substituted on the aromatic ring, for example, 2-nitrophenyl-β-D-galactoside (NPG) and 3-nitrophenyl-β-D-galactoside, as substrates of β-D-galactosidase. The phenols liberated by hydrolysis are determined photometrically in the UV range or, in the case of the nitrophenols, in the short-wave visible wavelength range. An oxidative coupling with aminoantipyrine can also follow as indicator reaction [see Analytical Biochem. 40, 281 (1971)]. Other substrates are described in the German Offenlegungsschrift 33 45 748 and the German Offenlegungsschrift 34 11 574. Typical concentration ranges of the main reagents for the enzymatic determination at 37 °C of sodium ions using a 10 µl sample of plasma or serum are: β-D-galactosidase 25 U/l to 7,500 U/l NPG 0.25 mmol/l to 5 mmol/l Kryptofix® 221 0 mmol/l to 10 mmol/l Buffer, pH 7 - 9.5 200 mmol/l to 500 mmol/l Mg 2+ 0.01 mmol/l to 10 mmol/l EGTA (lithium salt) 0 mmol/l to 20 mmol/l Serum albumin 0 g/l to 5 g/l EGTA means ethylenbis(oxyethylennitrilo)-tetraacetic acid. It will also be feasible to perform the analysis of sodium and potassium ions enzymatically in the same cuvette in the form of a twin test (see Example D). Example A Measurement of sodium ion concentration using β-D-galactosidase. Variation a: The final incubation mixture contains: 300 mmol/l Tris HCl, pH 8.7 (37 °C) 4 mmol/l Dithiothreitol 7.5 mmol/l Magnesium sulphate 16 mmol/l Lithium chloride 0.44 mmol/l EGTA (lithium salt) 460 mg/l Human serum albumin 760 U/l β-Galactosidase 1.5 mmol/l NPG 1.25 µmol/assay Kryptofix® 221 The reaction is monitored at 420 nm (or nearby) wavelength to determine the rate of formation of free 2-nitrophenol and hence the concentration of sodium ions in the original sample. Variation b: An alternative approach compared with Variation a would be to reduce the sample concentration tenfold by pre-dilution or to use a small sample volume, in which case the cryptand could be omitted. Variation c: Measurement in fluids of low sodium ion content (e.g. < 20 mmol/l) in which case the cryptand could be omitted. Example B Measurement of sodium ions with pyruvate kinase (direct stimulation of enzyme activity by sodium ions) under conditions where sensitivity of potassium ions is diminished. Variation a: Incubation mixture contains for a 10 µl plasma sample: 300 mmol/l Tris HCl, pH 8.7 (37 °C) 5 mmol/l MgCl2 2.6 mmol/l ADP (free acid) 2.9 mmol/l PEP (neutralized Tris salt) 0.34 mmol/l NADH 17000 U/l LDH (assayed at 25°C) 2000 U/l PK (from rabbit muscle, assayed at 37°C) 4 mmol/l KG 8600 U/l GDH in glycerol (assayed at 25°C) 1.25 µmol/assay Kryptofix® 222 Sodium ion calibration solutions contain 4 mmol/l potassium to compensate for the potassium activating effect of serum potassium ion on pyruvate kinase. Example C Measurement of sodium ions with pyruvate kinase (competitive binding assay) - potassium ion concentration known The final incubation mixture contains for a 10 µl plasma sample: 300 mmol/l Glycine, pH 9.8 5 mmol/l MgCl 2 2.6 mmo/l ADP (free acid) 2.9 mmol/l PEP (neutralized Tris salt) 0.34 mmol/l NADH 17000 U/l LDH (assayed at 25°C) 890 U/l PK from rabbit muscle (assayed at 37°C) 4 mmol/l KG 8500 U/l GDH (assayed at 25°C) 5 mmol/l KCl 2.5 µmol/assay Kryptofix® 221 Potassium chloride is added to this reagent and displaced stoichiometrically from Kryptofix® 221 by sodium ions, thus allowing the sodium ion concentration of the specimen to be quantified. Example D Measurement of potassium and sodium ion concentration in the same cuvette (Twin-Test) Sodium ion concentration is assayed first as in Example A except that the assay also contains: 2.6 mmol/l ADP (free acid) 2.9 mmol/l PEP (neutralized Tris salt) 0.4 mmol/l NADH 4.0 mmol/l KG 8600 U/l GDH Following the measurement of sodium ions by means of determination of the reaction rate, the pH ofthe incubation mixture is lowered to pH 7.4 with a hydrochloric acid aliquot. Then the following ingredients are added to achieve the final concentrations indicated: 17000 U/l LDH 890 U/l PK from Bacillus stearothermophilus 3.0 mmol/l MnCl 2 20.0 mmol/l LiCl The reaction rate may then be monitored at 340 nm but may also be measured at a slightly higher wavelength to minimize possible interference by the 2-nitrophenol liberated in the sodium ion indicator reaction.

Claims: Process for the determination of sodium ions in fluids, wherein the influence of ions on the activity of a transferase, a hydrolase or an oxidoreductase is measured. Process as claimed in claim 1, wherein the fluid is a biological or non-biological fluid. Process as claimed in claim 2, wherein the biological fluid is blood, serum, plasma, urine, sweat, cerebrospinal fluid, lymph or intestinal secretions, exudates or transudates and the non-biological fluid is water or an aqueous extract or mixture. Process as claimed in one of the claims 1-3, wherein the transferase is transferring phosphorous-containing groups and the hydrolase is a glycosidase or acting on peptide bonds. Process as claimed in claim 4, wherein the hydrolase is α- or β-D-galactosidase from Escherichia Coli, carboxypeptidase A from bovine pancreas, collagenase from Clostridium hystolicum, amylase from salvia or pancreas, calpain I, calpain II, phosphoglycolate phosphatase, dipeptidyl aminopeptidase I (cathepsin C). Process as claimed in claim 4, wherein the transferase is pyruvate kinase, hexokinase, adenylate kinase, pyridoxal kinase or acetate kinase and the hydrolase is α- or β-D-galactosidase from E. Coli or carboxypeptidase A from bovine pancreas. Process as claimed in one of the claims 1-3, wherein the oxidoreductase is acting on the CH-OH group of donor molecules or on the aldehyde or oxo group of donors. Process as claimed in one of the claims 1-3. wherein the oxidoreductase is glycerol dehydrogenase from Enterobacter aerogenes, acetaldehyde dehydrogenase from yeast, or tyrosinase (catechol oxidase). Process as claimed in one of the claims 1 - 8, wherein the sodium ions are determined by using the enzyme pyruvate kinase from rabbit muscle or β-D-galactosidase from E. Coli. Process as claimed in one of claims 1 - 9, wherein interfering ions are masked with a first binding agent and wherein the sodium ions to be determined are reduced in concentration to optimal levels for measurement by means of a second binding agent, if dilution of the sample is not feasible and/or the affinity of the enzyme to the ions to be determined needs to be decreased. Process as claimed in claim 10, wherein the interfering ions or analytes are bound by cryptands, coronands, podands, crown ethers, spherands, hemispherands, calixarens and combinations thereof, natural occurring ionophores, cyclic peptides, complexones and chelating agents, and derivatives thereof. Process as claimed in one of claims 1 - 11, whereby and in the determination of sodium ions the interfering potassium ions are bound by Kryptofix® 222. Process as claimed in claim 10, whereby in the determination of the sodium ions using β-galactosidase the concentration of free sodium ions is reduced with a second binding agent and/or the affinity of the enzyme to sodium ions is reduced with lithium ions. Process as claimed in one of the claims 10 and 13, whereby the second binding agent is Kryptofix® 221. Process as claimed in claim 10, wherein the second binding agents according to claim 11 are present, which form a complex with indicator ions and from which the indicator ions are displaced stoichiometrically by the sodium ions which are to be determined analytes and wherein the influence of the displaced indicator ions on the activity of the enzyme is assayed, thereby giving an indirect measure of the concentration of the sodium ions. Process as claimed in claim 15, wherein the enzyme is pyruvate kinase, the indicator ions are potassium, the second binding agent is Kryptofix® 221. Process as claimed in one of claims 1 - 12 or 14 - 16, wherein ions which are competitive inhibitors of the sensitive indicator enzyme are included in the assay. Process as claimed in claim 17, wherein the competitive inhibitor is lithium ion. Composition for the determination of sodium ions in fluids comprising a transferase, a hydrolase or an oxidoreductase, the activity of which is influenced by the sodium ions and a first binding agent which binds the interfering ions in the fluid and/or a second binding agent which lowers the concentration of the sodium ions to optimal levels for measurement. Composition as claimed in claim 19 for the determination of sodium ions comprising pyruvate kinase and a cryptand. Composition as claimed in claim 20, wherein the cryptand is Kryptofix® 222, Composition as claimed in claim 19 for the determination of sodium ions comprising β-D-galactosidase and a cryptand. Composition as claimed in claim 22, wherein the cryptand is Kryptofix® 221. Composition for the determination of sodium ions comprising: β-D-galactosidase 25 U/l to 7500 U/l NPG 0.25 mmol/l to 5 mmol/l Kryptofix® 221 0 mmol/l to 10 mmol/l Buffer, pH 7 - 9.5 200 mmol/l to 500 mmol/l Mg 2+ 0.01 mmol/l to 10 mmol/l EGTA (Lithium salt) 0 mmol/l to 20 mmol/l Serum albumin 0 g/l to 5 g/l Composition for the determination of sodium ions, which used preferably at 37°C comprising: PK (rabbit muscle) 50 U/l to 10000 U/l PEP (neutralized Tris salt) 0.3 mmol/l to 30 mmol/l Kryptofix® 221 1 mmol/l to 10 mmol/l NADH 0.01 mmol/l to 0.8 mmol/l Buffer, pH 9 - 10 50 mmol/l to 500 mmol/l Mg 2+ 1 mmol/l to 10 mmol/l ADP (free acid) 0.5 mmol/l to 10 mmol/l LDH (assayed at 25°C) 5000 mmol/l to 100000 U/l KCl 2 mmol/l to 10 mmol/l Serum albumin 0 g/l to 5 g/l GDH (assay at 25°C) 2500 U/l to 20000 U/l KG (free acid) 1 mmol/l to 10 mmol/l Measurement of potassium and sodium ion concentration in the same cuvette by assaying the sodium ion concentration as claimed in claim 17 by means of determination of the reaction rate, lowering the pH value of the incubation mixture and then measuring the potassium ion concentration. Measurement of potassium and sodium ion concentration in the same cuvette by assaying the sodium ion concentration as claimed in claim 17 by means of determination of the reaction rate, the assay containing Tris HCI, pH 8.7 (37°C) Dithiothreitol Magnesium sulphate Lithium chloride EGTA (lithium salt) Human Serum Albumin β-Galactosidase NPG Kryptofix® 221 ADP (free acid) PEP (neutralized Tris salt) NADH KG GDH lowering the pH value of the incubation mixture and adding the ingredients LDH PK MnCl 2 LiCl

Inventors: BERRY MICHAEL NATHANIEL PROF, HERRMANN UWE DR RER NAT, KRESSE GEORG-BURKHARDT DR RER, TOWN MICHAEL-HAROLD DR PHIL, BERRY, MICHAEL NATHANIEL, PROF., HERRMANN, UWE, DR. RER. NAT., KRESSE, GEORG-BURKHARDT, DR. RER. NAT., TOWN, MICHAEL-HAROLD, DR. PHIL.
Applicants: BOEHRINGER MANNHEIM GMBH, UNIV SOUTHERN AUSTRALIA, THE FLINDERS UNIVERSITY OF SOUTH AUSTRALIA
Application year: 1988
Main classification: G01N33
IPC codes: C12Q1, G01N33