DETERMINATION OF POTASSIUM IONS IN FLUIDS

Abstract: The invention concerns a process and a reagent for the determination of ions in fluids, wherein the influence of these ions on the activity of an enzyme is measured. The ions for example are sodium, potassium, calcium, magnesium, manganese, lithium, lead, zinc, copper, iron or other heavy metals or non-metallic ions comprising chloride, bicarbonate, protons, ammonium substances that give rise to ammonium. The enzymes which are used may be for example a transferase, a hydrolase, an oxidoreductase or a lyase. An essential part of the invention is a method to exclude interferences by ions by masking the interfering ions with a bindig 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 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 Hg²⁺ ions via diphenylcarbazone complex colorimetric determination of the rhodanide complex of iron, which is formed after dissociation of the mercury complexes upon precipitation of HgCl₂ [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 Cu²⁺ 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 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 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. Use is also made of the 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 of the 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. These general principles can best be illustrated in detail by showing their application to the determination of potassium, sodium, calcium, chloride and bicarbonate ions in plasma or serum. However, they are applicable to a wide spectrum of ions, for example cations such as magnesium, manganese, lithium, lead, zinc, copper, iron or other heavy metals. Examples of non-metallic ions that can be measured are protons, bicarbonate or ammonium ions or substances that give rise to the formation of ammonium such as urea in the presence of urease, or glutamine in the presence of glutaminase. Suitable Enzymes Enzymes which may be used can be for example (H. J. Evans et al., Ann. Rev. Plant Physiol. 17, 47; 1966): 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 thiol-dependent 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). Lyases like aldolase (from yeast) or carbonic anhydrase (from bovine erythrocytes). 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 quarternary tetrahedron compounds, macrocyclic bis-metal complexes, macrocycles with covalently incorporated lewis acid centers, protonated or alkylated quarternary 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(III) or silver nitrate. Function of Binding Agents: These binding agents are used for the following purposes: 1. The selective binding of interfering ions. 2. To reduce the concentration of the analytes to optimal measuring levels, 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 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 Mg²⁺, 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 Zn²⁺, the binding agent is Kryptofix® 221, and the analyte ion is a heavy metal ion; or the enzyme is α-amylase, the binding agent is a metal of the first and second side group (subgroups I b and II b) of the 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. 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. Non-biological 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 Potassium and Sodium Ions The essential requirements for a satisfactory method for the determination of potassium ions in serum or plasma on the basis of the sensitivity of an enzyme e.g pyruvate kinase, to potassium ions, is the overcoming of the interference by sodium and ammonium ions. According to the general principles embodied in this invention this can be achieved by one or more of the following procedures: 1. The selective binding of sodium ions with a suitable binding agent, for example Kryptofix® 221. 2. The selection of enzyme source e.g. Bacillus stearothermophilus, rather than rabbit muscle, as the source of pyruvate kinase since the bacterial enzyme has a sensitivity for potassium ions in relation to sodium ions twice as great as the muscle enzyme. 3. Inclusion of ions which are competitive inhibitors of the sensitive indicator enzyme in the assay, for example the use of lithium ions to compete with sodium ions and potassium ions. 4. Enzymatic removal of ammonium ions. Since lithium ions are less effective as a competitor against potassium ions, the net effect is to increase the relative sensitivity of pyruvate towards potassium ions a further 50 % as compared with sodium ions. Moreover, in the presence of lithium ions the effects of potassium and sodium ions on the activity of pyruvate kinase become additive, rather than co-operative. This allows the possibility of measurement of the concentration of either potassium or sodium ions, provided that the concentration of the other ions is known. By the use of procedures 2 and 3 in the absence of a binding agent it is possible to obtain a relative sensitivity of pyruvate kinase for potassium versus sodium ions in the order of 100 : 1. This means that even at extremely abnormal sodium ion concentrations of either 110 or 170 mmol/l, the error in measured potassium ion concentration will not exceed 0.3 mmol/l relative to a normal plasma sodium ion concentration of 140 mmol/l (Example A). This is not regarded as sufficiently accurate for many clinical purposes. However, if the true concentration of sodium ions in the plasma is known, accurate measurement of potassium ions down to ± 0.05 mmol/l is feasible (Example B). If procedures 1 - 3 are combined and a binding agent, e.g. Krypotfix® 221, is included, the relative sensitivity of pyruvate kinase for potassium ions with respect to sodium ions can be increased to > 500 : 1. Under these circumstances it is not necessary to know the sodium ion concentration to determine the plasma potassium ion concentration down to 0.05 mmol/l (Example C). These methods for potassium ion determination demonstrate the applicability of the general principles embodied in the invention in regard to reduction of interferences. On the other hand, 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). 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 1 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). Sodium ions may also be measured by means of pyruvate 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. 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 of pyruvate kinase in proportion to the plasma sodium ion concentration. 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. Detailed Description of Analytical Methods for Potassium Ions This section describes in more detail methods for potassium ion determination embodying the principles described in this invention. For the determination of potassium ions, a fluid, for example blood plasma, is incubated with a buffered mixture containing adenosine diphosphate (ADP), phosphoenolpyruvate (PEP), reduced nicotinamide adenine dinucleotide (NADH), pyruvate kinase (PK) and lactate dehydrogenase (LDH). The formation of pyruvate, and subsequently lactate in this mixture, in reactions catalysed by PK and LDH, is entirely dependent on the presence of appropriate cations, in the absence of which PK is virtually inactive. NADH absorbs strongly at 340 nm, whereas NAD does not. Under the conditions chosen for analysis which include the presence of manganese ions which are required by the bacterial PK, the rate of NADH oxidation is proportional to the concentration of potassium ions (see Examples A - C). Under these conditions the following reactions take place: The rate of reaction (1) is determined by the concentration of potassium ions present in the system, and this in turn limits the rate of reaction (2). There are several ways in which the rates of these reactions can be measured. A standard approach is the spectrophotometric measurement of the rate of disappearance of NADH in reaction (2). NADH absorbs strongly at 340 nm, whereas NAD does not. Accordingly, the fall in absorbance of the reaction mixture at 340 nm (or an alternative wavelength) provides a direct measure of the rate of the reaction and from this the concentration of potassium ions present in the mixture can be derived. Alternatively, advantage can be taken of the fact that both reactions (1) and (2) consume H⁺, thus lowering the proton concentration of the reaction mixture. The rate of fall in proton concentration can be measured with a pH meter, or by means of a titration procedure. In these latter cases the concentration of buffer employed will be much less than in the spectrophotometric technique. Other equipment such as fluorimeters or luminometers can be used to monitor the activity of pyruvate kinase. There are a number of other methods of detecting the accumulation of pyruvate associated with PK activity. These include any method for measuring the inorganic phosphate or oxygen consumed or the hydrogen peroxide; acetyl phosphate or carbon dioxide generated by the enzymatic action of pyruvate oxidase; the formation of the hydrazone with 2,4-dinitrophenylhydrazine; the measurement of the reactants or products of the enzymatic action of pyruvate carboxylase; pyruvate decarboxylase or pyruvate dehydrogenase; the use of flavine coupled systems; and isotopic methods for measuring minute concentrations of substrates [M. N. Berry et al., Analytical Biochem. 118, 344 - 352 (1981)]. In a survey of 200 serum samples good agreement has been obtained with other methods such as flame photometric or ion-selective electrode measurements. A significant interference with the method are ammonium ions which are generally present in serum or accumulate on standing. The possibility of ammonium ion interference can be completely avoided by including α-ketoglutarate (KG) and glutamate dehydrogenase (GDH) in the reaction mixture. Ammonium ions are removed in a preincubation according to the reaction: NH₄⁺ + KG + NADH ―> glutamate + NAD⁺ In solutions such as urine in which the ammonium ion content may be high a coupled reaction can be used: NH₄+ + KG + NADPH ―> glutamate + NADP Glucose-6-P + NADP ―> 6-phosphogluconate + NADPH The coupled method employs glucose-6-phosphate dehydrogenase. Provided that the added glucose-6-P and -KG are in excess of any ammonium ions present, all ammonium ions will be removed while preserving the NADH in the reagent. Typical concentration ranges of the main reagents for the enzymatic determination at 37 °C of potassium ions using a 10 µl sample of plasma or serum are: PK (B. stearothermophilus) 50 U/l to 10,000 U/l PEP (neutralized Tris salt) 0.3 mmol/l to 30 mmol/l Kryptofix® 221 0 mmol/l to 30 mmol/l NADH 0.01 mmol/l to 0.8 mmol/l Buffer, pH 7 - 8 50 mmol/l to 500 mmol/l Mn²⁺ or Mg²⁺ 1 mmol/l to 10 mmol/l LiCl 2 mmol/l to 100 mmol/l ADP (free acid) 0.5 mmol/l to 10 mmol/l LDH (assayed 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 Another example sensitive to potassium ions is glycerol dehydrogenase (E. C. C. Lin et al., B 235, 1820, 1960). Typical concentration ranges of the main reagents for the enzymatic determination at 37 °C of potassium ions using glycerol dehydrogenase are: Glycerol dehydrogenase 50 U/l to 1,000 U/l Glycerol 0.3 mol/l to 3 mol/l Kryptofix® 221 0 mmol/l to 30 mmol/l NAD 0.1 mmol/l to 5.0 mmol/l Buffer, pH 9 20 mmol/l to 500 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 Another enzyme sensitive to potassium ions is acetaldehyde dehydrogenase (S. Black, Arch. Biochem. Biophys. 34, 86, 1951). Typical concentration ranges of the main reagents for the enzymatic determination at 37 °C of potassium ions using acetaldehyde deydrogenase are: Acetaldehyde dehydrogenase 50 U/l to 10,000 U/l Glycolaldehyde 0.3 mmol/l to 30 mmol/l Kryptofix® 221 0 mmol/l to 30 mmol/l NAD 0.05 mmol/l to 2.0 mmol/l Buffer, pH 7 - 8 50 mmol/l to 500 mmol/l Dithiothreitol 0.1 mmol/l to 2 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 Acetaldehyde (0.02 mmol/l to 1 mmol/l) may be substituted for glycolaldehyde. Acetaldehyde dehydrogenase also exhibits esterase activity so that potassium ion concentration can be determined by monitoring the release of 4-nitrophenol from 4-nitrophenyl acetate. Typical concentration ranges of the main reagents for the enzymatic determination at 37 °C of potassium ions based on the esterase activity of acetaldehyde dehydrogenase are: Acetaldehyde dehydrogenase 5 U/l to 10,000 U/l 4-nitrophenyl acetate 0.1 mmol/l to 2 mmol/l Kryptofix® 221 0 mmol/l to 30 mmol/l NADH 0.001 mmol/l to 0.1 mmol/l Buffer, pH 7 - 8 50 mmol/l to 500 mmol/l Dithiothreitol 0.1 mmol/l to 2.0 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 Example A Measurement of potassium ion concentration using pyruvate kinase without a sodium ion-binding agent, sodium ion concentration unknown. The final incubation mixture contains: 175 mmol/l Tris-HCl buffer, pH 7.4 20 mmol/l Li⁺ [17 mmol/l LiOH, 3 mmol/l LiCl] 3.0 mmol/l MnCl₂ 2.6 mmol/l ADP (free acid) 2.9 mmol/l PEP (neutralized tris salt) 0.4 mmol/l NADH 17000 U/l LDH (assayed at 25 °C) 890 U/l PK from Bacillus stearothermophilus 4.0 mmol/l KG 8600 U/l GDH (in glycerol; assayed at 25 °C) 140 mg/l Human serum albumin Potassium ion standards (calibrating solutions) contain 140 mmol/l sodium ions to compensate for the stimulatory effect of sodium ions, present in plasma, on pyruvate kinase. Example B Measurement of potassium ion concentration using pyruvate kinase, without a sodium ion binding agent, sodium ion concentration known. The incubation mixture and calibrating solution are the same as for Example A. A correction may be made for the sodium ion concentration of the mixture by adding (or subtracting) 0.1 mmol/l potassium for every 10 mmol/l the sodium ion concentration is below (or above) 140 mmol/l sodium ions. However, this correction should be verified by analysing aqueous solutions containing known sodium and potassium concentrations. Example C Measurement of potassium ion concentration using pyruvate kinase the the presence of a sodium ion-binding agent. As for Example B, but human serum albumin is omitted and the medium contains in addition 6 µmol of Kryptofix® 221 per assay. A pH of 7.8 is selected to minimize variations in displacement of sodium ions from Kryptofix® 221 due to the differing potassium ion content of individual specimens. Example D Measurement of potassium and sodium ion concentration in the same cuvette (Twin-Test) Sodium ion concentration is assayed first as in Example D 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 of the 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 MnCl₂ 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 potassium ions in body fluids, wherein the influence of the potassium ions on the activity of a microbial transferase, a microbial oxidoreductase, a lyase or a hydrolase is measured. Process as claimed in claim 1, wherein the microbial transferase is a pyruvate kinase from Bacillus Stearothermophilus and the microbial oxidoreductase is a glycerol dehydrogenase or an actealdehyde dehydrogenase. Process as claimed in one of the claims 1 and 2, wherein interfering ions are masked with a binding agent. Process as claimed in one of the claims 1 - 3, wherein the ions to be determined are reduced in concentration to optimal levels for measurement by means of a binding agent, if dilution of the sample is not feasible and-or the affinity of the enzyme to the potassium ions needs to be decreased. Process as claimed in one of the claims 3 - 4, wherein the interfering ions are bound by cryptands, coronands, podands, crown ethers, spherands, hemispherands, calixarens and combinations thereof, natural occuring ionophores, cyclic peptides, complexones and chelating agents, and derivatives thereof. Process as claimed in one of the claims 1 - 5, whereby the interfering sodium ions are bound by Kryptofix® 221. Process as claimed in one of the claims 4 and 5, whereby the binding agent is Kryptofix® 222. Process as claimed in one of the claims 1 - 7, wherein the binding agents form a complex with indicator ions and the indicator ions are displaced stoichiometrically from the complex by the potassium ions 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 potassium ions. Process as claimed in one of the claims 1 - 8, wherein ions which are competitive inhibitors of the indicator enzyme are included in the assay, when the affinity of the enzyme to the potassium ions needs to be decreased. Process as claimed in claim 9, wherein the competitive inhibitors are lithium ions. Composition for the determination of potassium ions in body fluids comprising a microbial transferase or a microbial oxidoreductase the activity of which is influenced by the potassium ions. Composition for the determination of potassium ions in body fluids as claimed in claim 11, comprising microbial transferase or microbial oxidoreductase and a binding agent which binds the interfering ions in the fluid and/or lowers the concentration of the potassium ions to optimal levels for measurement and/or decreases the affinity of the microbial transferease or oxidorectuase to the potassium ions. Composition as claimed in claim 12 wherein the binding agent is cryptand. Composition as claimed in claim 12, wherein the cryptand is Kryptofix® 221 to bind interfering ions and Kryptofix® 222 to lower the concentration of the potassium ions and/or to decrease the affinity of the microbial transferase or the microbial oxidorectase. Composition for the determination of potassium ions in body fluids, which is used preferably at 37°C comprising: PK (B. stearothermphilus) 50 U/l to 10000 U/l PEP (neutralized Tris salt) 0.3 mmol/l to 30 mmol/l Kryptofix® 221 0 mmol/l to 30 mmol/l NADH 0.01 mmol/l to 0.8 mmol/l Buffer, pH 7 - 8 50 mmol/l to 500 mmol/l Mn²⁺ or Mg²⁺ 1 mmol/l to 10 mmol/l LiCl 2 mmol/l to 100 mmol/l ADP (free acid) 0.5 mmol/l to 10 mmol/l LDH (assayed at 25°C 5000 U/l to 100000 U/l Serum albumin 0 g/l to 5 g/l GDH (assayed at 25°C) 2500 U/l to 20000 U/l KG (free acid) 1 mmol/l to 10 mmol/l Composition for the determination of potassium ions, which is used preferably at 37°C comprising: Glycerol dehydrogenase (Enterobacter aerogenes) 50 U/l to 1000 U/l Glycerol 0.3 mol/l to 3 mol/l Kryptofix® 221 0 mmol/l to 30 mmol/l NAD 0.1 mmol/l to 5.0 mmol/l Buffer, pH 9 20 mmol/l to 500 mmol/l Serum albumin 0 g/l to 5 g/l GDH (assayed at 25°C) 2500 U/l to 20000 U/l KG (free acid) 1 mmol/l to 10 mmol/l Composition for the determination of potassium ions, which is used preferably at 37°C comprising: Acetaldehyde dehydrogenase yeast 50 U/l to 10000 U/l Glycolaldehyde 0.3 mmol/l to 30 mmol/l Kryptofix® 221 0 mmol/l to 30 mmol/l NAD 0.05 mmol/l to 2.0 mmol/l Buffer, pH 7 - 8 50 mmol/l to 500 mmol/l Dithiothreitol 0.1 mmol/l to 2 mmol/l Serum albumin 0 g/l to 5 g/l GDH (assayed at 25°C) 2500 U/l to 20000 U/l KG (free acid) 1 mmol/l to 10 mmol/l Composition for the determination of potassium ions, which is used preferably at 37°C comprising: Acetaldehyde dehydrogenase (yeast) 50 U/l to 10000 U/l 4-Nitrophenyl acetate 0.3 mmol/l to 30 mmol/l Kryptofix® 221 0 mmol/l to 30 mmol/l NAD 0.001 mmol/l to 0.1 mmol/l Buffer, pH 7 - 8 50 mmol/l to 500 mmol/l Dithiothreitol 0.1 mmol/l to 2 mmol/l Serum albumin 0 g/l to 5 g/l GDH (assayed at 25°C) 2500 U/l to 20000 U/l KG (free acid) 1 mmol/l to 10 mmol/l

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