DETERMINATION OF 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 binding agent.

Description: DETERMINATION OF IONS IN FLUIDS 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 anaiyte 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 presenc of other ions to which the enzyme is also sensitive. This invention aαdresses and solves these problems in diverse ways. In the practice of Clinical Biochemistry the measurement of ser electrolytes are the most common analytical tests performed wit hospitals. These measurements are requested not only for routin investigations but frequently for emergency and life-threatenin 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 easil performed near the bed-side 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 trie principle that certain atoms wnen 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 i directly proportional to the number of atoms excited in the flam 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 wou 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 c be performed spectrophotometrically and the clinical need for io 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 flu completion of formation of insoluble silver chloride product. Alternatively, potentiometric determinations may be used which a also very time consuming and involve additional instrumentation. For cnloriαe, in addition, there are a numoer of photometric an titrimetric methods, which e.g. include: titrimetric determination of free Hg *" ions via diphenyl- carbazone complex colorimetric determination of the rhodanide complex of iron wnich is formed after dissociation of the mercury complexes upon precipitation of HgCl- (Skeggs, Clin. Chem. 10, 1964, 918f.; Schmidt, Zentralblatt Pharm. 124 (9), 1985, 527f) colorimetric determination of chloranilic acid from the respective mercury salt (Renschler) determination of the coloured Cu + complex of diethyl- dithiocarbaminic acid from the colourless mercury salt (Ger Offenlegungsschrift 2137146) the rather common TPTZ-method (tripyridile-s-triazine) (R. Fried, Zeitschr. Klin. Chem., Klin. Bioch. 10, 1972, 280f; DOS 215 3387) which is similarily based on the formation of coloured metal complex upon dissociation of a mercury compl 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 reagents are unstable and calibration curves are non-linear (e. 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 Wo. 83/002670, the use toxic mercury compounds however is still a disadvantage. The on 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), 1975, 45if.; W. T. Law, Clin. Chem. 26 (15) 1980, 1874f.; US-4278440). considerable limitation of this method is the use of strongly aci reagents which are corrosive and therefore not compatible with mechanical pitetting systems. A further disadvantage is the interference D bilirubin in the samples. Calcium is a further example of an electrolyte which is routinel 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 fo calcium and depends on a careful handling of samples. The method has been succeeded in many laboratories by titrimetric and direc 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 metho however, involves the use of aggressive, highly alkaline solutio and toxic substances. It is particularly prone to interference b 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 calibrati curves are non-linear and the colour is greatly dependent on temperature. In K.H. Outlaw and O.H. Lowry, Analytical Biochemistry 92, 570- (1979) an enzyme-mediated assay for measuring potassium ions in tissues is described. The method employs pyruvate kinase, from . raobit muscle, which is activated by potassium ions and sodium ions, tne 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 whicn contains a thirty-fold excess of sodium ions. Therefore sodium ions cause unacceptable interference when usin the enzymatic photometric technique as described by Lowry et al to measure potassium in plasma or serum. A further problem is t ammonium ions give a similar activation to potassium ions. The above mentioned publication does not address or solve these critical problems in regard to the analysis of potassium ions i biological fluids such as serum, nor does it propose any method for the determination of sodium ions. Therefore, it is an object of the present invention to provide 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 o these ions on the activity of an enzyme is measured. A key feature of this invention is the use of selective 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 the invention is the use of competitive inhibitors of the releva analytical enzyme in order to reduce its sensitivity to the analyte, thereby permitting measurement of the latter at a highe concentration. This is especially useful, for example, where selective binding agents are not readily available or are unacceptady expensive. Another feature of the invention is that selective binding agent are employed to reduce the free concentrations of interfering io to levels where interference is no longer significant. Use is al 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 condition including the selection of an appropriate isoenzyme, such that t stimulatory or inhibitory effects of the analyte are substantial greater that those of the interfering ions. In addition, the action of the analyte and interfering ions on the activity of th analytical enzyme should be additive so that if the concentratio of interfering ions is known, the concentration of the analyte c readily be determined by difference.- Where an interfering ion is known to occur at a relatively constant concentration in the flu 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 analyt is the use of a competitive binding assay where the analyte displaces another ion from a binding agent and the effects of th released ion on the activity of an appropriate enzyme is 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 seru However, they are applicable to a wide spectrum of ions, for example cations such as magnesium, manganese, lithium, lead, zin copper, iron or other heavy metals. Examples of non-metallic ion that can be measured are protons or ammonium. Substances such as urea that give rise to ammonium can also be determined. Suitable Enzymes Enzymes which may used can be for example (Ff.J. Evans et al. Ann Rev. Plant Physiol. 17, 47, 1966): Transferases like phosphorous-containing group- transferring transferases . Sucn a transferase may be pyruvate kinase. In pla of pyruvate kinase other kinases such as adenylate kinase or hexokinase, sensitive to magnesium ion or manganous ion may σe employed. Another transferase is acetate kinase (from E. coli). Another example is pyridoxal kinase from brain which is sensitiv to zinc ions . Hydrolases like glycosidases , for example^- or tό-D-galactosidas (from Escherichia coli), carboxypeptidase A (from bovine pancreas), collagenase (from Clostridium hystolicum), amylase (from saliva or pancreas) or phosphoglycollate phosphatase. Also peptide hydrolases such as the cysteine or thiol dependent proteinases, specific examples of which are Calpain I and II (al called calcium activated neutral protease) described by Sasaki e al. in J. Biol. Chem. 259, 12489-12494, (1984). The latter enzym can be isolated and purified from a variety of animal tissues su as: rat liver and kidney, human and porcine erythrocytes, bovine brain, and rabbit skeletal muscle according to the method of A. Kitahara et al . , described in J. Biochem. 95, 1759-1766 (1984 A further example is dipeptidyi aminopeptidase I (E.C. 3.4.14.1, Cathepsin C), J. Ken Mc Donald, Bioch. Biophys. Res. Communicati 24(5), 66, 77 If. Another source for the enzymes are proteins obtained by gene recombination techniques. Oxidoreductases like glycerol dehydrogenase (from Enterobacter aerogenes), acetaldehydrogenase (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. Another source for the enzymes are proteins obtained gene recombination techniques. Selective Binding Agents: A wide variety of binding agents are available for the binding of analytes or interfering ions. Such binding or masking substances are cryptands, coronands, crown - ethers, podands, spheranαs, hemispnerands, calixarens and combinations thereof, natural 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; Konakte (Merck), 1977, No. 3, p. 56 ff; Phase Transfer Catalysts, Properties and Applications (Merck-Schucharαt) 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 edit., 399-415 (Clarendon Press) Oxford; F. Vδgtle 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, Ne York, 1982; J.M. Lehn et al., J.A . Chem. Soc. 97, 6700-6707 (1975); G. Schwarzenbach et al., Helv.Chi . Acta 28, 828 (1945); S.F.A. Kettle, Koordinationsverbindungen, Taschentext 3, Verlag Che ie, Weinheim/Bergstr . 1972; A.E. Martell et al., Die Chemie der Metallc elatverbindungen, 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 ultivalent ions, in particular bivalent cations are ethyleneglycol-bis-(2-aminoethyl ether)-N, N, N', N' -tetraacetic acid (referred to as EGTA) and (ethylenedinitrilo) tetraacetic acid (EDTA). While many binding agents exist that can Dind multivalent ions, e.g. EDTA and its derivatives, agents which bind monovalent ions are less common. Tetraphenylboron binds potassium ions. However-, 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 8 271-339). Special examples for cryptands are the Kryptofix^ compounds of Merck-Schuchardt , for example: 4 ,7 ,13 ,16 ,21-Pentaoxa-l,10-diazabicyclo[8.8.5]-tricosan, KryptofixS' 221, page 438, Merck-Schuchardt catalogue, dated 1987/88, no. 810646 (K 221). 4 ,7 ,13 ,16.21,24-Hexaoxa-l,10-diazabicyclo[8.8.8Jhexacosan, Kryptofix—' 222 , page 438, Merck-Schuchardt catalogue, dated 1987/1988, no. 810647 (K 222). As masking compounds for the elimination of interfering anions t following classes of substances may potentially be used: anioncryptandes , heterocyclophanes, catapinands and inorganic metalcomplexes 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 aikylated quarternary cryptands or catapinands (F. P. Schmidtche 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 hexachloro complex of Fe (III) or silver nitrate. Function of Binding Agents: These binding agents are used for th following purposes: 1. The selectivce binding of interfering ions 2. To reduce the concentration of the analytes to optimal measuring levels, if dilution of the sample is not feasible. 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 ar potassium, the binding agent is Kryptofii^ 221 and the ion to be determined is sodium; or the enzyme is a kinase, the indicator ion is Mg + , the binding agent is a chelating agent, e.g. EDTA, and the ion is a metal or the enzyme is pyridoxal "kinase, the indicator ion is Zn , the binding agent is Kryptofix (R y 2 1, and the ion is a heavy metal; or the enzyme is J-amylase, the binding agent is Ag or Hg and t ion to be determined is chloride; or the enzyme is collagenas 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, sweat, transudates or exudates or urine for example. Other examples of fluids are tap water or extracts of foodstuffs or fruits or fermented liquids such as w r ine. 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 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 bindin agent, for example Kryptofix^ 221. 2. The selection of 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 5. Inclusion of ions which are competitive inhibitors of the sensitive indicator enzyme in the assay, for example the us of lithium ions to compete with sodium ions and potassium i 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 kinase 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 mesurement of the concentration of either potassium or sodium ions, provided that the concentration of the other ions is known. By the use of procedure 2 and 3 in the absence of a binding agen it is possible to obtain a relative sensitivity of pyruvate kina for potassium versus sodium ions in the order of 100:1. This mea that even at extremely abnormal sodium ion concentrations of either 110 or 170 mmol/1, the error in measured potassium ion concentration will not exceed 0.3 mmol/1 relative to a normal plasma sodium ion concentration of 140 mmol/1 (Example A). This i not regarded as sufficiently accurate for many clinical purposes. However, if the true concentration of sodium ions in the plasma- i known, accurate measurement of potassium ions down to — 0.05 mmol/1 is feasible procedure 1-3 are combined and a binding agent, e 1 is included, the relative sensitivity of pyruvate kinase for potassium ions with respect to sodium ions can be increased to "7500:1. Under these circumstance it is not necessary to know the sodium ion concentration to determine the plasma potassium ion concentration down to 0.05 mmol/1 (Example C). These methods for potassium ion determination demonstrate the applicability of the general principles embodied ia the invention in regard to reduction of interferences. On the other hand the measurement of sodium ions in serum or plasma best illustrates th 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 ϋ-galactosidase (Kub et al., J. Am. Chem. Soc. 75, 890, 1953). However, the range of sodium ion concentration to which this enzyme is most sensitive i 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 i not feasible. 1. Use of a sodium ion binding agent such as Kryptof ix 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 (o-galactosidase , and the amount of binding aget can be manipula to minimise the signal for sodium ion concentrations below 110 mmol/1 while enhancing the signal in the usual analytical range (110-170 mmol/1) (Example D) . 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. Kryptofi *^ 222 , is included in the reaction mixture (Example E). In another method of determining plasma sodium ion concentratio the sodium ions are allowed to displace potassium ions from Kryptofι <^2 1 , the released potassium ions stimulating the activity of pyruvate kinase in proportion to the plasma sodium concentration (Example F). Other embodiments of the invention are compositions and reagent for the determination of ions in biological and non-biological fluids . The reagent according to the present invention can be present i 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, w 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 resi and fine meshes . The embodiments of the invention are described ereunαer in some detail but it will be seen that the invention need not necessaril be limited in any one or in a combination of the details described, and in particular, mechanical or chemical variations can be utilized besides those described in this embodiment. 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 inventio 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) an lactate dehydr genase (LDH). The formation of pyruvate, and subsequently lactate in this mixture, in reactions catalysed by P 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 n , whereas NAD does not. Under the conditions chosen for analysis which include the presence of manganese ions which are required by the bacterial PK tiie rate of NADH oxidation is proportional to the concentration o potassium ions (see Examples A-C). Under these conditions the following reactions take place: (1) PEP + ADP + H + PK -) Pyruvate + ATP (2) Pyruvate + NADH + H + LDH ) Lactate + NAD + 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 o 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 does ot. Accordingly, the fall in absorbance of the reaction mixture at 340 nm (or an alternative wavelength) provides a dir 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 reaction (1) and (2) consume H + , thus lowering the proton concentration of the reaction mixture. The rate of fall in prot 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 spectrophotometri 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 accumulatio of pyruvate associated with PK activity. These include any metho for measuring the inorganic phosphate or oxygen consumed or the hydrogen peroxide; acetyl phosphate or carbon dioxide generated the enzymatic action of pyruvate oxidase; the formation of- the hydrazone with 2 ,4-dinitrophenylhydrazine; the measurement of th reactants or products of the enyzmatic action of pyruvate carboxylase; pyruvate decarboxylase or pyruvate dehydrogenase; t use of flavine coupled systems; and isotopic methods for measuri minute concentrations of substrates (M.N. Berry et al . , Analytic Biochem. 118, 544-352 (1981)). In a survey of 200 serum samples good agreement has been obtaine with other methods such as flame photometric or ion-selective electrode measurements. A significant interference with the meth is ammonium ions which are generally present in serum or accumulate on standing. The possibilly of ammonium ion interference can be completely avoided by including <»-ketoglutarate (KG) and gluta ate dehydrogenase (GDH) in the reaction mixture. Ammonium ions are removed in a preincubation according to the reaction: NH, + KG -•- NADH ■ glutamate + NAD " In solutions such as urine in which the ammonium ion content may be high a coupled reaction can be used.: NH^ + 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 an 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 eennzzyymmaattiicc ddeetteerrmmiinnaattiioonn aatt 3377 ( °C of potassium, ions using a 10 ul sample of plasma or serum are; PK (B. stearothermophi .lus 50 U/l to 10 1 300 U/l PEP (neutralized Tris salt) 0.3 mmol/1 to 30 m o1/1 Kryptofix^ 221 0 mmol/1 to 30 mmo1/1 NADH 0.01 mmol/l ' to 0.8 mmo1/1 buffer, pH 7-8 50 mmol/l to 500 mmol/l M r ,n2+ or M..g2+ 1 mmol/l to 10 m ol/1 LiCl 2 mmol/l to 100 mmo1/1 ADP (free acid) 0.5 mmol/l to 10 mmol/l LDH (assayed at 25 °C) 5 000 U/l to 100 ( 300 U/l Serum albumin 0 g/1 to 5 g/1 GDH (assayed at 25 °C) 2 500 U/l to 20 1 300 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° of potassium ions using glycerol dehydrogenase are: Glycerol dehydrogenase 50 U/l to 1000 U/l Glycerol 0.3 mol/ 1 to 3 mol/1 Kryptofi ® 221 0 mmol/l to 50 mmol/l NAD 0.1 mmol/ 1 to 5.0 mmol/ 1 buffer, pH 9 20 mmol/l to 500 mmol/l Serum albumin 0 g/1 to 5 g/1 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 dehydrogenase are: Acetaldehyde dehydirogenase 50 U/l to 10 000 U/l Glycolaldehyde 0.3 mmol/l to 30 mmol/l Kryptofix 221 0 mmol/l to 50 mmol/l NAD 0.05 mmol/l to 2.0 mmol/ 1 buffer, pH 7-8 50 mmol/l to 500 mmol/l Dithiothreitol 0.1 mmol/l to 2 mmol/l Serum albumin 0 g/1 to 5 g/1 GDH (assayed at 25 C >α 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 . Acetaldenyoe dehydrogenase also exhibits esterase activity so tha potassium ion concentration can De determined by monitoring the release of 4-nitropnenol from 4-nitrophenyl acetate. Typical concentration ranges of the main reagents for the enzymatic determination at 57°C of potassium ions based on the esterase activity of acetaldehyde dehydrogenase are: Acetaldehyde dehydrogenase 50 U/ l to 10 000 U/l 4-nitrophenyl acetate 0 . 1 mmo l/ l t o 2 mmol/l Kryptofi 221 0 mmo l/ 1 to 30 mmol/l NADH 0 .001 mmo l/ l to 0.1 mmo1/1 buffer, pH 7-8 50 mmol/ 1 to 500 mmol/l Dithiothreitol 0 . 1 mmol/ l to 2.0 mmol/l Serum albumin 0 g/ 1 to 5 g/1 GDH (assayed at 25°C) 2 ! 500 U/ l to 20 000 U/l KG (free acid) 1 mmol/ l to 10 mmol/l 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 occu 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. IInn oonnee method the potassium ions are specifically boun with Kr ■ yyppttooffiixx**^^ 2222 and the effects of sodium ions on PK activity measured directly (Example E). 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 10 000 U/l PEP (neutralized Tris salt) 0.5 mmol/ 1 to 30 mmol/l Kryptofix® 222 0.4 mmol/ 1 to 4 mmol/l NADH υ.oi mmol/l to 0.8 mmol/l buffer, pH 7-9 50 mmol/l to 500 mmol/l Mg 1 mmol/ 1 to 10 mmol/l ADP (free acid) 0.5 mmol/l to 10 mmol/l L LDDHH ((aassssaayyeedd .at 25°C) 5 000 mmol/ 1 to 100 000 U/l Serum albumin 0 g/1 to 5 g/1 G GDDHH ((aassssaayyeedd .at 2S°C) 2 500 U/l to 20 000 U/l KG (free acid) 1 mmol/ 1 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 sodi ion concentration (Example F). 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 Krypto ' fi ® 221 are: PK (rabbit muscle) 50 U/l to 10 000 U/l PEP (neutralized Tris salt) 0.3 mmol/ 1 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) 5 000 mmol/l to 100 000 U/l KC1 2 mmol/l to 10 mmol/l Serum albumin 0 g/1 to 5 g/1 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-iϋ-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-selectiv 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 for analysis, which include the presence of moderately high concentrations of magnesium and lithium ions, the rate of 2-nitrophenol and galactose formation i 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 Km of the enzy'me for sodium ions. As subtrate 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 bein 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 phenyl-β-D-galactoside, as well as some further derivatives substituted on the aromatic ring, for example, 2-nithophenyl-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 U range or, in the case of the nitrophenols , in the short-wave visible wavelenght 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 ul sample of plasma or serum are: β-D-galactosidase 25 U/l to 7500 U/l NPG 0.25 mmol/l to 5 mmol/1 Kryptofix® 221 0 mmol/1 to 10 mmol/ 1 buffer, pH 7-9.5 200 mmol/1 to 500 mmol/1 ,M,g2 ώ + 0.01 mmol/ 1 to 10 mmol/1 EGTA (Lithium salt) 0 mmol/1 to 20 mmol/1 Serum albumin 0 g/1 to 5 g/1 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 twin test (see Example G). Determination of Calcium Ions For the measurement of calcium ions, a sample of blood plasma (o other body fluid) is incubated with a buffered mixture containin the peptide substrate succinyl-leucine-methionine-p-nitroanilide and the enzyme Calpain I. The reaction catalyzed by Calpain I is dependent on the presence of calcium ions and the rate of activi is a measure of the calcium ion concentration. A key feature of the method is the use of chelators capable of specifically bindi calcium ions in order to lower their concentration to a range ov which the enzyme is most sensitive. Under the conditions chosen for analysis, which include the presence of L-cysteine and 2-mercaptoethaπol, the rate of p-nitroaniline formation is virtually proportional to the concentration of calcium being measured. Preferred peptide substrates can oe described by the general formula whereby R represents acetyl, benzoyl, carbobenzoxy, succinyl , tert-butoxy carbonyl or 3-(2-furyl)acryloyl; P - P~ - P-, represents a peptide chain of a least 2 residues with a preferen for Tyr, Met, Lys or Arg in the P, position and a Leu or Val residue in the P 2 position; and X represents a chromogenic or fluorogenic group which is liberated by the action of the enzyme to yield a detectable change in colour or fluorescence. X can be nitrophenyl, naphthyl or thiobenzyl ester as well as a nitroaniline, naphthylamine or methylcoumarin gr.oup either with withou further substituents on the aromatic ring. Some suitable peptide derivatives have also been described by T. Sasaki et al . in J. Biol. Chem. 259, 12489-12494, examples are succinyl-Leu- Met-MCA (MCA = 4-methylcoumarin-7-amide) , succinyl-Leu-Tyr-MCA, succinyi-Leu-Leu-Val-Tyr-MCA and tert-butoxy carbonyl-Val-Leu- Lys-MCA. Further synthetic substrates are described in Berg eyer HU.(Ed) Methods of Enzymatic Analysis, 3rd Edition, Volume 5, p. 84-85 (1984). The concentration ranges of the compounds for such a determinati method and reagents are: Calpain I 1 000 U/ l to 40 000 U/ l* Suc-Leu-Met-p-nitroanilide 1 mmol/ l to 20 mmol/ l Chelator 0 .0 1 mmo l/ l to 1 mmol/ l L-Cysteine 1 mmol/ l to 10 mmol/ l 2-Mercaptoethanol 1 mmol/ l to 10 mmol/ l Buffer Imidazole-HCl 10 mmol/ l to 100 mmo l/ l pH 6 - 8 (preferred range 7-7.5) The asterisk * means that unit is defined as the quantity of enzyme which increases the absorba ce at 750 nm oy i .0 after 50 min of mcuDation at 30°C with casein as substrate ( . Yoshi ur et al., J. Biol. Chem. 258, 8883-8889, (1985)). Any ouffer having a pK in tne required pH range with a negligib Dinding capacity for calcium may be used in the assay. Many of Good-type buffers (N.E. Good et al., Biochem. 5, 467-477 (1966) such as Tris, HEPES, M0PS0, BES, TES and imidazole fulfil these requirements (see example H). Collagenase may be used instead o Calpain I, and assayed fluorometricaily at pH 6.5-7.5 with L- isoleucyl-L-analyiglycyl ethylester, 0.02 mmol/l to 0.2 mmol/ Determina ion of Chloride Ions For the determination of chloride in blood, plasma is incubated with a buffered mixture containing 0.01 mol/1 cysteamine, 4 mmo Gly-Phe-p-nitroanilide and 0.02 U/ml Cathepsin C. The formation p-nitroaniline is entirely dependent on the presence of the chloride anions. Selective binding agents may be added in addit in order to eliminate interference by bromide ions or to decrea the activity of chloride ions so as to adjust their concentrati to the optimal range of the enzyme. Under the conditions chosen for analysis (see example 5) the rate of formation of p-nitroaniline: Cathepsin C Gly-Arg-NH _ 7 Gly-Arg+H 2 N- N0- is proportional to the concentration of chloride ions in the sample. In this example the rate of the reaction is determined b measurement of the increase of absorption at 405 nm. The concentration ranges of the compounds of such a determination method are: citrate buffer 0.01 - 0.2 mol/1 pH 4 - 7 (preferred: 5.0 - 5.5) cys teamine 1 - 20 mmol/l gly- arg-p-ni troanil ide 1 - 20 mmol/l catheps in C 2 - 100 mU/ml Any buffer having a pK in the required pH-range and a negligible chloride concentration may be used in the assay. Examples of enzymes from all of the categories listed above (transferases, hydrolases, oxidoreductases and lyases) have been shown in the literature to have a chloride dependency, especially if the origi of these enzymes is from halophilic organisms. The chloride dependency of Enzymes of the peptidase type has been extensively described. For examples dipeptidylpeptidase I (Cathepsin C), EC 3.4.14.1 (J. Ken Mc Donald, Bioch. Bioph. Res. Communication, 24 (5) 1966, 771f) or dipeptidylpeptidase III EC. 3.4.14.3 (J. Ken Mc Donald, Journal of Biol. Chem. 241 (7) 1966, 1494f), both catalyzing the hydrolysis of oligopeptide derivatives from the amino terminal end. Another example is the angiotensin convertin enzyme EC 3.4.15.1 which is a dipeptidylcarboxypeptidase that catalyses the hydrolytic release of dipeptides from the carboxyl terminus of a oroad range of oligopeptides (P. Bunning et al., Bioch. 26, 1987, 3374f; R. Shapiro et al., Bioch. 22, 1983, 3850f Instead of Gly-Arg-p-nitroanilide different other dipeptide- or oligopeptide substrates may be used. Preferred peptide substrate can be described by the general formula -P n -P L -X whereby for the dipeptidyl peptidase enzymes R = H and P - P, represents a peptide chain of at least 2 residues. X represents chromogenic-. or fluorogenic group which is liberated by the acti of the enzyme to yield a detectable change in colour or fluorescence. X can be a nitrophenyi- , naphthyl- or thiobenzyl- ester as well as a nitroaniline , naphthyla ine or methylcoumari group either with or without further substituents on the aromat ring. In the case of dipetidylcarboxypeptidase enzymes X represents the amino terminal and of the peptide cnain an R is chromogenic or fluorogenic group which is liberated by the acti of the genzyme. R can be a N-2-furanacryloyl- or benzoyl-group either with or without further substituents (see example I). As a further example of the enzymatic determination of chloride ione (Example J), plasma is incubated with a buffered mixture containing 4 ,6-ethylidene(G 7 ) -p-nitrophenyl(G-, )- ,D-maltohep- taoside (4 ,6-ethylidene-G 7 PNP) (5 mmol/1) , -amylase (0.60 U/ml and < -glucosidase (= 30 U/ml). The formation of p-nitrophenol i entirely dependent on the presence of chloride anions, once aga using either predilution, small sample volumenes or selective binding agents to adjust chloride ion concentration to the opti range of the enzyme. The test principle is summarised below, an the rate of the reaction is determined by the ' measurement of th increase in absorption at 405 nm. 5 ethylidene-G 7 PNP + 5H 2 0 ^ ~a rcylase 2 ethylidene-G.. + 2 G„PNP + 2 ethylidene-G. + 2 G,PNP + 5 2 4 ethylidene-G + G PNP 2 G 2 PNP + G 3 PNP + 10 H 2 0 - lucos i dase 4 pNp + 10 G (PNP = p-nitrophenol; G = glucose). The concentration ranges of compounds used for such a determination are: Hepes or alternative chloride free Duffer 0.01-0.5 mmol/l pH 6.5-7.5 -amylase 60-6000 U/l £.-glucosidase 3000-300 000 U/l 4,6-ethylidene-G 7 PNP 0.5-10 mmol/l Assay variations discussed above for Cathepsin C (Example I) also apply to Example J. Determination of Heavy Metal Ions These metals b'ind tightly to cryptands such as Kryptofix^ 221 and Kryptofix*^ 222. They will therefore displace other metals that are more loosely bound. An example of a metal ion readily displaced is zinc. Zinc ions are present in very low concentratio in plasma. Thus it is feasible to add zinc ions complexed to K 22 to a buffered serum mixture. If a heavy metal is present the zinc ions will be liberated and their presence can be detected by stimulation of pyridoxal kinase (from sheep brain) an enzyme whic is highly sensitive to zinc ions. Many other similar competitive binding assays are feasible, and those described are given oγ way of example and not of limitation. Determination of Bicarbonate Ions Bicarbonate ions can be measured using a variation of the principles embodied in the invention. Many ligands, e.g. the cryptands, are pH sensitive, and this property can be exploited t measure bicarbonate. Essentially, advantage is taken of the ability of bicarbonate to neutralize protons. It can be shown th tne pH of a very lightly buffered serum sample, to which hydrochloric acid has been added will vary as a function of the bicarbonate concentration. The final pH is detected by the amou of free sooium ions (as detected with ii-galactosidase J present the presence of a pH-sensitive ion-binding agent such as Krypto i xQJ 221, and this is a function of the original bicarbonate concentration. In essence, serum is acidfied to pH approx, with an equal volume of HC1 (75 mmol/l) to convert all bicarbonate to hydroxyl ions, and then reacted with an assay system at pH 7.5-7.8, using a dilute Tris buffer (5 mmol/l) whi incorporates an ion-selective enzyme, such as β-galactosidase a appropriate pH-sensitive ion binding agent. The sodium ion concentration of the sample of the sample must be known to obta accurate results since a correction is necessary based on the quantity of sodium ions in the sample. The method is substantia more sensitive than procedures using chromogenic indicators as detectors (Example K). Typical concentration ranges of the main reagents for the. enzymatic determination at 37°C of bicarbonate ions using a 10 ul sample of plasma or serum are: β-D-galactosidase 250 U/l to 7 500 U/l NPG 0.25 mmol/1 to 5 mmol/ 1 KryptofiiS 221 0.2 mmol/ 1 to 5 mmol/ 1 Duffer, pH 7.5-7.8 1 mmol/l to 10 mmol/1 Mg2+ 0.01 mmol/l to 10 mmol/ 1 EGTA (Li salt) 0.1 mmol/l to 5 mmol/ 1 Serum albumin o g/i to 5 g/1 The need to correct for sodium ion concentration can be aDoided using an enzyme (e.g. pyridoxal kinase) sensitive to trace metal (e.g. zinc ions) normally present in plasma in ulcromolar concentration. Provided that the binding of the trace metal to i binding agent is pH sensitive and possesses a similar affinity to sodium ions for Krypto i *^ 221, bicarbonate ions can be measured by including the zinc ions in the reaction mixture in concentrations sufficiently in excess of those that can be encountered in plasma. Hence endogenous zinc ions will not interfere. The methods as described above are simple, very rapid, accurate and precise and can be performed with inexpensive apparatus. The laboratory hazard of inflammable gases can be avoided as can the many problems associated with ion-selective electrodes. The metho can be adapted for use with large equipment performing multiple analyses, yet can also be employed with inexpensive stand-alone instruments for emergency use close to the bedside. The packagin of the method in kit form is straightforward. Moreover, the determination of potassium and sodium ion concentration can be performed sequentially in the same cuvette (Example G). It is als intended that the method be useful for Doctors 1 offices with a machine employing dry chemistries. Although these methods have been developed using automatic spectrometers, they are readily adaptable to automated or manual laboratory equiment such as fluorimeters, lumino eters, isotope counters etc. The present invention will now be described in more detail with reference to the following examples, on the basis of a serum or plasma sample of 10 μl. These examples are given by way of illustration and not of limitation. In the following Examples a small volume of sample (10 μl except where otherwise indicated in the case of serum or plasma) is mixe with Reagent 1, containing buffered substrate and certain cofactors, and incubated for a period of time, generally 0.1-5 min. Absorbance readings are normally taken at regular intervals during this Incubation period. Reagent 2, containing the indicato enzyme is then added and the reaction rate monitored. In some Examples, Reagent 1 contains the indicator enzyme and Reagent 2 the appropriate substrate. The Examples show the final reaction mixture after the sample, Reagent 1 and Reagent 2 have been mixe Example A Measurement of Potassium Ion Concentration Using Pyruvate Kinas 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, 5 mmol/l LiCl] 3.0 mmol/l MnCl 2 2.6 mmol/ 1 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/1 Human serum albumin Potassium ion standards (calibrating solutions) contain 140 mmo 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 Kno 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 ever 10 mmol/l the sodium ion concentration is below (or above) 140 mmol/l sodium ions. However, this correction should be verified analysing aqueous solutions containing known sodium and potassiu concentrations . Example C Measurement of Potassium Ion Concentration Using Pyruvate Kinase in 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 minimise variations in displacement o sodium ions from Kryptofix^ 221 due to the differing potassium ion content of Individual specimens. Example D Measurement of Sodium Ion Concentration Using β-D-galactosidase Variation a: The final incubation mixture contains: 300 mmol/l Tris HC1, pH 8.7 (37°C) 4 mmol/l Di thiothr eitol 7.5 mmol/l Magnesium sulphate 16 mmol/l Lithium chloride 0.44 mmol/l EGTA (lithium salt) 460 mg/1 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 ten-fold 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 E 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 ul plasma samp 300 mmol/l Tris HC1, pH 8.7 (37°C) 5 mmol/l gCl 2 2.6 mmol/l ADP (free acid) 2.9 mmol/l PEP (neutralized Tris salt) 0.34 mmol/l NADH 17 000 U/l LDH (assayed at 25°C) 2 000 U/l PK (from rabbit muscle, assayed at 37°C 4 mmol/l KG 8 600 U/l GDH in glycerol (assayed at 25°C) 1.25 μmol/assay Kryptofix® 222 Sodium ion calibrating solutions contain 4 mmol/l potassium to compensate for the potassium activating effect of serum potassiu ions on pyruvate kinase. E xamp le F 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: 500 mmol/l Glycine, pH 9.8 5 mmol/l MgCl 2 2.6 mmol/l ADP (free acid) 2.9 mmol/l PEP (neutralized tris salt) 0.34 mmol/l NADH 17 000 U/l LDH (assayed at 25°C) 890 U/l PK from Rabbit muscle (assayed at 37°C) 4 mmol/l KG 8 500 U/l GDH (assayed at 25°C) 5 mmol/l KC1 2.5 μmol/assay Krypto ix® 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 G Measurement of Potassium and Sodium Ion Concentration in the Sam 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 8 600 U/l GDH Following the measurement of sodium ions by means of determinatio fo the reaction rate, the pH of the incubation mixture is lowere to pH 7.4 with a hydrochloric acid aliquot. Then the following ingredients are added to achieve the final concentrations indicated: 17 000 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 minimise possible Interference by the 2-nitrophenol liberated in the sodium ion indicator reaction. E xampl e H Measurement of Calcium Ion Concentration Using Calpain I In this emoodiment, a small sample of blood is centrifuged to obtain plasma. For the measurement of calcium ions the sample w incubated with a mixture containing 50 mmol/l imidazole-HCl buffer, pH 7.3, 5 mmol/l L-cysteine, 2.5 mmol/l 2-mercaptoethan 0.1 mmol/l EGTA and 5 mmol/l Suc-Leu-Met-p-nitroanilide at 30°C The increase of absorbance at 405 nm was monitored over at 5 mi interval. The rate is proportional to the calcium ion concentration of the sample. Example I Measurement of chloride ion concentration using cathepsin C In this embodiment a small sample of blood is centrifuged to obtain plasma (5μl). For the measurement of chloride ion the incubation mixture contains 0.05 mol/1 citrate buffer pH-5.0, 10mmo1/1 cysteamine and 4 mmol/l gly-arg-p-nitroanilide and 0.01 U/ml cathepsin C. The latter has been dialyzed against 10 mmol/l sodiumphosphate buffer (pH - 6.8) and 43 % (v/v) of glycerol in order to remove chloride ions. A typical calibration curve obtained this way is shown in Fig. 1. Example J Measurement of chloride ion concentration using -amylase Inhibition mixture contains for a 5 ul plasma sample: Hepes or alternative chloride free buffer 100 mmol/l pH 7.1 <_L-Amylase 600 U/l < *-Glucos idase 30 000 U/l ,6-Ethylidene-G 7 PNP 5 mmol/l The reaction is monitored at 405 nm (or nearby wavelength to determine the rate of formation of free 4-nitrophenol and hence the concentration of chloride ions in the original sample. Measurement of bicarbonate ion concentration using Variation a: The final incubation mixture contains: 5 mmo 1/ 1 Tris HC1, pH 7.8 (37°C) 4 mmol/l Dithiothreitol 7.5 mmo 1/ 1 Magnesium sulphate 16 mmol/l Lithium chloride 0.44 mmol/l EGTA (lithium salt) 460 mg/1 Human serum albumin 1500 U/l β-Galactosidase EGTA means Ethylenbis(oxyethylennitrilo)- etraacetic acid. The sample is pre-incubated with an equivalent volume of HCl (f plasma, 75 mmol/l) to reduce sample pH to 4.5. The reaction is- then monitored at 420 nm (or nearby) wavelength to determine th rate of formation of free 2-nitrophenol and hence to concentrat of bicarbonate ions in the original sample. A correction is mad for sodium ion concentration in the original specimen. Variaton b: Pyridoxal kinase, pyridoxal and zinc ions are substituted for β-galactosidase and NGP .

Claims: Patent claims 1. Process for the determination of ions in fluids, wherein the influence of these ions on the activity of an enzyme Is measured. 2. Process as claimed in claim 1, wherein the fluid is a biological or non-biological fluid. 3. Process as claimed in claim 2, wherein the biological fluid i blood, serum, plasma, urine, sweat, cerebrospinal fluid, lymp or intestinal secretions, exudates or transudates and the non-biological fluid is water or an aqueous extract or mixtur 4. Process as claimed in one of the claims 1-3, wherein the enzyme is a transferase, a hydrolase, an oxidoreductase or a lyase. 5. Process as claimed in claim 4, wherein the transferase is transferring phosphorous containing groups and the hydrolase is a glycosidase or acting on peptide bonds. 6. Process as claimed ' in claim 5, wherein the hydrolase is J- or β-D-galactosidase, carboxypeptidase A, collagenase, amylase, calpain I, calpain II, dipeptidyl a inopeptidase I (cathepsin C) or phosphoglycollate phosphatase. 7. Process as claimed in claim 5, wherein the transferase is pyruvate kinase, hexokinase, adenylate kinase, pyridoxal kinase or acetate kinase and the hydrolase is < - or β-D-galactosidase or carboxypeptidase A. 8. Process as claimed in claim 4 , wherein the oxidoreductase i acting on the CH-OH group of donor molecules or on the alde or oxo group of donors. 9. Process as claimed in claim 7, wherein the oxidoreductase is glycerol dehydrogenase, acetaldehyde dehydrogenase or tyrosi (catechol oxidase). 10. Process as claimed in claim 4, wherein the lyase is a carbon carbon lyase or a carbon-oxygen lyase. 11. Process as claimed in claim 9, wherein the lyase is aldolase carbonic anhydrase. 12. Process as claimed in one the claims 1-11, wherein the ion i potassium, sodium, calcium, magnesium, mangenese, lithium, l zinc, copper, iron or other heavy metals or non-metallic ion comprising protons, bicarbonate, chloride, ammonium or substances that give rise to ammonium. 13. Process as claimed in one of the claims 1-11, wherein interfering ions are masked with a binding agent. 14. Process as claimed in one of the claims 1-12, wherein the io oe determined is reduced in concentration to optimal levels measurement by means of a binding agent, if dilution of the sample is not feasible. 15. Process as claimed in one of the claims 1-14, wherein potass ions are determined using pyruvate kinase, glycerol dehydrogenase or acetaldehyde dehydrogenase. 16. Process as claimed in claims 1-14, wherein sodium ions are determined by using the enzyme pyruvate kinase or lu-D-galactosidase. 17. Process as claimed in one of the claims 15 and 14, wherein th interfering ions or analytes are bound by cryptands, coronand podands, crown ethers, spherands, hemispherands , calixarens a combinations thereof, natural occurring ionophores, cyclic peptides, complexones and chelating agents, and derivatives thereof, metals of the 1. and 2. side group of the periodic system. 18. Process as claimed in one of the claims 11-17, whereby in the determination of potassium ions the interfering sodium ions a bound by Kryptofix^ 221 and in the determination of sodium ions the interfering potassium ions are bound by Krypto- fix® 222. 19. Process as claimed in claim 14, whereby in the determination sodium ions using β-galactosidase the concentration of free sodium ions is reduced with a binding agent. 20. Process as claimed in claim 14, whereby the binding agent is Kryptofix^ 221. 21. Process as claimed in claim 14, wherein the binding agents according to claim 17 are present, which form a complex with Indicator ions and from which the indicator ions are displace stoichiometrically by the 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 indirect measure of the concentration of the analytes. 22. Process as claimed in claim 21, wherein the enzyme is pyruvat kinase, the indicator ions are potassium, the binding agent i Kryptofix^ 221 and the ion to be determined is sodium. 23. Process as claimed in claim 21, wherein the enzyme is a kinas the indicator ion is Mg , the binding agent is a chelating . agent and the ion is a metal. 24. Process as claimed in claim 25, wherein the enzyme is pyrid kinase, the indicator ion is Zn , the binding agent is Kryptofix® 221 and the ion is a heavy metal. 25. Process as claimed in claim 21 whereby the ions to be measu cause a change in the pH value which displaces the indicato ions from a complex with a pH-sensitive binding agent. 26. Process as claimed in claim 25 wherein the enzyme is β-galactosidase, the pH-sensitive binding agent is Kryptofi 221, the indicator ions are sodium, and the ion to be mesur bicarbonate . 27. Process as claimed in one of the claims 14 and 17, wherein enzyme is ς ^-amylase or cathepsin C, the binding agent is Ag Hg and the ion to be determined is chloride. 28. Process as claimed in in one of the claims 14 and 17, where the enzyme is collagenase or calpain I or calpain II, the binding agent is a chelating agent and the ion to be determi is calcium. 29. Process as in claims 1-13, wherein ions which are competitiv inhibitors of the sensitive indicator enzyme are included in assay. 30. Process as claimed in claim 29, wherein the ions to be determined are sodium or potassium and the competitive inhib is lithium ion. 31. Composition for the determination of ions in fluids comprisi an enzyme, the activity of which is influenced by the ion. 52. Composition for the determination of ions in fluids as claim in claim 51, comprising an enzyme, the activity of which is influenced by the Ion, and a binding agent which binds the- interfering ions in the fluid or lowers the concentration of analyte ions to optimal levels for measurement. 53. Composition as claimed in claim 32 for the determination of potassium ions comprising pyruvate kinase and a cryptand. 34. Composition as claimed in claim 35, wherein the cryptand is Kryptofix® 221. 35. Composition as claimed in claim 32 for the determination of sodium ions comprising pyruvate kinase and a cryptand. 36. Compositio'n as claimed in claim 35, wherein the cryptand is Kryptofix 222. 37. Composition as claimed in claim 32 for the determination of sodium ions comprising β-D-galactosidase and a cryptand. 38. Composition as claimed in claim 37 wherein the cryptand is Kryptofijfi 221. 39. Composition as claimed in claim 31 comprising: PK (B. stearother ophilus 50 U/ l to 10 ι D00 U/ l PEP (neutralized Tris salt) 0 . 3 mmol/ l to 50 mmol/ 1 Kryptofi& 221 0 mmo l/ l to 30 mmo 1/ 1 NADH 0 .01 mmol/ l to 0 .8 mmo l/ l LICl 2 mmol/ l to 100 mmol/ 1 ADP (free acid) 0 .5 mmo 1/ 1 to 10 mmol/ l LDH (assayed at 25°C) 5 1 300 U/ l to 100 1 300 U/ l Serum albumin 0 g/ 1 ■ to 5 g/ 1 GDH (assayed at 25°C) 2 : 500 U/ l to 20 ι 000 U/ l KG (free acid) 1 mmo l/ l to 10 mmo l/ 1 40. Composition as claimed in claim 51 comprising: Glycerol dehydrogenase 50 U/l to 1000 U/l. Glycerol 0.3 mol/1 to 3 mol/1 Kryptofijβ' 221 0 mmol/l to 30 mmol/l NAD 0.1 mmol/l to 5.0 mmol/ 1 buffer, pH 9 20 mmol/l to 500 mmol/l Serum albumin 0 g/1 to 5 g/1 GDH (assayed at 25 °C) 2 500 U/l to 20 000 U/l KG (free acid) 1 mmol/l to 10 mmol/l 41. Composition as claimed in claim 31 comprising: Acetaldehyde dehydirogenase ' 50 U/l to 10 000 U/l Glycolaldehyde 0.3 mmol/ 1 to 50 mmol/l Kryptofix® 221 0 mmol/l to 30 mmol/l NAD 0.05 mmol/l to 2.0 mmol/ 1 buffer, pH 7-8 50 mmol/l to 500 mmol/l Dithiothreitol 0.1 mmol/l to 2 mmol/l Serum albumin 0 g/1 to 5 g/1 GDH (assayed at 25 C 'O 2 -500 U/l to 20 000 U/l KG (free acid) 1 mmol/l to 10 mmol/l 42. Composition as claimed in claim 31 comprising: Acetaldehyde dehydrogenase 50 U/l to 10 I 300 U/l 4-nitrophenyl acetate 0.1 mmol/ 1 to 2 mmol/ 1 ΓD"\ ' Kryptofix ' 221 0 mmol/l to 30 mmol/ 1 NADH 0.001 mmol/l to 0.1 mmol/ 1 buffer, pH 7-8 50 mmol/l to 500 mmol/ 1 Dithiothreitol 0.1 mmol/ 1 to 2.0 mmol/ 1 Serum albumin 0 g/1 to 5 g/1 GDH (assayed at 25 ( d O 2 500 U/l to 20 000 U/l KG (free acid) 1 mmol/l to 10 mmol/ 1 ' 43. Composition as claimed in claim 51 comprising: β-D-galactosidase 25 U/l to 7500 U/l NPG 0.25 mmol/l to 5 mmol/l Kryptofi& ffiy 221 0 mmol/l to 10 mmol/l buffer, pH 7-9.5 200 mmo1/1 to 500 mmol/ 1 \ MΛrg- + 0.01 mmol/ 1 to 10 mmol/ 1 EGTA (Lithium salt) 0 mmo1/1 to 20 mmol/l Serum albumin 0 g/1 to 5 g/1 44. Composition as claimed in claim 31 comprising: PK (rabbit muscle) 50 U/l to 10 000 U/l PEP (neutralized Tris salt) 0.3 mmol/l to 30 mmol/l Kryptofi* fa^ 221 1 mmol/l to 10 mmol/l NADH 0.01 mmo1/1 to 0.8 mmol/l buffer, pH 9-10 50 mmol/ 1 to 500 mmol/l Mg 1 mmol/ 1 to 10 mmol/l ADP (free acid) 0.5 mmol/l to 10 mmol/l LDH (assayed at 25°C) 5 000 mmol/ 1 to 100 000 U/l KC1 2 mmol/l to 10 mmol/l Serum albumin 0 g/1 to 5 g/1 GDH (assayed at 25°C) 2 ! 500 U/l to 20 000 U/l KG (free acid) 1 mmo1/1 to 10 mmol/l 45. Composition as claimed in claim 31 comprising: Calpain I 1 000 U/l to 40 000 U/l Suc-Leu-Met-p-nitroanilide 1 mmol/l to 20 mmol/l Chelator 0.01 mmol/l to 1 mmol/l L-Cysteine 1 mmol/l to 10 mmol/l 2-Mercaptoethanol 1 mmol/l to 10 mmol/l Buffer Imidazole-HCl 10 mmol/l to 100 mmol/l pH 6 - 8 (preferred range 7-7.5) 46. Composition as claimed in claim 51 comprising: Hepes or alternative chloride free buffer 0.01-0.5 mmol/l pH 6.5-7.5 J -amylase 60-6000 U/l rS-glucosidase 5000-300 000 U/l 4 ,6-ethylidene-G PNP 0.5-10 mmol/l 47. Composition as claimed in claim 51 comprising: citrate buffer 0.01 - 0.2 mol/1 pH 4 - 7 (preferred: 5.0 - 5.5) cysteamine 1 - 20 mmol/l gly-arg-p-nitroanilide 1 - 20 mmol/l cathepsin C 2 - 100 mU/ml 48. Composition as claimed in claim 31 comprising: β-D-galactosidase 250 U/l to 7500 U-/1 NPG 0.25 mmol/1 to 5 mmol/l Kryptofix® 221 0.2 mmol/1 to 5 mmol/l buffer, pH 7.5-7.8 1 mmol/l to 10 mmol/l ,M,g2+ 0.01 mmol/l to 10 mmol/l EGTA (Li salt) 0.1 mmol/l to 5 mmol/l Serum albumin o g/1 to 5 g/1

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