AU651712B2 - Determination of ions in fluids - Google Patents

Description

I-

Name of Applicant: 651712

AUSTRALIA

PATENTS ACT 1990

ORIGINAL

COMPLETE SPECIFICATION THE FLINDERS UNIVERSITY OF SOUTH

AUSTRALIA

and BOEHRINGER MANNHEIM GmbH Sturt Street, Bedford Park, South Australia 5042, Australia and Sandhofer Strasse 116, D-6800 Mannheim 41, Germany, respectively.

Michael Nathaniel BERRY; Michael Harold TOWN; Georg-Burkhard KRESSE and Uwe

HERRMANN.

DAVIES COLLISON CAVE, Patent Attorneys, 1 Little Collins Street, Melbourne, 3000.

Address of Applicant: Actual Inventor(s): Address for Service: iI i-i

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1 i i i i i.' Complete Specification for the invention entitled: DETERMINATION OF IONS IN FLUIDS The following statement is a full description of this invention, including the best method of performing it known to us: -1- 920323,jdatO 10,FLindersIBoehringer, 1 -I i ~~III 11 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 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.

o In the practice of Clinical Biochemistry the measurement of 1serum electrolytes are the most common analytical tests performed within hospitals. These measurements are requested not only for routine investigations, but frequently for i 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.

L I lr 2 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 1 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 prdperty that would allow it to respond to only one ion. In practice this is not the case and interfering ions exist for all ionselective 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 Cor 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.

SSimilarily, 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 on completion of formation of insoluble silver chloride product. Alternatively, 3 potentiometric determinations may be used which are also very time-consuming and involve additional instrumentation.

For chloride, in addition, there are a number of photometric and titrimetric methods, which e. g. include: titrimetric determination of free Hg 2 ions via diphenylcarbazone complex colorimetric determination of the rhodanide complex of iron, which is formed after dissociation of the mercury complexes upon precipitation of HgC1 2 [Skeggs, Clin.

Chem. 10, 1964, 918f.; Schmidt, Zentralblatt Pharm. 124 1985, 527f] colorimetric determination of chloranilic acid from the respective mercury salt [Reuschler, Klin.

Wochenschrift 40, 489 (1962)] determination of the coloured Cu 2 complex of diethyldithiocarbaminic acid from the colourless I mercury salt (German Offenlegungsschrift 2137146) S- the rather common TPTZ-method (tripyridile-s-triazine) Fried, Zeitschr. Klin. Chem., Klin. Bioch. 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.

I A major drawback of these methods is the use of solutions containing highly toxic substances. Some of the methods are complicated and imprecise the titration method). Many of the reagents are unstable and calibration curves are nonlinear the rhodanide method). Some of these methods in addition need a pretreatment in order to eliminate interferences by the protein content of the sample.

4 4 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 Hoppe, Ther. Ggw. 110 1971, 554f., H. Mahner, Zeitschr. Klin. Chem., Klin. Biochem. 11 1973, 451f.; SW. 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.

i One of the earliest methods for the determination of calcium was that described by Tisdall 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 -t SL_ u r

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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 ii the colour is greatly dependent on temperature.

In W. H. Outlaw and 0. 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 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 Li problem is that ammonium ions give a similar activation to potassium ions. The above mentioned publication does neither i: o address or solve these critical problems in regard to the i' analysis of potassium ions in biological fluids such as i 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 a-amylase.

I I I_ ~-rcci 6 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 determinationof 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.

I_ I_ 7 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 n, i L-- Z L 8 give rise to the formation of ammonium such as urea in the presence of urease, or glutamin in the presence of glutaminase.

Suitable Enzymes Enzymes which may be used can be for example 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.

Hvdrolases like glycosidases, for example a- or B-Dgalactosidase (from Escherichia coli), carboxypepti-ase A (from bovine pancreas), collagenase (from Clostridium hystolicum), amylase (from saliva or pancreas) or phosphoglycolate phosphatase.

Also peptide hydrolases such as the cysteine or thioldependent proteinases, specific examples of which are Calpain I and II (also called calcium activated neutral protease) are described by Sasaki et al. in J. Biol.

Chem. 259, 12489 12494, (1984). The latter enzymes can be isolated and purified from a variety of animal tissues such as: rat liver and kidney, human and porcine erythrocytes, bovine brain, and rabbit skeletal muscle according to the method of A. Kitahara et al., J. Biochem. 95, 1759 1766 (1984). A further example is dipeptidyl aminopeptidase I 3.4.14.1, Cathepsin J. Ken McDonald, Bioch. Biophys. Res. Communication 24(5), 66, 771f.

Oxidoreductases like glycerol dehydrogenase (from Enterobacter aerogenes), acetaleehyde dehydrogenase (from yeast) or tyrosinase (catechol bxidase).

Lvases 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. Vogtle 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, 10 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-(2aminoethylether)-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 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-l,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-l,10-diazabicyclol([8.8]hexacosan, Krypotfixg 222, page 438, Merck-Schuchardt catalogue, i 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 rl r 11 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 P. Schmidtchen, Nachrichten Chem. Techn.

lab. 36 1988, S. 8f; E. Graf, J. Amer. Chem. Soc. 98 1976, 6403f; C. H. Park, J. Amer. Chem. Soc. 90 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 selective binding agent to lower the concentration of the ion to be determined into the appropriate range for enzymatic analyses serves to substantially increase the sensitivity of the method.

So 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 Sgiving an indirect measure of the concentration of ii 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 Sdetermined is sodium; or the enzyme is a kinase, the indicator ion is Mg 2 the binding agent is a chelating agent, e.g. EDTA, and the analyte ion is a metal ion or the enzyme is pyridoxal kinase, the indicator ion is 25 12 Zn 2 the binding agent is Kryptofix* 221, and the analyte ion is a heavy metal ion.

Fluids for Analysis: The biological fluids in which the measurement of analytes is made are blood, serum, plasma, urine, sweat, cerebrospinal fluid, lymph or intestinal secretions, exudates or transudates for example. Nonbiological fluids are water or aqueous extracts or mixtures, like extracts of foodstuffs or fruits or fermented liquids such as wine.

Application of General Principles to the Determination of Potassium and Sodium Ions SThe 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: 4; 1. The selective binding of sodium ions with a suitable binding agent, for example Kryptofix* 221.

-J c i 13 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.

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 .i 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 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 If *-f i i -e L' i 14 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 Ithe applicability of the general principles embodied in the J 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 S use an enzyme whose activity is sensitive to sodium ions. An example of such an enzyme is 8-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 i 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 Bgalactosidase, thereby decreasing the sensitivity of the enzyme to sodium ions.

L 1- IIII~ i 15 The combination of procedures 1 and 2 readily allows the determination of sodium ions in plasma or serum, using 8galactosidase, and the amount of binding agent can be manipulated to minimize the signal for sodium ion concentrations below 110 iumol/l while enhancing the signal in the usual analytical range (110 170 mmol/l) (Example D).

SSodium ions may also be measured by means of pyruvate kinase, provided that conditions are chosen whereby the i stimulation of enzyme activity by potassium ions is reduced, and a potassium ion-binding agent, e.g. Kryptofix® 222, is included in the reaction mixture (Example E).

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 (Example

F).

Other embodiments of the invention are compositions and o° reagents for the determination of ions in biological and L o non-biological fluids.

The reagent according to the present invention can be present in dissolved or dry form. It can be present l impregnated on an appropriate carrier. A diagnostic agent in 1| 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 i I yl;gun~r~-s-i~lil' 1 i I 16 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: PEP ADP H+ PK Pyruvate ATP Pyruvate NADH H LDH Lactate NAD+ I ii -l L---i 17 The rate of reaction is determined by the concentration of potassium ions present in the system, and this in turn limits the rate of reaction 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 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 and consume H thus I 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 Sequipment such as fluorimeters or luminometers can be used to monitor the activity of pyruvate kinase.

I There are a number of other methods of detecting the SI <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 Sor carbon dioxide generated by the enzymatic action of pyruvate oxidase; the formation of the hydrazone with 2,4dinitrophenylhydrazine; 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 N. Berry et al., Analytical Biochem. 118, 344 352 (1981)].

L; i j~C i_ L 1 ;1 18 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 possibilly of ammonium ion interference can be completely avoided by including a-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:

NH

4 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 Al sample of plasma or serum are: r 1; i -u_-1II1III~LI~

I

19 PK stearothermophilus) PEP (neutralized Tris salt) Kryptofix* 221

NADH

Buffer, pH 7 8 Mn 2 or Mg 2 LiCl ADP (free acid) LDH (assayed at 25 "C) Serum albumin GDH (assayed at 25 "C) KG (free acid) 50 0.3 0 0.01 50 1 2 0.5 5,000 0 2,500 1 U/1 mmol/l mmol/l mmol/l mmol/l mmol/l mmol/l mmol/l U/1 g/l U/l mmol/1 10,000 30 30 0.8 500 10 100 10 100,000 5 20,000 10 U/l mmol/1 mmol/1 mmol/l mmol/l mmol/l mmol/1 mmol/1 U/1 g/ l U/i mmol/1 Another example sensitive to potassium ions is glycerol dehydrogenase 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 Glycerol Kryptofix® 221

NAD

Buffer, pH 9 Serum albumin GDH (assayed at 25 "C) KG (free acid) 50 U/1 to 1,000 U/1 0.3 mol/l to 3 mol/l 0 mmol/l to 30 mmol/1 0.1 mmol/1 to 5.0 mmol/l 20 mmol/l to 500 mmol/l 0 g/l to 5 g/l 2,500 U/1 to 20,000 U/1 1 mmol/l to 10 mmol/l Another enzyme sensitive to potassium ions is acetaldehyde dehydrogenase 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: i i~x~ih~ I~ i 20 Acetaldehyde dehydrogenase 50 U/1 to 10,000 U/1 Glycolaldehyde 0.3 mmol/l to 30 mmol/1 Kryptofix® 221 0 mmol/l to 30 mmol/l NAD 0.05 mmol/1 to 2.0 mmol/l Buffer, pH 7 8 50 mmol/l to 500 mmol/l Dithiothreitol 0.1 mmol/1 to 2 mmol/l Serum albumin 0 g/l to 5 g/l GDH (assayed at 25 2,500 U/1 to 20,000 U/1 KG (free acid) 1 mmol/l to 10 mmol/1 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/1 to 10,000 U/1 K 4-nitrophenyl acetate 0.1 mmol/l to 2 mmol/l S° Kryptofix® 221 0 mmol/l to 30 mmol/l NADH 0.001 mmol/l to 0.1 mmol/l SBuffer, pH 7 8 50 mmol/l to 500 mmol/1 Dithiothreitol 0.1 mmol/l to 2.0 mmol/l o Serum albumin 0 g/1 to 5 g/1 GDH (assayed at 25 2,500 U/1 to 20,000 U/1 KG (free acid) 1 mmol/l to 10 mmol/l L- i- I 21 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 il, to sodium ion, depending on incubation conditions as described above.

Hence even though sodium ions occur in the plasma at a concentration some 30 times that of potassium ions, the latter can interfere with sodium ion measurement.

According to the general principles espoused in this invention, lithium ions are omitted, PK from rabbit muscle is preferred to the bacterial enzyme, and magnesium ions may be substituted for manganese. In one method the potassium ions are specifically bound with Kryptofix® 222 and the effects of sodium ions on PK activity measured directly (Example E).

Typical concentration ranges of the main reagents for the enzymatic determination at 37 'C of sodium ions using pyruvate kinase are: 0 PK (rabbit muscle) PEP (neutralized Tris salt) Kryptofix* 222

NADH

Buffer, pH 7 9 Mg 2 ADP (free acid) LDH (assayed at 25 *C) Serum albumin GDH (assayed at 25 *C) KG (free acid) 50 U/1 to 0.3 mmol/1 to 0.4 mmol/1 to 0.01 mmol/l to 50 mmol/l to 1 mmol/l to 0.5 mmol/l to 5,000 U/1 to 0 g/1 to 2,500 U/l to 1 mmol/l to 10,000 U/1 30 mmol/1 4 mmol/1 0.8 mmol/1 500 mmol/l 10 mmol/l 10 mmol/l 100,000 U/1 5 g/l 20,000 U/1 10 mmol/l L. 22 In another method the sodium ions are allowed to displace potassium ions from Kryptofix® 221, the released potassium ions stimulating the activity of PK to a degree dependent on the sodium ion concentration (Example 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 ions from Kryptofix® 221 are: of potassium PK (rabbit muscle) PEP (neutralized Tris salt) Kryptofix® 221l

NADH

Buffer, pH 9 10 Mg 2 ADP (free acid) LDH (assayed at 25 *C) KC1 Serum albumin GDH (assayed at 25 "C) KG (free acid) 50 1 to 0.3 mmol/l to 1 mmol/1 to 0.01 mmol/1 to 50 mmol/l to 1 mmol/l to 0.5 mmol/1 to 5,000 U/1 to 2 mmol/l to 0 g/l to 2,500 U/l to 1 mmol/l to 10,000 U/1 30 mmol/l 10 mmol/l 0.8 mmol/l 500 mmol/l 10 mmol/l 10 mmol/l 100,000 U/1 10 mmol/l 5 g/l 20,000 U/1 10 mmol/l ioo A more accurate and precise embodiment for determining uses a sodium ion-dependent enzyme such as B-galactosidase (Example D).

Blood plasma is incubated with a buffered mixture containing 2-nitrophenyl-B-D-galactopyranoside (NPG) and the enzyme 8- D-galactosidase. The reaction catalysed by 8-D-galactosidase is dependent on the presence of sodium ions and the rate of activity is a measure of sodium ion concentration. A key feature of this method is the use of an appropriate amount of sodium ion-selective binding agent Kryptofix® 221) for measurements in serum or other biological fluids where j ~_5C -I i 23 the sodium ion concentration may exceed 100 mmol/1, 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 is virtually proportional to the coiicentration of sodium ions being measured. Magnesium ions are required for optimal 8galactosidase activity. Lithium ions are competiti. with sodium ions and therefore raise the Michaelis constant Km of the enzyme for sodium ions.

As substrate for B-D-galactosidase many other compounds are suitable. Quite generally, the galactosidase-containing sample is mixed with an appropriate B-D-galactosidase Ij substrate, the substrate being split by the enzyme, one of the fission products then 'bing 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. 133, 209 (1960), there are described phenyl-8-D-galactoside, as well as some further derivatives substituted on the aromatic ring, for example, 2-nitrophenyl-B-D-galactoside (NPG) and 3nitrophenyl-B-D-galactoside, as substrates ot B-Dgalactosidase. The phenols liberated by hydrolysis are determined photometrically in the UV range or, in the case of the nitrophenols, in the short-wave visible wavelength range. An oxidative coupling with aminoantipyrine can also follow as indicator reaction [see Analytical Biochem. 281 (1971)]. Other substrates are described in the German Offenlegungsschrift 33 45 748 and the German Offenlegungsschrift 34 11 574.

-r 24 Typical concentration ranges of the main reagents for the enzymatic determination at 37 'C of sodium ions using a Al sample of plasma or serum are: 8-D-galactosidase 25 U/l to 7,500 U/1 NPG 0.25 mmol/l to 5 mmol/l KryptofixO 221 0 mmol/l to 10 mmol/l Buffer, pH 7 9.5 200 mmol/l to 500 mmol/l Mg 2 0.01 mmol/l to 10 mmol/l EGTA (lithium salt) 0 mmol/l to 20 mmol/l Serum albumin 0 g/l to 5 g/l EGTA means ethylenbis(oxyethy ennitrilo)-tetraacetic acid.

It will also be feasible to perform the analysis of sodium and potassium ions enzymatically in the same cuvette in the form of a twin test (see Example G).

Determination of Calcium Ions For the measurement of calcium ions, a sample of blood plasma (or other body fluid) is incubated with a buffered mixture containing the peptide substrate succinyl-leucine- 0 methionine-p-nitroanilide and the enzyme Calpain I. The reaction catalysed by Calpain I is dependent on the presence of calcium ions and the rate of activity is a measure of the calcium ion concentration. A key feature of the method is the use of chelators capable of specifically binding calcium S. ions in order to lower their concentration to a range over which the enzyme is most sensitive. Under the conditions chosen for analysis, which include the presence of Lcysteine and 2-mercaptoethanol, the rate of p-nitroaniline formation is virtually proportional to the concentration of calcium being measured.

L L- I~ 25 Preferred peptide substrates can be described by the general formula: R Pn P 2

P

1

X

whereby R represents acetyl, benzoyl, carbobenzoxy, succinyl, tert-butoxy carbonyl or 3-(2-furyl)acryloyl; Pn

P

2

P

1 represents a peptide chain P of at least 2 residues when n 0, with a preference for Tyr, Met, Lys or Arg in the P 1 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 a nitrophenyl, naphthyl or thiobenzyl ester as well as a nitroaniline, naphthylamine or methylcoumarin group either with or without 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, succinyl-Leu-Leu-Val-Tyr-MCA and tertbutoxy carbonyl-Val-Leu-Lys-MCA. Further synthetic substrates are described in Bergmeyer, Methods of Enzymatic o °0 Analysis, 3rd Edition, Volume 5, p. 84 85 (1984).

The concentration ranges of the compounds for such a S0" determination method and reagents are: Calpain I 1,000 U/1 to 40,000 U/l* Suc-Leu-Met-p-nitroanilide 1 mmol/1 to 20 mmol/l SChelator 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 ~r n nc~- 1 I i-l- 26 The asterisk means that the unit is defined as the quantity of enzyme which increases the absorbance at 750 nm by 1.0 after 30 min of incubation at 30 "C with casein as substrate Yoshimura et al., J. Biol. Chem. 258, 8883 8889 (1983)].

Any buffer having a PK in the required pH range with a negligible binding capacity for calcium may be used in the assay. Many of the Good-type buffers E. Good et al., Biochem. 5, 467 477 (1966)] such as Tris, HEPES, MOPSO, BES, TES and imidazole fulfil these requirements (see example Collagenase may be used instead of Calpain I, and assayed fluorometrically at pH 6.5 7.5 with Lisoleucyl-L-analylglycyl ethylester, 0.02 mmol/l to 0.2 mmol/l. as a standard, 0.2% collagen, 6.7 pmol/l as substrate and fluorescamine, 36 pmol/1 as indicator.

Determination of Chloride Ions For the determination of chloride in blood, plasma is Sincubated with a buftered mixture containing 0.01 mol/l cysteamine, 4 mmol/l Gly-Phe-p-nitroanilide and 0.02 U/ml Cathepsin C. The formation of p-nitroaniline is entirely ^'dependent on the presence of the chloride anions. Selective o binding agents may be added in addition in order to eliminate interference by bromide ions or to decrease the activity of chloride ions so as to adjust their concentration to the optimal range of the enzyme. Under the conditions chosen for analysis (see example 5) the rate of t formation of p-nitroaniline: Cathepsin C Gly-Arg-NH- -NO 2 Gly-Arg H 2 N- -NO 2 i. 27 is proportional to the concentration of chloride ions in the sample. In this example the rate of the reaction is determined by 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/l pH 4 7 (preferred: 5.0 Cysteamine 1 20 mmol/l Gly-Arg-p-nitroanilide 1 20 mmol/l Cathepsin C 1 100 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 origin of these enzymes is from halophilic organisms. The chloride dependency of enzymes of the peptidase type has been extensively described. For example dipeptidylpeptidase I (Cathepsin EC 3.4.14.1 Ken McDonald, Bioch. Bioph. Res. Communication, 24 1966, B 771f] or dipeptidylpeptidase III, EC. 3.4.14.3 S[J. Ken McDonald, Journal of Biol. Chem. 241 1966, 1494f], both catalysing the hydrolysis of oligopeptide derivatives from the amino terminal end. Another example is the angiotensin converting enzyme EC 3.4.15.1 which is a dipeptidylcarboxypeptidase that catalyses the hydrolytic release of dipeptides from the carboxyl terminus of a broad range of oligopeptides Bunning et al., Bioch. 26, 1987, 3374f; R. Shapiro et al., Bioch. 22, 1983, 3850f).

1. IC 28 Instead of Gly-Arg-p-nitroanilide different other dipeptide or oligopeptide substrates may be used. Preferred peptide substrates can be described by the general formula R P X whereby for the dipeptidyl peptidase enzymes R H and P represents a peptide chain of at least 2 residues. 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 a nitrophenyl-, naphtyl- or thiobenzylester group as well as a nitroaniline, naphthylamine or methylcoumarin group either with or without further substituents on the aromatic ring. In the case of dipeptidylcarboxypeptidase enzymes X represents the amino terminal end of the peptide chain and R is a chromogenic or fluorogenic group which is liberated by the action of the enzyme. 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 ion (example plasma is incubated with a buffered mixture containing 4,6-0-benzylidine-a-4o a nitrophenyl-a-D-maltoheptaoxide (4,6-ethylidene-G 7

PNP)

mmol/l), a-amylase (0.60 U/ml) and a-glucosidase 30 U/ml). The formation of p-nitrophenol is entirely dependent on the presence of chloride anions, once again 0 using either predilution, small sample volumes or selective Sbinding agents to adjust chloride ion concentration to the optimal range of the enzyme. The test principle is summarised below, and the rate of the reaction is determined by the measurement of the increase in absorption at 405 nm.

t YLI--. I L 29 ethylidene-G 7 PNP 5 H 2 0 a-amylase Cl 2-ethylidene-G 5 2 G 2 PNP 2 ethylidene-G 4 2 G 3 PNP ethylidene-G 3

G

4

PNP

2 G 2 PNP G 3 PNP 10 H 2 0 a-glucosidase> 4 PNP 1 G (PNP p-nitrophenol; G glucose) The concentration ranges of compounds used for such a determination are: <j

S*

Hepes or alternative chloride free buffer pH a-Amylase a-Glucosidase 4,6-Ethylidene-G 7

PNP

0.01 0.5 mmol/1 6.5 60 6,000 U/1 3,000 300,000 U/1 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 bind 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 concentration in plasma. Thus it is feasible to add zinc ions complexed to K 221 to a buffered serum mixture. If a heavy metal is present the zinc ions will be liberated and their presence can be detected by -e 1 C C- 30 stimulation of pyridoxal kinase (from sheep brain) an enzyme which is highly sensitive to zinc ions. Many other similar competitive binding assays are feasible, and those described are given by 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 I cryptands, are pH sensitive, and this property can be r exploited to measure bicarbonate. Essentially, advantage is taken of the ability of bicarbonate to neutralize protons.

It can be shown that the 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 amount of free sodium ions (as detected with B-galactosidase) present in the presence of a pHsensitive ion-binding agent such as Kryptofix* 221, and this is a function of the original bicarbonate concentration. In H essence, serum is acidified to pH 4.5, with an equal volume of HC1 (75 mmol/l) to convert all bicarbonate to hydroxyl Sions, and then reacted with an assay system at pH 7.5 7.8, using a dilute Tris buffer (5 mmol/l) which incorporates an ion-selective enzyme, such as 8-galactosidase and appropriate pH-sensitive ion-binding agent. The sodium ion concentration of the sample must be known to obtain accurate results since a correction is neceisary based on the quantity of sodium ions in the sample. The method is substantially more sensitive than procedures using chromogenic indicators as pH detectors (example K).

Typical concentration ranges of the main reagents for the enzymatic determination at 37 "C of bicarbonate ions using a pl sample of plasma or serum are: _~T5F C 31 250 U/1 to 7,500 U/1 NPG 0.25 mmol/l to 5 mmol/l Kryptofix® 221 0.2 mmol/l to 5 mmol/l Buffer, pH 7.5 7.8 1 mmol/1 to 10 mmol/l Mg2+ 0.01 mmol/1 to 10 mmol/l EGTA (Li salt) 0.1 mmol/l to 5 mmol/l Serum albumin 0 g/l to 5 g/l The need to correct for sodium ion concentration can be avoided by using an enzyme pyridoxal kinase) sensitive to trace metals zinc ions) normally present in plasma in micromolar concentration. Provided that the binding of the trace metal to its binding agent is pH-sensitive and possesses a similar affinity to sodium ions for o00 oo o ~Kryptofix® 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 04 encountered in plasma. Hence endogenous zinc ions will not interfere.

.o 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 o avoided as can the many problems associated with ion- 00, 0selective electrodes. The method can be adapted for use with large equipment performing multiple analyses, yet can also be employed with inexpensive stand-alone instruments for o emergency use close to the bedside. The packaging 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 also intended that the method be useful for doctors' offices with a machine employing dry chemistries. Although these methods have been developed using automatic 32 spectrometers, they are readily adaptable to automated or manual laboratory equipment such as fluorimeters, luminometers, 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 4l. These examples are given by way of illustration and not of limitation.

In the following examples a small volume of sample (10 1 except where otherwise indicated in the case of serum or plasma) is mixed 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 indicator enzyme is then added and the reaction rate monitored. In some examples, o.0' 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 mixed.

I B6 0 a

.A

t-.

33 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 mmol/l mmol/1 2.6 mmol/l 2.9 mmol/l 0.4 mmol/l 17000 U/1 890 U/1 mmol/1 8600 U/1 140 mg/l Tris-HC1 buffer, pH 7.4 Li [17 mmol/1 LiOH, 3 mmol/1 LiC1] MnCl 2 ADP (free acid) PEP (neutralized tris salt)

NADH

LDH (assayed at 25 °C) PK from Bacillus stearothermophilus

KG

GDH (in glycerol; assayed at 25 *C) Human serum albumin i,.

nooi o Ir rr cccri cc o oeri 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 1 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.

c- 34 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 Amol 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.

Qa a oat or 35 Example D Measurement of sodium ion concentration using B-Dgalactosidase.

Variation a: The final incubation mixture contains: 300 mmol/1 4 mmol/l mmol/1 16 mmol/1 0.44 mmol/1 460 mg/1 760 U/1 1.5 mmol/l 1.25 Amol/assay Tris HC1, pH 8.7 (37 "C) Dithiothreitol Magnesium sulphate Lithium chloride EGTA (lithium salt) Human serum albumin B-Galactosidase

NPG

Kryptofixe 221 c pDO O I O~C1

OI

r) tl ~100 a

OO

COOd 8' The reaction is monitored at 420 nm (or nearby) wavelength to determine the rate of formation of free 2-nitrophenol and hence the concentration of sodium ions in the original sample.

Variation b: An alternative approach compared with Variation a would be to reduce the sample concentration tenfold by pre-dilution or to use a small sample volume, in which case the cryptand could be omitted.

Variation c: Measurement in fluids of low sodium ion content 20 mmol/l) in which case the cryptand could be omitted.

'II

36 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 i plasma sample: 300 mmol/l Tris HC1, pH 8.7 (37 *C) mmol/l MgCl 2 2.6 mmol/l ADP (free acid) 2.9 mmol/l PEP (neutralized Tris salt) 0.34 mmol/l NADH 17000 U/1 LDH (assayed at 25 'C) 2000 U/1 PK (from rabbit muscle, assayed at 37 "C) 4 mmol/l KG 8600 U/1 GDH in glycerol (assayed at 25 "C) 1.25 Amol/assay Kryptofix® 222 Sodium ion calibration solutions contain 4 mmol/l potassi m to compensate for the potassium activating effect of serum potassium ion on pyruvate kinase.

I 4 I 37- Measureent of sodium ions with pyruvata kinasra (competitive binding assay) potassium ion concentration known The final incubation' mixture contains for a 10 A~l plasma sample: 300 mmol/l mm01./1 2.6 mmo 1/ 1 2.9 mind/I 0.34 minol/i 17000 U/1 890 U/i 4 mm01/1 8500 U/i mOl/i amol/assay Glycine, pH 9.8 MgCl 2 ADP (f~ree acid) PEP (neutr&-tized Tris-- salt)

NADH

LDII (assayed at 25 'C) PK from rabbit muscle (assayed at 37

KG

GDH (assayed at 25 *C) KCI1 Kryptofix'O 221 Potassium chloride is added to this reagent and displaced stoichiometrically from KryptofixO 221 by sodium ions, thus allowing the sodium ion concentration of the specimen to be quantified.

6- Z.

:1 38 Example G 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 2.9 mmol/l 0.4 mmol/l mmol/l 8600 U/i ADP (free acid) PEP (neutralized Tris salt)

NADH

KG

GDH

oooae~

I

r 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: o as a~os o oo s aa Ii~sls ooJ~ ooa i 17000 U/1 890 U/1 3.0 mmol/1 20.0 mmol/l

LDH

PK from Bacillus stearothermophilus MnC12 LiC1 The reactiLi rate may then be monitored at 340 nm.

39 ExamDle H Measurement of calcium ion concentration using Calpain I In this embodiment, a small sample of blood is centrifuged to obtain plasma. For the measurement of calcium ions the sample was incubated with a mixture containing 50 mmol/l imidazole-HCl buffer, pH 7.3, 5 mmol/l L-cysteine, mmol/l 2-mercaptoethanol, 0.1 mmol/l EGTA, 0.1 U/ml Calpain I and 5 mmol/l Suc-Leu-Met-p-nitroanilide at 30 *C.

The increase of absorbance at 405 nm was monitored over a min interval. The rate is proportional to the calcium ion concentration of the sample.

n, ier

D

r a 00 q ao Ia 004.

Example I Measurement of chloride ion concentration using Cathepsin C In this embodiment a small sample of blood is centrifuged to obtain plasma (5 il). For the measurement of chloride ion the incubation mixture contains 0.05 mol/l citrate buffer pH 5.0, 10 mmol/l cysteamine and 4 mmol/l gly-arg-pnitroanilide and 0.01 U/ml cathepsin C. The latter has been dialyzed against 10 mmol/l sodium phosphate buffer (pH 6.8) and 43 of glycerol in order to remove chloride ions.

L 40 ExsaMpj&.

Measurement of chloride ion concentration using 'x-amylase Inhibition mixture contains for a 5 4l plasma sample: Hepes or alternative chloride free buffer pH a -Amy.ase a-Glucosidase 4, 6-Ethylidene-G 7

PNP

100 mmol/l 7.1 600 U/')1 30000 U,,1 5 mmol/1 o 0 00 00 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 originalsample.

Examnle K Measurement of bicarbonate ion concentration using B-Dgalactosidase Variationq a: The final incubation mixture contains: 4 16 0.44 460 1500 mmol/l mmol/1 mmol/l mmol/l mmol/l mg/ 1 U/1.

mmol./l Aimol/ as say Tris HCl, pH 7.8 (37 *C) Dithiothreitol Magnesium sulphate Lithium chloride EGTA (lithium salt) Human serum albumin B-Ga lactos idase

NPG

KryptofixO 221 41 EGTA means ethylenbis(oxyethylennitrilo)-tetraacetic acid.

The sample is pre-incubated with an equivalent volume of HC1 (for plasma, 75 mmol/l) to reduce sample pH to Subsequently the incuation mixture containing 8-galactosidase and a pH-sensitive ion binding agent (kryptofix 221) is added and incubated for 5 min at proportinal to the concentration of bicarbonate ions in the original sample is determined by monitoring the reaction at 420nm (or nearby) wavelength. A corretion is made for the sodium ion concentration in the original specimen which can be determined according to Example D.

Variation b: Pyridoxal kinase, pyridoxal and zinc ions are substituted for 8-galactosidase and NGP.

o 00 o e Loca L~ ii -42- Example L Measurement of potassium ion concentration using a lyase, carbonic anhydrase (EC 4.2.1.1).

The following stock solutions were prepared: Substrate solution: 0.1 mmol/l 4-nitrophenylacetate Buffer solution: 0.1 mmol/l Tris-sulphate, pH Enzyme solution: mg carbonic anhydrase from bovine erythrocytes (1.4 U/mg) dissolved in 1 ml mmol/l sodium phosphate, pH 1.0 ml substrate solution, 0.3 ml buffer and 1.7 ml Ssample were mixed at 25'C. 20 pl enzyme solution was *added and the mixture was incubated for 2.5 min at The absorbance at 405 nm was measured at the beginning and end of this 2.5 min incubation period. The change in absorbance (AA) for samples containing different concentrations of potassium is shown as follows: *0 Effect of different potassium concentrations on the activity of carbonic anhydrase Potassium concentration Absorbance change in the sample (AA) re(mmol/1) 0 0.206 S100 0.247 500 0.305 The results of this experiment clearly show that the carbonic anhydrase is dependent on the potassium concentration in the sample.

Example M Measurement of potassium ion concentration using the lyase, tryptophanase Evans et al., 1966, supra.) 920728,gsrlet083,div.flinders,42 -43- The tryptophanase catalyses the following reaction: trytophanase tryptophan pyridoxal 5-phosphate indole pyruvate NH 4

K'

The ammonium ions generated in this reaction are removed with glutamate dehydrogenase: glutamace dehydrogenase (ii) NH 4 +2-oxoglutarate+NADPH K glutamate NADP Endogenous ammonium ions are also first removed in reaction Following removal of endogenous ammonium ions, the additional ammonium ions generated in the reaction catalysed by tryptophanase in reaction are likewise removed by glutamate dehydrogenase. The rate of o removal, corresponding to the K' dependent activity of 20 tryptophanase is monitored by the decrease in absorbance at 340 nm and is a function of potassium ion concentration.

For the performance of this test, plasma or serum 5-20 pl is mixed with 300 pl of reagent 1 and incubated for 5-10 S" min at 37 0 C to ensure removal of endogenous ammonium ions. Reagent 2 (100 pl) is then added, the solutions mixed, and the reaction monitored fro a further 5-10 min.

o i 30 Reagent 1 contains 0.1 M triethanolamine, tris, or HEPES buffer, pH 7.8; 10 mM 2-oxoglutarate; 0.25 mM NADPH; 0.1 M pyriodoxal 5-phosphate; glutamate dehydrogenase, Reagent 2 contains tryptophanase 100 U/L in buffer, pH 7.6.

A linear curve between 2 and 7 mM K+ can be obtained by this method. The interference of sodium ions can be overcome in two ways. One method is to saturate the 920728,gsrlet083,div.flinders,43 L--L~L .1 1 1- -44system with sodium ions by including 40 mM NaCl in Reagent 1. Under these conditions the absorbance change is 0.005 A per min for each mmol/L K. Alternatively, Kryptofix® 221, at a final concentration of 25 mM can be included in Reagent 1. This latter approach gives greater sensitivity.

Example N Measurement of potassium ion concentration using the hydrolase, P-galactosidase.

The final incubation mixture contains: 325 mmol/l Tris-HCL buffer, pH 9.0 (37°C) 16 mmol/l Lithium chloride 3.8 mmol/l Sodium chloride 7.5 mmol/l Magnesium sulphate O 10 g/l Bovine serum albumin 0.7 U/ml p-galactosidase mmol/l 0-nitrophenyl-p-galactosidase 3.9 mmol/l Kryptofix® 221 350 pl of the incubation mixture is added to 10 pl of sample and the reaction (at 370C) is monitored at 420 nm 0 (or a nearby wavelength) to determine the rate of formation of free 2-nitrophenol which is inversely V 25 proportional to the concentration of potassium ions as shown in the following table: Dependence of the reaction rate of p-galactosidase on the potassium ion concentration Potassium concentration Rate of change (mmol/l in the sample) of absorbance (mA/min at 405) 0 449.3 2 448.0 4 444.2 6 441.7 8 439.5 439.0 920728,gsretO83,div.flinders,44 L- I~)i ni C L

Claims (19)

1. Process for the determination of potassium ions in fluids, wherein the influence of these ions on the activity of a lyase or a hydrolase 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 is blood, serum, plasma, urine, sweat, cerebrospinal fluid, lymph or intestinal secretions, exudates or transudates and the non- biological fluid is water or an aqueous extract or mixture. 0

4. Process as claimed in any one of claims 1-3, wherein the enzyme is a hydrolase. 20 5. Process as claimed in claim 4, wherein the hydrolase is a glycosidase or acting on peptide bonds.

6. Process as claimed in claim 5, wherein the hydrolase is a- or p-D-galactosidase.

7. Process as claimed in any one of claims 1-3 wherein .0.4 the enzyme is a lyase.

8. Process as claimed in claim 7, wherein the lyase is aldolase or carbonic anhydrase. -i

9. Process as claimed in claim 7, wherein the lyase is tryptophanase.

10. Process as claimed in any one of claims 1-9, wherein interfering ions are masked with a binding agent. 920728gsrlet083,div.flinders,45 c~c~~ -46-

11. Process as claimed in any one of claims 1-10, wherein the potassium 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 ions to be determined needs to be decreased.

12. Process as claimed in claim 10 or claim 11, wherein the interfering ions or analytes are bound by cryptands, coronands, podands, crown ethers, spherands, hemispherands, calixarens and combinations thereof, natural occurring ionophores, cyclic peptides, complexones and chelating agents, and derivatives thereof.

13. Process as claimed in any one of claims 1-12, wherein interfering sodium ions are bound by KryptofixO 221.

14. Process as claimed in claim 10 or claim 11, wherein the binding agent is KryptofixO 222. Process as claimed in claim 10 or claim 11, wherein the binding agents form a complex with indicator ions and the indicator ions are displaced stoichiometrically from the complex by the potassium ions which are to be determined and wherein the influence of the displaced indicator ions on the activity of the enzymes is assayed, thereby giving an indirect measure of the concentration of the potassium ions.

16. Process as claimed in any one of claims 1-15, wherein ions which are competitive inhibitors of the sensitive indicator enzyme are included in the assay, when the affinity of the enzyme to the 920728,gsrletO83,div.finders,46 L1 L I 47 potassium ions needs to be decreased.

17. Process as claimed in clair 16, wherein the competitive inhibitors are lithium ions.

18. Composition for the determination of potassium ions in fluids when used in a process or claimed in any one of claims 1 to 17, comprising a lyase or a hydrolase, the activity of which is influenced by the potassium ions.

19. Composition as claimed in claim 18, comprising a lyase or a hydrolase, 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 lyase or hydrolase to the potassium ions. Compositions as claimed in claim 19, wherein the binding agent is a cryptand.

21. Composition as claimed in claim 20, wherein the cryptand is Kryptofix® 221 to bind interfering ions, or Kryptofix® 222 to lower the concentration of the potassium ions and/or to decrease the affinity of the ~lyase or hydrolase. S' 22. Process as claimed in claim 1, substantially as herein described with reference to the Examples.

23. Composition as claimed in claim 18, substantially as herein described with reference to the Examples. Dated this 13th day of May, 1994 The Flinders University of South Australia AND Boehringer Mannheim GmbH By their Patent Attorneys Davies Collison Cave 940513,q:\oper\jms,20601-po.133,47 i: O nl-. -I -48 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 oxodoreductase 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. o o ,t 920728,gsrleLO83,div.flinders,48