REAGENT FOR ASSAYING MAGNESIUM IONS

Abstract: A reagent for assaying magnesium ion by utilizing the fact that a complex of a magnesium ion with adenosine 5'-triphosphate serves as a substrate for glucokinase originating in a bacterium of the genus Zymomonas . This reagent has such a wide range of assay that it can assay magnesium ions present in a variety of specimens, such as bodily fluids of living organisms and foods, and magnesium ions contained in a low concentration in the specimen reliably with high accuracy. Further it has a good operability because of a reduction in the step of diluting the specimen, so that it has a satisfactory performance in the routine assay of magnesium ion.

Description: The present invention relates to a reagent for the measurement of magnesium ions used for determining the content of magnesium ions in various samples, which is useful for the fields of medical sciences, food analysis and environmental preservation. The measurement of magnesium ions in biological fluids (e.g. blood, etc.), foods, environment, such as rivers, and the like is frequently done and is considered to be important. A method for the measurement of magnesium ions is classified into a determining method by an instrumental analysis method (e.g. atomic absorption spectrophotometry, etc.) and a determining method by a chemical method (e.g. xylidyl blue method, etc.). According to the former method, comparatively accurate results can be obtained. However, expensive instruments are especially required and the operation is complicated, whereby great skill is required. Further, the latter method is a simple method, however, it has the problem that results with large error factors are obtained because it lacks in specificity for magnesium ions and the influence of coexisting substances in the sample cannot be avoided. In order to solve these problems, there have been proposed various reagents for the measurement of magnesium ions which utilize an enzyme reaction. These reagents are those which utilize the fact that various kinase enzymes such as hexokinase (hereinafter abbreviated as HK) act on a complex of magnesium ions with adenosine 5'-triphosphate (hereinafter abbreviated as ATP) as a substrate to manifest their activities. Examples thereof include a reagent utilizing a coupling reaction of HK with glucose-6-phosphate dehydrogenase (hereinafter abbreviated as G6PDH) [Methods of Enzymatic Analysis, 3rd edition, pages 592-597, 1985: U.S. Patent Publication No. 4,657,854]; a reagent utilizing a coupling reaction of glucokinase (hereinafter abbreviated as GlcK) having high specificity to glucose, which is similar to HK in the enzymatic reaction, with G6PDH [summaries of the 25th annual meeting of Japan Society of Clinical Chemistry, page 75, 1985: U.S. Patent Publication No. 4,778,754];a reagent utilizing a coupling reaction of three enzymes, glycerol kinase (hereinafter abbreviated as Glyk), glycerol-3-phosphate dehydrogenase (hereinafter abbreviated as G3PDH) and peroxidase (hereinafter abbreviated as POD) [Clinical Chemistry, Vol. 32, pages 629-632, 1986] and the like. In WO88/08137, there is described that adenylate kinase or HK which is sensitive to magnesium ions can be used, however, there is no disclosure about the accurate measurement. The composition for measurement utilizing these enzyme reactions has the advantage that a simple chemical method can be used and that the measurement is accurate. However, ATP can form a complex with divalent metal ions (e.g. manganese, calcium, zinc, etc.) in addition to magnesium and the complex can also be a substrate of an enzyme and, therefore, there is a risk that errors occur on determination of magnesium ions when a large amount of these metal ions is also present in a sample. It has also been proposed that these error factors are avoided by masking a divalent metal ion other than a magnesium ion with a chelating agent (see Japanese Patent Kokai No. 1-51100). Among various enzymatic measuring methods, a method utilizing a coupling reaction of GlcK with G6PDH is most practical from the viewpoints of stability of the reagent, accuracy of the measurement and the like. However, in this method, the measuring range is comparatively narrow, particularly when a sample containing magnesium ions in high concentration is used. Furthermore, an operation of diluting the sample whenever the measurement is made is required and the measurement becomes complicated. Therefore, regarding a sample containing magnesium ions in high concentration such as urea and the like, an error of the measurement may be caused by the complicated operation. The object of the present invention is to provide a reagent for the measurement of magnesium ions, which employs a measuring range wherein the determination can be made without dilution even in a sample containing high concentration of magnesium ions. The above object is accomplished by using GlcK derived from bacteria of the genus Zymomonas as GlcK. The composition for measurement of magnesium ions of the present invention has a high enough performance to be widely used. That is according to the present invention, there is provided a reagent for the measurement of magnesium ions wherein the magnesium ions are allowed to form a complex with adenosine 5'-triphosphate and then a quantitative determination is made by employing the complex as a substrate of an enzyme reaction, characterized in that glucokinase derived from bacteria of the genus Zymomonas is used. The reagent for the measurement of magnesium ions comprises ATP, nicotinamide adenine dinucleotide (phosphate) (hereinafter abbreviated as NAD(P)) and glucose as main ingredients, in addition to the above GlcK and G6PDH. It may also contain additives such as chelating agents, accelerators, excipients or preservatives. The concentration of the main ingredients in the reagent of the present invention is as follows. The concentration of GlcK, G6PDH, ATP, NAD(P) and glucose is 0.1 to 50 units/ml, 0.1 to 50 units/ml, 0.01 to 20 mM, 0.1 to 20 mM and 1 to 100 mM, respectively. Preferably, the concentration of Glck, G6PDH, ATP, NAD(P) and glucose is 0.1 to 10 units/ml, 0.2 to 10 units/ml, 0.05 to 10 mM, 0.2 to 10 mM, 1 to 50 mM, respectively. The concentration of the chelating agent is preferably about 0.001 to 10 mM. The concentration of the salts is preferably about 1 to 300 mM. GlcK used in the present invention must be those derived from a bacteria of the genus Zymomonas . The origin of GlcK is not limited to a specific one and examples thereof include bacteria of the genus Zymomonas such as Zymomonas mobilis or Zymomonas anaerobia . Among them, GlcK derived from bacteria of the species Zymomonas mobilis is particularly preferred. Examples thereof include Zymomonas mobilis ATCC strains, 31821, 31823, 35000, 35001, 10988, 29191 and 29192. In order to obtain GlcK from the above bacteria of the genus Zymomonas , the bacteria are cultured on a culture medium which is normally used for culturing , followed by isolating and purifying it from the resulting cells by a conventional method. The purification and characterisation of GlcK from Zymomonas mobilis is described in Biochem J 228 (1985) 627. In the present invention, G6PDH is also used as a coupling enzyme, in addition to GlcK. However, the origin of G6PDH is not limited to a specific one (e.g. microorganisms such as yeast, animals, etc.). Preferably, as the coenzyme, there is used G6PDH which acts on nicotinamide adenine dinucleotide (hereinafter abbreviated as NAD) as well as NAD(P) [e.g. those derived from microorganisms that belong to the genus Leuconostoc such as Leuconostoc mesenteroides , the genus Pseudomonas such as Pseudomonas fluorescens or the genus Zymomonas such as Zymomonas mobilis , etc.]. More preferably, G6PDH derived from thermophilic bacteria having excellent stability and storage stability, wherein both NAD(P) and NAD can be used as the coenzyme is preferred. As chelating agent agents may be used which possess a numerical difference of two or more in the chelate stability constant between magnesium and other divalent metal ions [see Rikagaku Jiten, 3rd edition, page 61, 1981, published by Iwanami Shoten; Kagaku Daijiten, 1st edition, pages 496-497, 1963, published by Kyoritsu Shuppan]. The chelate stability constant is usually expressed as the logarithm of the inverse number of the dissociation constant of the chelate complex with the metal ion. Examples thereof include ethylenediaminetetraacetic acid, glycoletherdiamine-N,N,N',N'-tetraacetic acid (hereinafter abbreviated as GEDTA), 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (hereinafter abbreviated as BAPTA), trans-1,2-cyclohexanediamine-N,N,N',N'-tetraacetic acid, dihydroxyethylglycine, ethylenediamine-N,N'-dipropionic acid, hydroxyethyliminodiacetic acid and nitrilotriacetic acid. As the salts, normal salts such as potassium chloride can be used. In the measurement according to the present invention, the content of magnesium ions is not directly determined, but a complex is once formed between magnesium ions and ATP and then the enzyme reaction using the complex as a substrate is utilized. The actual reaction can be shown as follows. That is, an increasing rate in absorbance at 340 nm due to the formation of NAD(P)H in the above coupling reaction of the formulas (1) and (2) is measured. In order to measure magnesium ions in a sample using the reagent for measurement of the present invention, for example, a reagent for measurement is charged in a cell of a spectrophotometer which cell chamber is thermally controlled, which is maintained at a measuring temperature (any temperature of 20 to 45°C) for about 3 minutes. Thereafter, a sample is added to the mixture and then the increase in absorbance at 340 nm is measured. Further, the reaction time for the measurement after the addition of the sample can be optionally selected from the range between 30 seconds and 30 minutes. When an autoanalyser is used, the conditions suitable for the measuring conditions of the autoanalyser may be selected. The following Examples and Comparative Examples further illustrate the present invention in detail but are not to be construed to limit the scope thereof. Example 1 and Comparative Example 1 Zymomonas mobilis ATCC29191 was cultured according to a method described in The Biochemical Journal, Vol. 228, pages 627-634, 1985 followed by isolating and purifying GlcK and G6PDH, respectively. It was cultured in a medium wherein biotin (1 mg), calcium pantothenate (2 mg) and ammonium iron sulfate hexahydrate (20 mg) were added to 1 liter of a solution containing 15 % glucose, 0.5 % yeast extract, 0.05 % potassium monophosphate and 0.05 % magnesium sulfate. After culturing, the collected cells were disrupted and allowed to adsorb on a Scarlet MX-G Sepharose™ column (Scarlet MX-G is available from ICI Japan Co. under the trade name of Procion Scarlet MX-G). GlcK was eluted by increasing the pH and G6PDH was eluted by adding NAD(P), respectively, and then they were subjected to ammonium sulfate fractionation. Further, both enzymes were subjected to gel filtration on a Sephacryl™ S-200 column to obtain a purified enzyme. By using GlcK and G6PDH purified as described above, a reagent for the measurement of magnesium ions was prepared. A reagent comprising 1 unit/ml of GlcK, 0.75 units/ml of G6PDH, 2.5 mM ATP [available from Boehringer Mannheim Yamanouchi Co.], 0.625 mM NAD(P) [available from Boehringer Mannheim Yamanouchi Co.], 0.5 mM GEDTA, 18.75 mM potassium chloride and 125 mM imidazole-hydrochloric acid buffer solution (pH 8.0) was prepared (Example 1). In addition, a 50 mM glucose solution was also prepared. For comparison, by using GlcK derived from Bacillus stearothermophilis [available from Seikagaku Kogyo Co.] and G6PDH derived from Leuconostoc mesenteroides [available from Boehringer Mannheim Yamanouchi Co.], a reagent for the measurement of magnesium ions having the following composition was prepared (Comparative Example 1, see Japanese Patent Kokai No. 1-51100). A reagent comprising 0.44 units/ml of GlcK, 0.75 units/ml of G6PDH, 12.5 mM ATP, 0.625 mM NAD(P), 0.5 mM GEDTA, 18.75 mM KCl and 125 mM trishydrochloric acid buffer solution (pH 8.5) was prepared. Then, by using magnesium chloride, various solutions having magnesium ion concentrations of 0, 4, 8, 12, 16, 20, 24, 28, 32, 36 and 40 mg/dl were prepared as a sample. The reagent for measurement prepared above (2.4 ml) was charged in a cell having an optical path length of 1 cm and the sample prepared above (0.03 ml) was added. While the cell chamber was maintained at 37°C, the reaction was initiated by adding a glucose solution (0.6 ml) and the change in absorbance at 340 nm was measured to determine the magnesium content in the sample from the change in absorbance per minute. The results are shown in Fig. 1. In Example 1, the change in absorbance per minute linearly increased until the magnesium content reached about 30 mg/dl. On the other hand, in Comparative Example 1, the change in absorbance per minute linearly increased until the magnesium content reached about 10 mg/dl only. It became clear that the reagent for measurement of the present invention exhibits a wide measuring range. Example 2 By using GlcK prepared in Example 1 and G6PDH derived from Leuconostoc mesenteroides [available from Boehringer Mannheim Yamanouchi Co.], a reagent for the measurement of magnesium ions was prepared. A reagent comprising 1 unit/ml of GlcK, 0.75 units/ml of G6PDH, 2.5 mM ATP, 0.625 mM NAD(P), 0.625 mM BAPTA, 18.75 mM potassium chloride and 125 mM imidazole-hydrochloric acid buffer solution (pH 8.0) was prepared (Example 2). According to the same manner as that described in Example 1, each solution having various magnesium content was measured to examine a dependence of change in absorbance per one minute on the magnesium content. As a result, it was found that the change in absorbance per minute has a linear relation to the magnesium content until about 30 mg/dl. It was shown that the reagent for measurement of the present invention has a wide measuring range. Example 3 A reagent similar to that of Example 1 was prepared. By using a serum having a magnesium content of about 0.6 mg/dl as a sample, the same sample was measured 30 times to examine the accuracy of the measurement. As a result, the measured average value was 0.61 mg/dl, the standard deviation was 0.01 mg/dl and the coefficient of variation indicating accuracy of the measurement was 1.64 %. Therefore, it was shown that it is an extremely accurate reagent. It was found that the reagent for measurement of the present invention has a wide measuring range and, at the same time, is a reagent by which accurate measuring can be made even at a low concentration range of magnesium ions. Example 4 A reagent similar to that of Example 2 was prepared and stored in a refrigerator for about one month. By using three kinds of reagents, the magnesium content in each sample was measured every 10 days. The results are shown in Table 1. As described above, it was found that the reagent for measurement of the present invention is a stable reagent the measuring range of which is maintained for a long period of time. Example 5 By using GlcK and G6PDH as prepared in Example 1, a reagent for the measurement of magnesium ions was prepared. A reagent comprising 1.0 unit/ml of GlcK, 0.75 units/ml of G6PDH, 12.5 mM glucose, 0.625 mM NAD(P), 0.5 mM GEDTA, 18.75 mM HCl, 12.5 mM N-acetylcysteine and 100 mM Bicine buffer solution (pH 7.8) was prepared. In addition, a reagent comprising 16 mM ATP, 80 mM ammonium sulfate and 20 mM Bicine solution (pH 7.8) was also prepared. Then according to the same manner as that described in Example 1, sample solutions with magnesium concentrations of 0, 4, 8, 12, 16, 20, 24, 28, 32, 36 and 40 mg/dl were prepared, respectively, using magnesium chloride. The reagent for measurement prepared above (2.4 ml) was charged in a cell having an optical path length of 1 cm and the sample prepared above (0.03 ml) was added. While the cell chamber was maintained at 37°C, the reaction was initiated by adding a reagent containing ATP (0.6 ml). It was found that the change in absorbance per minute at 340 nm linearly increases until the magnesium content in the sample reached about 32 mg/dl. It was shown that the reagent for measurement of the present invention exhibits a wide measuring range. Example 6 By using GlcK prepared in Example 1 and G6PDH derived from Leuconostoc mesenteroides , a reagent for the measurement of magnesium ions was prepared. A reagent comprising 1 unit/ml of GlcK, 1.5 units/ml of G6PDH, 12.5 mM glucose, 4 mM NAD(P), 0.5 mM GEDTA, 18.75 mM KCl, 12.5 mM N-acetylcysteine and 100 mM Bicine buffer solution (pH 7.8) was prepared. In addition, a reagent comprising 16 mM ATP, 80 mM ammonium sulfate and 20 mM Bicine buffer solution (pH 7.8) was also prepared. These two kinds of reagents were stored in a refrigerator for about one month. By using three kinds of samples containing magnesium, the magnesium content in each sample was measured every 10 days. The results are shown in Table 2. (Change in measuring range on the storage) Magnesium measured value (mg/dl) Storage period of time (days) 0 10 20 30 Sample 1 0.50 0.50 0.49 0.50 Sample 2 15.1 15.0 15.0 15.1 Sample 3 28.0 28.1 27.9 27.9 As described above, it was found that the reagent for measurement of the present invention is a stable reagent the measuring range of which is maintained for a long period of time. Example 7 By using GlcK prepared in Example 1 and G6PDH derived from Leuconostoc mesenteroides , a reagent for the measurement of magnesium ions was prepared. A reagent comprising 1 unit/ml of GlcK, 1.5 units/ml of G6PDH, 12.5 mM glucose, 4 mM NAD(P), 0.5 mM GEDTA, 18.75 mM KCl, 20 mM ammonium sulfate, 12.5 mM N-acetylcysteine and 100 mM Bicine buffer solution (pH 7.8) was prepared. In addition, a reagent comprising 16 mM ATP and 20 mM Bicine buffer solution was also prepared. Then, solutions having a magnesium content of 0.5 mg/dl, 10 mg/dl or 30 mg/dl were prepared, and the same sample was measured 30 times to examine accuracy of the measurement. As a result, the measured average value was 0.51 mg/dl, 10.1 mg/dl and 29.9 mg/dl; the standard deviation was 0.01 mg/dl, 0.15 mg/dl and 0.30 mg/dl; and the coefficient of variation indicating the accuracy of the measurement was 1.06 %, 1.49 % and 1.00 %, respectively. Therefore, it was shown that it is an extremely accurate reagent. It was found that the reagent for measurement of the present invention has a wide measuring range and, at the same time, is a reagent by which accurate measuring can be made even at a low concentration range of magnesium ions. Brief Explanation of the Drawing Fig. 1 is a graph showing the measuring range of the reagent for the measurement of magnesium ions of the present invention, wherein the ordinate indicates the measured values of the magnesium ions (mg/dl) in a sample and the abscissa indicates the dilution rate of the sample. The mark () indicates the measured results of Example 1 and the mark (○) indicates those of Comparative Example 1.

Claims: A reagent for the measurement of magnesium ions wherein the magnesium ion is allowed to form a complex with adenosine 5'-triphosphate and then a quantitative determination is made by employing the complex as a substrate of an enzyme reaction, characterized in that the reagent comprises glucokinase derived from bacteria of the genus Zymomonas . The reagent for the measurement of magnesium ions according to claim 1, wherein glucokinase is derived from Zymomonas mobilis . The reagent for the measurement of magnesium ions according to claim 1 which comprises 0.1 to 50 units/ml of glucokinase derived from bacteria of the genus Zymomonas , 0.1 to 50 units/ml of glucose-6-phosphate dehydrogenase, 0.01 to 20 mM adenosine 5'-triphosphate, 0.1 to 20 mM nicotinamide adenine dinucleotide phosphate and 1 to 100 mM glucose.

Inventors: KAWAHARA KAZUE, KONDOU HITOSHI, NAGATA KAZUHIKO, KAWAHARA, KAZUE, KONDOU, HITOSHI, NAGATA, KAZUHIKO
Applicants: IATRON LAB, UNITIKA LTD, IATRON LABORATORIES, INC., UNITIKA LTD.
Application year: 1992
Main classification: C12Q1
IPC codes: C12Q1, G01N33