NOVEL PHOSPHORIC ESTER AND A METHOD FOR QUANTITATIVE ANALYSIS OF AN ALKALINE PHOSPHATASE USING THE SAME AS A SUBSTRATE

Abstract: New phosphoric ester having the following formula (I) and a method for the analysis of an alkaline phosphatase, using the phosphoric ester as a substrate; A - N = -N - B -O(OM)₂ (I) wherein M is a hydrogen atom or an alkali metal atom; A is phenyl, naphthyl, 2- or 4-pyridyl, 2- or 4-pyrimidyl, pyrazinyl, sym-triazinyl, 2-imidazolyl, 2-oxazolyl, 2-thiazolyl, or sym­ thiadiazolyl group which may or may not be substituted with other substituent(s) or may form a condensed ring together with a benzene ring, if possible; B is paraphenylene group or may form naphthylene group by condensation together with a benzene ring, said groups may or may not be substituted with other substituent(s).

Description: This invention relates to phosphoric acid esters which are hydrolyzed by the action of alkaline phosphatase to release an azo dye and a method for the analysis of alkaline phosphatase, using the esters as a substrate. It is very important for clinical examination to determine the activity of alkaline phosphatase in human body fluids. It is well known that when the activity of alkaline phosphatase is high, metabolism in the liver or bones is generally under unfavourable conditions and therefore, symptons of liver disease, rickets, osteosarcoma or dysthyroidism appear. Accordingly various methods for the determination of alkaline phosphatase activity have been proposed. It is required that a method for the quantitative determination of alkaline phosphatase used in an ordinary clinical examination is simple in operation and has good reproducibility. For this purpose colorimetric and fluorometric determinations have been proposed. The colorimetric determination is a method of quantitative analysis in which a compound, which on hydrolysis by the action of alkaline phosphatase releases a dye or a dye-precursor, is used as a substrate for the enzymatic reaction, and the dye released in the enzymatic reaction is colormetrically measured or the dye-precursor released in the enzymatic reaction is reacted with a reagent thereby being converted to the dye which is then colorimetrically measured to determine the activity of the alkaline phosphatase. The colormetric determination has the advantage that a colorimeter or a spectrophotometer which are common analytic devices may be conveniently used to carry out the analysis. On the other hand, the fluorometric determination is a method of quantitative analysis in which a compound which on hydrolysis by the action of alkaline phosphatase releases a fluoroscent substance, is used as a substrate for the enzymatic reaction, and the fluorescent intensity of the fluorescent substance released in the enzymatic reaction is measured to thereby determine the activity of the alkaline phosphatase. The fluorometric determination has a higher sensitivity as compared with the colorimetric one, however it has some disadvantages that it is apt to be disturbed by traces of fluorescent substances, which may coexist therewith and that fluorometers are not so common. This invention relates to phosphoric acid esters which may be used as a suitable substrate in a method for the quantitative analysis of alkaline phosphatase by a colorimetric determination. In the past, there have been used the following methods for the quantitative analysis of alkaline phosphatase by colorimetric determination. (1) The method in which disodium p-nitrophenyl phosphate is used as a substrate and p-nitrophenyl which is released on hydrolysis of the substrate by the action of alkaline phosphatase is colorimetrically measured to determine the activity of the alkaline phosphatase. This method is specifically described in O. A. Bessey, O. H. Lowry and M. J. Brock, "Journal of Biological Chemistry", Vol. 164, p. 321 (1946). (2) The method in which phenolphthalein phosphate is used as a substrate and phenolphthalein wich is released on hydrolysis of the substrate by the action of alkaline phosphatase is colorimetrically measured to determine the activity of the alkaline phosphatase. This method is specifically described in A. L. Babson, S. J. Greeley, C. M. Coleman, and G. E. Phillips, "Clinical Chemistry", Vol. 12, p. 482 (1966). (3) The method in which thymolphthalein phosphate is used as a substrate and thymolphthalein which is released on hydrolysis of the substrate by the action of alkaline phosphatase is colorimetrically measured to determine the activity of the alkaline phosphatase. This method is specifically described in C. M. Colemann, "Clinical Chemistry", Vol. 13, p.401 (1966). (4) The method in which thymolblue mono-phosphate is used as a substrate and thymolblue released on hydrolysis of the substrate by the action of alkaline phosphatase is colorimetrically measured to determine the activity of the alkaline phosphatase. This method is specifically described in Japanese Patent (OPI) No. 136662/1976. (5) The method in which phenyl phosphate is used as a substrate and phenol released on hydrolysis of the substrate by the action of alkaline phosphatase is oxidatively condensed with 4-amino-antipyrine in the presence of potassium ferricyanide to form a red quinone which is colorimetrically measured to determine the activity of the alkaline phosphatase. This method is specifically described in P. R. N. Kind and E. J. King, "Clinical Pathology", Vol. 7, p.322 (1954); K. Watanabe et al., "RHINSHO-BYORI or Clinical Pathology", Vol. 15, p.708 (1967); T. Nakayama et al., "RINSHO-BYORI or Clinical Pathology", Vol. 23 (a supplementary volume in the general meeting), p.85 (1975). Among the methods in the prior art described above, the method (1) is most widely used for the quantitative analysis of alkaline phosphatase, however, the colorimetric determination is carried out at 410 nm and therefor in this method interferences with colored substances such as bilirubin or hemoglobin contained in serum may occur and thus, it requires a blank test which makes operations more complicate. The prior methods (2) to (5) tentatively overcome the disadvantage of method (1) because the colorimetric determination in the methods (2) to (5) is conducted in a longer wavelength region than in method (1). However, it is difficult to synthesize highly pure mono-phosphoric acid esters of phthaleins which are used as a substrate in methods (2) to (4). In other words, these mono-phosphoric acid esters may be obtained by hydrolysis of monophosphorodichloridate of the corresponding phthalein which may be prepared by reacting one hydroxyl group of the phthalein molecule with phosphorus oxychloride. In this reaction, a higher molar ratio of phosphorus oxychloride to phthalein causes phosphorylation of more than one hydroxyl group, while a lower molar ratio thereof causes formation of two or three kinds of products since one molecule of phosphorus oxychloride reacts with two or three molecules of phthalein. In either case, compounds unsuitable for the quantitative analysis of alkaline phosphatase are produced as by-products. Accordingly, the isolation and purification of the product after the reaction require a troublesome operation and as the result, it is difficult to obtain products having a high degree of purity. On the other hand, method (5) of the prior art comprises a two-step reaction of an enzymatic, phenol-release reaction and a subsequent coloring reaction, which require complicated operations. Thus, it has been expected that an improved method for the quantitative analysis of alkaline phosphatase without the disadvantages described above would appear. The hydrolysis of p-phenylazophenyl phosphate, e.g. in the form of the Ba-salt of the formula is disclosed in The Journal of Biological Chemistry Vol. 247 (1972), 6007-6010, The Journal of Biological Chemistry Vol. 249 (1974), 7192-7202, and in Biochemistry Vol. 11 (1972), 201-205. Since the above compound has a low solubility, it can be used only in the concentration of less than 10⁻⁴ mol/l as shown in Biochemistry, loc. cit. Accordingly, it is an object of the present invention to provide a new substrate to be used for the quantitative analysis of alkaline phosphatase, the substrate being excellent in operations at the time of measurement, having no susceptability to interference with colored substances such as pigments in blood, having a high sensitivity, and being easily synthesized and purified. It has been found that the above mentioned object can effectively be accomphished by the novel phosphoric acid esters of the general formulas (A) - (D): The Figures are graphs showing a relationship between the absorbance (ΔOD) measured by the Bessey-Lowry method, which is shown on the abscissa, and that measured by the method in which a compound of the present invention is used as substrate, which is shown on the ordinate. The compounds of the invention are more soluble than the known p-phenylazophenyl phosphate and give easily solutions with a concentration of about 10⁻² mol/1 (e.g. example 1, where the solubility is 7.8 x 10⁻³ mol/l). The compounds represented by the general formulas (A) - (D) will now be described with reference to the following specific examples. Compound 1: Compound 2: The same as Compound 1 except that K is substituted for Na. Compound 3: Compound 4: The same as Compound 3 except that K is substituted for Na. Compound 9: Compound 10: The same as Compound 9 except that K is substituted for Na. Compound 25: Compound 26: Compound 27: Compound 28: Compound 30: Compound 32: Now, a method for the synthesis of the compounds will be explained. First, a conventional diazo coupling reaction between an aromatic amine (III) and a phenol derivative H-B-OH is conducted to produce an azo dye (IV). In cases where the radical A has a strong electron withdrawing group of which the Hammett's sigma constant is above 0.5 such as a nitro or a methanesulfonyl group or where the sum of the Hammett's sigma constants of the substituents on the radical A is above 0.5, preferably, the so-called nitrosyl sulfuric acid process of which a specific example is described by S.R. Sandler and W. Karo in "Organic Functional Group Preparations", Vol. 2, p. 295, Academic Press, (1971), may be used for the synthesis of the diazonium salt formed by diazotization of compound (III). The azo dye (IV) is then reacted with a large excess of phosphorus oxychloride in the presence of pyridine to produce phosphorodichloridate (V) which is then reacted with water to obtain phosphoric monoester (I′). In the reaction in which compound (V) is obtained from compound (IV), it has been found that the compound (IV) can be converted to compound (V) with almost no by-products by dropping a solution of compound (IV) in pyridine, pyridine-containing acetone, pyridine-containing trimethyl phosphate, or a mixed solvent of pyridine-containing acetone and trimethyl phosphate to a 1/2 to 1/50, preferably 1/5 to 1/10 solution of more than 3 moles, preferably from 3 to 10 moles of phosphorus oxychloride based on one mole of compound (IV) in acetone, trimethyl phosphate, or a mixed solvent thereof, at from - 15°C to 10°C preferably from -10C to 0°C. Instead of pyridine, a tertiary amine having a higher basicity than that of pyridine, such as triethylamine, may be added to the solution of compound (IV). The formation of compound (V) can be confirmed by a silica gel thin layer chromatography in which a spot having a Rf value smaller than that of compound (IV) appears. In the past, an aryl phosphorodichloridate such as p-nitrophenyl phosphorodichloridate was synthesized by reacting phosphorus oxychloride with phenol under refluxing in the presence of potassium chloride a specific example of which is described by Mukaiyama and Hashimoto in "Bulletin of the Chemical Society of Japan". Vol. 44, 196 (1971 ). However, compounds (V) of the present invention were inefficiently produced under such severe conditions. The conversion of compound (V) into compound (I′) may be carried out by the addition of water to the reaction liquid without the isolation of compound (V). Under acidic conditions compound (I′) may be occasionally decomposed and therefore, in the reaction beween compound (V) and water, a tertiary amine such as pyridine may be added to capture the hydrogen chloride production therein, or a water-immiscible solvent such as ethyl acetate may be added to form a two-layer reaction system of water and oil. Such modifications of the reaction may bring about good results. The compound (I′) thus obtained is dissolved in an aqeuous solution of sodium or potassium hydroxide. After adjusting the pH of the resulting solution to around 10, insolubles are removed by filtration, the resulting solution is dropped to ethanol having a volume of about 10 to 20 times as much as that of the solution with agitation, and then crystals of compound (I) are obtained. The compound (I) thus obtained is dissolved in a small amount of water. The resulting solution is poured into ethanol, and the purified compound (I) is crystallized out. The crystalline compound (I) obtained by the process described above often contains one or a few molecules of water of crystallization. The compounds of the present invention thus synthesized are an excellent substrate to be used for the quantitative analysis of alkaline phosphatase. In other words, the compounds have the following advantages: (1) the compounds are hydrolyzed by the action of alkaline phosphatase at a rate being in proportion to the activity of the alkaline phosphatase to release an azophenol dye, (2) the dye shows the maximum absorption of visible light in a longer wavelength region as compared with p-nitrophenol which is a dye released in the conventional method in which sodium p-nitrophenol phosphate is used as a substrate. Therefore there is less interference with colored substances, such as pigments of the blood, and (3) the compounds may be synthesized with no difficulty, as shown in Synthetic Example 1, and may be purified by a normal crystallization, which is in striking contrast to the fact that the syntheses of phthalein phosphate, thymolphthalein phosphate, or thymolblue monophosphate require complicated operations for the isolation and the purification thereof. In the quantitative measurement of alkaline phosphatase according to the present invention, the compounds of the present invention may be used instead of di-sodium p-nitrophenyl phosphate in the Bessey-Lowry method, wich is described in "Journal of Biological Chemistry", Vol. 164, p. 321 (1946), and the quantity of the azo dye A-N = N-B-OH released from the compound of the present invention may be measured instead of p-nitrophenol formed by the enzymatic reaction of the Bessey-Lowry method. For example, a compound is dissolved in a buffer solution having a pH of around 10 to prepare a substrate solution. An enzyme activator, such as magnesium chloride, may preferably be added to the substrate solution. Then, a solution containing the alkaline phosphatase to be measured is added to the substrate solution to carry out the reaction. At this time, the compound may preferably be contained in the substrate solution in the concentration of from 1 to 20 mmol/l. Preferably, the compound may be present in large excess as compared with the alkaline phosphatase, that is, in a molar ratio of more than 10. The reaction described above may be conducted at a temperature ranging from 10 to 50°C, preferably from 30 to 40°C, for a period ranging from 10 to 60 minutes. Then, if necessary, the pH of the reaction solution may be raised or lowered by the addition of base or acid, and an organic solvent such as DMF or Methyl Cellosolve (trademark) may sometimes be added in order to stop the reaction. The density of the azo dye formed may then be measured by a spectrophotometer to determine the activity of the alkaline phosphatase, if needed, on the calibration curve. The wavelength to be used in the colorimetric determination may be set in the region of the absorption maximum wavelength of the dye. In the colorimetric measurement, the addition of cyclodextrins, preferably a-cyclodextrin may shift the absorption maximum wavelength toward the longer wavelength. The amount of cyclodextrin added at this time, is in the range of from 0.1 to 20 mg, preferably from 1 to 10 mg per 1 ml of the solution to be colorimetrically measured. Next, Synthesis Example 1 will be described with respect to typical compounds of the present invention. Other compounds than those described hereinafter may also be synthesized with no difficulty according to the aforementioned synthetic process or the following descriptions. Synthesis Example 1 Synthesis of Compound 3 Four ml of phosphorus oxychloride was mixed with 20 ml of acetone to prepare a solution which was then cooled to -8°C. To the solution was dropped a solution of 2 g of 4-(2-methanesulfonyl-4-nitrophenylazo) phenol in 20 ml of acetone, 10 ml of trimethyl phosphate, 8 ml of pyridine and 0.5 ml of triethylamine over a period of 25 minutes. During the addition, external cooling was conducted to maintain the temperature of the reaction liquid at below 0°C. Further agitation was continued for 30 minutes at below 0°C, and 30 ml of water was then dropped thereto. The resulting solution was agitated for 10 minutes at room temperature, and was then poured into 200 ml of a mixture of a saturated NaCl aqueous solution and ice, and the crystals formed were collected by filtration. The crystals were dissolved in 20 ml of 1N-NaOH aqueous solution, and the resulting solution was adjusted to pH of about 10. Insolubles were removed by filtration, and the resulting filtrate was poured into 150 ml of ethanol. The crystals thus formed were collected by filtration, washed with ethanol, and dried in vacuo. Yield 2 g, λ 0,02N-NaOH max 382 nm Example 1 Preparation of a substrate solution: Crystals of Compound 3 (35.05 mg) were dissolved in 10 ml of a glycine buffer (pH 10, 0.5 m mole/l of magnesium chloride contained therein). Preparation of an enzyme solution: About 6 mg alkaline phosphatase Type I (produced by Sigma Chemical Co.) was dissolved in 20 ml of distilled water to prepare an enzyme solution. The solution was diluted to 1/2 and 1/4 respectively and thus, three kinds of enzyme solutions were prepared. Procedure: The substrate solution (1 ml) was charged in a test tube and heated at 37°C for 3 minutes. The enzyme solution (0.1 ml) was added to the test tube and heated at 37°C for 30 minutes to carry out the reaction and then, 10 ml of 0.02 N-NAOH aqueous soltion was added thereto to stop the reaction. A blank test solution was prepared by the same procedure as described above except that distilled water was substituted for the enzyme solution. Absorbance (ΔOD) was measured at 520 nm using the blank test solution as a control. The same procedure as described above was conducted with respect to each of three kinds of the enzyme solutions. Another absorbance (ΔOD) was measured at 410 nm with respect to each of the same enzyme solutions by the Bessey-Lowry method, in which the blank test solution also was used as a control. The procedure of the Bessey-Lowry method is the same as described above except that 20 mg of sodium p-nitrophenyl phosphate is substituted for 35.05 mg of the crystals of Compound 3 and the density of color is colorimetrically measured at 410 nm. A relationship between the values of ΔOD obtained by these two methods is given in Figure 1. Figure 1 discloses the following: (1) The ΔOD value measured by the present method varies in proportion to the enzyme concentration or the ΔOD value measured by the Bessey-Lowry method and thus, the former ΔOD value may be used to determine the activity of the enzyme. (2) The ΔOD value measured by the method in which Compound 3 is used as a substrate is higher than the ΔOD value measured by the Bessey-Lowry method (the slope of the straight line is above 1) and thus, the sensitivity of the method in which Compound 3 is used as a substrate is higher than that of the Bessey-Lowry method. (3) In the case where Compound 3 is used as a substrate, the colorimetric determination may be conducted at 520 nm and thus, this method has less susceptibility to interference with colored substances such as blood pigments than the Bessey-Lowry method in which the colorimetric determination is conducted at 410 nm. Thus, it can be seen that the compound of the present invention is suitable for the quantitative analysis of alkaline phosphatase. Example 2 The procedure of Example 1 was repeated except that 48.18 mg of the crystals of Compound 32 was substituted for 35.05 mg of the crystals of Compound 3 of Example 1, and the measurement of the absorbance was conducted at 644 nm instead of 520 nm. The results are given in Figure 2. Figure 2 discloses the following: (1) The ΔOD value measured by the present method varies in proportion to the enzyme concentration or the ΔOD value measured by the Bessey-Lowry method and thus, the former ΔOD value may be used to determine the activity of the enzyme. (2) In the case where Compound 32 is used as a substrate the colorimetric determination may be conducted at 644 nm and thus, this method has less susceptibility to interference with colored substances such as blood pigments than the Bessey-Lowry method in which the colorimetric determination is conducted at 410 nm. Thus, it can be seen that the compound of the present invention is suitable for the quantitative analysis of alkaline phosphatase.

Claims: 1. Phosphoric acid esters of formulae (A) to (D) wherein M is Na or K, and R is H or Cl, wherein M is Na or K, Wherein M is Na, R¹ is H or CH₃, and R² is Cl; or R¹ is OH or and R² is H, wherein M is H, and R³ is CH₃ or 3-sulfamoylphenyl. 2. The use of the phosphoric acid esters according to claim 1 as a substrate for the quantitative analysis of alkaline phosphatase.

Inventors: INAGAKI, YOSHIO, OKAZAKI, MASAKI
Applicants: FUJI PHOTO FILM CO., LTD.
Application year: 1982
Main classification: C07F9
IPC codes: G01N33, C07F9, G01N21, C12Q1