METHOD FOR DETERMINING ENZYMATIC ACTIVITY

Abstract: This invention relates to a substrate to be used in determining the activities of N-acetyl- beta -D-glucosaminitase and N-acetyl- beta -D-hexosaminitase both terminated with a 3,4-dinitrophenoxy group, alkaline and acidic phosphatases, beta -D-galactosidase and beta -D-glucuronidase, and to a method and reagents for determining these activities by using said substrate. This substrate is excellent in the stability and solubility in the working pH range (neutral or acidic) of the enzyme and functions to bring the working pH of the enzyme close to the color developing pH.

Description: The present invention relates to an enzyme activity assay. More specifically, it relates to a method of assaying enzyme activity which involves using a substrate having a 3,4-dinitrophenoxy group at the end of a molecule, for assaying the activity of N-acetyl-β-D-glucosaminidase or N-acetyl-β-D-hexosaminidase, the activity of an acidic phosphatase, β-D-glucuronidase or β-D-galactosidase. In the method, the indication (absorbance) of the 3,4-dinitrophenol formed from the substrate is measured. To carry out an assay of the activity of N-acetyl-β-D-glucosaminidase or N-acetyl-β-D-hexosaminidase (hereinafter abbreviated as NAGase), which are indispensable for an early diagnosis of various kidney diseases such as uriniferous tubule disorders and diabetic kidney disorders, the prior art method uses p-nitrophenyl-N-acetyl-β-D-glucosaminide having a p-nitrophenoxy group at the end of a molecule as the substrate (with p-nitrophenol as aglycon), and quantitates by colorimetry the p-nitrophenol (aglycon) liberated by the action of the enzyme NAGase at a high pH region (see Method in Enzymology, Vol. 28, p. 772 - 776). Also, a method is employed in which m-cresol-sulfophthaleinyl-N-acetyl-β-D-glucosaminide is used as the substrate (Japanese Unexamined Patent Publication (Kokai) No. 58-994, Rinsho Byori (Clinical pathology) XXXI: 2179 (1983)). Nevertheless, although the optimum pH for NAGase enzyme action is around 5 in the acidic side region, p-nitrophenol and the like do not indicate a required color formation unless in the alkali side region having a pH of 8 or higher. Accordingly, to make an assay of the activity of the enzyme NAGase, in all cases the operations of mixing the enzyme and the substrate to initiate the reaction, stopping the reaction by an addition of an alkali solution after a certain time, and subjecting the reaction mixture to a spectroscopic photometer must be carried out, and thus a drawback arises in that a rate analysis by monitoring under suitable conditions (i.e., rate assay) cannot be performed. Accordingly, to perform a rate assay, attempts have been made to develop an N-acetyl-β-D-glucosaminide substrate having a compound (aglycon) convertible to p-nitrophenol bound thereto, which forms color at around pH 5 on the acidic side where the action of the NAGase is adequate. For this purpose, the substrates with the following nitro compounds as the aglycon have been proposed (Japanese Unexamined Patent Publication (Kokai) No. 61-177999): (a) 2,4-dinitrophenol (J. Biol. Chem., Vol. 255, p. 11861 - 11869, 1980); (b) 2-chloro-4-nitrophenol (Japanese Unexamined Patent Publication (Kokai) No. 61-112092; (c) 2,5-dinitrophenol; (d) 2,6-dichloro-4-nitrophenol; (e) 2,6-dibromo-4-nitrophenol; (f) 2,3,6-trichloro-4-nitrophenol; (g) 2,3,5,6-tetrabromo-4-nitrophenol; (h) 2,3,5,6-tetrachloro-4-nitrophenol; (i) 7-sulfo-2,4-dinitro-1-naphthol (Japanese Unexamined Patent Publication (Kokai) No. 61-177999). Since all of the above compounds have a bulky electron-attracting group substituted at the position adjacent to the phenolic hydroxyl group, to provide the properties of intensifying the acidity and ionization at a low pH to effect a color formation, the substrates having these bound (as aglycon) at the end of a molecule were expected to compensate for the above-mentioned drawback possessed by p-nitrophenol. Nevertheless, the synthetic substrates obtained by synthesizing these acidic aglycons and N-acetyl-β-D-glucosamine have problems in that they are all difficult to dissolve in water, do not exhibit a satisfactory ionized color formation at a low pH, are per se unstable, and cause problems in enzyme reactions due to the bulky substituents at the 2-position. For example, 2-chloro-4-nitrophenyl-N-acetyl-β-D-glucosaminide is difficult to dissolve in water, and therefore, the substrate cannot be sufficiently dissolved, and thus the procedure of preparing the reagent and the assay accuracy are markedly adversely affected and a satisfactory assay method cannot be obtained for practical application. Also, a 2,4-dinitrophenyl-N-acetyl-β-D-glucosaminide substrate can be synthesized only with difficulty, and its yield is at most only 5%, even when using a synthetic method considered most suitable as a result of an investigation of various reaction conditions: This is due to the instability of this substance, and the utmost care must be taken to prevent hydrolysis even during the recrystallization at the final stage ( supra , J. Biol. Chem., Vol. 255, p. 11861 - 11869, 1980). As described above, the problems concerning the substrate of NAGase were not solved by using the various nitrophenol compounds convertible to p-nitrophenol and proposed to date. The enzymes and substrates thereof to which the state of the art as described above is applicable are shown in Table 1. Enzyme Substrate N-acetyl-β-D-glucosaminidase (hexosaminidase) p-nitrophenyl-N-acetyl-β-D-glucosaminide (hexosaminide) α-D-glucosidase p-nitrophenyl-α-D-glucopyranoside β-D-glucosidase p-nitrophenyl-β-D-glucopyranoside β-D-galactosidase o-nitrophenyl-β-D-galactoside phosphatase p-nitrophenylphosphoric acid β-D-glucuronidase p-nitrophenyl-β-D-glucuronide esterase p-nitrophenylacetic acid allylsulphatase p-nitrophenylsulfuric acid An object of the present invention is to provide a method in which the substrate is sufficiently soluble in water and is stable when used as the substrate for a rate assay of NAGase or phosphatase, β-galactosidase, and β-glucuronidase, and further, exhibits an excellent color formability and quantitativity in the acidic pH region, and in which enzyme activity assay method an indication (absorbance) of 3,4-dinitrophenol formed from the substrate is employed. According to the invention there is provided a method of assaying the activity of any one of the enzymes N-acetyl-β-D-glucosaminidase or N-acetyl-β-D-hexosaminidase activity, an acidic phosphatase, β-D-galactosidase or β-D-glucuronidase, which comprises measuring the color indication (absorbance) of 3, 4-dinitrophenol formed by the action of the enzyme on a substrate molecule having a 3,4-dinitrophenoxy group at the end of the molecule. Examples of the method of the invention include an assay of N-acetyl-β-D-glucosaminidase or N-acetyl-β-D-hexosaminidase activity, or the activity of acidic phosphatase in a sample taken from a living body, and an assay of the activity of β-galactosidase or phosphatase to be used as a suitable labelling enzyme, and which makes use of a substrate having a 3,4-dinitrophenoxy group at the end of a molecule. The accompanying drawing, figure 1, is a graph showing the correlation between the enzyme (NAGase) amount and absorbance change in Example 3. The present inventors carried out intensive investigations into the electron state of a benzene ring, and the stereospecific reactivity to obtain a nitrophenol compound instead of the above-described p-nitrophenol, and consequently, found that 3,4-dinitrophenol is suitable for the above-mentioned purpose from the standpoint that it is a derivative having a necessary acidity and does not have a bulky substituent at a position adjacent to the phenolic hydroxyl group, and synthesized various substrates with a 3,4-dinitrophenol bound to the end of a molecule to thereby accomplish the present invention. The 3,4-dinitrophenol per se to be used in the present invention is a known compound, but no report has been made public about the synthesis of N-acetyl-β-D-glucosaminide having this compound (bound) at the end of a molecule, and the properties are also unknown. Further, no report at all has been made about the utilization thereof for the substrate for an activity assay of enzymes in a living body, such as NAGase and phosphatase, and the substrate for measuring the activity of a labeled enzyme such as β-D-galactosidase. The 3,4-dinitrophenol to be used in the present invention is easily dissolved when bound to the end of a substrate molecule, and can be used as the enzyme substrate without the need for a dissolving auxiliary agent. Namely, when 3,4-dinitrophenol is bound to the end of N-acetyl-β-D-glucosaminide, an aqueous solution with a concentration having an extremely high practical applicability of 12 mM can be prepared, but the solution is very stable when stored at low temperatures. In contrast, for example, 2-chloro-4-nitrophenol has an acidity of pk = 5.5, but 2-chloro-4-nitrophenyl-N-acetyl-β-D-glucosaminide prepared therefrom is difficult to dissolve, and when introducing same into an assaying kit, a sufficient solubility must be obtained by an addition of a solubilizing auxiliary agent such as cyclodextrin and crown ether, and therefore, the performance thereof as the reagent for the assay is not satisfactory. By introducing a 3,4-dinitrophenyl group instead of the substrate having a p-nitrophenoxy group, at the end of a molecule, as in the prior art (with e.g. p-nitrophenol as the aglycon), the present invention (i) improves to a great extent the solubility and stability (storability) of the substrate in an aqueous solution of the acting pH of enzyme (neutral or acidic side), and (ii) makes the indication (absorbance measurement) pH of the measurement index substance (3,4-dinitrophenol) derived from of the enzyme reaction identical to or close to the acting pH of enzyme, and thus a correct and precise rate analysis is obtained. Examples The present invention is described with reference to the Examples, but of course the scope of the present invention is not limited to these Examples. Example 1 Synthesis of substrate of NAGase, 3,4-dinitrophenyl-N-acetyl-β-D-glucosaminide According to the method of Holleman (Receuil des Travaux Chimiques des Pays-Bas, Vol. 21, p. 432 - 447, 1902), starting from 20 g of m-nitrophenol, via nitration with conc. nitric acid and separation from the side reaction products, 12 g of a pure 3,4-dinitrophenol was synthesized. The melting point was 134°C, which coincided with the value given in the literature supra . Then, from 1 g of the 3,4-dinitrophenol synthesized above, a powder of potassium 3,4-dinitrophenolate was prepared and was refluxed together with 2 g of 2-acetamide-2-deoxy-3,4,6-tri-O-acetyl-α-1-chloro-D-glucosaminide in dry acetone for about 3 hours, the acetone was distilled off, the condensed product dissolved in chloroform, and an unreacted substance 3,4-dinitrophenolate removed with a dil. alkali to give 1.5 g of the condensed product, which was purified by recrystallization from ethanol. Finally, the purified condensed product was dissolved in methanol, and deacetylated with a small amount of sodium methylate as the catalyst, to give substantially quantitatively 3,4-dinitrophenyl-N-acetyl-β-D-glucosaminide, which was purified by recrystallization from hot ethanol. The 3,4-dinitrophenyl-N-acetyl-β-D-glucosaminide (hereinafter also called 3,4-DNG) was found to have sufficient water solubility to be used as the substrate, and an aqueous solution with a concentration of 12 mM sufficient for a NAGase activity assay was easily prepared. Also, the aqueous solution was found to be stable under ice-cooling, with only about a 1% decomposition occurring when left to stand at room temperature overnight, and the decomposition ratio under enzyme reaction conditions (pH 5, 37°C) for one hour being only 1.5%. The spectrum had a strong absorption at UV-ray 290 nm, but exhibited substantially no absorption at a visible portion 400 nm, and thus was optically remarkably distinguishable. Example 2 Synthesis of 3,4-dinitrophenyl-β-D-aalactoside, 3,4-dinitrophenyl-β-D-glucuronide and 3,4-dinitrophenylphosphoric acid, and enzyme reaction Using 3,4-dinitrophenol synthesized according to the same method as in Example 1, and 2,3,4,6-tetra-O-acetyl-α-1-chloro-D-galactoside and 2,3,4-tri-O-acetyl-α-1-chloro-D-glucuronide-6-methyl ester, 1.3 g and 0.7 g of the condensed products were obtained, respectively, and after recrystallization from ethanol, deacetylated in the same manner as in Example 1, followed by purification by a recrystallization again from ethanol. Similarly, 3,4-dinitrophenol was reacted with 2 g of each of POCl₃ and H₃PO₄ in pyridine, to obtain 1.5 g of the condensed product, which was purified by recrystallization as sodium salt. Both of the above had solubilities of at least 10 mM, and the respective enzyme activities were assayed with urine and serum as the test samples, using substrates for an activity assay of β-D-galactosidase, β-D-glucuronidase, and phosphatase in a 20 mM phosphate buffer (pH 6.5) for galactosidase (derived from E. Coli), and in a 50 mM acetate buffer (pH 5.0) and a 40 mM citrate buffer (pH 5.1) for glucuronidase and acidic phosphatase. Example 3 Assay of NAG activity A reaction mixture containing 1.5 ml of a 0.2 M citrate buffer (pH 5) and 1.2 mM of 3,4-DNG (made up to a volume of 3 ml by an addition of an enzyme solution during the reaction) was equilibrated at 37°C, set in a spectrophotometer, and then 100 µl of an NAGase solution (enzyme of human placenta manufactured by Sigma, with activity values according to the standard assay method, Method in Enzymology, Vol 28, p. 772 - 776, prepared to 5 kinds of 0, 50 U/liter, 100 U/liter, 200 U/liter and 300 U/liter) was added, and the change with lapse of time of the absorbance at the wavelength of 400 nm was immediately monitored, to obtain the results shown in Fig. 1. A proportional relationship can be seen between the reaction rate corrected with the control, and the enzyme amount, thus indicating that the rate assay was made correctly. The change with a lapse of time of the absorbance without an enzyme addition (control) showed a high reproducibility, and the subsequent assay was continued after once taking the control. Example 4 Assay method of NAGase using a reagent containing 3,4-DNG (1) Preparation method 3,4-DNG 1.2 mM Citrate buffer (pH 5.0) 0.1 M (2) Operational method Into 150 µl of a test sample was added 3 ml of the reagent heated at 37°C for 5 minutes, to carry out the reaction, and the absorbance change at 400 nm per unit time was measured with the reagent blank as a control. (3) Results The calibration curve when the standard products were employed exhibited a linearity up to at least 200 U/liter, and the correlation with the 2-chloro-4-nitrophenyl-N-acetyl-β-D-glucosaminide substrate method also was good. The NAGase activity assay values in living body fluids were as shown in Table 2. Method of Invention 2-CNP-NAG method Urine-1 5.9 U/liter 5.2 U/liter Urine-2 8.5 U/liter 8.3 U/liter Urine-3 2.8 U/liter 2.6 U/liter Urine-4 10.4 U/liter 10.0 U/liter Note: 2-CNP-NAG method refers to 2-chloro-4-nitrophenyl-N-acetyl-β-D-glucosaminide substrate method. Example 5 Acidic phosphatase activity assay method (1) Preparation of reagent 3,4-Dinitrophenylphosphoric acid 10 mM Citrate buffer (pH 5.1) 40 mM (2) Operational method Into 20 µl of a test sample was added 200 µl of the reagent heated at 37°C for 5 minutes, to carry out the reaction, and the absorbance change at 400 nm per unit time was measured with the reagent blank as a control. (3) Results The calibration curve when the standard products were employed exhibited a linearity up to at least 200 U/liter, and the correlation with the p-nitrophenylphosphoric acid substrate method also was good. The acidic phosphatase activity assay values in living body fluids were as shown in Table 3. Method of Invention PNP-phosphoric acid method Serum-1 3.8 U/liter 3.5 U/liter Serum-2 7.2 U/liter 7.1 U/liter Serum-3 14.2 U/liter 13.5 U/liter Serum-4 2.5 U/liter 2.5 U/liter Note: PNP-phosphoric acid method refers to p-nitrophenylphospholic acid substrate method Example 6 Comparison of assay precision (prepared as reagent for autoanalyzer) (1) Reagents used (A) 3,4-DNG method (Method of Present Invention) (1) Preparation of reagent R-1 Citrate buffer (pH 5.0) 0.1 M R-2 0.1 M citrate buffer (pH 5.0) containing 6 mM 3,4-DNG (2) Operational method After 20 µl of the test sample and 0.4 ml of R-1 were heated at 37°C for 5 minutes, 0.1 ml of R-2 was added to carry out the reaction, followed by a measurement of the change in absorbance at 400 nm. (B) 2-Chloro-4-nitrophenyl-N-acetylglucosaminide method Mayassay NAG-R R (sold by Sanko Junyaku) was used in accordance with the manufacturer's instructions. (C) Results (comparison as CV value) Method A Method B 2 U/liter 10.8% 83.4% 51 U/liter 3.7% 4.9% A great difference appears in the assay precision near the normal value (several U/ml) of NAGase in urine, and the assay results show the advantage of the method of the present invention having an excellent substrate solubility. The assay reagent compositions in the above examples are general, and various applications are possible for those skilled in the art. For example, it is possible to conveniently add various surfactants to prevent turbidity in the test sample, to add antiseptics, stabilizers, and further, to add cyclodextrin or various derivatives thereof, for enhancing the color formation sensitivity, relatively reducing the reagent blank value, migrating the 3,4-dinitrophenol pKa value to the acidic side, and increasing the implemented S/N ratio. Example 7 Sensitization effects with various cyclodextrins (CD) The cyclodextrins (CD) shown below were prepared to a final concentration of 2.4 mM, and the sensitizing effects thereof on 3,4-DNP in a 0.1 M citrate buffer (pH 5.0) are shown. CD Sensitization ratio (%) No addition 100 α-CD's 103 - 104 β-CD's 111 - 146 γ-CD's 106 For β-CD with a high sensitization efficiency, the NAGase is measured based on Example 3, to obtain a similar good sensitization effect. Particularly, for DEAE(diethylaminoethyl)-β-CD with a high water solubility and high sensitization effect, a 2-fold or higher elevation of the sensitivity is exhibited at a final concentration of 24 mM. The essence of the present invention resides in an assay of activity of enzymes as listed in Claim 1 which makes use of substrates having at the end of a molecule a 3,4-dinitrophenol group, which is little affected by the pH and temperature and exhibits an excellent solubility as aglycon and further, includes the applications described above. The substrate for an enzyme reaction in a sample taken from a living body, the enzyme activity assay method, and the reagent for use in the assay according to the present invention, as described above, have a particular effect of greatly improving the precision of a rate assay of NAGase and phosphatase, β-D-glucuronidase, and β-D-galactosidase.

Claims: A method of assaying the activity of any one of the enzymes N-acetyl-β-D-glucosaminidase or N-acetyl-β-D-hexosaminidase, an acidic phosphatase, β-D-galactosidase or β-D-glucuronidase, which comprises measuring the color indication (absorbance) of 3,4-dinitrophenol formed by the action of the enzyme on a substrate molecule having a 3,4-dinitrophenoxy group at the end of the molecule. A method according to claim 1 wherein the enzyme activity to be assayed is that of N-acetyl-β-D-glucosaminidase or N-acetyl-β-D-hexosaminidase in a sample taken from a living body, and the substrate is 3,4-dinitrophenyl-N-acetyl-β-D-glucosaminide or 3,4-dinitrophenyl-N-acetyl-β-D-hexosaminide. A method according to claim 1 wherein the enzyme activity to be assayed is an acidic phosphatase activity, and the substrate is 3,4-dinitrophenylphosphoric acid or a salt thereof. A method according to claim 1, wherein the enzyme activity to be assayed is an activity of β-D-glucuronidase in a sample taken from a living body, and the substrate is 3,4-dinitrophenyl-β-D-glucuronide. A method according to claim 1, wherein the enzyme activity to be assayed is an activity of β-D-galactosidase, and the substrate is 3,4-dinitrophenyl-β-D-galactoside. A method of assaying an enzyme activity as claimed in any one of claims 1 to 5, carried out in the presence of cyclodextrin.

Inventors: HISADA RYUKI, SHIBATA HIDETO, YAGI TATSUHIKO, HISADA, RYUKI, SHIBATA, HIDETO, YAGI, TATSUHIKO
Applicants: IATRON LAB, IATRON LABORATORIES, INC.
Application year: 1990
Main classification: C12Q1
IPC codes: C12Q1