LDH1 assay

Abstract: A process for the determination of an LDH₁ fraction in a sample is disclosed which comprises inhibiting LDH₂, LDH₃, LDH₄ and LDH₅ in the sample with a protease in the presence of a protein-denaturating agent and then determining the remaining uninhibited LDH₁.

Description: The present invention relates to a process for the determination of an LDH₁ fraction in a sample. LDH (lactate dehydrogenase) has five isozymes, that is LDH₁ through LDH₅. Each organ has its own composition of these isozymes. For example, LDH₁ is present in the myocardium in the largest amount. Since LDH₁ escapes from the myocardium into the blood during myocardial infarction, the increase of the LDH₁ serum level can be used for the diagnosis of such a disease. Therefore, an LDH₁ isozyme assay is clinically significant. In the most common LDH isozyme assay, LDH₁, LDH₂, LDH₃, LDH₄ and then LDH₅ are fractionated in the order of electrophoretic mobility. An immunological LDS assay is also known. In another LDH assay disclosed in Japanese Patent Kokai Publication No. 6477/1983, a coenzyme derivative is used. In addition, LDH assays wherein a sample is treated under alkaline conditions are described in Japanese Patent Publication No. 28280/1985 and Japanese Patent Kokai Publication No. 278997/1987. The above-mentioned electrophoretic or immunological assay is unsuitable for clinical autoanalysis because of the complicated process and a long operation time. In addition, the LDH isozymes may be insufficiently fractionated by the electrophoretic method. Strictly speaking, it is impossible to assay the LDH isozyme fractions by the method wherein the coenzyme derivative is used, while the ratio of the subunit to the M subunit in the enzyme can be suitably determined by the method. In the method comprising the alkaline treatment of the sample, more than 50 % of LDH₁ may be inactivated during the inhibition of the other isozymes. In addition, the process is difficult and time-consuming. Thus, the above-mentioned conventional LDH assays are unsatisfactory for clinical examination and diagnosis. Thus, an object of the present invention is to provide an LDH₁ assay useful for the simple, fast and reliable clinical examination or diagnosis including autoanalysis. Accordingly, the present invention provides a process for the determination of an LDH₁ fraction in a sample, which comprises inhibiting LDH₂, LDH₃, LDH₄ and LDH₅ in the sample with a protease in the presence of a protein-denaturating agent and then determining the remaining uninhibited LDH₁. The present process is suitable for the determination of the LDH₁ fraction in a clinical sample such as serum or plasma. The protease used in the present invention may be any protease suitable for the purpose of the invention. For example, α-chymotrypsin, trypsin, thrombin, elastase, endoprotease, subtilisin, bromelain, papain, carboxypeptidase, pronase, pepsin or cathepsin can be used. The proteases disclosed in Japanese Patent Kokai Publication No. 149399/1985 may also be used. Although the amount of the protease is not critical in the present invention and varies with other assay conditions, α-chymotrypsin, for example, can be used in a concentration in the range of from 10 to 1,000 units/ml. In the present invention, any protein-denaturating agent having an activity suitable for the process can be used. For example, cholic acid, deoxycholic acid, taurocholic acid, taurodeoxycholic acid, guanidine (hydrochloride), trichloroacetic acid, urea, thiourea or thiocyanate are preferably used as the protein-denaturating agent as such or in the form of an admixture. Although the amount of the protein-denaturating agent is not critical and varies with other assay conditions, for example 0.05 to 5 M guanidine can be used. The LDH₁ isozyme remaining uninhibited can be determined according to an enzyme assay selected from a lot of conventional assays widely used in clinical examinations. Such an assay used in the present process should be easily carried out within a short time with high precision, sensitivity and accuracy. In a preferred embodiment of the present process, the LDH₁ isozyme remaining uninhibited is determined by an enzyme assay which comprises catalytically developing a chromogen or dye precursor with the isozyme and then measuring absorption in the visible light range. It is also possible to measure ultraviolet absorption of coenzyme NADH reduced by a catalytic effect of the LDH₁ isozyme. In another preferred embodiment, the LDH₁ isozyme is determined according to the Wroblewski method which is a well-known enzyme assay performed under neutral conditions. It is found that the inhibition of the isozymes LDH₂₋₅ can be effected in a wide pH range according to the present invention (cf. Example 1). Therefore, when the determination of the LDH₁ isozyme remaining uninhibited is performed under a neutral condition, the present assay can be advantageously carried out under the same neutral condition throughout the assay, while complicated pH correction is necessary in the above-mentioned conventional assay comprising the alkaline treatment of the sample. The present invention is further illustrated by the following Examples. However, the invention is not limited to these Examples. Example 1 To an authentic sample (0.04 ml) of each of the isozymes LDH₁₋₅, which was obtained by purifying human blood, a buffer (0.2 ml) containing α-chymotrypsin (450 units/ml) and 0.65 M guanidine was added and it was incubated at 37°C for 5 minutes. The buffers used and the pH values thereof were as shown in the following Table 1. Then, after the addition of a 0.1 M Tris buffer (2.5 ml) containing 2 mM sodium pyruvate and 0.2 mM NADH, by which the pH value was regulated at 7.8, the changing rate of the absorbance per minute was measured at 340 nm. Residual activity of each isozyme was calculated from the measured value. The results are shown in the following Table 1. It was found that only the LDH₁ fraction among the isozymes LDH₁₋₅ could be determined by the present assay and that the LDH₁ isozyme remained uninhibited at any pH value during the inhibition of the other isozymes. Therefore, when the LDH₁ isozyme remaining uninhibited is determined under neutral conditions, the present assay can be advantageously carried out under these neutral conditions throughout the assay without complicated pH correction. pH (Buffer) Residual activity (%) of LDH₁ LDH₂ LDH₃ LDH₄ LDH₅ 6 (MES) 83 1 1 0 0 7 (PIPES) 85 2 0 1 0 8 (Tris) 83 2 1 0 0 9 (CHES) 80 1 0 0 0 10 (CHES) 74 1 0 0 0 11 (CAPS) 69 0 0 0 0 Example 2 To the same sample (0.04 ml) as used in Example 1, a 0.1 M Tris buffer (pH 7.8; 0.2 ml) containing subtilisin (5 units/ml) and sodium deoxycholate (1.5 %) was added and incubated at 37 °C for 5 minutes. Then, after the addition of a 0.1 M Tris buffer (pH 7.8; 2.5 ml) containing 2 mM sodium pyruvate and 0.2 mM NADH, the residual activity of each isozyme was determined in the same way as in Example 1. The results are shown in Table 2. It was found that the present LDH₁ assay could be effectively carried out at pH 7.8. Residual activity (%) of LDH₁ LDH₂ LDH₃ LDH₄ LDH₅ 83 0 0 0 0 Example 3 As the sample, 15 human sera (each 0.04 ml) were used. The total LDH activity of each sample was known as shown in the following Table 3. In the same way as in Example 1, the changing rate of the absorbance per minute was measured. Then, the LDH₁ activity shown in Table 3 was determined with using a calibration curve. Separately, the sera were subjected to a conventional immunological assay for the LDH₁ isozyme (with using Isomune-LD; Rosh). The results are also shown in Table 3. The results of the present process well corresponded with those of the conventional immunological assay. Sample Total LDH activity (units/l) LDH₁ activity (units/l) determined by the present process the conventional immunological assay 1 386 84 87 2 290 85 89 3 305 89 94 4 242 45 44 5 342 113 118 6 2147 1113 1138 7 239 73 69 8 623 181 181 9 387 70 74 10 423 113 109 11 1812 500 483 12 322 79 73 13 906 247 262 14 485 117 115 15 1011 288 287 Example 4 To the same sample (10 µl) as used in Example 1, a 50 mM MES buffer (pH 6.0; 75 µl) containing 0.5 M guanidine hydrochloride, 2 % Triton X-405 and a protease shown in the following Table 4 was added. After incubating at 37 °C for 5 minutes, a 0.1 M Tris buffer (PH 8.0; 400 µl) containing 2 mM sodium pyruvate and 0.2 mM NADH was added. Then, the residual activity of each isozyme was determined in the same way as in Example 1. The results are shown in the Table 4. It was found that the LDH₁ fraction could be determined with any protease, preferably with carboxypeptidase Y, cathepsin C or α-chymotrypsin. Residual activity (%) of LDH₁ LDH₂ LDH₃ LDH₄ LDH₅ Pepsin 41 22 5 0 0 Carboxypeptidase P 96 63 60 60 6 Carboxypeptidase Y 100 1 0 0 0 Cathepsin 80 2 1 1 1 α-Chymotrypsin 99 11 2 2 1 Example 5 To the same sample (0.04 ml) as used in Example 1, a Tris buffer (pH 8.0; 0.2 ml) containing either α-chymotrypsin (450 units/ml) or 0.65 M guanidine was added and it was incubated at 37 °C for 5 minutes. Then, after the addition of a 0.1 M Tris buffer (2.5 ml) containing 2 mM sodium pyruvate and 0.2 mM NADH, by which the pH value was regulated at 7.8, the residual activity of each isozyme was determined in the same way as in Example 1. The results are shown in Table 5. It was found that the LDH₂₋₅ fractions were insufficiently inactivated. Therefore, it is essential in the present assay to use a protease in the presence of the protein-denaturating agent. Residual activity (%) of LDH₁ LDH₂ LD H₃ LD H₄ LD H₅ When guanidine was added without α-chymotrypsin 96 87 79 45 40 When α-chymotrypsin was added without guanidine 100 78 29 2 0

Claims: A process for the determination of an LDH₁ fraction in a sample, which comprises inhibiting LDH₂, LDH₃, LDH₄ and LDH₅ in the sample with a protease in the presence of a protein-denaturating agent and then determining the remaining uninhibited LDH₁. The process of claim 1, wherein the sample is a clinical sample of serum or plasma. The process of claim 1 or 2, wherein the protease is α-chymotrypsin, trypsin, thrombin, elastase, endoprotease, subtilisin, bromelain, papain, carboxypeptidase, pronase, pepsin or cathepsin. The process of claim 3, wherein a-chymotrypsin is used as the protease in a concentration in the range of from 10 to 1,000 units/ml. The process of any one of claims 1 to 4, wherein the protein-denaturating agent is cholic acid, deoxycholic acid, taurocholic acid, taurodeoxycholic acid, guanidine (hydrochloride), trichloroacetic acid, urea, thiourea or thiocyanate or a mixture thereof. The process of claim 5, wherein 0.05 to 5 M guanidine is used as the protein-denaturating agent. The process of any one of claims 1 to 6, wherein the LDH₁ isozyme remaining uninhibited is determined according to an enzyme assay which comprises catalytically developing a chromogen or dye precursor with the isozyme and then measuring the absorption in a visible light range or which comprises measuring the ultraviolet absorption of the coenzyme reduced by the catalytic activity of the isozyme. The process of claim 7, which is carried out under neutral conditions throughout the assay without pH correction.

Inventors: ISSHIKI KENJI, SHIRAHASE YASUSHI, WATAZU YOSHIFUMI, ISSHIKI, KENJI, SHIRAHASE, YASUSHI, WATAZU, YOSHIFUMI
Applicants: INT REAGENTS CORP, INTERNATIONAL REAGENTS CORPORATION
Application year: 1989
Main classification: C12Q1
IPC codes: C12Q1