Selective proteolysis of cytosolic aspartate aminotransferase by a new microbial protease

Proteinase K inactivation of cytosolic aspartate aminotransferase isoenzyme for measurement of human serum mitochondrial aspartate aminotransferase

We studied a new proteinase K assay method for human serum mitochondrial aspartate aminotransferase. We found that proteinase K showed no inactivation of human mitochondrial aspartate aminotransferase isoenzyme and complete inactivation of cytosolic aspartate aminotransferase. Previous studies have shown that selective proteolytic measurement for mitochondrial aspartate aminotransferase in serum using the protease 401 cleaved peptide bond at Leu 20 from the amino-terminal bond shows complete inactivation of cytosolic aspartate aminotransferase and slight inactivation of mitochondrial aspartate aminotransferase isoenzyme, depending on protease concentration. In this investigation, we found that the proteinase K method does not depend on protease concentration. The proteinase K enzyme inactivation of cytosolic aspartate aminotransferase is caused by the cleavage of the peptide bond at Ileu 21 from the aminoterminal bond. In studies with various animal cytosolic aspartate aminotransferase isoenzymes, proteinase K almost completely inactivated cytosolic aspartate aminotransferase. Precision and correlation using proteinase K for measurement of serum mitochondrial aspartate aminotransferase in human showed a good coefficient of variation (within-run < 4.45%) and a coefficient of correlation of r = 0.985 (N = 125).

A homogeneous assay system of aspartate aminotransferase iso-enzymes using proteases and application for clinical evaluation of myocardial infarction

We designed a rapid, homogeneous assay for human aspartate aminotransferase (AST) isoenzymes, by a selective proteolysis of soluble AST (s-AST), using chymotrypsin and protease 401. The linearity of mitochondrial (m-AST) was elongated up to 4000 U/l. m-AST values from the human liver, and determined by a homogeneous assay using protease 401 or chymotrypsin, were relative to those obtained using an immunoprecipitation method. In perioperative patients or those with an acute myocardial infarction, the peaks of s-AST and m-AST values were noted 13 h and at 57 h after ictus, respectively, whereas the peak of ratio between was seen 6 h after ictus. In the case of Budd-Chiari syndrome, the maximum levels of the two AST activities were evident 14 days after hospitalization and the peak of ratio between them was seen after 7 days. We propose that this homogeneous assay can serve as a diagnostic tool for early phase detection of myocardial infarction and of Budd-Chiari syndrome.

Proteolytic measurement of mitochondrial aspartate aminotransferase in human serum

A new proteolytic measurement of serum mitochondrial aspartate aminotransferase was evaluated using cytosolic aspartate aminotransferase inactivating protease. Some of the proteases, such as, alpha-chymotrypsin, subtilisin and cytosolic aspartate aminotransferase inactivating protease 401 from Streptomyces species, also specifically inactivated cytosolic aspartate aminotransferase, but not mitochondrial, aspartate aminotransferase. The protease 401 was the most heat stable for storage and showed a higher inactivation rate for cytosolic aspartate aminotransferase--up to 7000 IU/L--more than 200-fold the upper limit. The coefficient of variation of the proteolytic method was less than 10%. Results by the present method correlated with those by the immunochemical method (r = 0.970) and the regression curve was Y = 0.95X + 1.60 (Y: immunochemical method; X: proteolytic method). In the present assay system, reference values for mitochondrial aspartate aminotransferase activity in 500 healthy people ranged from 2.0-7.2 U/L (mean 3.8 U/L).

Optimal conditions for protease use in the assay of serum mitochondrial aspartate aminotransferase

The optimal conditions for selective proteolytic inactivation of cytosolic aspartate aminotransferase (c-AST) to determine mitochondrial aspartate aminotransferase (m-AST) in serum were studied. Protease 401 was found to be effective over a pH range of 6.0-10.0. A pH of 9.5 with 0.5% albumin in the reagent mixture was determined to be optimal for inactivation of c-AST and preservation of m-AST, lactic dehydrogenase (LDH), and malic dehydrogenase (MDH) in the assay procedure. The presence of serum endogenous protein inhibitors such as alpha 1-antitrypsin and alpha 2-macroglobin did not inhibit protease 401.

Determination of human aspartate aminotransferase isoenzymes by their differential sensitivity to proteases

The effect of various proteases (trypsin, chymotrypsin, subtilisin, protease 401, and thermolysin) on the mitochondrial isoenzyme (m-AST) and cytoplasmic isoenzyme (c-AST) of human and swine aspartate aminotransferase (AST;EC 2.6.1.1) was evaluated. All procedures including the reaction with proteases and the subsequent determination of the AST activity were carried out in an automatic analyzer. The mammalian c-AST was efficiently inactivated by chymotrypsin, subtilisin and protease 401 while m-AST activity decreased very slowly with these proteases. Thermolysin and trypsin showed much less effect on c-AST activity. Especially, chymotrypsin at concentrations of 0.5-1.0 g/L inactivated human c-AST almost completely but showed no detectable inactivating effect on m-AST. Thus chymotrypsin appears to be the most suitable protease for the differential determination of AST isoenzymes in human serum. Further studies on the effects of proteases with AST from other species showed that Escherichia coli AST resembled mammalian m-AST while Pseudomonas AST resembled c-AST.