THE REGULATION OF ALANINE AND ASPARTATE-AMINOTRANSFERASE BY DIFFERENTAMINOTHIOLS AND BY VITAMIN-B-6 DERIVATIVES

We examined the effects on alanine aminotransferase and aspartate amin otransferase of different aminothiols (L-cysteine, D-cysteine, cysteam ine, L-cysteine ethyl ester, L-cysteine methyl ester) and several vita min B-6 derivatives (pyridoxal, pyridoxamine, pyridoxol, pyridoxol 5'- phosphate), before and after treatment with KOCN, which transforms the se molecules into the corresponding carbamoyl derivatives. Only GPT, a nd not GOT, was specifically inhibited by L-cysteine and, to a lesser extent, by D-cysteine. The association reaction: PLP + apo GPT <-> hol e GPT was inhibited by the vitamin B-6 derivatives, and this inhibitio n was prevented by pretreatment of the vitamin B-6 derivatives with KO CN. All the observed effects occurred at pH 7, 37 degrees C, at mM and even lower concentrations of reagents. Hence, they all potentially pl ay a physiological role, in the regulation of the PLP dependent enzyme s and of the vitamin B-6 levels in the cell.