We examined the effects on alanine aminotransferase and aspartate aminotransferase of different aminothiols (L-cysteine, D-cysteine, cysteamine, L-cysteine ethyl ester, L-cysteine methyl ester) and several vitamin B-6 derivatives (pyridoxal, pyridoxamine, pyridoxol, pyridoxol 5'-phosphate), before and after treatment with KOCN, which transforms these molecules into the corresponding carbamoyl derivatives. Only GPT, and not GOT, was specifically inhibited by L-cysteine and, to a lesser extent, by D-cysteine. The association reaction: PLP + apo GPT<-->holo GPT was inhibited by the vitamin B-6 derivatives, and this inhibition was prevented by pretreatment of the vitamin B-6 derivatives with KOCN. All the observed effects occurred at pH 7, 37 degrees C, at mM and even lower concentrations of reagents. Hence, they all potentially play a physiological role, in the regulation of the PLP dependent enzymes and of the vitamin B-6 levels in the cell.