STUDIES ON CONVERSION OF MULTIPLE FORMS OF TYROSINE AMINOTRANSFERASE IN RAT-LIVER

At least 3 separable forms of the hepatic enzyme, tyrosine aminotransferase [EC 2.3.1.8] exist. The studies reported in this paper demonstrate the existence of a heat-labile, pH- and temperature-dependent, nondialyzable component associated predominantly with the lysosomal and mitochondrial fraction of rat liver which catalyzes the conversion of form II to forms III and IV of the enzyme. The activity of this conversion factor is not significantly affected by F-, [MoO4]2-, or 2 inhibitors of proteases. Cyanate ion completely inhibits the conversion of form II to forms III and IV of tyrosine aminotransferase, as do iodoacetate and oxidized glutathione. p-Chloromercuribenzoate also markedly inhibits the conversion. Kinetic studies suggest that the shift from one form to another follows the pathway: II to III to IV. Titration of the available SH groups of the 3 forms of the enzyme demonstrates that form II possesses 16-17 titratable SH groups/molecule while forms III and IV possesses 15 and 13 or 14, respectively. The possible catalytic mechanism by which the conversion of the multiple forms of tyrosine aminotransferase is accomplished is discussed.