STUDIES ON SOLUBLE AND MITOCHONDRIAL TYROSINE AMINOTRANSFERASE - EVIDENCE FOR A PHYSICAL CHANGE IN CYTOSOL ENZYME DURING INDUCTION

Tyrosine aminotransferase acts on tyrosine or monoiodotyrosine (MIT) as substrate. The ratio of the initial velocities for the reactions with tyrosine and monoiodotyrosine has been measured under various conditions in vivo. When the enzyme is induced by cortisol, this ratio increases due to the reduction of the relative rate of MIT transamination. In contrast, purification of the cytosol enzyme decreases the tyrosine:MIT ratio. The mitochondrial enzyme, by its utilization of oxaloacetate as amino acceptor and its selective inhibition by aspartate, can be distinguished from the cytosol enzyme. Since the mitochondrial enzyme is not responsible for the change in ratio of activities with the two substrates, induction in vivo may involve the formation of physical conformers of the cytosol tyrosine aminotransferase. © 1969.