Evidence for Reversible Inactivation of Induced Tyrosine Aminotransferase in Rat Liver in Vivo

Induction of tyrosine aminotransferase (EC 2.6.1.5) with cortisol is followed by a rapid phase of inactivation of the enzyme, during which administration of puromycin causes a sharp increase in enzyme activity. The increase is not due to puromycin-resistant synthesis of protein, since precipitation of the enzyme with antibodies to tyrosine aminotransferase after pulse labeling inhibited incorporation of radioactive amino acids. Reactivation of the enzyme was specific, since the structurally similar dimethylaminopurine and purine aminonucleoside were ineffective. An action of puromycin other than its capacity to inhibit protein synthesis was required for reactivation, since cycloheximide did not demonstrate such an effect. In vitro studies with free and bound polysomes, isolated during the phase of inactivation, indicated that reactivation by puromycin was not due to release of nascent tyrosine aminotransferase peptides. We conclude that the enzyme is initially reversibly inactivated; therefore, the overall degradation of this intracellular protein is a multistep phenomenon.