Insulin selectively slows the degradation rate of tyrosine aminotransferase.

Glucocorticoids induce a selective 5to lo-fold increase in the rate of synthesis of tyrosine aminotransferase in rat hepatoma cells in tissue culture. Addition of insulin to steroid-induced cells stimulates a further 2to 3-fold increase in the cellular concentration of this enzyme. The induction by insulin occurs after a lag of only 6 min and does not require concomitant RNA synthesis. Insulin also causes a 35% increase in total protein synthesis and a 10 to 25% decrease in the rate of total protein degradation. Therefore, insulin might induce transaminase by selectively increasing its rate of synthesis, or decreasing its rate of degradation, or both. In order to distinguish among these alternatives, we have measured rates of synthesis and degradation of tyrosine aminotransferase by specific radioimmunoprecipitation techniques using sheep anti-rat liver transaminase antiserum. The enzyme was identified by electrophoresis of the solubilized immunoprecipitate on sodium dodecyl sulfate-polyacrylamide gels. Under conditions in which insulin stimulated a 100% increase in the cellular concentration of transaminase, it caused only a 35% increase in the rate of synthesis of this enzyme as well as of total protein. In contrast, insulin caused a selective 2-fold decrease in the rate of degradation of tyrosine aminotransferase relative to total protein under these same conditions. We conclude that insulin induces tyrosine aminotransferase in rat hepatoma cells primarily by selectively slowing the rate of degradation of the enzyme. The a-fold increase in cellular concentration of this enzyme reflects a a-fold increase in its half-life rather than a selective increase in its rate of synthesis.