The regulation of hepatic tyrosine aminotransferase.

Tyrosine aminotransferase induction has been studied in hepatocytes from untreated, partially and fully glucocorticoid-induced rats: enzyme activities were initially 12.9 +/- 1.7 (n = 16), 41.4 +/- 3.2 (n = 6) and 117.9 +/- 10.5 (n = 7) munits/mg protein, respectively. Untreated or fully induced hepatocytes maintain initial levels, whereas partially induced hepatocytes increase their tyrosine aminotransferase activity even in the presence of actinomycin D. Fully induced hepatocytes possess a normal protein synthetizing machinery and the mechanisms to degrade selectively tyrosine aminotransferase. The effect of progesterone treatment is consistent with these cells retaining a high dexamethasone level. Glucagon induces tyrosine aminotransferase via its second messenger, cyclic AMP. This induction decreases dramatically with in vivo glucocorticoid treatment. Time courses and effects of inhibitors are consistent with these in vivo and in vitro treatments being alternative methods of inducing tyrosine aminotransferase by the same basic pretranslational step.