Regulation of tyrosine transaminase degradation in the isolated perfused rat liver by cycloheximide and insulin

In the isolated perfused rat liver, synthesis of the enzyme tyrosine transminase (EC 2.6.1.5) is stimulated by either insulin or glucocorticoid hormone. The antibiotic cycloheximide blocks both synthesis and degradation of tyrosine transminase in insulin-treated perfused livers, whereas it blocks only synthesis of the enzyme in glucocorticoid-treated or untreated livers. When insulin is given in additions to cycloheximide, degradation of the enzyme is blocked in glucocorticoid-treated livers as well. The inhibitory effects on degradation are enzyme-specific, since tryptophan oxygenase (EC 1.13.1.12), the synthesis of which is stimulated by glucocorticoid hormone but not by insulin, is degraded at the same rate under all conditions. It is suggested that both cycloheximide and insulin affect tyrosine transminase degradation indirectly, bvy augmenting the amounts of amino acids or other metabolites that inhibit degradation.