The functional significance of changes in activity of the enzymes, tryptophan pyrrolase and tyrosine transaminase, after induction in intact rats and in the isolated, perfused rat liver.

The possible functional significance of induction of the hepatic enzymes, tryptophan pyrrolase and tyrosine transaminase, has been studied in intact normal and adrenalectomized adult male rats and in the isolated perfused rat liver. In intact, normal rats, several-fold increases in tryptophan pyrrolase activity induced by treatment with hydrocortisone or tryptophan, or both, were associated with at most a 20% increase in the percentage dose of DL-tryptophan-3-14C converted to 14C02. However, under conditions in which the activity of tryptophan pyrrolase did not increase further, for example, with increasing the tryptophan load from 17.5 to 35 mg, the conversion of labeled tryptophan to 14C02 was substantially increased. In the adrenalectomized rat, hydrocortisone elicited a 6-fold increase in tryptophan pyrrolase activity without changing oxidation of a tracer dose of DMryptophan-3J4C to wo*. Although hydrocortisone added to the labeled tryptophan load led to a further 4-fold increase in tryptophan pyrrolase activity, no change in oxidation of tryptophan to 14COz was seen. In the adrenalectomized rat the percentage dose L-tyrosine1-W converted to 14COZ increased substantially with a IO-mg or 30-mg load of L-tyrosine in spite of the fact that the level of hepatic tyrosine transaminase remains unchanged; in contrast, treatment with hydrocortisone leads to a 7to 14-fold increase of enzyme activity, yet the increase in oxidation of L-tyrosine-l-14C to 14C02 is trivial at several load levels. In the isolated perfused rat liver induction of tryptophan