Hormonal and substrate induction of tryptophan pyrrolase in regenerating rat liver.

The induction of tryptophan pyrrolase by hydroeortisone has been studied in rats following 70% hepatectomy. An alteration in the rate of induction was observed in both sham-operated and 70% hepatectomized rats early in the postoperative period. Normal maximal values were eventually reached, however. Later in the postoperative period, induction patterns were similar to those observed in unoperated rats. During the period in which DNA synthesis is known to occur following 70% hepatectomy, the induction of tryptophan pyrrolase by hydrocortisone was depressed in the 70% hepatectomized but not in the shamoperated rats. At this latter time period, induction of tryptophan pyrrolase by L-tryptophan was not diminished in the regenerating liver. It has been suggested by other workers that alterations in enzyme control mechanisms, such as the absolute or relative failure of tryptophan pyrrolase induction by cortisone in minimaldeviation hepatomas, may represent a fundamental biochemical alteration in tumors. This study shows that a depressed response to hormonal enzyme induction also occurs in non-neoplastic hepatic cells preparing to divide.