Regulation of Tyrosine α-Ketoglutarate Transaminase in Rat Liver

Abstract

Abstract Synthesis of the messenger RNA that codes for tyrosine α-ketoglutarate transaminase (l-tyrosine:2-oxo-glutarate aminotransferase, EC 2.6.1.5) was blocked by actinomycin and its stability was determined in cultured hepatoma cells by measuring rates of transaminase synthesis. By this criterion the transaminase messenger RNA is degraded with a half-life of 2 to 3 hours in both H-35 and HTC cell lines; this rate is not changed by hydrocortisone. Transaminase synthesis is decelerated by high and low concentrations of actinomycin; high concentrations of this antibiotic block transaminase degradation as well. At low concentrations of actinomycin, transaminase induction by hydrocortisone is blocked but induction by insulin is unaffected for a period of time consistent with messenger stability. Preliminary treatment with hydrocortisone primes the cells to respond to insulin with a rapid increase in enzyme, but preliminary treatment with insulin does not alter the response to hydrocortisone. With the use of experimentally derived rates of degradation of both the transaminase and its messenger RNA, a mathematical model was developed which satisfactorily predicts kinetic behavior of the enzyme level upon addition and withdrawal of inducing hormones. The data are consistent with the conclusion that hydrocortisone acts transcriptionally while insulin acts on some posttranscriptional or translational step in enzyme synthesis. We find no indication of posttranscriptional mechanisms in the steroid-mediated induction. A discussion of the mathematical model used is appended.