Effects of Estrogen on the Turnover Rates of Ornithine Aminotransferase in Rat Liver and Kidney

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Abstract
The turnover rates of ornithine aminotransferase in the liver and kidney of control rats and those treated with estrogen were determined by injecting L-[14C]leucine (U) and following the decay of specific radioactivity incorporated into immunoprecipitates from the partially purified enzymes. The half-life of ornithine aminotransferase [EC 2.6.1.13] in the liver (t1/2=0.95 days) was significantly different from that of the kidney enzyme (t1/2=4.0 days). Studies on the incorporation of radioactive leucine into ornithine aminotransferase in the kidney under steady-state conditions showed that the rate of synthesis of this enzyme after treatment with estrogen was 5 to 6 times higher than that in untreated animals. The rate constant of degradation of kidney ornithine aminotransferase under steady-state conditions induced by estrogen treatment was not significantly different from that under control conditions. The results showed that the increased amount of kidney ornithine aminotransferase in estrogen-treated rats is due to an increase in the rate of biosynthesis, not to a decrease in the rate of degradation. No significant changes in the rates of biosynthesis and degradation of liver ornithine aminotransferase were observed after administration of estrogen.