Abstract
The mitochondrial matrix enzyme, ornithine aminotransferase, is induced in rat liver by the administration of a diet high in protein and by glucagon. The rate of synthesis of the enzyme is increased 100-fold in the livers of rats maintained on a 60% relative to a 0% protein diet, whereas the levels of functional and hybridizable mRNA measured by in vitro translation and through the use of a cloned cDNA probe increased by only 2- to 6-fold and 2- to 3-fold, respectively. Under conditions of glucagon induction that resulted in a 10- to 12-fold increase in the rate of enzyme synthesis, the relative level of functional ornithine aminotransferase mRNA increased by only 2-fold, and the level of hybridizable mRNA actually decreased. The rate of polypeptide chain elongation and the relative number of ornithine aminotransferase nascent chains on polysomes were 2-fold and 23-fold greater, respectively, in hepatocytes derived from 60% relative to 0% protein-fed rats. Using these data, a 23-fold increase in the translational efficiency of the mRNA was calculated. This increase, along with a 2-fold increase in the mRNA level, completely account for the 40-fold increase in the rate of ornithine aminotransferase synthesis observed in hepatocytes derived from 60% protein-fed rats. We conclude that ornithine aminotransferase synthesis is regulated at both a translational and a pretranslational level in rat liver.
Highlights
0%protein diet, whereas the levels of functional and and increases the translationalefficiency of a basal level hybridizable mRNA measured by in vitro translation of mRNA, depending on the administration protocol
We recently reported the cloning of DNA complementary to ornithineaminotransferase mRNA (14)
Respectively, in hepatocytes derived from60%relative does not increase in proportionto the raotef enzyme synthesis to 0%protein-fed rats
Summary
Rate of synthesis, and functional mRNA levels in liver. Rats were maintained on the respective diets for 6 days. Enzyme activities and ratesof synthesis were determined as described under “Experimental Procedures.”Values given aremean 2 S.E. of determinations (3 ratlivers). Total liver poly(A+) RNAwas isolated from 2 rats in each experimentalgroup. The RNA was translated in therabbit reticulocyte lysatesystemandthepercentage of [35S] methionine incorporated into ornithine aminotransferase relative to total trichloroacetic acid-precipitable protein determineddeasscribed under “Experimental Procedures.”. The different experiments represent results with independent RNA preparations. Glucagon (0.5 mg/ 100 g, body w t ) was administered 8 h before killing
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