Regulation of ornithine aminotransferase by cyclic AMP and glucose in primary cultures of adult rat hepatocytes.
Hepatic ornithine aminotransferase (EC 2.6.1.13) (OAT) is a mitochondrial matrix enzyme that plays a role in amino acid catabolism and in gluconeogenesis. In rats, the synthesis of hepatic OAT is regulated by glucagon, dietary protein, and glucose. Serum-free primary cultures of adult rat hepatocytes were used to demonstrate that glucagon, cyclic AMP, and glucose are able to alter OAT synthesis by a direct action on hepatocytes. The rates of OAT synthesis were measured by immunoprecipitation of pulse-labeled OAT with an affinity-purified monospecific antibody. Ten hours after cyclic AMP addition to the culture medium, the relative rate of OAT synthesis reached a peak value that was six- to eightfold above the control rate. OAT activity accumulated more slowly, reaching a level that was approximately threefold above the control by 24 h. The inclusion of glucose in the culture medium inhibited the increases in OAT synthesis and activity in a dose-dependent manner. Although synthesized as a precursor (pOAT), no pOAT was detected under control, induced, or carbohydrate-inhibited conditions; this suggests that pOAT processing may not be a regulatory site of OAT expression. By following the loss of labeled OAT, a half-life of 34 h in these cultures under all of the above conditions was observed. Regulation of OAT levels in cultured hepatocytes appears to be achieved primarily through changes in the rate of OAT synthesis.
Merrill
M J
MJ
Pitot
H C
HC
D000818:Animals; D003994:Bucladesine / Q000494:pharmacology; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000502:physiology*; D004044:Dietary Proteins / Q000494:pharmacology; D004790:Enzyme Induction / Q000187:drug effects; D005632:Fructose / Q000494:pharmacology; D005934:Glucagon / Q000494:pharmacology; D005947:Glucose / Q000494:pharmacology / Q000502:physiology*; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D008297:Male; D009953:Ornithine-Oxo-Acid Transaminase / Q000096:biosynthesis*; D051381:Rats; D000637:Transaminases / Q000096:biosynthesis*
D004044:Dietary Proteins; D005632:Fructose; D003994:Bucladesine; D005934:Glucagon; D000242:Cyclic AMP; D000637:Transaminases; D009953:Ornithine-Oxo-Acid Transaminase; D005947:Glucose