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.