Regulation of malic enzyme and mitochondrial alpha-glycerophosphate dehydrogenase by thyroid hormones, insulin, and glucocorticoids in cultured hepatocytes.

Hepatocytes isolated from normal adult rats were cultured under serum-free conditions. Induction of mitochondrial alpha-glycerophosphate dehydrogenase (glycerol 3-phosphate dehydrogenase) (EC 1.1.99.5; sn-glycerol-3-phosphate: (acceptor) oxidoreductase) and cytosolic malic enzyme (EC 1.1.1.40; L-malate-NADP+ oxidoreductase (decarboxylating)) by 3,3'-5-triiodo-L-thyronine (triiodothyronine) in the culture medium follows the same time course as the in vivo response to thyroid hormones. The addition of 1 microM cycloheximide blocks the triiodothyronine response. Thyroxine is also capable of stimulating the activities of both enzymes. Although increases in alpha-glycerophosphate dehydrogenase and malic enzyme activities are observed when triiodothyronine is added to the culture medium for 3 days (62% and 36%, respectively), in the presence of insulin and cortisol the response is significantly greater. Dexamethasone is more potent than cortisol in increasing triiodothyronine action. In the presence of bovine serum albumin, to prevent metabolism of triiodothyronine, hepatocytes show increased enzyme activity at concentrations as low as 10(-10) M triiodothyronine.