The outer carnitine palmitoyltransferase and regulation of fatty acid metabolism in rat liver in different thyroid states.

The activity of the outer carnitine palmitoyltransferase (EC 2.3.1.21) and the carnitine-dependent oxidation of palmitoyl-CoA was increased 3-4 fold in liver mitochondria from hyperthyroid rats as compared with hypothyroid rats. The inhibitory effect of malonyl-CoA on carnitine-dependent fatty acid oxidation was preserved in all thyroid states, but decreased in hyperthyroid mitochondria. Fasting for 24 h increased the activity of the outer carnitine palmitoyltransferase about 50% in hypothyroid liver mitochondria, whereas it had no significant effect in hyperthyroid mitochondria. The thyroid state had no significant effect on total carnitine palmitoyltransferase in liver mitochondria. Fasting stimulated fatty acid oxidation 3-4-fold in isolated hepatocytes from hypothyroid rats, whereas it had no effect in hyperthyroid rats. Feeding 0.3% clofibrate to euthyroid rats about doubled the activity of the total carnitine palmitoyltransferase, whereas it had no effect on the outer transferase. The regulation of fatty acid oxidation in the liver is discussed.