1. Hepatic carnitine palmitoyltransferase activity was measured over a range of concentrations of palmitoyl-CoA and in the presence of several concentrations of the inhibitor malonyl-CoA. These measurements were made in mitochondria obtained from the livers of fed and starved (24 h) normal rats and of fed and starved thyroidectomized rats. 2. In the fed state thyroidectomy substantially decreased overt carnitine palmitoyltransferase activity and also decreased both the Hill coefficient and the s0.5 when palmitoyl-CoA concentration was varied as substrate. Thyroidectomy did not appreciably alter the inhibitory effect of malonyl-CoA on the enzyme. 3. Starvation increased overt carnitine palmitoyltransferase activity in both the fed and the thyroidectomized state. In percentage terms this response to starvation was substantially greater after thyroidectomy. In both the hypothyroid and normal states starvation decreased sensitivity to inhibition by malonyl-CoA.

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