The efficiency of oxidative phosphorylation and the rapid control by thyroid hormone of nicotinamide nucleotide reduction and transhydrogenation in intact rat liver mitochondria.

In confirmation of previous work enhancement of the fluorescence emission of reduced nicotinamide nucleotides in intact rat liver mitochondria was found to depend on incubation conditions. Under standard conditions the enhancement is constant at 4.8-fold in states 3 and 4 and is not altered by thyroidectomy of the animal 6 weeks prior to experiment. The ADP-induced (state 4----state 3----state 4) fluorescence changes are significantly different in intact mitochondria from normal and hypothyroid animals and reflect the decreased rate and efficiency of oxidative phosphorylation after thyroidectomy. Incubation of liver homogenates in vitro for 15 min with 1 microM triiodothyronine before isolating mitochondria significantly restores their ADP response towards normal. Direct addition of hormone to isolated mitochondria was ineffective. Enzymatic measurement of mitochondrial extracts shows that thyroidectomy leads to increases in the contents of NAD(H) by 22% and NADP(H) by 33%. With glutamate as substrate ADP-induced changes in the reduced/oxidized ratio of NAD+ are not significantly altered in hypothyroid preparations. By contrast the NADP+ ratio remains substantially more reduced in state 3 than it does in normal mitochondria. The hypothesis is advanced that the decreased efficiency of hypothyroid preparations in phosphorylating ADP may be the result of increased energy-linked transhydrogenase activity. This is needed to supply NADPH via the glutathione peroxidase for reducing endogenously formed peroxides. Direct reduction of mitochondrial glutathione with dithiothreitol had no substantial effect on ADP/O ratios or on ADP-induced redox cycles in either normal or thyroidectomised preparations. This decisively eliminates the possibility that lowered phosphorylation efficiency is the result of a leak of reducing equivalents via glutathione peroxidase.

Effects of thyroidectomy and starvation on the activity and properties of hepatic carnitine palmitoyltransferase.

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.