The uncoupling of oxidative phosphorylation in rat-liver mitochondria by thyroxine, triiodothyronine and related substances

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Abstract
The action of L-thyroxine and related substances on oxidative phosphorylation by rat-liver mitochondria was studied in a system where phosphorylation quotients were measured by the rate of incorporation of P32 into adenosine triphosphate (ATP) according to Krebs et al. (1953). L-Thyroxine and L-triiodothyronine uncoupled phosphorylation provided there was preliminary contact for 30-40 minutes with the mitochondria at 0[degree]. L-Thyroxine was a more effective uncoupling agent. About 5 x 10-5[image] L-thyroxine and 10-4[image] L-triiodothyronine were required to give over 90% uncoupling. D-Thyroxine, DL-thyroxine and L-diiodothyronine all uncoupled phosphorylation less readily than the same concentration of L-thyroxine. L-Thyronine and L-diiodotyrosine were without activity at a concentration of 2 x 10-4[image]. Potassium iodide had no effect on phosphorylation, but 2 x 10-4[image] iodine uncoupled phosphorylation and inhibited respiration. These results are in agreement with the findings of other workers, and are discussed in relation to the physiological action of thyroxine and triiodothyronine.