The effect of thyroxine (T4) on oxidation of succinate and of malate by frog myocardial homogenates was assessed following in vivo or in vitro administration of the hormone. In vitro, T4 enhanced the oxidation of succinate and inhibited oxidation of malate, a response similar to that observed in the myocardium of the rat. Following in vivo administration of T4, no enhancement of succinate oxidation could be demonstrated, and malate oxidation was slightly stimulated. The significance of the succinoxidase response as a manifestation of the action of thyroid hormone and its relationship to the deiodination of T4 are discussed.

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