Role of Peroxidase and Catalase in the Physiological Deiodination of Thyroxine1

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Abstract
Studies in several tissues from the rat and in tissue-free media have afforded evidence that physiological deiodination of thyroxine (T4) and 3,5,3′-triiodothyronine (T3) is mediated by a tissue peroxidase and inhibited by catalase. First, H2O2 alone was found capable of deiodinating T4, both in tissue preparations and in tissue-free media. Second, H2O2 overcame the absolute requirement of tissue systems for oxygen in deiodinating T4. Third, catalase inhibited, often completely, the deiodination of T4 which occurred in otherwise unsupplemented homogenates of all tissues studied. Fourth, a source of H2O2 (glucose-glucose oxidase) was found, which, in appropriate concentrations, stimulated deiodination of both T4 and T3 in unboiled, but not in boiled, homogenates of some tissues and which had no effect in tissue-free media. Furthermore, catalase inhibited the increased deiodination of T4 induced by glucose- glucose oxidase. In addition, the glucose oxidase system also potentiated deiodination of T4 in more organized systems, such as slices of liver and skeletal muscle. TPN, DPN, ATP and ADP also stimulated deiodination of T4, and their effects were inhibited by catalase, as were the stimulatory effects of hemoglobin and Fe++. Evidence is presented which suggests that hemoglobin may enhance deiodination by functioning as a peroxidase and that, in the presence of oxygen, Fe++ may lead to H2O2 formation.