Effect of Catalase Inhibitors on the Deiodination of Thyroxine

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Abstract
Previous evidence suggests that the physiological deiodination of thyroxine (T4) may be mediated by a hydrogen peroxide-peroxidase system in which catalase acts as an inhibitor. In the present experiments, the effects of several inhibitors of tissue catalases on the in vitro and in vivo deiodination of T4 were assessed. 3-Amino- 1,2,4-triazole, whether exhibited in vitro or in vivo, decreased catalase activity and enhanced deiodination of T4 in homogenates of liver and myocardium of mice. No effects were noted in brain and muscle, tissues in which the activity of catalase is very low. However, amino-triazole did not significantly increase the deiodination of T4 in rats in vivo, nor did it increase deiodination in homogenates of kidney, although catalase activity in the latter system was inhibited. Bacterial endotoxin, administered in vivo, decreased catalase activity and increased deiodination of T4 in homogenates of mouse kidney and enhanced the deiodination of T4 in vivo in rats. Azide, in vitro, decreased catalase activity and increased deiodination of T4 in homogenates of mouse liver and myocardium and had no effect on either activity in muscle and brain. In general, the results suggest that endogenous catalases act as inhibitors of the physiological deiodination of T4 in some tissues. (Endocrinology74: 627, 1964)