ROLE OF CATALASES AND PEROXIDASES IN THE DEIODINATION OF THYROXINE IN FROG LIVER.

Abstract

Previous data indicate that, in the frog, the transition from the larval to the adult stage is associated with a decrease or loss of the ability to deiodinate the thyroid hormones. Subsequent evidence suggests that the physiological deiodination of thyroxine (T4) is mediated by a hydrogen peroxide-peroxidase system in which tissue catalase acts as an inhibitor. In the present study, the role of catalases and peroxidases as determinants of the deiodinating activity of frog liver was assessed. Frog liver was shown to contain far more catalase activity than tadpole liver; considerable pyrogallol peroxidase activity was also observed. Comparative studies in the frog, rat, mouse and tadpole revealed that hepatic deiodinating activity varied inversely with catalase activity. Deiodinating activity was induced in homogenates of frog liver supplemented with ferrous ion or with an H2O2-generating system (glucose-glucose oxidase). 3-Amino-l,2,4-triazole, either administered in vivo or added in vitro, decreased catalase activity and induced significant T4- deiodinating activity in homogenates of frog liver. The stimulating effect of glucose oxidase was far greater in homogenates of liver from the amino-triazole treated frogs than from control animals. Azide added in vitro also decreased catalase activity and induced T4- deiodinating activity in homogenates of frog liver. Catalase activity was also greatly reduced by storing homogenates of frog liver for at least 24 hr. Significant deiodinating activity was found in these stored preparations. Dialysis had no greater effect than storage on the activity of hepatic catalase and T4- deiodinase. The results suggest that the H2O2 content of homogenates of frog liver is inadequate for the deiodination of T4. This is attributed to the relatively high concentration of hepatic catalases and to a limited generation of H2O2. (Endocrinology76: 479,1965)