PROPERTIES OF AN FMN-DEPENDENT THYROXINE DEIODINASE OF RAT LIVER MITOCHONDRIA

Some properties of an FMN-dependent thyroxine deiodinase were studied, employing rat liver as an enzyme source. The enzyme was present concentrated within mitochondria. Ready inactivation by boiling, inactivity under anaerobic conditions and cyanide-sensitivity were observed. Optimum pH was found between 7.0 and 7.5. Substrate specificity was rather low; T4 and TA4 were deiodinated similarly. The enzyme was inhibited by T3 probably through competition. The mitochondrial enzyme was far more prominent than any other thyroid hormone deiodinase ever reported. From the enzymatic deiodination, the chemical deiodination caused by FMN alone was distinguished. Riboflavin and FAD had a similar effect to FMN.On the basis of these facts, it was considered that the physiologically important role of the liver in deiodinating thyroid hormones should be ascribed to the FMN-dependent deiodinase of mitochondrial origin.