Metabolism of thyroid hormones. The deiodination of thyroxine and triiodothyronine in vitro

Abstract

Two aerobic partially heat-resistant systems which deiodinate L-thyroxine and triiodo-L-thyronine were demonstrated in homogenates from the liver and other tissues of the rat. "N" systems, demonstrated with non-radioactive thyroxine and triiodothyronine, were effective at pH 3.5 and at pH 9.5. They were inhibited by cyanide and by exclusion of air but not by citrate or by antithyroxine compounds. "R" systems, demonstrated with Il31-labeled thyroxine, were effective at pH 6 and were inhibited by cyanide, exclusion of air, citrate and anti-thyroxine compounds. Evidence was obtained for the conversion of thyroxine into triiodothyronine by "R" systems. The significance of these results is briefly discussed.