Sequential deiodination of thyroxine in rat liver homogenate

Abstract

Rat liver homogenate was incubated at 37.degree. C with thyroxine, 3,3'',5-triiodothyronine, 3,3'',5''-triiodothyronine or 3,3''-diiodothyronine. The degradation or accumulation of these compounds was measured by specific radioimmunoassays. Production of 3,3'',5-triiodothyronine from thyroxine was highest at pH 6.0-6.5 and was markedly stimulated by the addition of dithiothreitol and effectively inhibited in the presence of 6-propyl-2-thiouracil. Accumulation of 3,3'',5''-triiodothyronine on incubation of thyroxine with homogenate was only observed above pH 8.5. Otherwise the product was converted into 3,3''-diiodothyronine too rapidly to allow its measurement. By measuring 3,3''-diiodothyronine it was deduced that 5-deiodination of thyroxine was most effective at approx. [approximately] pH 8.0. Dithiothreitol powerfully stimulated this reaction and 6-propyl-2-thiouracil strongly inhibited it. Monodeiodination of the tyrosine ring of 3,3'',5-triiodothyronine was the slowest reaction, was optimal at pH 8.0 and was less affected by dithiothreitol and 6-propyl-2-thiouracil than the above reactions. 5''-Deiodination of 3,3'',5''-triiodothyronine was extremely rapid, with a pH optimum probably at about 6.5. Owing to the high reaction rate under the conditions used it was not possible to assess the effects of dithiothreitol and 6-propyl-2-thiouracil.