THE METABOLISM OF I131-LABELED THYROXINE, TRIIODOTHYRONINE AND DIIODOTYROSINE BY ANISOLATED PERFUSED RABBIT LIVER1

Isolated rabbit livers were perfused for 12 to 15 hours with I131-labclcd thyroxine,triiodothyronine and diiodotyrosine in order to evaluate the hepaticmetabolism of these substances. The livers were able to concentrate those, amino acids in appreciable quantities showing somewhat greater accumulation in livers perfused with labeled triiodothyronine than with thyroxinc. Triidothyronine and thyroxine were metabolized by the perfused liver at rates identical in ratio to the intact rabbit but faster by a factor of almost six. Diiodotyrosine, however, was deiodinated at a rate similar to that in the intact animal. Measurements of biliary excretion of radioactivity showed that this pathway proved more important over the period studied for livers perfused with triiodothyronine than for those perfused with thyroxine. Twenty-six per cent of initial radioactivity was excreted over 12 hours in the bile of the former and 8% inthe latter. Less than 3% of the initial perfusate radioactivity appeared in the bile and over 12 hours when perfusion with labeled diiodotyrosine was carried out.