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

Abstract

Isolated rabbit livers were perfused for 12 to 15 hours with I131 -labeled thyroxine, triiodothyronine and diiodotyrosine in order to evaluate the hepatic metabolism of these substances. The livers were able to concentrate these amino acids in appreciable quantities showing somewhat greater accumulation in livers perfused with labeled triiodothyronine than with thyroxine. 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 8% in the 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. The perfused liver proved capable of producing triiodothyronine by the deiodination of thyroxine in the majority of studies. Several unidentified compounds appeared in both the bile and the plasma of the perfusate following perfusion with both thyroxine and triiodothyronine. It is likely that the most abundant of these components is a glucuronide conjugate of thyroxine and triiodothyronine.