Metabolism of I131-Labeled Thyroxine—Studies With Isolated, Perfused Rat Liver

Abstract

I131-labeled L-thyroxine of high specific activity was introduced into the perfusate of isolated livers. The perfused liver produced 0.5 ml. or more bile/hr. The injected I131 appeared rapidly in the bile obtained from the perfused liver, as much as 20% being excreted in 5 hrs. By filter paper chromatography it was demonstrated that the I131 in the bile was primarily in the form of a conjugated thyroxine compound, which has been designated Compound U. Compound U has previously been shown to be the major excretion product of thyroxine in the bile of the intact rat. The rate of conversion of thyroxine to Compound U by the isolated, perfused liver is adequate to account for the total formation of Compound U in the intact rat. It is concluded, therefore, that the liver is the principal site of this conversion in the normal rat. Rat liver slices, but not rat liver homogenates, showed some activity in converting thyroxine to Compound U. Spleen slices were completely inactive.