THE MECHANISM OF PROTEIN IODINATION DURING THE METABOLISM OF THYROID HORMONES BY PERIPHERAL TISSUES1

Abstract

During the metabolism of I¹³¹-labeled thyroxine and its analogues by tissue preparations, two major products are formed: inorganic iodide and a material which remains at the point of application during paper chromatography. The nature of this “origin material” and the mechanism of its formation have been investigated. The I¹³¹ in origin material was TCA-precipitable, non-dialyzable and was not extractable with lipid solvents, and was therefore thought to be an iodinated protein. Except in the presence of serotonin, the formation of origin material during the metabolism of thyroid hormones was invariably associated with the appearance of inorganic iodide. However, origin material was not found when inorganic iodide was the initial substrate. Incubation in complete darkness had no consistent effect on the generation of either iodide or origin material from thyroxine. Furthermore, anaerobiosis and goitrogens failed to prevent the formation of origin material from labeled thyroxine. It was thus concluded that origin material is not formed by the re-oxidation of iodide liberated during hormonal deiodination. An alternative mechanism is proposed and its implications with regard to hormonal action are discussed.