THE PHOTOCHEMICAL ACTIVATION OF THYROXINE

Abstract

Recent evidence suggests that thyroxine is not the active form of the thyroid hormone. Since thyroxine's phenolic hydroxyl group appears to be quite labile, changes in this group may be involved in converting thyroxine to its active form. Light was found to catalyze an apparent oxidation of the hydroxyl group, and this photosensitivity suggested the possibility that exposure to light might hasten thyroxine activation. Accordingly, Qo² was measured in liver slices and in diaphragm from normal and thyroxine injected rats, with half the tissues in each group exposed to brilliant illumination. Both tissues exhibited an increased response to thyroxine when subjected to the light, although the effect on liver was considerably greater than on muscle.It is postulated that thyroxine activation may require oxidation of the phenolic hydroxyl group and that a free radical may represent the active form of this hormone.