Thyroxine Degradation. Iii. Competitive Inhibition of Thyroxine Degradation; Relationship of Structure to Inhibition.

Few attempts have been made to relate the structure of substances deiodinated by the systems in vitro which degrade thyroxine to specific structural requirements necessary for the reaction. In general, it has been found that several thyronine derivatives may be degraded by the rabbit muscle or rat liver microsomal thyroxine-degrading systems, but that none is more efficiently degraded than thyroxine itself (1, 2). The present study was undertaken in attempt to delineate the critical structure necessary for competition with thyroxine for initial reactions in the rat liver microsomal thyroxine-degrading system. These’ investigations indicate that 1,4-hydroquinone is the simplest reactive structure and that the initial reaction involves microsomal binding of substrate distinct from the later reaction of deiodination.