THYROXINE METABOLISM BY AMPHIBIAN SKIN DURING METAMORPHOSIS AND MOLTING,

Both intact segments and crude homogenates of frog and toad skin rapidly form tetraiodothyroacetic acid from thyroxine. Substrate and product addition inhibits the activity in frog skin, but the system in toad skin is not appreciably inhibited by concentrations of thyroxine or of tetraiodothyroacetic acid up to 2600 ug per cent. Most, if not all, of the activity is localized in the epidermis. Although heat-labile deiodination of thyroxine is undetectable in toad skin and dissociated from oxidative deamination in frog skin, tetraiodothyroacetic acid is partially deiodinated in intact animals. During metamorphosis, heat-labile deiodinating activity markedly decreases in amphibian liver whereas a phase of increased activity occurs in the skin and tail during the formation of hind leg buds. Tetraiodothyroacetic acid does not appear until metamorphosis is complete. Finally, both regional differences and differences during various days of the molting cycle are observed in deiodination and oxidative deamination by frog skin. Author