Iodothyronine 5'-deiodinase activity in the amphibian Rana catesbeiana at different stages of the life cycle.

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In a previous study, 5' deiodinase (5'D) activity was detected in skin but not in other tissues of premetamorphic tadpoles. When these tadpoles were stimulated to metamorphic climax by treatment with thyroid hormone, activity was increased in skin and was found also in gut and tail tissue. In the present study, 5'D activity was studied at various stages of spontaneous development from early prometamorphosis through metamorphic climax and including the adult frog. 5'D activity was detected in gut and skin at all stages studied and in metamorphosing tail tissue. It was minimal or absent in all other tissues examined. Activity in prometamorphic skin was comparable to that previously observed in premetamorphic skin but increased significantly during metamorphic climax and remained at this level in the adult. 5'D activity was not observed in tail tissue until resorption commenced and when present could not be correlated with tadpole stage or tail length. In both skin and tail, the 5'D activity exhibited values for Michaelis-Menten constant (Km) for rT3 and T4 in the nanomolar range. During prometamorphosis, the 5'D activity in gut exhibited a value for the rT3 Km in the micromolar range. As the tadpoles entered metamorphic climax, the rT3 Vmax of this activity in gut decreased and a second type of 5'D activity, with a Km for rT3 in the nanomolar range, was detected in this tissue. By stage XXI, only the latter component was observed, and its activity increased significantly during metamorphic climax. In the adult frog, 5'D activity in gut was comparable to that seen at the onset of climax. The adult frog also exhibited T3 5D activity in all tissues studied. These observations, together with those reported in the previous study, indicate that gut and skin are the principal T3-generating organs in anuran amphibia and suggest that increased activity of the 5'D system, at the time in the life cycle that the tissues require increased amounts of thyroid hormone, is an important part of thyroid hormone economy in this species.