The overall objective of this research project is to determine the physiological parameters and biochemical mechanisms which regulate the metabolism and action of thyroid hormones. Studies are focused specifically on the mechanisms whereby thyroid hormones and other factors regulate the activity of iodothyronine 5'-deiodinase (I5'D), which catalyzes the conversion of thyroxine (T4) to the metabolically more active 3,5,3'-triiodothyronine (T3). Considerable progress has been made toward the specific aims detailed in the original proposal. The characteristics of I5'D activity in GH3 rat pituitary tumor cells and NB41A3 mouse neuroblastoma cells has been defined. Of major importance has been the discovery that 15'D activity in these cell lines, as well as in normal rat pituitary and cerebral cortex, is regulated by a unique mechanism of thyroid hormone action which involves the specific and rapid inactivation and/or degradation of the enzyme. In the present application, a hypothesis is put forth to explain this phenomenum and a number of studies are detailed to provide a more complete understanding of this process. The specific aims of the current proposal are: a) To characterize further the regulatory effects of thyroid hormones, competitive inhibitors, and propylthiouracil on type I I5'D activity in liver, kidney, thyroid, and cultured heptatoma cells; b) To investigate in whole animals, GH3 pituitary tumor cells, and Reuber FA0 hepatoma cells the influence of the cellular thiol:disulfide balance on the regulation of I5'D activity; c) To determine the effects of substrates, competitive inhibitors, and sulfhydryl reagents on the thermolability of type II I5'D in crude microsomal preparations; d) To determine if type III I5D in rat brain and fetal chick liver is subject to substrate-induced inactivation and/or degradation in a manner analogous to types I and II I5'D; e) To develop methodology for the labeling and identification of iodothyronine deiodinases in tissue preparations and cell culture systems; f) To investigate the cellular mechanism(s) whereby substrates induce inactivation of I5'D. These studies will enhance our understanding of thyroid hormone action and allow a more rational approach to the treatment of clinical disorders characterized by alterations in thyroid hormone metabolism. Indeed, this knowledge may suggest novel approaches to the therapy of other disorders where thyroid hormones have been implicated in the pathophysiology.