The effects and interactions of substrates, inhibitors, and the cellular thiol-disulfide balance on the regulation of type II iodothyronine 5'-deiodinase.

Thyroid hormones rapidly inhibit type II iodothyronine 5'-deiodinase (5'DII) activity in the rat central nervous system, anterior pituitary gland, and GH3 pituitary tumor cells. To gain insight into the cellular mechanisms responsible for this down-regulation, the effects of substrates, inhibitors, and alterations in the cellular thiol-disulfide balance on 5'DII regulation were investigated. The results demonstrate that in vitro competitive inhibitors, such as iopanoic acid, as well as iodothyronine substrates induce a rapid and irreversible loss of enzyme activity in rat cerebral cortex and anterior pituitary tissue. The potency of these agents in down-regulating this enzymatic process in intact GH3 cells is directly related to their competitive inhibitory effects on 5'DII activity in vitro. Additional studies demonstrated that treatment of intact GH3 cells with the sulfhydryl-oxidizing agent diamide mimicks the effect of substrate and results in the rapid inactivation of 5'DII. In contrast, preincubation of cells with the sulfhydryl-reducing agent dithiothreitol renders the enzyme less susceptible to the down-regulatory effects of substrate. Sulfhydryl-reducing agents thus appear to play a dual role in the 5'DII process by serving as cosubstrates and by modulating the enzymes susceptibility to substrate-induced down-regulation. These findings suggest that the in vivo inhibition of 5'DII by thyroid hormones involves a unique mechanism of enzyme regulation whereby the binding of ligand to the active site induces the rapid and irreversible inactivation and/or degradation of the enzyme. This inactivation of 5'DII initiated by substrate binding may be mediated by alterations in the sulfhydryl state of the enzyme as it progresses through the catalytic cycle.