Inhibition of rat hepatic thyroxine 5′-monodeiodinase by propylthiouracil: relation to site of interaction of thyroxine and glutathione

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Abstract
When rat liver cytosol, dialyzed free of glutathione, was chromatographed on Sephadex G-100 after incubation with 35S-propylthiouracil, 2 peaks of bound radioactivity were observed, 1 of which contained nearly all the thyroxine 5′-monodeiodinase activity in rat liver cytosol. Binding of propylthiouracil to this peak was inhibited by glutathione but not by thyroxine. Approximately 25% of 35S -propylthiouracil initially bound to the thyroxine 5′-monodeiodinating activity peak remained bound after dialysis, precipitation with trichloroacetic acid, and multiple extractions with ethanol, methanol, and chloroform, suggesting that binding was at least in part covalent. Dialysis studies showed that the presumed covalent binding of 35S -propylthiouracil to the thyroxine 5′-monodeiodinase peak could be inhibited by glutathione, dithioerythritol, and unlabelled propylthiouracil but not by oxidized glutathione or thyroxine. Conversely, thyroxine binding was unaffected by thiol compounds. We studied the kinetics of thyroxine 5′-monodeiodi-nation by radioimmunoassay techniques using rat liver homogenates as source of enzyme and observed the dependence of enzymic reaction upon glutathione (Km = 2.4 mM). Propylthiouracil inhibited the reaction and this inhibition could be overcome with increasing glutathione concentrations. We conclude that the thiol-dependent thyroxine 5′-monodeiodinase is inhibited by propylthiouracil through its covalent binding, probably as mixed disulfide, to a site on the enzyme at which glutathione interacts either as a cosubstrate or reducing agent. This binding site is separate from the site at which thyroxine binds.