title: Substrate requirement for inactivation of iodothyronine-5'-deiodinase activity by thiouracil.
abstract: Preincubation of rat liver microsomal fraction with 1 microM 2-thiouracil and 0.01-1 microM 3,3',5'-triiodothyronine or 3',5'-diiodothyronine, 0.1-10 microM thyroxine or 3,5-diiodothyronine led to a progressive, irreversible and concomitant decrease in subsequently assayed 3,3',5'-triiodothyronine- and 3',5'-diiodothyronine-5'-deiodinase activity. Preincubation with thiouracil alone, with iodothyronines alone or with thiouracil and 10 microM thyronine or 3,5-diiodotyrosine had no or virtually no effect. The results indicate that (1) a previously proposed ping-pong mechanism for thyroid hormone deiodination, involving the formation of an enzyme-sulphenyl iodide intermediate, is correct; (2) thyroxine, 3,3',5'-triiodothyronine and 3',5'-diiodothyronine are substrates for a common 5'-deiodinase; (3) this 5'-deiodinase is not fully specific as regards the position of the iodine substituents in the substrate, since it also appears to catalyse the 5-deiodination of 3,5-diiodothyronine.
mesh_terms: D000818:Animals; D004104:Diiodothyronines; D066298:In Vitro Techniques; D007453:Iodide Peroxidase / Q000037:antagonists & inhibitors*; D008862:Microsomes, Liver / Q000201:enzymology; D010544:Peroxidases / Q000037:antagonists & inhibitors*; D051381:Rats; D013379:Substrate Specificity; D013889:Thiouracil / Q000494:pharmacology*; D013974:Thyroxine; D014284:Triiodothyronine
chemical_list: D004104:Diiodothyronines; D014284:Triiodothyronine; D013889:Thiouracil; D010544:Peroxidases; D007453:Iodide Peroxidase; D013974:Thyroxine