title: Thionamides and arsenite inhibit specific T3 binding to the hepatic nuclear receptor.
abstract: Methimazole (MMI) and propylthiouracil (PTU) are widely used for the treatment of Graves' disease. However, no studies have been reported on the action of these drugs on binding of L-triiodothyronine (T3) to the nuclear receptor. T3 receptors of rat liver nuclei, prepared by differential centrifugation, were extracted with 0.4 M KCl and 5 mM dithiothreitol (DTT). In the assessment of T3 binding to the DTT-reduced receptor, the hepatic nuclear extract was chromatographed on Superose 6 to remove DTT and isolate proteins of relative mass approximately 50,000 (chromatographed nuclear receptors (CNRs)), prior to the addition of [125I]T3 of high specific activity (3300 microCi/micrograms; 1 Ci = 37 GBq). MMI or PTU at 2 mM reduced specific T3 binding to CNR by 84% and 85%, respectively. The inhibitory effects of these reagents and 2 mM sodium arsenite (which complexes dithiols) were additive. Scatchard analyses indicated that neither MMI nor PTU (at 2 mM) significantly altered the affinity constant (Ka) (from 2.41 x 10(9) to 1.74 x 10(9) M-1 for PTU and 1.79 x 10(9) M-1 for MMI), while they both decreased (p less than 0.02) maximal binding capacity (from 0.36 +/- 0.02 to 0.19 +/- 0.02 pmol/mg protein for MMI and 0.17 +/- 0.02 pmol/mg protein for PTU). Dose-response curves showed that 50% inhibition was attained at 0.6 mM PTU or 1.0 mM MMI with approximately 25% inhibition by both at 0.1 mM. Artefactual binding effects by MMI and PTU on [125I]T3 were excluded by chromatography experiments. Similar results were obtained using nuclear receptors prepared from livers of hyperthyroid rats.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Thyroid Research Laboratory, Mount Sinai Hospital Research Institute, Toronto, Ont., Canada.
authors.forename: S
authors.initials: S
authors.lastname: Takagi
authors.forename: B C
authors.initials: BC
authors.lastname: Hummel
authors.forename: P G
authors.initials: PG
authors.lastname: Walfish
mesh_terms: D000818:Animals; D001151:Arsenic / Q000494:pharmacology*; D018053:Arsenites; D002467:Cell Nucleus / Q000378:metabolism; D002853:Chromatography, Liquid; D004229:Dithiothreitol / Q000494:pharmacology; D008099:Liver / Q000378:metabolism*; D008713:Methimazole / Q000494:pharmacology*; D008741:Methyl Methanesulfonate / Q000031:analogs & derivatives / Q000037:antagonists & inhibitors / Q000494:pharmacology; D011441:Propylthiouracil / Q000494:pharmacology*; D051381:Rats; D011988:Receptors, Thyroid Hormone / Q000187:drug effects* / Q000378:metabolism; D017670:Sodium Compounds; D013439:Sulfhydryl Reagents / Q000494:pharmacology*; D014284:Triiodothyronine / Q000378:metabolism*
chemical_list: D018053:Arsenites; D011988:Receptors, Thyroid Hormone; D017670:Sodium Compounds; D013439:Sulfhydryl Reagents; D014284:Triiodothyronine; C014674:methyl methanethiosulfonate; C017947:sodium arsenite; D008713:Methimazole; D011441:Propylthiouracil; D008741:Methyl Methanesulfonate; D001151:Arsenic; D004229:Dithiothreitol