title: Evidence that type II 5'-deiodinase is not a selenoprotein.
abstract: Brain type II 5'-iodothyronine deiodinase and liver type I 5'-iodothyronine deiodinase activities are decreased in rats fed a Se(2+)-deficient diet suggesting that both enzymes are Se(2+)-dependent proteins. Since serum thyroxine (T4) concentrations are twice normal in the Se(2+)-deficient animals, it is unclear whether the Se2+ deficiency or the increased circulating T4 account for the decrease in the brain enzyme. In order to separate these two possibilities, the effects of Se2+ on 5'-deiodinase in glial cells (type II) and LLC-PK1 cells (type I) were examined. LLC-PK1 and glial cells were grown in serum-free defined medium containing 0, 1 pM, 10 nM, and 40 nM Se2+ for 3-5 days or in medium containing 75Se2+ for 24 h. Deiodinase isozymes were determined by measuring catalytic activity and by quantification of the BrAc[125I]T4 affinity-labeled substrate binding subunits. Se2+ deficiency was confirmed by measuring the activity of the selenoprotein, glutathione peroxidase. Se2+ caused a concentration-dependent increase in glutathione peroxidase activity in both cell types, as well as in the type I enzyme, but had no effect on the type II enzyme. LLC-PK1 cells contained multiple 75Se(2+)-labeled proteins including the 27-kDa substrate binding subunit of the type I 5'-deiodinase. Glial cells contained seven 75Se(2+)-labeled proteins ranging in size from 12 to 62 kDa, none of which corresponded to the type II substrate binding subunit. these data show that, unlike the type I enzyme, the type II enzyme does not contain a selenocysteine or selenomethionine, further emphasizing the differences between these two isozymes.
authors.affiliation: Molecular Endocrinology Laboratory, University of Massachusetts Medical School, Worcester 01655.
authors.forename: M
authors.initials: M
authors.lastname: Safran
authors.forename: A P
authors.initials: AP
authors.lastname: Farwell
authors.forename: J L
authors.initials: JL
authors.lastname: Leonard
mesh_terms: D000818:Animals; D001665:Binding Sites; D002460:Cell Line; D004173:Dipodomys; D005979:Glutathione Peroxidase / Q000378:metabolism; D066298:In Vitro Techniques; D007453:Iodide Peroxidase / Q000737:chemistry* / Q000378:metabolism; D007527:Isoenzymes / Q000378:metabolism; D008970:Molecular Weight; D009457:Neuroglia / Q000201:enzymology; D011506:Proteins / Q000378:metabolism*; D051381:Rats; D012643:Selenium / Q000378:metabolism* / Q000494:pharmacology; D051140:Selenoproteins
chemical_list: D007527:Isoenzymes; D011506:Proteins; D051140:Selenoproteins; D007453:Iodide Peroxidase; D005979:Glutathione Peroxidase; D012643:Selenium