title: Interactions of sodium selenite, glutathione, arsenic species, and omega class human glutathione transferase.
abstract: Human monomethylarsenate reductase [MMA(V) reductase] and human glutathione S-transferase omega 1-1 (hGSTO1-1) [because MMA(V) reductase and hGSTO1-1 are identical proteins, the authors will utilize the designation "hGSTO1-1"] are identical proteins that catalyze the reduction of arsenate, monomethylarsenate [MMA(V)], and dimethylarsenate [DMA(V)]. Sodium selenite (selenite) inhibited the reduction of each of these substrates by the enzyme in a concentration-dependent manner. The kinetics indicated a noncompetitive inhibition of the MMA(V), DMA(V), or arsenate reducing activity of hGSTO1-1. The inhibition of the MMA(V) reducting activity of hGSTO1-1 by selenite was reversed by 1 mM DL-dithiothreitol (DTT) but not by reduced glutathione (GSH), which is a required substrate for the enzyme. Neither superoxide anion nor hydrogen peroxide was involved in the selenite inhibition of hGSTO1-1. MALDI-TOF and MS/MS analysis demonstrated that five molecules of GSH were bound to one monomer of hGSTO1-1. Four of the five cysteines of the monomer were glutathionylated. Cys-32 in the active center, however, exists mostly in the sulfhydryl form since it was alkylated consistently by iodoacetamide. MALDI-TOF mass spectra analysis of hGSTO1-1 after reaction with GSH and sodium selenite indicated that selenium was integrated into hGSTO1-1 molecules. Three selenium were found to be covalently bonded to the monomer of hGSTO1-1 with three molecules of GSH. It is proposed that the reaction products of the reduction of selenite inhibited the activity of hGSTO1-1 by reacting with disulfides of glutathionylated cysteines to form bis (S-cysteinyl)selenide and S-selanylcysteine and had little or no interaction with the sulfhydryl of Cys-32 in the active site of the enzyme.
authors.affiliation: Department of Molecular and Cellular Biology, The University of Arizona, Tucson, Arizona 85721-0106, USA.
authors.forename: Robert A
authors.initials: RA
authors.lastname: Zakharyan
authors.forename: George
authors.initials: G
authors.lastname: Tsaprailis
authors.forename: Uttam K
authors.initials: UK
authors.lastname: Chowdhury
authors.forename: Alba
authors.initials: A
authors.lastname: Hernandez
authors.forename: H Vasken
authors.initials: HV
authors.lastname: Aposhian
mesh_terms: D000595:Amino Acid Sequence; D001152:Arsenicals / Q000494:pharmacology*; D003545:Cysteine / Q000737:chemistry; D004229:Dithiothreitol / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D005978:Glutathione / Q000494:pharmacology*; D005982:Glutathione Transferase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008969:Molecular Sequence Data; D010084:Oxidation-Reduction; D017382:Reactive Oxygen Species; D011994:Recombinant Proteins / Q000737:chemistry; D018038:Sodium Selenite / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; D013237:Stereoisomerism; D013438:Sulfhydryl Compounds / Q000737:chemistry / Q000378:metabolism
chemical_list: D001152:Arsenicals; D004791:Enzyme Inhibitors; D017382:Reactive Oxygen Species; D011994:Recombinant Proteins; D013438:Sulfhydryl Compounds; C490476:GSTO1 protein, human; D005982:Glutathione Transferase; D005978:Glutathione; D018038:Sodium Selenite; D003545:Cysteine; D004229:Dithiothreitol