title: A novel antioxidant mechanism of ebselen involving ebselen diselenide, a substrate of mammalian thioredoxin and thioredoxin reductase.
abstract: The antioxidant mechanism of ebselen involves recently discovered reductions by mammalian thioredoxin reductase (TrxR) and thioredoxin (Trx) forming ebselen selenol. Here we describe a previously unknown reaction; ebselen reacts with its selenol forming an ebselen diselenide with a rate constant of 372 m(-1)s(-1). The diselenide also was a substrate of TrxR forming the selenol with K(m) of 40 microm and k(cat) of 79 min(-1) (k(cat)/K(m) of 3.3 x 10(4) m(-1)s(-1)). Trx increased the reduction because of its fast reaction with diselenide (rate constant 1.7 x 10(3) m(-1)s(-1)). Diselenide stimulated the H2O2 reductase activity of TrxR, even more efficiently with Trx present. Because the mechanism of ebselen as an antioxidant has been assumed to involve glutathione peroxidase-like activity, we compared the H2O2 reductase activity of ebselen with the GSH and Trx systems. TrxR at 50 nm, far below the estimated physiological level, gave 8-fold higher activity compared with 1 mm GSH; addition of 5 microm Trx increased this difference to 13-fold. The rate constant of ebselen selenol reacting with H2O2 was estimated to be faster than 350 m(-1)s(-1). We propose novel mechanisms for ebselen antioxidant action involving ebselen selenol and diselenide formation, with the thioredoxin system rather than glutathione as the predominant effector and target.
authors.affiliation: Medical Nobel Institute for Biochemistry, Department of Medical Biochemistry and Biophysics, Karolinska Institutet, SE-171 77 Stockholm, Sweden.
authors.forename: Rong
authors.initials: R
authors.lastname: Zhao
authors.forename: Arne
authors.initials: A
authors.lastname: Holmgren
mesh_terms: D000818:Animals; D000975:Antioxidants / Q000494:pharmacology; D001393:Azoles / Q000737:chemistry* / Q000494:pharmacology; D001549:Benzamides / Q000737:chemistry* / Q000494:pharmacology; D002417:Cattle; D004229:Dithiothreitol / Q000494:pharmacology; D004926:Escherichia coli / Q000378:metabolism; D005978:Glutathione / Q000378:metabolism; D006801:Humans; D006861:Hydrogen Peroxide / Q000737:chemistry; D054833:Isoindoles; D007700:Kinetics; D008956:Models, Chemical; D016566:Organoselenium Compounds / Q000737:chemistry* / Q000494:pharmacology; D010920:Placenta / Q000378:metabolism; D011485:Protein Binding; D013050:Spectrometry, Fluorescence; D013880:Thioredoxin-Disulfide Reductase / Q000378:metabolism*; D013879:Thioredoxins / Q000737:chemistry / Q000378:metabolism*; D013997:Time Factors
chemical_list: D000975:Antioxidants; D001393:Azoles; D001549:Benzamides; D054833:Isoindoles; D016566:Organoselenium Compounds; C067619:bis(2-(N-phenylcarboxamido)phenyl)diselenide; C042986:ebselen; D013879:Thioredoxins; D006861:Hydrogen Peroxide; D013880:Thioredoxin-Disulfide Reductase; D005978:Glutathione; D004229:Dithiothreitol