The Oxidation of Ebselen Metabolites to Thiol Oxidants Catalyzed by Liver Microsomes and Perfused Rat Liver

Abstract

The oxidation of 2-(methylseleno)benzanilide and 2-selenylbenzanilide, metabolites of the antioxidant drug ebselen, was examined in reactions catalyzed by rat, pig, and guinea pig liver microsomes and in perfused rat liver. Microsomes from all three species catalyzed NADPH- and oxygen-dependent oxidation of the selenide and the selenol to thiol-reactive metabolites. The oxidation product of the selenide was similar in properties to the chemically synthesized selenoxide [2-(methylseleninyl)benzanilide]. The selenoxide oxidized GSH and thiocholine at rate constants of 1.2 × 102 and 7.2 × 102 M−1 s−1, respectively at pH 7.4, 37°C. n-Octylamine stimulated the oxidation of the ring-opened metabolites of ebselen catalyzed by pig and guinea pig liver microsomes but it had essentially no effect on these activities in rat liver microsomes. The selenoxidase activity of microsomes from all three species was partially (30-50%) sensitive to N-benzylimidazole. The effects of n-octylamine and the imidazole suggest that the oxidation of the selenide was catalyzed primarily by enzymes with the properties of flavin-containing and P450-dependent monooxygenases, but the nature of enzymes responsible for a small fraction of the N-benzylimidazole-sensitive activity was not fully resolved. The 2-(methylseleno)benzanilide oxidase activity of pig liver microsomes sensitive to N-benzylimidazole was only partially sensitive to antisera to pig liver NADPH-cytochrome P450 reductase, While neither 2-(methylseleno)benzanilide nor ebselen affected bile flow, the biliary efflux of GSSG was stimulated about fourfold in rat liver perfused with either of these selenium compounds. The increased GSSG efflux produced by 5 μM ebselen or its methyl metabolite was abolished by N-benzylimidazole.