NADPH-DEPENDENT OXIDATION OF REDUCED EBSELEN, 2-SELENYLBENZANILIDE, AND OF 2-(METHYLSELENO)BENZANILIDE CATALYZED BY PIG-LIVER FLAVIN-CONTAINING MONOOXYGENASE

The selenazole ring-opened metabolites of ebselen, 2-selenylbenzanilid e and 2-(methyl-seleno)benzanilide, are substrates for flavin-containi ng monooxygenase from pig liver. The K(m) values were 25 and 3-mu-M, r espectively, measured at 37-degrees-C, pH 7.4, in the presence of 1 mM GSH. The V(max) values were 390 mU/mg of protein, similar to those ob tained with methimazole or other substrates for FMO1. Although ebselen also appears to be a substrate in the absence of GSH, it progressivel y inactivates the enzyme, apparently by binding covalently to essentia l enzyme thiols. The oxidation products of the selenol and methylselen o derivatives are rapidly reduced by GSH, regenerating the parent subs trates. Rapid reduction of the selenide oxide by GSH was unexpected an d suggests that, unlike S-oxidation of sulfides, Se-oxidation of selen ides may be a route for bioactivation. The data show that in the prese nce of FMO1 micromolar amounts of either of these ring-opened metaboli tes establish a futile cycle catalyzing the oxidation of GSH to GSSG b y NADPH and oxygen.