Liver Microsome and Flavin-Containing Monooxygenase Catalyzed Oxidation of Organic Selenium Compounds

Abstract

Eight commercially available selenides, a selenol, and selenourea were examined for substrate activity with purified pig liver flavin-containing monooxygenase (FMO). While none of the aromatic heterocyclic selenides tested showed detectable activity, all dialkyl- and alkylaryl-selenides free from ionic groups stimulated NADPH- and FMO-dependent oxygen uptake. With limiting substrate the molar ratio of oxygen reduced to selenide added was 1:1. Analysis of products from N-[2-(methylseleno)ethyl]benzamide demonstrated that the selenide was quantitatively oxidized to selenoxide. Further oxidation of either the FMO-generated or synthetic selenoxides was not detected. The dialkyl- and alkylaryl-selenoxides are potent thiol oxidants and reaction rates for the oxidation of thiols by methylphenylselenoxide and N-[2-(methylseleninyl)ethyl]benzamide followed second order kinetics with rate constants from 3-4 × 10 3 M −1 s −1 at pH 7.4, 37°C. The rapid oxidation of thiols by these selenoxides demonstrates that the oxidation of selenides to selenoxides catalyzed by crude tissue preparations can be measured by following selenide-dependent oxidation of thiocholine by the procedure described earlier for the oxidation of thiourea (WXA Guo and D. M. Ziegler, 1991, Anal. Biochem. 198, 143-148). Activity measurements by this procedure indicated that the oxidation of dialkyl selenides by rat, guinea pig, or pig liver microsomes was catalyzed primarily by a monooxygenase with the properties of FMO. However, from 20 to 40% of the microsome-catalyzed oxidation of selenides bearing aryl substituents was sensitive to N-benzylimidazole, suggesting that P450-dependent monooxygenases also contribute, at least in part, to the oxidation of these xenobiotics.