Oxidative Metabolism of Seleno-l-methionine to l-Methionine Selenoxide by Flavin-Containing Monooxygenases

Abstract

The roles of flavin-containing monooxygenases (FMOs) in the oxidation of seleno-L-methionine (SeMet) to L-methionine selenoxide (MetSeO) were investigated using cDNA-expressed human FMOs, purified rat liver FMOs, and rat liver microsomes. MetSeO and the N-2,4-dinitrophenylderivatives of SeMet and MetSeO were synthesized and characterized by 1 H-NMR and ESI/MS. These reference compounds were then used to develop a sensitive HPLC assay to monitor SeMet oxidation to MetSeO. Formation of MetSeO in rat liver microsomes was time-, protein concentration-, SeMet concentration-, and NADPH-dependent. The microsomal activity exhibited a SeMet K m value (mean ±S.D.; n=4) of 0.91 ± 0.29 mM and a V max value of 44 ± 8.0 nmol MetSeO/ mg protein/min. Inclusion of 1-benzylimidazole, superoxide dismutase or deferoxamine caused no inhibition of the rat liver microsomal activity. Because these results suggested the involvement of FMOs in the oxidation of SeMet in rat liver microsomes, formation of MetSeO was also examined using cDNA-expressed human and purified rat FMOs. The results showed that both rat and human FMO1 and FMO3 but not FMO5 can catalyze the reaction. The SeMet kinetic constants were obtained with purified rat liver FMO3 (K m = 0.11 mM, V max = 280 nmol/mg protein/min) and rat liver FMO1 (K m = 7.8 mM, V max = 1200 nmol/mg protein/min). Because SeMet has anti-cancer, chemopreventive, and toxic properties, the kinetic results suggest FMO3 is likely to play a role in the biological activities of SeMet at low exposure conditions.

Antioxidant and Anticancer Properties and Mechanisms of Inorganic Selenium, Oxo-Sulfur, and Oxo-Selenium Compounds

“…Clearly, the observed anticancer properties of MSeA may arise from a combination of several different mechanisms, and more work is required to develop a complete understanding of its biological activity. The naturally occurring amino acid, selenomethionine (SeMet) is found in many proteins in place of its sulfur analog, methionine [266,267]. Similar to methionine oxidation to its corresponding sulfoxide, SeMet is also oxidized by peroxynitrite and enzymes such as flavincontaining monooxygenases (FMOs) to methionine selenoxide (MetSeO) [218,219,266].…”

“…The naturally occurring amino acid, selenomethionine (SeMet) is found in many proteins in place of its sulfur analog, methionine [266,267]. Similar to methionine oxidation to its corresponding sulfoxide, SeMet is also oxidized by peroxynitrite and enzymes such as flavincontaining monooxygenases (FMOs) to methionine selenoxide (MetSeO) [218,219,266]. A significant amount of research has investigated Met/MetSO interconversion, but little work has investigated the properties of MetSeO.…”

“…Apart from its role in protein function, SeMet metabolism by the methionine transsulfuration pathway produces selenocysteine, an essential amino acid for the function of several antioxidant enzymes including glutathione peroxidase and thioredoxin reductase [266,269]. Interestingly, FMOs also have been shown to oxidize other selenium compounds, including the selenium containing drug ebselen, to their corresponding selenoxides [270].…”

A rapid and sensitive CE method with field‐enhanced sample injection and in‐capillary derivatization for selenomethionine metabolism catalyzed by flavin‐containing monooxygenases

“…Recent studies showed that SeMet Xin Hai 1 Thomas Nauwelaers 1 Roger Busson 2 Erwin Adams 1 metabolites are responsible for its anticancer activity [6]. Oxidation of SeMet can be catalyzed by FMO, but not CYP to yield selenomethionine selenoxide (SeOMet), which is quite reactive and may be reduced back to SeMet by glutathione [7,8]. This redox cycling is unique to SeOMet since methionine sulfoxide is not reduced to methionine under similar conditions.…”

“…Previously, an HPLC method for determination of SeMet and SeOMet was reported, which requires 24 h in the dark for pre-column derivatization using a highly toxic derivatization reagent, 1-fluoro-2,4-dinitrobenzene [7]. Here, we describe a rapid, sensitive and harmless EMMA-FESI method for in-capillary derivatization of SeMet and SeOMet to investigate Se-oxygenation catalyzed by FMO.…”