Selenoxidation by flavin-containing monooxygenases as a novel pathway for beta-elimination of selenocysteine Se-conjugates.
Previously, it was shown that beta-elimination of selenocysteine Se-conjugates by rat renal cytosol leading to pyruvate formation was not solely catalyzed by pyridoxal phosphate-dependent enzymes. It was hypothesized that selenoxidation of the selenocysteine Se-conjugates, followed by syn-elimination, may be an alternative mechanism for pyruvate formation. In this study, selenoxidation of selenocysteine Se-conjugates was studied using rat liver microsomes and recombinant human oxidative enzymes. For all six selenocysteine Se-conjugates that were tested, it was found that rat liver microsomal incubations led to the formation of pyruvate, whereas the corresponding selenoxides were not observed. Microsomal pyruvate formation from Se-benzyl-L-selenocysteine (SeBC) was NADPH-dependent, but only marginally inhibited by several P450 inhibitors. Inhibition by methimazole and by heat pretreatment and stimulation by n-octylamine indicated that flavin-containing monooxygenases are mainly responsible for pyruvate formation from the selenocysteine Se-conjugates in rat liver microsomes. In the case of S-benzyl-L-cysteine, the sulfur analogue of SeBC, pyruvate formation was not observed. For this substrate, a chemically stable sulfoxide could be observed, as previously described. By using recombinantly expressed human flavin-containing monooxygenases and P450 enzymes, it was delineated that SeBC is selenoxidized by human FMOs, but not by human P450s. The k(cat)/K(m) of selenoxidation was 3.8-fold higher for FMO-1 than for FMO-3. In conclusion, selenoxidation of selenocysteine Se-conjugates catalyzed by FMOs and subsequently syn-elimination has taken place as an alternative route for the formation of pyruvate from selenocysteine Se-conjugates. Although selenoxides are known to be easily reduced by thiol compounds, microsomal pyruvate formation from SeBC was only 75% inhibited in the presence of an excess of glutathione. This indicates that even in the presence of physiological concentrations of reducing thiol compounds, selenoxides of selenocysteine Se-conjugates may undergo syn-elimination to some extent. Whether selenoxides and/or selenenic acids that are formed are involved in the activity of chemopreventive selenocysteine Se-conjugates remains to be established.
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Leiden/Amsterdam Center for Drug Research (LACDR), Division of Molecular Toxicology, Department of Pharmacochemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands.
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D000818:Animals; D003545:Cysteine / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism / Q000493:pharmacokinetics; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D006801:Humans; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology / Q000378:metabolism; D010084:Oxidation-Reduction; D010105:Oxygenases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D019289:Pyruvic Acid / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism; D018036:Selenium Compounds / Q000378:metabolism; D064588:Selenium Oxides; D017279:Selenocysteine / Q000031:analogs & derivatives / Q000378:metabolism* / Q000493:pharmacokinetics
D065607:Cytochrome P-450 Enzyme Inhibitors; D011994:Recombinant Proteins; D018036:Selenium Compounds; D064588:Selenium Oxides; D017279:Selenocysteine; D019289:Pyruvic Acid; D003577:Cytochrome P-450 Enzyme System; C002206:S-benzylcysteine; D010105:Oxygenases; C020066:dimethylaniline monooxygenase (N-oxide forming); D003545:Cysteine