Krause Renee J, Elfarra Adnan A
Department of Comparative Biosciences and the Center for Molecular and Environmental Toxicology, University of Wisconsin-Madison, Madison, WI 53706, United States.
Biochem Pharmacol. 2009 Jan 1;77(1):134-40. doi: 10.1016/j.bcp.2008.09.022. Epub 2008 Sep 27.
Seleno-L-methionine (SeMet) can be oxidized to L-methionine selenoxide (MetSeO) by flavin-containing monooxygenase 3 (FMO3) and rat liver microsomes in the presence of NADPH. MetSeO can be reduced by GSH to yield SeMet and GSSG. In the present study, the potential reduction of MetSeO to SeMet by other cellular components and antioxidants was investigated. Besides GSH, other thiols (L-cysteine, or N-acetyl-L-cysteine) and antioxidants (ascorbic acid and methimazole) also reduced MetSeO to SeMet. This reduction is unique to MetSeO since methionine sulfoxide was not reduced to methionine under similar conditions. The MetSeO reduction by thiols was instaneous and much faster than the reduction by ascorbic acid or methimazole. However, only one molar equivalent of ascorbic acid or methimazole was needed to complete the reduction, as opposed to two molar equivalents of thiols. Whereas the disulfides produced by the reactions of MetSeO with thiols are chemically stable, methimazole disulfide readily decomposed at pH 7.4, 37 degrees C to yield methimazole, methimazole-sulfenic acid, methimazole sulfinic acid, methimazole S-sulfonate, 1-methylimidazole (MI) and sulfite anion. Collectively, the results demonstrate reduction of MetSeO to SeMet by multiple endogenous thiols, ascorbic acid, and methimazole. Thus, oxidation of SeMet to MetSeO may result in depletion of endogenous thiols and antioxidant molecules. Furthermore, the novel reduction of MetSeO by methimazole provides clear evidence that methimazole should not be used as an alternative FMO substrate when studying FMO-mediated oxidation of SeMet.
在NADPH存在的情况下，含黄素单加氧酶3（FMO3）和大鼠肝脏微粒体可将硒代-L-蛋氨酸（SeMet）氧化为L-蛋氨酸亚硒酸盐（MetSeO）。MetSeO可被谷胱甘肽（GSH）还原生成SeMet和氧化型谷胱甘肽（GSSG）。在本研究中，研究了其他细胞成分和抗氧化剂将MetSeO还原为SeMet的可能性。除了GSH外，其他硫醇（L-半胱氨酸或N-乙酰-L-半胱氨酸）和抗氧化剂（抗坏血酸和甲巯咪唑）也可将MetSeO还原为SeMet。这种还原是MetSeO所特有的，因为在类似条件下蛋氨酸亚砜不会还原为蛋氨酸。硫醇对MetSeO的还原是即时的，且比对抗坏血酸或甲巯咪唑的还原快得多。然而，完成还原只需一摩尔当量的抗坏血酸或甲巯咪唑，而硫醇则需要两摩尔当量。虽然MetSeO与硫醇反应产生的二硫化物在化学上是稳定的，但甲巯咪唑二硫化物在pH 7.4、37℃时容易分解，生成甲巯咪唑、甲巯咪唑亚磺酸、甲巯咪唑磺酸、甲巯咪唑S-磺酸盐、1-甲基咪唑（MI）和亚硫酸根阴离子。总的来说，结果表明多种内源性硫醇、抗坏血酸和甲巯咪唑可将MetSeO还原为SeMet。因此，SeMet氧化为MetSeO可能导致内源性硫醇和抗氧化分子的消耗。此外，甲巯咪唑对MetSeO的新型还原提供了明确的证据，表明在研究FMO介导的SeMet氧化时，甲巯咪唑不应用作替代的FMO底物。
