Photo-oxidation-induced inactivation of the selenium-containing protective enzymes thioredoxin reductase and glutathione peroxidase

Singlet oxygen (1O2) is a reactive oxygen species generated during photo-oxidation, inflammation, and via peroxidase-catalyzed reactions (e.g., myeloperoxidase and eosinophil peroxidase). 1O2 oxidizes the free amino acids Trp, Tyr, His, Cys, and Met, and those species present on peptides/proteins, with this resulting in modulation of protein structure and function. Impairment of the activity of antioxidant enzymes may be of relevance to the oxidative stress observed in a number of pathologies involving either light exposure or inflammation. In this study, the effects of 1O2 on glutathione peroxidase (GPx) and thioredoxin reductase (TrxR) activity, including the mechanisms of their inactivation, were investigated. Exposure of GPx or TrxR, either as purified proteins or in cell lysates, to Rose Bengal and visible light (an established source of 1O2) resulted in significant, photolysis time-dependent reductions in enzyme activity (10–40%, P<0.05). More extensive inhibition (ca. 2-fold) was detected when the reactions were carried out in D2O, consistent with the intermediacy of 1O2. No additional inhibition was detected after the cessation of photolysis, eliminating a role for photo-products. Methionine, which reacts rapidly with 1O2 (k∼107 M−1 s−1), significantly reduced photo-inactivation at large molar excesses, presumably by acting as an alternative target. Reductants (NaBH4, DTT, GSH, or NADPH) added after the cessation of 1O2 formation were unable to reverse enzyme inactivation, consistent with irreversible enzyme oxidation. Formation of nonreducible protein aggregates and/or fragments was detected for both photo-oxidized GPx and TrxR by SDS-PAGE. An oxidant concentration-dependent increase in protein carbonyls was detected with TrxR but not GPx. These studies thus demonstrate that the antioxidant enzymes GPx and TrxR can be irreversibly inactivated by 1O2.

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► We hypothesized that 1O2 may oxidize protein selenocysteine (Sec) residues.
► Inactivation of glutathione peroxidase and thioredoxin reductase was detected.
► Damage was enhanced by D2O, and not reversed by reducing agents.
► Protein aggregation/fragmentation and protein carbonyls are detected.
► 1O2-mediated irreversible inactivation of GPx and TrxR may exacerbate oxidative stress.