Elsevier

Free Radical Biology and Medicine

Volume 51, Issue 12, 15 December 2011, Pages 2288-2299
Free Radical Biology and Medicine

Original Contribution
Catalytic activity of selenomethionine in removing amino acid, peptide, and protein hydroperoxides

https://doi.org/10.1016/j.freeradbiomed.2011.09.027Get rights and content

Abstract

Selenium is a critical trace element, with deficiency associated with numerous diseases including cardiovascular disease, diabetes, and cancer. Selenomethionine (SeMet; a selenium analogue of the amino acid methionine, Met) is a major form of organic selenium and an important dietary source of selenium for selenoprotein synthesis in vivo. As selenium compounds can be readily oxidized and reduced, and selenocysteine residues play a critical role in the catalytic activity of the key protective enzymes glutathione peroxidase and thioredoxin reductase, we investigated the ability of SeMet (and its sulfur analogue, Met) to scavenge hydroperoxides present on amino acids, peptides, and proteins, which are key intermediates in protein oxidation. We show that SeMet, but not Met, can remove these species both stoichiometrically and catalytically in the presence of glutathione (GSH) or a thioredoxin reductase (TrxR)/thioredoxin (Trx)/NADPH system. Reaction of the hydroperoxide with SeMet results in selenoxide formation as detected by HPLC. Recycling of the selenoxide back to SeMet occurs rapidly with GSH, TrxR/NADPH, or a complete TrxR/Trx/NADPH reducing system, with this resulting in an enhanced rate of peroxide removal. In the complete TrxR/Trx/NADPH system loss of peroxide is essentially stoichiometric with NADPH consumption, indicative of a highly efficient system. Similar reactions do not occur with Met under these conditions. Studies using murine macrophage-like J774A.1 cells demonstrate a greater peroxide-removing capacity in cells supplemented with SeMet, compared to nonsupplemented controls. Overall, these findings demonstrate that SeMet may play an important role in the catalytic removal of damaging peptide and protein oxidation products.

Graphical abstract

Highlights

► Peptide and protein hydroperoxides are key intermediates in protein oxidation. ► These peroxides are poorly detoxified by classical peroxidases and protective enzymes. ► Selenomethionine reacts stoichiometrically with hydroperoxides to give a selenoxide ► The selenoxide is re-reduced by GSH or thioredoxin reductase/thioredoxin /NADPH. ► Cells supplemented with selenomethionine have greater peroxide reduction capacity.

Section snippets

Materials

SeMet, Met, GSH, l-ascorbic acid, bovine lactalbumin, and soybean trypsin inhibitor were obtained from Sigma–Aldrich (Australia). N-Ac-Trp-OMe, Gly-His-Gly, and Gly-Tyr-Gly were from Bachem (Bubendorf, Switzerland). H2O2, bovine serum albumin (BSA; Cohn faction V, essentially fatty acid free), and NADPH were obtained from Roche (Castle Hill, Australia). Escherichia coli Trx and TrxR, and rat recombinant TrxR, were obtained from IMCO Corp. (Stockholm, Sweden). Blood was obtained from multiple

SeMet reduces H2O2 and amino acid, peptide, and protein hydroperoxides in a dose- and time-dependent manner

Incubation of H2O2 or hydroperoxides present on N-Ac-Trp-OMe, Gly-His-Gly, Gly-Tyr-Gly, or BSA with varying molar excesses of SeMet for 3 min at 22 °C resulted in a significant loss of hydroperoxides (Fig. 1). The extent of hydroperoxide loss was markedly affected by the structure of these species, with the overall order of reactivity being Gly-His-Gly hydroperoxides > Gly-Tyr-Gly hydroperoxides ≥ N-Ac-Trp-OMe hydroperoxides > BSA hydroperoxides > H2O2. The extent of hydroperoxide removal was

Discussion

Selenium is an essential micronutrient, with a deficiency in this element reported to increase the sensitivity of tissues and cells to oxidative damage [26]. One major mechanism through which selenium may afford protection is via up-regulation of the expression and activity of seleno-antioxidants such as glutathione peroxidase and thioredoxin reductase [13], [14], [27]. Both sodium selenite and SeMet have been widely used in in vitro and animal studies to assess the antioxidant benefit of

Acknowledgments

The authors are grateful to Dr. Philip E. Morgan for assistance with the HPLC experiments in this study. This work was supported financially by the Australian Research Council, through the Centres of Excellence (CE0561607) and Discovery (DP0988311) Programs, and the National Heart Foundation (Grant in Aid: G09S4313).

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