Polyphenol autoxidation and prooxidative activity induce protein oxidation and protein-polyphenol adduct formation in...

Polyphenols are well-known for their antioxidant properties, but their prooxidative activity remain less understood. This study quantitatively examined the formation of hydrogen peroxide (H₂O₂) during the autooxidation of nine different polyphenols in model systems, investigating how it impacts protein oxidation and protein-polyphenol covalent adduct formation. Polyphenols (4 mM) generated H₂O₂ in the range of 0.2-242 μM, depending on type of polyphenol, incubation time, temperature, and pH, but no clear relationship between polyphenol structure and H₂O₂ production was observed. The presence of free amino acids and proteins (bovine serum albumin and β-lactoglobulin) inhibited H₂O₂ formation, with Cys completely scavenging H₂O₂. Met was highly susceptible to oxidation with a 25-75% loss, forming methionine sulfoxide through a two-electron oxidation pathway. Trp and Tyr were oxidized to produce dioxindolyl-ʟ-alanine, kynurenine, 3,4-dihydroxyphenylalanine, N'-formylkynurenine, and 5-hydroxytryptophan in the nmol/mol-mmol/mol amino acid range. Furthermore, autoxidation of polyphenols resulted in >177 distinct amino acid/protein-polyphenol adducts as identified using LC-Orbitrap-MS/MS analysis.