Protein oxidation in meat is considered to decrease meat tenderness due to protein disulfide cross-link formation of thiol-containing amino acid residues. An LC-MS method for detection of thiol-quinone adducts (RS-QH(2)) in myofibrillar proteins was developed to investigate the interaction between phenols, as protective antioxidants, and proteins from meat under oxidative conditions using aqueous solutions of (i) cysteine (Cys), (ii) glutathione (GSH), (iii) bovine serum albumin (BSA), or (iv) a myofibrillar protein isolate (MPI). The aqueous solutions were incubated at room temperature (30 min) with 4-methyl-1,2-benzoquinone (4MBQ) prepared from oxidation of 4-methylcatechol (4MC) by periodate resin or incubated at room temperature (5 h) with 4MC and Fe(II)/H(2)O(2). GSH, BSA, and MPI were hydrolyzed (6 N HCl, 110 °C, 22 h) after incubation, and the cysteine-quinone adduct, Cys-QH(2) (m/z 244.2) was identified according to UV and mass spectra after separation on an RP-C18 column. The thiol-quinone adduct was present in all thiol systems after incubation with 4MBQ or 4MC oxidized by Fe(II)/H(2)O(2). Direct reaction with 4MBQ resulted in each case in increased Cys-QH(2) formation compared to simultaneous oxidation of thiol source and 4MC with Fe(II)/H(2)O(2). The covalent bonds between quinones and thiol groups may act as a potential antioxidant by inhibiting disulfide protein cross-link formation.
Plant-derived polyphenolic compounds have received much attention for their ability to sequester high-energy free radicals in a great variety of food-related and biological systems, protecting those systems from oxidative change. The ability of these compounds to scavenge free radicals has always been attributed to their phenolic functionality, from which a hydrogen atom can be easily abstracted. In this study, the cinnamates and the ubiquitous hydroxycinnamates were found to equally suppress the formation of oxidation products in wine exposed to the Fenton reaction (catalytic Fe(II) with hydrogen peroxide). Mechanistic investigations led to the unexpected discovery that the α,β-unsaturated side chain of cinnamic acids could efficiently trap 1-hydroxyethyl radicals, representing a newly discovered mode of antioxidant radical scavenging activity for these broadly occurring compounds in a food system. The proposed pathway is supported by prior fundamental studies with radiolytically generated radicals.
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