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The antioxidant activity of caseins and casein-derived peptides was evaluated by using three free radical producing reactions-the lipoxygenase- and AAPH-catalyzed oxidation of linoleic acid and the hemoglobin-catalyzed oxidation of linoleic acid hydroperoxide. Caseins and casein-derived peptides were able to inhibit enzymatic and nonenzymatic lipid peroxidation, suggesting they were preferred targets for the free radical intermediates. The antioxidative feature was not lost with the dephosphorylation or the proteolysis of the proteins. The fractionation of the tryptic beta-casein digest yielded peptides with antioxidant activity. A structure-function relationship between the amino acid sequence and the antioxidant capacity and effectiveness is proposed. In addition, indirect evidence suggested that the trapping of free radicals by the proteins/peptides was accompanied by the oxidation of proteins/peptides, according to a sequence-specific mechanism.

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