Antioxidative activities of 28 synthetic peptides, which were designed based on an antioxidative peptide (Leu-Leu-Pro-His-His) derived from proteolytic digests of a soybean protein, against the peroxidation of linoleic acid in an aqueous system were measured by the ferric thiocyanate method. The results for the hydroperoxide levels derived from linoleic acid agreed with those obtained by reversed-phase high-performance liquid chromatography. The deletion of the C-terminal His decreased the activity, whereas the deletion of the N-terminal Leu had no effect. In the peptide sequence, His and Pro played important roles in the antioxidative activity and, among the peptides tested, Pro-His-His was the most antioxidative. The activity decreased on substitution of the second His with d-His. Introduction of Tyr to the positions of Pro or His did not increase the activities of the corresponding peptides. Antioxidative peptides showed synergistic effects with nonpeptidic antioxidants as observed in soybean protein hydrolysates. The magnitude of the effects, however, did not correlate with the antioxidative activities of the peptides. Keywords: Antioxidative peptides; antioxidant; soybean protein hydrolysate; ferric thiocyanate method; synergistic effect

The properties of 22 synthetic peptides containing histidine, which were designed on the basis of the antioxidative peptide (Leu-Leu-Pro-His-His) derived from proteolytic digests of a soybean protein, were examined with regard to their antioxidative activity against the peroxidation of linoleic acid and the scavenging effects on active oxygen and free radical species. The antioxidative activities of these peptides in an emulsion oxidation system using 2,2'-azobis(2-amidinopropane) dihydrochloride as a radical initiator correlated well within an aqueous system. Although the histidine-containing peptides had a quenching activity on singlet oxygen, they did not show antioxidative activity in an 2,2'-azobis(2,4-dimethylvaleronitrile)-induced oxidation system or scavenging effects on 1,1-diphenyl-2-picrylhydrazyl radical and superoxide. The metal-ion chelating activities and the hydrophobicities of these peptides showed no direct correlation with their antioxidative activities. Leu-Leu-Pro-His-His was modified with a hydroxyl radical in an aqueous ethanol system during the peroxidation of linoleic acid.

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