EPR Spin-Trapping Studies of the Hydroxyl Radical Scavenging Activity of Carnosine and Related Dipeptides

The electron paramagnetic resonance technique of spin trapping was employed to study the interaction of hydroxyl radicals with carnosine and related His-containing dipeptides. His and His-containing dipeptides were able to quench 49.1-94.9% of hydroxyl radicals produced by Fe²⁺ and H₂O₂. The hydroxyl radical scavenging activity of dipeptides was in the order β-Ala < γ-aminobutyric acid (GABA) < Gly < His < homocarnosine < carnosine < Gly-His. Nonhydroxyl radicals spin trapped by 5,5-dimethylpyrroline N-oxide with similar splitting constants of α^N= 15.6 G and α^Hβ= 18.8 G were detected from homocarnosine, carnosine, and Gly-His in the presence of the hydroxyl radical generation system. The ability of dipeptides and amino acids to inhibit hydroxyl radical catalyzed oxidation of phosphatidylcholine liposomes was in the order β-Ala < Gly = GABA < Gly-His < His < homocarnosine < carnosine. These data suggest that the hydroxyl radical scavenging and antioxidant activities of the dipeptides are related to the presence of the peptide bond and the amino acid composition of the dipeptides.