Antioxidative activity of nonenzymatically browned proteins produced in oxidized lipid/protein reactions

The antioxidative activity of nonenzymatically browned proteins was studied to analyze the contribution of oxidized lipid/amino acid reaction products (OLAARP) to the antioxidative activities observed for proteins and protein hydrolysates. Bovine serum albumin (BSA) was incubated overnight with (E)-4,5-epoxy-(E)-2-heptenal at 37 degrees C and later fractionated on Sephacryl S-200-HR. Two modified protein fractions were obtained, which corresponded to monomeric and dimeric modified BSA (MBSA and DBSA, respectively). Both MESA and DBSA exhibited decreased basic amino acid residues and the presence of OLAARP residue epsilon-N-pyrrolylnorleucine. BSA, MBSA, DBSA, and butylated hydroxytoluene (BHT) (added at 10, 30, and 50 ppm) were tested for antioxidative activity in soybean oil using the thiobarbituric acid-reactive substances (TEARS) assay, and all of them significantly (p < 0.05) decreased TEARS formation. The order of effectiveness obtained was ESA < MESA < DBSA < BHT and was parallel to the number of OLAARP residues in the protein. These results suggest that OLAARP formation conferred to the modified protein their antioxidative activities.