Structural Characterization and Antioxidant Activity of Walnut Peptides

This study prepared and structurally characterized peptides with different molecular masses from walnut meal protein by enzymatic hydrolysis with a mixture of two proteases followed by ultrafiltration, and it also explored the antioxidant activities of walnut peptides and their protective effects on oxidative damage in HepG2 cells. The results showed that the antioxidant activity of walnut protein hydrolysate (WPH) with molecular mass < 1 kDa was the strongest, with half-maximal inhibitory concentration (IC₅₀) of 11.47, 35.67 and 49.72 mg/mL for hydroxyl radical, 1,1-diphenyl-2-picrylhydrazyl (DPPH) and 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical cation scavenging capacity, respectively. Moreover, the < 1 kDa walnut peptide fraction could reduce the reactive oxygen species (ROS) content and increase the superoxide dismutase (SOD), catalase (CAT) and glutathione reductase (GSH-Rx) and glutathione peroxidase (GSH-Px) activity in HepG2 cells. The walnut peptide was irregular in shape and had a smooth surface and dense structure with numerous rough patterns and pores. The walnut peptide had a maximum absorption peak at 225 nm wavelength. The most abundant secondary structures of the walnut peptide were random coil (36.5%) and β-sheet (36.6%). These results indicate that the < 1 kDa walnut peptide has a protective effect against oxidative damage in HepG2 cells.