Purification and antioxidant activities of peptides from sea squirt (Halocynthia roretzi) protein hydrolysates using pepsin hydrolysis

Sea squirt (Halocynthia roretzi) are widely used as a food material in Korea. Sea squirt protein (SSP) were isolated by alkaline solubilization followed by acidic precipitation. SSP were further hydrolyzed using pepsin hydrolysis at pH 2 for 2 h to release antioxidant peptides. SSP hydrolysates (SSPH) showed DPPH radical scavenging activity of 31 ± 1% at 1 mg/mL, ORAC of 576 ± 1 μM trolox equivalent (TE)/mg SSPH, and ABTS+ radical scavenging activity of 38 ± 1 μM TE/mg SSPH. Three antioxidant peptides LEW, MTTL, and YYPYQL were purified and identified from SSPH using chromatography and tandem mass spectrometry (LC-MS/MS). The antioxidant activities of LEW, MTTL, and YYPYQL were evaluated using DPPH and ABTS+ radical scavenging activity, Fe2+ chelating activity, ORAC, and reducing power. Among the three peptides, LEW showed the highest DPPH radical scavenging activity of 75% at 5 mM. The ORAC value of LEW (77.1 μM TE/50 μM LEW) and YYPYQL (84.6 μM TE/50 μM YYPYQL) are higher than glutathione (18.5 μM TE/50 μM GSH), as a positive control. The three antioxidant peptides showed higher Fe2+ chelating activity than GSH, but ABTS+ radical scavenging activity and reducing power of LEW, MTTL, and YYPYQL were lower than GSH. Taken together, these results suggest that SSPH and/or the purified antioxidant peptides may be useful as a functional food additive.