Antioxidant activities of the rice endosperm protein hydrolysate: identification of the active peptide

The defatted rice endosperm protein (REP) was digested using five different proteases (Alcalase, Chymotrypsin, Neutrase, Papain, and Flavorase) to produce the antioxidative peptide. The degree of hydrolysis of REP by Neutrase (20.00%) was slightly lower than that of Chymotrypsin, but higher than those of other enzymes. The Neutrase hydrolysate from rice endosperm protein (NHREP) took on 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity similar to α-tocopherol. Also, the reaction condition of Neutrase hydrolysis was moderate (pH of 7.0 and temperature of 37 °C). Therefore, Neutrase was chosen to be the optimum enzyme for producing the antioxidative peptide from REP. In succession, the antioxidant activities of NHREP were further evaluated. The median effective concentration (EC50) value of NHREP for hydroxyl radical scavenging activity was 2.0 mg/mL, while its superoxide radical scavenging activity did not surpassed 50% even at 6 mg/mL. The percentage inhibition of autooxidation in linoleic acid system by NHREP was 82.09%, similar to that of α-tocopherol (86.59%) on day 5 at the same concentration. NHREP displayed 89.15% chelating effect on ferrous ion at a concentration of 1000 μg/mL, and a high correlation was also observed between the reducing power and antioxidant activity of NHREP (r2 = 0.99678). Consequently, NHREP was purified and identified, and the sequence determination by MALDI-TOF/TOF MS/MS revealed that the active constituent contained eight amino acids in its sequence (Lys-His-Asn-Arg-Gly-Asp-Glu-Phe), with the molecular mass of 1002.5217 Da. After sequence interpretation and database searching, the MS/MS spectrum was matched to glutelin protein f (460–465).