A novel trypsin inhibitor from sweet potato (Ipomoea batatas Lam.) leaves and its synthesized peptides with antioxidant activities in vitro

Recombinant SPLTI-a [sweet potato leaf trypsin inhibitor-a] overproduced in E. coli (M15) was purified by Ni-chelated affinity chromatography. The molecular mass of SPLTI-a is ca. 8000 Da as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). SPLTI-a was examined using different antioxidative models (Total antioxidant status, reducing power method, Fe-chelating ability, ferric thiocyanate (FTC) method, and protecting calf thymus DNA against hydroxyl radical-induced damage). The SPLTI-a protein with a concentration of 100 μg/mL exhibited highest activity (expressed as 2.12 ± 0.02 mM Trolox equivalent antioxidative value, TEAC) in total antioxidant status test. Like total antioxidant status, the reducing power, Fe-chelating ability, FTC activity and protecting calf thymus DNA against hydroxyl radical-induced damage all showed that SPLTI-a polypeptide has significant antioxidant activities. It was found that the antioxidant activity increased after 24 h hydrolysis of SPLTI-a by trypsin from 18% (0 h) to about 35% (24 h). Accumulation of shorter peptides increased along the longer trypsin incubation. The obtained VR, STIEK, ITDGK, and EYIFDR showed IC50 (concentration for 50% inhibition) values of 5.83, 3.75, 2.65, and 0.73 mM, respectively, when scavenging activity of DPPH radicals (%) was measured. These findings mean that tyrosine residue is most important in antiradical activities. It was suggested that SPLTI-a possess antioxidant activities.