Defensin protein from sweet potato (Ipomoea batatas [L.] Lam 'Tainong 57') storage roots exhibits antioxidant activities in vitro and ex vivo

This study was designed to investigate the antioxidant activities of sweet potato defensin (SPD1) in vitro and ex vivo. Antioxidant status [2,2′-azinobis[3-ethylbenzthiazoline-6-sulphonic acid (ABTS) assay], scavenging activity against DPPH (1,1-dipheny-2-picrylhydrazyl) radical method, reducing power method, Fe²⁺-chelating ability, FTC (ferric thiocyanate) method, and protection of calf thymus DNA against hydroxyl radical-induced damage were studied in vitro. The ex vivo experiments revealed that SPD1 could decrease the production of intracellular peroxide in HepG2 cells. Four peptides, namely GFR, GPCSR, CFCTKPC and MCESASSK for testing antioxidative activity, were synthesized according to tryptic hydrolysis simulation. In the TEAC assay CFCTKPC performed the best (13.5 ± 0.3 μmol TE/g dw), even better than reduced glutathione (7.3 ± 0.2 μmol TE/g dw). In the DPPH radical assay (%), [IC₅₀ (μM) (the concentration required for scavenging 50% activity)] CFCTKPC again had the highest antioxidant activity (IC₅₀ is 11.3 ± 3.2 μM) even better than reduced glutathione (IC₅₀ is 74.3 ± 2.4 μM). In the lipid peroxidation assay, once again CFCTKPC performed the best, with an IC₅₀ value of 0.5 ± 0.0 μM better than reduced glutathione (1.2 ± 0.1 μM). These findings mean that cysteine residue is most important in antioxidant activities. It was suggested that SPD1 might contribute its antioxidant activities against hydroxyl and peroxyl radicals.