Storage proteins of two cultivars of sweet potato (Ipomoea batatas L.) and their protease hydrolysates exhibited antioxidant activity in vitro

The major root storage proteins of sweet potato (Ipomoea batatas Lam.), trypsin inhibitors (TIs), were purified from tuberous roots of two cultivars, Tainong 57 (T57) and T65, by trypsin-Sepharose 4B affinity column. The major protein was 28 kDa TI from two sweet potato cultivars. The purified TIs from T57 and T65 showed dose-dependent DPPH and hydroxyl radical-scavenging activities with glutathione as a control, and casein or bovine serum albumin were used for comparisons. In average, scavenging capacity for superoxide radical of 100 μg TN57 TIs and TN65 TIs were equivalent to 10.49 and 6.13 units of recombinant human superoxide dismutase, respectively. TN65 TIs had activity in the protection of Cu ²⁺-induced human LDL peroxidation, about three-fold that of TN57 TIs. The capacity of TN65 TIs to protect calf thymus from hydroxyl radical-induced DNA damage was about 10-fold that of TN57 TIs. The TN57 TIs showed dose-dependent (1-8 mg/ml) protection effect on dihydrorhodamine 123 (DHR) against peroxynitrite-mediated oxidation. Modifications of denatured TIs by iodoacetamide, or N-bromosuccinimide showed that Trp (tryptophan) residues mainly contributed scavenging activity against hydroxyl radical, while Cys (cysteine) residues mainly contributed scavenging activity against DPPH radical. Putting the above data together we propose that the 3D conformations of native molecular forms of both TN65 TIs and TN57 TIs are important factors that determine the relative capacities of the samples when different antioxidant test systems with different pH, ionic strength, chemical composition and temperature are used. Pepsin treatment or sequential treatments of pepsin and chymotrypsin for various times showed that the DPPH radical-scavenging activity of TI hydrolysates increased during hydrolysis.