Antioxidant and Hypoglycemic Activities of the Soluble and Nondigestible Chinese Yam (<i>Dioscorea opposita</i> Thunb.) Polysaccharides with Covalent Se-Grafting

Yam (Dioscorea opposita Thunb.) is cultivated as one of the staple food crops, and its edible tubers are found to contain various bioactive substances, especially the so-called soluble and nondigestible yam polysaccharides (YPs). However, whether covalent Se-grafting of YP could lead to changed antioxidant and hypoglycemic activities remains unknown. YPs were thus separated from fresh Chinese yam tubers using water extraction and enzymatic treatment and then chemically selenized to yield two products, namely, YPSe-I and YPSe-II. These products acquired covalent Se-grafting in the form of selenite and had different Se contents (0.795 and 1.48 versus 0.035 g/kg) or selenization extents. The results obtained from the determination of antioxidant capacity indicated that the two Se-grafted products possessed a higher antioxidant activity compared to YP because they were more effective in scavenging both OH and ABTS radicals or reducing Fe3+ to Fe2+. Meanwhile, the two Se-grafted products, especially YPSe-II, had a higher capacity than YP to inhibit the activities of two digestive enzymes, namely, α-amylase and α-glucosidase. The results derived from cell experiments indicated that YP, YPSe-I, and YPSe-II were not cytotoxic to the normal BRL-3A liver cells and established insulin-resistant BRL-3A cells. However, YPSe-I and particularly YPSe-II exhibited a hypoglycemic effect in the model cells that was higher than that of YP, resulting in increased glucose consumption and enhanced oxidase activities in superoxide dismutase, catalase, and glutathione peroxidase. Data comparison results also manifested that YPSe-II consistently demonstrated higher antioxidant and hypoglycemic activities than YPSe-I. It was thus concluded that the covalent Se-grafting and high selenization extent endowed the Se-grafted YP products with enhanced beneficial functions, such as antioxidant and hypoglycemic effect. Meanwhile, this covalent Se-grafting could potentially serve as a suitable chemical approach to modify the two beneficial functions of natural polysaccharides.