Chemical composition of lipids in freeze-dried tubers of Chinese yam harvested in Hokkaido and their growth inhibition of human colon cancer cells

Abstract

In the present study, we report on the lipid classes, including fat-soluble antioxidants, and fatty acid and sterol profiles of freeze-dried Chinese yam (Dioscorea opposita Thunb., nagaimo) tubers harvested in Hokkaido. The effect of the unsaponifiables on human colon cancer Caco-2 cells was also examined. In addition to acylglycerols and free fatty acids, five glycolipids, four phospholipids and two neutral sterol lipids were found in the tubers, which had, in general, a low lipid content (0.4%). However, diosgenin, a steroidal saponin with various physiological functions, was not detected. The major lipid classes were phosphatidic acid, diglycosyldiacylglycerol, phosphatidylcholine and phosphatidylethanolamine. In relation to components showing DPPH-radical scavenging activity, α-, β- and γ- tocopherols were detected; the total amount was approximately 2 μg/100 g. At least 17 fatty acids were identified. Among these, linoleic, palmitic and α-linolenic acids were predominant, with significant amounts of oleic and cisvaccenic acids. The unsaturation index of the fatty acid constituents in the major lipid classes was high in glyceroglycolipids and low in phosphatidylinositol and acyl sterylglycoside, the same tendency as found in potato tuber lipids. The principal sterol components were usually sitosterol and campesterol. Chinese yam unsaponifiables exhibited a growth-inhibitory effect against Caco-2 cells in a concentration-dependent manner, and the active principle was confirmed to be mainly sitosterol. It was also suggested that the unsaponifiables induced apoptosis in Caco-2 cells.