Withanolides from the Rhizomes of Dioscorea japonica and Their Cytotoxicity

Edible yams are tropical crops that serve as important staple foods in many parts of the world. The rhizome of Dioscorea japonica, well-known as “Japanese yam”, is a food and medicinal source known as “San Yak” in Korea. Bioassay-guided fractionation and chemical investigation of the extract of this yam resulted in the identification of two new withanolides, named dioscorolide A (1) and dioscorolide B (2). The structures of these new compounds were determined by spectroscopic methods, including 1D and 2D nuclear magnetic resonance (NMR) techniques, high-resolution mass spectrometry (HRMS), and chemical methods. The cytotoxic activities of the isolates (1 and 2) were evaluated by determining their inhibitory effects on four human tumor cell lines (A549, SK-OV-3, SK-MEL-2, and HCT15) and a human normal cell line (HUVEC) using a sulforhodamine B (SRB) bioassay. Compounds 1 and 2 showed cytotoxicity against tumor cell lines (A549, SK-OV-3, SK-MEL-2, and HCT15) with IC₅₀ values ranging from 6.3 to 26.9 μM and exhibited lower activity against the normal cell line (HUVEC) with IC₅₀ values ranging from 27.1 to 28.8 μM, suggesting selective toxicity among tumor and normal cells.