Four new ginsenosides from red ginseng with inhibitory activity on melanogenesis in melanoma cells

During a search for novel melanogenesis inhibitors originating from nature sources, four new ginsenosides, including three dammarane-type triterpenoid saponins, 20(S)-ginsenoside-Rf-1a (1), 20Z-ginsenoside-Rs4 (2), 23-O-methylginsenoside-Rg11 (3), and one oleanane-type saponin, ginsenoside-Ro-6'-O-butyl ester (4) were isolated from red ginseng (the steamed ginseng) to evaluate their protective effects against melanogenesis. Compounds 2 and 3 exhibited potent inhibitory effects against both melanin synthesis and tyrosinase activity in a dose-dependent manner in the α-MSH-stimulated B16 melanoma cells, and were more potent than the positive control arbutin, a well-known tyrosinase inhibitor. The results indicated that just the two carbon-20(22) double-bond-type ginsenosides showed strong inhibiting activity on melanogenesis through reducing tyrosinase activity. Thus, ginsenosides with such similar chemical structure in red ginseng may be potential natural products as tyrosinase inhibitors against malignant melanoma.