Ginsenosides, known as saponins, are the major components of ginseng (Fig. 1). Ginsenosides have a steroidal skeleton with a modified side chain at C-20 (7,8). They differ from one another by the type of sugar moieties, their number, and their site of attachment. Among the saponins, the genuine sapogenins 20(S)-protopanaxa-diol and -triol have been identified as 20(S) 12p-hydroxyl-and 20(S) 6a, 12p-dihydroxy-dammarenediol-II, respectively (9). Some partly deglycosylated saponins such as ginsenoside Rh1, Rh2, and Rg3 are obtained from red ginseng as artifacts produced during steaming. Stepwise deglycosylated compounds such as compound K and 20(S)-protopanaxadiol can be generated through metabolic transformation by human intestinal bacteria. Ginsenoside Rg1 is converted into 20(S)-protopanaxatriol via ginsenoside Rh1. The binding of the sugar has been shown to influence biological activity. Rh1 and Rh2 are structurally similar, but have different activity. Ginsenoside Rh2 has been shown to decrease growth of B16-BL6 melanoma cells, and stimulates melanogenesis and cell-to-cell adhesiveness. However, Rh1 has no effect on cell growth and cell-to-cell adhesiveness, but stimulates melanogenesis (10). Although both Rh2 and Rh3 induce differentiation of promyelocytic leukemia HL-60 cells into morphological and
functional granulocytes, the potency of Rh2 is higher (11). Another factor that contributes to structural difference between ginsenosides is stereochemistry at C-20. Although both 20(S)- and 20(R)-ginsenoside Rg2 inhibit acetycholine-evoked secretion of catecholamines from cultured bovine adrenal chromaffin cells, the 20(S) isomer has a greater inhibitory effect (12). Structural changes after oral administration also contribute to diversity. Certain ginsenosides such as Rbi and Rgi are poorly absorbed after injection (13). Rbi was hydrolyzed to compound K by intestinal flora (14); compound K was shown to increase the cytotoxicity of antineoplastic drugs (15) and to induce apoptosis in B16-BL6 melanoma cells (16).
The other major bioactive ingredient of ginseng is polysaccharide. Acidic polysaccharides found in ginseng such as ginsan (17,18), ginsenan S-IA, and ginsenan S-IIA (19) have been reported to have potential immunological activities. Ginseng's immunomodulatory effects may be one of its antitumor mechanisms. Ginseng polysaccharide GH1 reduces blood glucose and liver glycogen of mice (20). A high-output inducible nitric oxide synthase (iNOS) was shown in female BALB/c mice administered intraperitoneally (i.p.) with acidic polysaccharide from ginseng (21). Rhamnogalacturonan II from the leaves of P. ginseng C. A. Meyer acts as a macrophage Fc receptor expression-enhancing polysaccharide (22).
These structure-activity relationships are useful for understanding previously identified, bioactive components of ginseng, and to isolate and develop new therapeutic ingredients of ginseng.
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