Relationship between haemolytic and adjuvant activity and structure of protopanaxadiol-type saponins from the roots of Panax notoginseng

Four protopanaxadiol-type saponins (PDS), ginsenosides-Rb1, -Rd, notoginsenosides-K, -R4 isolated from the roots of Panax notoginseng were evaluated for their haemolytic activities and adjuvant potentials on the cellular and humoral immune responses of ICR mice against ovalbumin (OVA). The effect of the substitution pattern of these PDS on their biological activities was investigated and structure–activity relationships were established. Among four PDS, the ranking of the haemolytic activity was K > R4 > Rb1 > Rd (P < 0.01 or <0.001). Rd, Rb1, and K could significantly enhance mitogen- and OVA-induced splenocyte proliferation in the OVA-immunized mice (P < 0.001), with the order in terms of stimulation index being Rd > Rb1 > K > R4. OVA-specific IgG, IgG1, IgG2a and IgG2b antibody levels in the OVA-immunized mice were significantly enhanced by four PDS. Adjuvant potentials of Rd on antibody responses were higher than those of other three PDS. Meanwhile, Rd also significantly enhanced the production of the Th1 and Th2 cytokines in OVA-immunized mice (P < 0.05 or <0.01). The structure–activity relationship studies suggested that the length of sugar side chains at position C-20 and the linkage of glucose moiety at position C-3 of protopanaxadiol could affect the haemolytic and adjuvant activities of PDS. The information about this structure/function relationship might be useful for developing semisynthetic tetracyclic triterpenoid saponin derivatives with immunological adjuvant activity, as well as a reference to the distribution of the functional groups composing the saponin molecule.