Structure-function relationship of the saponins from the roots ofPlatycodon grandiflorumfor hemolytic and adjuvant activity

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To assess the contribution of the aglycone and sugar chain to the biological activity of saponins from Platycodon grandiflorum, seven structurally consecutive saponins, platycodin D (PD), D2 (PD2), D3 (PD3), platycoside A (PA), E (PE), deapioplatycoside E (DPE), and polygalacin D2 (PGD) were compared for their hemolytic activities and adjuvant potentials on the immune responses to Newcastle disease virus-based recombinant avian influenza vaccine (rL-H5) in mice. Among seven compounds, the order of the hemolytic activity was PGD≈PD>PD2>PA>PD3>PE>DPE. PD, PD2, PA, and PGD significantly not only promoted concanavalin A (Con A)-, lipopolysaccharide (LPS)- and antigen-induced splenocyte proliferation, but enhanced the NK cell activity in mice immunized with rL-H5. PD and PD2 increased the antigen specific IgG, IgG1, IgG2a, and IgG2b antibody titers, while PA and PGD only induce the IgG and IgG1 antibody responses in the immunized mice. However, the other three saponins were not observed for adjuvant activity. The results suggested that the sugar chains attached to C-3, the glycidic moiety at C-28 of aglycone, as well as aglycone affect their biological activities. Interestingly, their hemolytic and adjuvant activities increased with the retention time by reverse phase HPLC analysis. The retention time may be useful for primary estimation of fundamental adjuvanticity of saponin with the same aglycone.

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