Relationship between adjuvant activity and amphipathic structure of soyasaponins.

A correlation between adjuvant activity and amphipathic structure of saponin was first demonstrated on an experimental basis using structurally consecutive analogues. To clarify the physicochemical factors regulating the adjuvanticity of saponin, we compared the profile of the antibody response against chicken ovalbumin (OVA) in mice and hydrophile-lipophile balance (HLB) of eight purified soyasaponins. Soyasaponins bearing sugar chain(s) showed adjuvanticity stimulating anti-OVA total-IgG and IgG1 antibody responses, while their corresponding aglycones soyasapogenols A and B, did not. Among bisdesmosidic soyasaponins, soyasaponin A(1) (HLB: 26.9) with a long sugar side chain induced stronger total-IgG and IgG1 antibody responses than soyasaponin A(2) (HLB: 21.4). For monodesmosidic soyasaponins, the ranking in terms of antibody response was soyasaponin I (which has the highest HLB value (13.6) among the monodesmosidic soyasaponins) > soyasaponin II (HLB: 12.2) > soyasaponin III (HLB: 10.0). The adjuvant activity increased with the HLB value. The length, the number, and the composition of sugar side chains affecting the HLB value would give the overall conformation of each saponin molecule, and the amphipathic structure may define the fundamental adjuvanticity of saponins.