Evaluation of the total oxidant scavenging capacity of saponins isolated from Platycodon grandiflorum

The antioxidant activity of saponins isolated from Platycodon grandiflorum (PG; Balloon flower) was determined using the total oxidant-scavenging capacity (TOSC) assay. Platycodigenin, polygalacic acid, platycodin D, platycoside E and deapioplatycoside E were isolated and their structures were characterised based on their physical and spectral properties and by comparison of these results with similar data in the literature. Platycodin D showed the greatest TOSC value against peroxyl radicals, followed (in decreasing order) by polygalacic acid, platycodigenin, deapioplatycosides E and platycoside E. Although the TOSC value of the saponins against peroxyl radicals was less than that of glutathione (GSH) and Trolox used as positive controls. However, TOSC value of platycodigenin, deapioplatycoside E, platycodin D or platycoside E against peroxynitrite was 2.35-, 1.27-, 1.02- or 0.75-fold of GSH, respectively, while polygalacic acid exhibited no scavenging capacity of peroxynitrites. These results suggest importance of the presence of hydroxyl group at carbon 24 in platycodigenin in peroxynitrite scavenging. As the number of attached sugar residues in the saponin glycosides is increased, the scavenging capacity of peroxyl radical, but not peroxynitrite was significantly decreased. These results showed that PG saponins have potent antioxidant activities, which is different according to the structure of aglycones and the number of attached sugar residues.