Anti-inflammatory activity of extracts and a saponin isolated from Melilotus elegans

The crude methanol extract of Melilotus elegans Ser. (Fabaceae), a plant widely used in Ethiopian traditional medicine for the treatment of asthma, haemorrhoid and lacerated wounds showed a significant anti-inflammatory activity against carrageenin-induced rat paw oedema. At a dose corresponding to 333.3 mg per kg body weight of dry plant material, the methanol extract displayed a strong inhibitory effect that was comparable to the inhibitory effect of 1 mg/kg of indomethacin in the same test system. Bioassay guided fractionation of the alcoholic extract led to the isolation of an oleanene-type triterpene saponin identified as azukisaponin V (1) ((3-O-[alpha-L-rhamnopyranosyl-(1 --> 2)-beta-D-glucopyranosyl(1 --> 2)-beta-D-glucuronopyranosyl]-soyasapogenol B). The structure of the compound was identified by using MS and extensive one- and two-dimensional NMR experiments (1H, 13C, COSY, HMQC, HMBC and NOESY). One hour after injection of carrageenin, inhibition of oedema exerted by 1 was approximately ten times higher than that of indomethacin on a molar basis.