Spirostane-type steroidal saponin from Allium hookeri roots with mushroom tyrosinase inhibitory activity

Allium hookeri (Liliaceae) has been received the increasing attention as a bioactive resource due to its potent biological activities including anti-oxidant, anti-obesity, anti-microbial and lipid-regulating activities. The beneficial effects of A. hookeri are known contributed from the high content of organosulfur compounds in A. hookeri. Though a variety of articles demonstrated that A. hookeri contains 'saponin' as a bioactive constituent, the scientific evidence to prove it was limited. In the present study, we have attempted to identify saponin contained in A. hookeri through chromatographic isolation and NMR spectroscopic methods. As a result, a spirostane-type steroidal saponin (1) has been successfully isolated from the methanolic extract of A. hookeri roots. The structure of 1 was elucidated by extensive 1D and 2D spectroscopic methods including H-1-NMR, C-13-NMR, H-1-H-1 COSY, HSQC, HMBC and NOESY; identified as (3 beta, 22R, 25S)-spirost-5-en-3yl O-6-deoxy-alpha-L-mannopyrano-syl-(1 -> 4)-O-6-deoxy -alpha-L-mannopyranosyl-(1 -> 4)-O-[6-deoxy-alpha-L-mannopyranosyl-(1 -> 2)]-beta-D-gluco pyranoside. 1 showed the significant inhibitory activity on mushroom tyrosinase with IC50 values of 248.7 mu M while the inhibition on alpha-glucosidase was not significant.