Anti-depressive activities and biotransformation of timosaponin B-III and its derivatives

Timosaponin B-III (TB-III) is a steroidal saponin isolated from the rhizome of Anemarrhenae asphodeloides (Liliaceae). The biotransformation of TB-III by β-glucosidase was investigated. Three biotransformation products were isolated and their structures were identified as timosaponin B-III-a (M1), (20R,25S)-5β-spirostane-3β-ol-3-O-β-d-glucopyranosyl-(1 → 2)-β-d-galacopyanoside (M2) and timosaponin AIII (M3). Then the four compounds were evaluated for their anti-depressive activity in mice by the open field test, tail suspension test and forced swimming test. As a result, TB-III, M1 and M3 exhibited modest anti-depressive activity. Structure–activity relationships were investigated and the preliminary conclusions are summarised as follows: the glycosyl at C-3 and C-26 can increase the activity, the double bond between C-20 and C-22 might be important for the anti-depressive activity, the R-configuration at C-22 and S-configuration at C-20 are necessary for its anti-depressive activity.