Inhibition of monoamine oxidase by stilbenes from Rheum palmatum

Seven stilbenes and one catechin were bioactivity-guidedly isolated from the rhizomes of Rheum palmatem. Their structures were identified as piceatannol(1), resveratrol(2), piceid(3), rhapontigenin(4), piceatannol-3'-O-β-D-glucopyranoside(5), rhaponticin(6), catechin(7) and desoxyrhapontigenin(8). Anti-monoamine oxidase (MAO) activities of compounds 1–8 were tested. Compounds 1 and 8 showed significant MAO inhibitory activities with IC50 values 16.4±1.5 μM and 11.5±.1.1, respectively, when the IC50 value of iproniazid as a standard was 6.5 ±0.5 μM. The selectivity of compounds 1-8 against MAO-A and MAO-B were also evaluated. The results showed that compounds 4、6、8 preferred to inhibit MAO-A rather than MAO-B with selectivity values ([IC50 of MAO-B]/ [IC50 of MAO-A]) of 4.74, 10.01 and 9.42, respectively . The preliminary structure–activity relationships (SARs) of these compounds were discussed and the molecular modeling was also performed to explore the binding mode of inhibitors at the active site of MAO-A and MAO-B.