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 IC(50) values 16.4 ± 1.5 μM and 11.5 ± 1.1, respectively, when the IC(50) 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 ([IC(50) of MAO-B]/ [IC(50) 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.