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-beta-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 mu M and 11.5 +/- 1.1, respectively, when the IC50 value of iproniazid as a standard was 6.5 +/- 0.5 mu M. 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.