Lipoxygenase inhibitory constituents from rhubarb

Phytochemical study on the ethanol extract of rhubarb led to the isolation of fifteen compounds, including five anthraquinones: chrysophanol (1), physcion (2), emodin (7), chrysophanol-8-O-beta-D-glucopyranoside (9) and emodin-8-O-beta-D-glucopyranoside (15), and ten stilbenes: desoxyrhaponticin (3), rhaponticin (4), resveratrol (5), desoxyrhapotigenin (6), rhapontigenin (8), piceatannol-3'-O-beta-D-glucopyranoside (10), piceid (11), epsilon-viniferin (12), ampelopsin B (13) and isorhaponticin (14). Their structures were identified by comparing the physicochemical data with those of published papers. Among the isolated compounds, stilbene derivatives (3-6, 8 and 10-14) showed remarkable inhibitory effect on lipoxygenase with IC50 values ranging from 6.7 to 74.1 mu M. The inhibition kinetics analyzed by Lineweaver-Burk plots found that they were competitive inhibitors with the linoleic acid at the active site of lipoxygenase. In addition, stilbenes exhibited significantly free radical scavenging activity against ABTS(center dot+) with trolox equivalent activity capacity (TEAC) values ranging from 1.16 to 4.64. Whereas, anthraquinone derivatives (1-2, 7, 9 and 15) neither inhibited lipoxygenase nor scavenged free radical ABTS(center dot+). These results indicated that stilbene derivatives were considerate to be mainly lipoxygenase inhibitor and free radical scavenger constituents of rhubarb.