Antioxidant constituents from rhubarb: Structural requirements of stilbenes for the activity and structures of two new anthraquinone glucosides

The methanolic extracts from five kinds of rhubarb were found to show scave nging activity for DPPH radical and .O-2(-). Two new anthraquinone glucosid es were isolated from the rhizome of Rheum undulatum L. together with two a nthraquinone glucosides, a naphthalene glucoside, and 10 stilbenes. In the screening test for radical scavenging activity of rhubarb constituents, sti lbenes and a naphthalene glucoside showed activity, but anthraquinones and sennosides did not. In addition, most stilbenes inhibited lipid peroxidatio n of erythrocyte membrane by tert-butyl hydroperoxide. Detailed examination of the scavenging effect on various related compounds suggested the follow ing structural requirements; 1) phenolic hydroxyl groups are essential to s how the activity; 2) galloyl moiety enhances the activity; 3) glucoside moi ety reduces the activity; 4) dihydrostilbene derivatives maintain the scave nging activity for the DPPH radical, but they show weak activity for .O-2(- ) In addition, several stilbenes with both the 3-hydroxyl and 4'-methoxyl g roups inhibited xanthine oxidase. (C) 2000 Elsevier Science Ltd. All rights reserved.