- Antioxidant constituents from rhubarb: Structural requirements of stilbenes for the activity and structures of two new anthraquinone glucosides
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The methanolic extracts from five kinds of rhubarb were found to show scavenging activity for DPPH radical and ·O2-. Two new anthraquinone glucosides were isolated from the rhizome of Rheum undulatum L. together with two anthraquinone glucosides, a naphthalene glucoside, and 10 stilbenes. In the screening test for radical scavenging activity of rhubarb constituents, stilbenes and a naphthalene glucoside showed activity, but anthraquinones and sennosides did not. In addition, most stilbenes inhibited lipid peroxidation of erythrocyte membrane by tert-butyl hydroperoxide. Detailed examination of the scavenging effect on various related compounds suggested the following structural requirements; 1) phenolic hydroxyl groups are essential to show the activity; 2) galloyl moiety enhances the activity; 3) glucoside moiety reduces the activity; 4) dihydrostilbene derivatives maintain the scavenging activity for the DPPH radical, but they show weak activity for ·O2-. In addition, several stilbenes with both the 3-hydroxyl and 4′-methoxyl groups inhibited xanthine oxidase.
- Matsuda, Hisashi,Morikawa, Toshio,Toguchida, Iwao,Park, Ji-Young,Harima, Shoichi,Yoshikawa, Masayuki
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