Radical scavenging and xanthine oxidase inhibitory activity of phenolic compounds from Bridelia ferruginea stem bark

Bridelia ferruginea Benth. (Euphorbiaceae) is a subtropical medicinal plant widely used in traditional African medicine against various diseases, including rheumatic pains. Seven of its constituents (3‐O‐methylquercetin (1), 3,7,3′,4′‐tetra‐O‐methylquercetin (rutisin, 2), myricetin (3), 3′,4′,5′‐tri‐O‐methylmyricetin (ferrugin, 4), 3,3′,4′,5′‐tetra‐O‐methylmyricetin (5), quercetin 3‐O‐glucoside (6), and a biflavanol gallocatechin‐[4′‐O‐7]‐epigallocatechin (7)) have been evaluated in‐vitro in the xanthine‐xanthine oxidase enzymatic system for inhibition of xanthine oxidase and radical scavenging activity. Results indicated that compounds 1, 3, 4 and 6 exhibited, at different levels, xanthine oxidase inhibiting and superoxide scavenging activity at micromolar concentrations, whereas compound 7 showed scavenging activity only. Compounds 2 and 5 were inactive in both cases. Study of the structure‐activity relationship demonstrated that for flavonoids the xanthine oxidase inhibitory activity was reduced by methylation of the hydroxyl functionality at C‐3 and in rings A and B. These results may partly explain and support the use of B. ferruginea stem bark for the treatment of rheumatic pains in traditional medicine.