Characterisation of the antioxidant activity of flavonoids

Inhibition of the superoxide anion (O2−) generation catalysed by xanthine oxidase using certain flavonoids was examined to determine their antioxidant effects. All of the flavonoids and their glycosides, except for kaempferol-3-glucoside, considerably and markedly inhibited O2− generation. Flavonoids also inhibited uric acid formation, whereas their glycosides did not. The flavonoids and their glycosides with scavenging activity against the 1,1-diphenyl-2-picrylhydrazyl radical had more than one conjugated en-diol group and exhibited more potent inhibition of the O2− generation than the uric acid formation catalysed by xanthine oxidase. The results show that the inhibition of O2− generation with flavonoids without any conjugated en-diol group was due to competitive inhibition of uric acid formation, while the inhibition with flavonoids having more than one conjugated en-diol group was due to the reduced form of the enzyme.