Flavonols and flavan-3-ols as modulators of xanthine oxidase and manganese superoxide dismutase activity

Experiments were performed to assess the dose-dependent effects of quercetin, kaempferol, (+) catechin, and (?) epicatechin on superoxide radical production through the modulation of manganese superoxide dismutase and xanthine oxidase activities. The experiments were carried out at flavanoid concentrations ranging from 1?µM to 100?µM. This investigation highlighted that flavonols induced opposite effects on superoxide radical production at different doses, i.e. pro-oxidant at the highest concentration (100?µM) and anti-oxidant at the lowest concentration (1?µM). Similar behaviors were observed for xanthine oxidase with flavan-3ols. The diastereoisomer (the catechin) acted as a stronger radical scavenger than the epicatechin. However, flavan-3ols were less pro-oxidant than flavonols: in fact, the addition of the superoxide dismutase enzyme was able to cancel the flavan-3ols’ pro-oxidant effect. This study also shows that the absence of the 4-carbonyl group conjugated with the 2–3 double bonds in the heterocyclic ring cancelled the pro-oxidant effect of flavan-3ols. The opposite dose-dependent effects of flavonols suggest that they may be used as either a pro-oxidant or antioxidant.