Antioxidant and pro-oxidant actions of the plant phenolics quercetin, gossypol and myricetin

The plant-derived phenolic compounds gossypol, quercetin and myricetin are powerful inhibitors of iron-induced lipid peroxidation in rat liver microsomes, under all five experimental conditions tested and at low micromolar concentrations (IC50 ⩽ 1.5 μM). However, they greatly accelerate the generation of hydroxyl radicals (.OH) from H2O2 in the presence of Fe3+-EDTA at pH 7.4, as measured by the deoxyribose assay. At 100 μm, the three phenolic compounds enhanced · OH formation up to eight-fold. The hydroxyl radical generation was inhibited by catalase and Superoxide dismutase, suggesting a mechanism in which the phenols oxidize to produce Superoxide radical, which then assists · OH generation from H2O2 in the presence of Fe3+-EDTA. At concentrations up to 75 μM, quercetin and myricetin also accelerate bleomycin-dependent DNA damage in the presence of Fe3+, possibly by reducing the Fe3+-bleomycin-DNA complex to the Fe2+ form. Hence these naturally-occurring substances can have pro-oxidant effects under some reaction conditions and cannot be classified simplistically as “antioxidants”.