Superoxide scavenging by polyphenols: effect of conjugation and dimerization

The superoxide anion scavenging capacity of two flavonols (quercetin and kaempferol) and some of their conjugates (quercetin-3-rhamnoglucoside, quercetin-3-sophoroside, quercetin-3-sulphate, quercetin-3-glucuronide, kaempferol-3-sophoroside, kaempferol-3-glucuronide) and of several hydroxycinnamic acids (caffeic acid, ferulic acid, 5-5 diferulic acid, 8-O-4 diferulic acid and 8-8 diferulic acid) were studied. Superoxide anions were generated non-enzymatically in a phenazine methosulphate-NADH system and assayed by reduction of nitro-blue tetrazolium. Among the flavonols examined, the most effective scavengers of superoxide anions were the sophoroside, glucuronide and rhamnoglucoside conjugates. Conversely, quercetin-3-sulphate and the flavonol aglycones, exhibited some pro-oxidant activity at the range of concentrations tested (0.5-10 microM). These results show that conjugation has a marked effect on the scavenging capacity of flavonols and that the type of conjugate at the 3-position determines the final superoxide scavenging capacity. Caffeic acid and ferulic acid showed no effect on the generation of superoxide anions by phenazine methosulphate-NADH. However, dimerization of ferulic acid enhanced the superoxide scavenging capacity of this hydroxycinnamic acid, but this depended on the type of linkage between the monomers. The order, from highest to lowest, of superoxide radical scavenging capacity for the dimers of ferulic acid was: 5-5-diferulic acid > 8-O-4-diferulic acid > 8-8-diferulic acid.