The contribution of the pyrogallol moiety to the superoxide radical scavenging activity of flavonoids

Sixteen flavonoids including flavonols, flavones, flavanonol and catechins, and five aromatic compounds were examined for their ability to scavenge superoxide radical (O-2(radical anion)) generated enzymaticallyin a xanthin-xanthinoxidase system and non-enzymatically in a phenazine methosulfate-NADH system. Pyrogallol, gallic acid and its ester, were much more efficient in scavenging O-2(radical anion) than catechol. The superiority,of pyrogallol over catechol in the flavonoidal nucleus is apparent from the much higher O-2(radical anion) scavenging activity of myricetin and epigallocatechin, which contain 3',4',5'-trihydroxyl substitution in the B-ring, compared to quercetin and epicatechin, which contain 3',4'-dihydroxyl substitution, respectively. The strong O-2(radical anion) scavenging ability of pyrogallol appears to function even in the A-ring, as in baicalein, and also in the form of a pyrogalloyl ester at the C-3 position in the C-ring, as in epicatechin gallate and epigallocatechin gallate. It can be concluded that the pyrogallol moiety is an active component of flavonoids for displaying high O-2(radical anion) scavenging activity. Flavonoids and aromatics were also examined to correlate their O-2(radical anion) scavenging activity with their oxidizability, which was measured on the basis of electrochemical redox potential and the reducing ability of the Cu2+ ion. Aromatics such as pyrogallol, gallic acid and its ester, and flavonoids such as baicalein, epicatechin gallate and epigallocatechin gallate, in which, the O-2(radical anion) scavenging activity is enhanced by the presence of a pyrogallol moiety which does not belong to the B-ring, reduced the correlation between the higher O-2(radical anion) scavenging activity and the lower redox potential. The O-2(radical anion) scavenging activity was well correlated with the Cu (2+) reducing ability of flavonoids and aromatics.