In vitro anti-nitrosative,antioxidant, and cytotoxicity activities of plant flavonoids: a comparative study

The ability of eight flavonoids to react with the biologically relevant reactive nitrogen species, nitric oxide, peroxynitrite, and nitrous acid were investigated in vitro, for the first time. All the investigated flavonoids were found to be potent reactive nitrogen and oxygen species scavengers and resulted in a significant inhibition of 3-nitrotyrosine (3-NT) formation in a dose-dependent manner. All the IC₅₀s were found at the μM level. The results reveal that, the presence of C₅–OH and C_4’–OH, in the flavonoid skeleton, enhanced the anti-nitrosative activity. The absence of the C₂=C₃ double bond and/or C₄-carbonyl group resulted in slight decrease of the anti-nitrosative activity. However, the presence of C₆- and/or C_3’-methoxy groups together with C_4’–OH dramatically decrease the inhibition capacity. Especially the free hydroxyl groups at 3’ and 4’ of the catechol containing ring B flavonoids as well as both hydroxyl groups at C₃ and C₅ are necessary for better antioxidant activity. The absence of both C₃–OH and/or C_3’–OH groups resulted in a remarkable decrease in the antioxidant activity. The cytotoxicity results indicate that the investigated flavonoids are safe to be used up to a concentration of 100 μM and may be utilized as promising sources of therapeutics.