Inhibition of Peroxynitrite‐Mediated Oxidation of Dopamine by Flavonoid and Phenolic Antioxidants and Their Structural Relationships

The interaction between peroxynitrite and dopamine and the inhibition of this reaction by plant-derived antioxidants have been investigated. Peroxynitrite promoted the oxidation of dopamine to 6-hydroxyindole-5-one as characterised by HPLC and photodiode array spectra, akin to the products of the tyrosinase-dopamine reaction, but no evidence of dopamine nitration was obtained. Although peroxynitrite did not cause nitration of dopamine in vitro, the catecholamine is capable of inhibiting the formation of 3-nitrotyrosine from peroxynitrite-mediated nitration of tyrosine. The plant-derived phenolic compounds, caffeic acid and catechin, inhibited peroxynitrite-mediated oxidation of dopamine. This effect is attributed to the ability of catechol-containing antioxidants to reduce peroxynitrite through electron donation, resulting in their oxidation to the corresponding o-quinones. The antioxidant effect of caffeic acid and catechin was comparable to that of the endogenous antioxidant, glutathione. In contrast, the structurally related monohydroxylated hydroxycinnamates, p-coumaric acid and ferulic acid, which inhibit tyrosine nitration through a mechanism of competitive nitration, did not inhibit peroxynitrite-induced dopamine oxidation. The findings of the present study suggest that certain plant-derived phenolics can inhibit dopamine oxidation.