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Reaction of Peroxynitrite With the Melanin Precursor, 5,6-Dihydroxyindole-2-Carboxylic Acid

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The reaction of peroxynitrite with the melanin precursor, 5,6-dihydroxyindole-2-carboxylic acid (DHICA), was investigated by the stopped-flow technique under deaerated conditions. The reaction is fast and proceeds through the intermediacy of unstable intermediates with absorption in the UV and visible regions which finally gave brownish-black polymeric products. The apparent second-order rate constant for the oxidation of DHICA by peroxynitrite is (6.3 ± 0.1) x 103 M-1s-1 at pH 7.4 and 25 °C. LC-MS analysis of the reaction mixture of DHICA and peroxynitrite did not show the presence of nitrated products but dimers and trimers of DHICA as reaction products. Thus, peroxynitrite, which nitrates phenolic compounds, did not produce nitro-derivatives of DHICA, but oxidized ones. The oxidation of DHICA by peroxynitrite produced indole-5,6-quinone-2-carboxylic acid (DHICA-Q) as evidenced by its characteristic absorption in the 380-400 nm range followed by its transformation to a melanin like polymer as the final stable product. The rate of build-up of the absorption at 380 nm increased linearly with the concentration of DHICA. Overall second-order kinetics is observed for the peroxynitrite-mediated oxidation of DHICA suggesting that both DHICA and peroxynitrite be involved in the rate-determining step. The dependence of the apparent second-order rate constant on pH displayed a bell-shaped curve with a maximum near pH 7.0. The observed pH dependence clearly indicates the involvement of two acid-base equilibria in this reaction, those of peroxynitrite and DHICA respectively. This novel reaction may have physiological relevance in the biosynthesis of melanin and in the understanding of the molecular mechanisms of UVB- induced melanogenesis. Since the precursors of peroxynitrite, nitric oxide and superoxide, are produced by melanocytes in response to UVB radiation it is possible that peroxynitrite could play a role in UV induced pigmentation.

Affiliations: 1: Organic Division, Unilever Research, U.S, 45 River Road, Edgewater, New Jersey 07020 USA


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