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Formation of Nitroapocynin and Diapocynin in Reactions of Apocynin with Peroxynitrite/CO2: Implications for the use of Apocynin as a Selective Inhibitor of NADPH Oxidase System

Peroxynitrite (PN) is an elusive oxidant formed in biological systems from the reaction of nitric oxide (•NO) with superoxide (O2•–). In bicarbonate‐rich physiological fluids, PN reacts predominantly with CO2 forming nitrogen dioxide (•NO2) and carbonate radicals (CO3 •–) in yields up to 40 mol% which, in turn, react with target molecules. In the present study, we examined the reactions of PN/CO2 with apocynin (Apo), a classical inhibitor of NADPH oxidase system with potential for treating inflammatory and degenerative conditions. At pH 7.2, Apo readily reacted in PN/CO2 system with an over all yield of oxidation of 36 ± 2 mol% (based on the initial concentration of PN employed). About 85% of the oxidation of Apo could be accounted for by the formation nitroapocynin (NitroApo; 5.4 ± 0.2 mol%) and diapocynin (DiApo; 12.8 ± 0.7 mol%). When pH was varied between 5.76 and 8.34, the yield of NitroApo increased until pH 6.74 and then tended to decline. The yield of DiApo, on the other hand, decreased with increase in the pH. At a given pH, the yield of NitroApo but not DiApo decreased with increase in the concentration of Apo. These results are consistent with the reactions of CO3▸ and •NO2 with the phenol and phenolate forms of Apo and their 1‐electron oxidation product, Apo phenolate radical. The implications of Apo reaction in the PN/CO2 system will be discussed in relation to the specificity of Apo as a probe for NADPH oxidase system and the possible metabolism of xenobitics in general by ▸NO‐derived oxidants.