Oxidation of DOPAC by nitric oxide: effect of superoxide dismutase

Abstract

This study aimed to characterize the redox interaction between 3,4-dihydroxyphenylacetic acid (DOPAC) and nitric oxide (·NO), and to assess the reductive and oxidative decay pathways of the DOPAC semiquinone originating from this interaction. The reaction between DOPAC and ·NO led to the formation of the DOPAC semiquinone radical, detected by electron paramagnetic resonance (EPR) and stabilized by Mg2+, and the nitrosyl anion detected as nitrosylmyoglobin. The EPR signal corresponding to the DOPAC semiquinone was modulated as follows: (i) it was suppressed by glutathione and ascorbic acid with the formation of new EPR spectra corresponding to the glutathionyl and ascorbyl radical, respectively; (ii) it was enhanced by Cu,Zn-superoxide dismutase; the enzyme also accelerated the decay of the semiquinone species to DOPAC quinone. These results are interpreted as a one-electron oxidation of DOPAC by ·NO; the reductive decay of the semiquinone back to DOPAC was facilitated by reducing agents, such as glutathione and ascorbate, whereas the oxidative decay to DOPAC quinone was facilitated by superoxide dismutase. The latter effect is understood in terms of a reversible conversion of nitrosyl anion to ·NO by the enzyme. The biological relevance of these reactions is also discussed in terms of the reactivity of peroxynitrite towards DOPAC as a model with implications for aerobic conditions.