Dopamine Oxidation Generates an Oxidative Stress Mediated by Dopamine Semiquinone and Unrelated to Reactive Oxygen Species

Dopamine (100μm, 10–30 min) inhibits/inactivates the MgATP-dependent generation of a transmembrane proton electrochemical gradient in chromaffin granule ghosts. The dopamine dependent inhibition was enhanced by adding soluble dopamineβ-monooxygenase (DBM, 0.2 U/ml) and completely prevented by ascorbate (1 mm), dithiothreitol (2 mm) and approximately 80% by the DBM inhibitor fusaric acid (10μm). This indicates that the inhibition is caused by the dopamine semiquinone free radical generated during DBM-dependent dopamine oxidation. Catalase, superoxide dismutase or both did not prevent the inhibition, and DBM-catalysed dopamine oxidation did not change the basal level of lipid peroxidation, excluding the involvement of reactive oxygen species as being responsible for the inhibition. N-ethylmaleimide-sensitive ATPase activity (i.e. the proton translocating ATPase) in the vesicle membranes was inhibited during dopamine incubation, indicating that the toxic metabolite (dopamine semiquinone) inhibits proton pumping by inhibiting the endogenous vacuolar H+-ATPase. As this proton pump represents the driving force for the vesicular uptake and storage of catecholamines, the dopamine dependent inhibition, if taking placein vivo, may inhibit dopamine uptake in storage vesicles in sympathetic neurons, e.g. as observed in the myopathic hamster heart.