Facile Biological Oxidation of Dopamine to 6-Hydroxydopamine p-Quinone in a Sequential Two-Step Process: Implications for Parkinson’s Disease

6-Hydroxydopamine (6-OHDA), a hydroxyl-derivative of the endogenous neurotransmitter dopamine, can selectively induce Parkinsonian symptoms in animal models. At present, most researchers consider 6-OHDA a man-made neurotoxicant, due to the lack of strong evidence for its presence and/or formation in biological systems. The present study aims to determine whether 6-OHDA can be formed under physiologically relevant conditions. Here, we report in the Fenton reaction system (containing 15 μM Fe²⁺, 142 μM ascorbic acid and 80 μM EDTA in 50 mM phosphate buffer, pH 7.4), dopamine can undergo facile oxidation to 6-OHDA p-quinone (a stable, oxidized form of 6-OHDA) in a sequential two-step process: the first step involves dopamine oxidation to its o-quinone (DAQ), and this process is facilitated by oxidants like transition metal ions Fe^2+/3+ and Mn^2+/3+; and the second step involves the further oxidization of DAQ to 6-OHDA p-quinone by hydroxyl radical or hydrogen peroxide. The chemical mechanism by which H₂O₂ oxidizes DAQ to 6-OHDA p-quinone likely results from the attack of H₂O₂-derived ^-OOH at the C-6 position of DAQ. We also demonstrate that while catalase abolishes 6-OHDA p-quinone formation by removing hydrogen peroxide or hydroxyl radical, glutathione and cysteine provide effective protection by forming conjugates with DAQ and 6-OHDA p-quinone. The results of this study demonstrate that 6-OHDA can be readily formed from dopamine under physiologically relevant conditions, and thus provide important tangible support for the long-held speculation that 6-OHDA is an intrinsic etiological factor in Parkinson's disease.