Chemical evidence for 6-hydroxydopamine to be an endogenous toxic factor in the pathogenesis of Parkinson's disease

The presence of 5-Hydroxydopamine (5-OHDA) and 6-Hydroxydopamine (6-OHDA) in the urine of parkinsonian patients on levodopa medication was reported by Andrew et al. (1993). To answer the question about the putative relevance of 6-OHDA endogenously formed in the brain for the pathogenesis of Parkinson's disease (PD), the chemical mechanisms leading to dopamine-coordinative complexes were investigated in vitro. Kinetic studies of the reaction of dopamine (DA) with dioxygen over the pH range 7.0-9.0, where it reacts spontaneously without the necessity of metal-ion analysis, show that stoichiometric amounts of H2O2 are produced. Pink dopaminochrome, another oxidation product, is not stable and further reacts - without the consumption of dioxygen - to form the insoluble polymeric material known as melanin. Based on these results, the in vitro chemistry of the reactions of DA, 5-OHDA, and 6-OHDA in the presence of Fe3+ and dioxygen are studied. A mechanism for the initiation of a chain reaction is suggested by which excess Fe3+ could arise, and its relevance for the degeneration of dopaminergic neurons in PD is discussed. Detailed studies on the release of ferritin bound iron (0.2-1.4 mu M Fe3+) by synthetic DA (200 mu M) may provide further insight into the pathogenesis of PD,but further studies are warranted to elucidate the molecular basis of this neurodegenerative disorder of the extrapyramidal system.