Iron-Mediated Generation of the Neurotoxin 6-Hydroxydopamine Quinone by Reaction of Fatty Acid Hydroperoxides with Dopamine: A Possible Contributory Mechanism for Neuronal Degeneration in Parkinson's Disease

Abstract

Exposure of dopamine to an excess of linoleic acid 13-hydroperoxide (13-hydroperoxyoctadecadienoic acid) in the presence of ferrous ions in Tris buffer, pH 7.4, resulted in a relatively fast, oxygen-independent reaction exhibiting first-order kinetics with respect to both catecholamine and metal concentrations. Product analysis in the early stages revealed the presence of significant amounts of the quinone of the neurotoxin 6-hydroxydopamine, together with some aminochrome and ill-defined melanin-like material. Quinone formation required the presence of iron, either in the ferrous or ferric form, and was unaffected by peroxidase, catalase, and hydroxyl radical scavengers, e.g. mannitol, as well as biologically relevant antioxidants, like ascorbate and glutathione. Hydrogen peroxide proved as effective as linoleic acid hydroperoxide in inducing dopamine oxidation and conversion to 6-hydroxydopamine quinone. Metal chelators, including EDTA and bipyridyl, markedly suppressed quinone formation without, however, inhibiting dopamine oxidation. These and other results are consistent with a hydroxyl radical independent hydroxylation/oxidation mechanism basically different from the Fenton reaction, which involves direct interaction of the peroxide with a dopamine−Fe(III) chelate generated during the process.