Formation of dopamine-mediated α-synuclein-soluble oligomers requires methionine oxidation

α-Synuclein is the major component of the intracellular Lewy body inclusions present in Parkinson disease (PD) neurons. PD involves the loss of dopaminergic neurons in the substantia nigra and the subsequent depletion of dopamine (DA) in the striatum. DA can inhibit α-synuclein fibrillization in vitro and promote α-synuclein aggregation into soluble oligomers. We have studied the mechanism by which DA mediates α-synuclein aggregation into soluble oligomers. Reacting α-synuclein with DA increased the mass of α-synuclein by 64 Da. NMR showed that all four methionine residues were oxidized by DA, consistent with the addition of 64 Da. Substituting all four methionines to alanine significantly reduced the formation of DA-mediated soluble oligomers. The 125YEMPS 129 motif in α-synuclein can modulate DA inhibition of α-synuclein fibrillization. However, α-synuclein ending before the 125YEMPS 129 motif (residues 1–124) could still form soluble oligomers. The addition of exogenous synthetic YEMPS peptide inhibited the formation of soluble oligomers and resulted in the YEMPS peptide being oxidized. Therefore, the 125YEMPS 129 acts as an antioxidant rather than interacting directly with DA. Our study defines methionine oxidation as the dominant mechanism by which DA generates soluble α-synuclein oligomers and highlights the potential role for oxidative stress in modulating α-synuclein aggregation.