Metal-Catalyzed Oxidation of α-Synuclein in the Presence of Copper(II) and Hydrogen Peroxide

Abstract

α-Synuclein is a component of abnormal protein depositions of Lewy bodies and senile plaques found in Parkinson's and Alzheimer's diseases, respectively. By using chemical coupling reagents such as dicyclohexylcarbodiimide or N-(ethoxycarbonyl)-2-ethoxy-1,2-dihydroquinoline, the protein was shown to experience self-oligomerization in the presence of either copper(II) or Aβ25-35. The oligomers which appeared as a ladder on a 10–20% Tricine/SDS–PAGE have been suggested to participate in the formation of protein aggregations by possibly providing a nucleation center. Since oxidatively modified protein could increase its own tendency toward protein aggregation, metal-catalyzed oxidation of α-synuclein has been examined with copper(II) and hydrogen peroxide in the absence of the coupling reagent. Intriguingly, the protein was also self-oligomerized into an SDS-resistant ladder on the gel. This biochemically specific copper-mediated oxidative oligomerization was shown to be dependent upon the acidic C-terminus of α-synuclein because the C-terminally truncated proteins such as α-syn114 and α-syn97 were not affected by the metal and hydrogen peroxide. More importantly, the oxidative oligomerization was synergistically enhanced by the presence of Aβ25-35, indicating that the peptide interaction with α-synuclein facilitated the copper(II) binding to the acidic C-terminus and subsequent oxidative crosslinking. It has been, therefore, suggested that abnormalities in copper and H₂O₂ homeostasis and certain pathological factors functionally similar to the Aβ25-35 could play critical roles in the metal-catalyzed oxidative oligomerization of α-synuclein, which may lead to possible protein aggregation and neurodegenerations.