Oxidized quercetin inhibits α-synuclein fibrillization

Background: alpha-Synucein is a small (14 kDa), abundant, intrinsically disordered presynaptic protein, whose aggregation is believed to be a critical step in Parkinson's disease (PD). Oxidative stress is reported to be a risk factor for dopamine cell degeneration in PD. Flavonoids are suggested to be important antioxidant against oxidative stress. Flavonoids were reported to inhibit fibrillization and disaggregate the preformed fibrils of alpha-synucein, but the molecular mechanism was still not clear. Methods: Quercetin, a well-recognized flavonoid antioxidant, was tested for its inhibition of a-synucein aggregation by thioflavin T assay, light scattering measurement, size-exclusion high performance liquid chromatography, atomic force microscopy, etc. Results: The pre-incubated quercetin exhibited a noticeably stronger inhibition behavior to the fibril formation than that of the freshly prepared. The inhibition is significant in the presence of ortho- and para-benzenediol isomers and inconsiderable in the presence of meta-isomer. The oxidized quercetin species (i.e., chalcantrione, benzyfuranone, quercetinchinone, and other derivatives) cause stronger inhibition than quercetin does because of the elevated polarity and hydrophilicity. Presence of quercetin disaggregates a-synucein fibrils, rather than oligomers and amorphous aggregations. Conclusions: Instead of the antioxidant activity, the 1:1 covalent binding of quercetin with a-synucein, and the increased hydophilicity of the covalently modified a-synucein oligomers or monomers, account for the inhibition of a-synucein fibrillation. General significance: Clarification of the molecular mechanism of the inhibition and disaggregation may help to screen safer and more effective flavonoid therapeutic in combating PD. (C) 2012 Published by Elsevier B.V.