At Low Concentrations, 3,4-Dihydroxyphenylacetic Acid (DOPAC) Binds Non-Covalently to α-Synuclein and Prevents Its Fibrillation

Abstract

Several studies have shown that catecholamines can inhibit the fibrillation of α-synuclein, a small presynaptic protein whose aggregation is believed to be a critical step in the etiology of Parkinson's disease and several other neurodegenerative disorders. However, the mechanism of this inhibition is uncertain. We show here that substoichiometric concentrations of DOPAC (3, 4-dihydroxyphenylacetic acid), a normal product of the metabolism of dopamine, can inhibit the fibrillation of α-synuclein (α-Syn), due to non-covalent binding of DOPAC to α-Syn monomer. Intriguingly, the presence of α-synuclein accelerates the spontaneous oxidation of DOPAC, and the oxidized form of DOPAC (the quinone) is responsible for the fibrillation inhibition. In addition, the presence of DOPAC leads to the oxidation of the methionine residues of α-Syn, probably due to the H2O2 production as a by-product of DOPAC oxidation. The lack of fibrillation results from the formation of stable oligomers, which are very similar to those observed transiently at early of the α-Syn fibrillation. A possible explanation for this phenomenon is that DOPAC stabilizes the normally transient oligomers and prevents them from subsequent fibril formation. The analysis of the α-synuclein Y39W variant suggests that DOPAC binds non-covalently to the same N-terminal region of α-Syn as lipid vesicles, probably in the vicinity of residue 39. In contrast to the compounds with 1,2-dihydroxyphenyl groups (DOPAC, catechol), their 1,4-dihydroxyphenyl isomers (hydroquinone, homogentisic acid) are able to modify α-Syn covalently, probably due to the less steric hindrance in the Michael addition.

3, 4-Dihydroxyphenylacetic Acid (DOPAC) Impairs α-Synuclein Interaction with Lipids

“…It was shown that DOPAC binds to α-Syn and inhibits its fibrillation [45]. In agreement with these earlier observations, Fig.…”

“…The area of the monomer peak remained relatively constant for the first 20 hours and then decreased. These data agree well with the results of ThT assays which indicated that stable insoluble fibrils were formed after 20 hours (lag time) [45], suggesting that the oligomers were formed before fibrils. These oligomers dissociated to monomer on the SEC HPLC column due to the strong dilution.…”

“…We have shown that DOPAC (3, 4-dihydroxyphenyl acetic acid, a normal product of the metabolism of dopamine), similar to dopamine, can bind to α-Syn, stabilize its oligomers and prevents this protein from fibrillation [45]. Here we further show that DOPAC binding to α-Syn decreases its interaction with lipids, indicating potential loss of α-Syn function due to the DOPAC binding.…”

“…Earlier we also established that the incubation of α-Syn alone or with DOPAC was accompanied by the formation of oligomeric species [45]. When α-Syn was incubated with agitation alone, these highly soluble oligomers accumulated during the first 20 hours (unlike fibrils, such oligomers were not pelletable by the centrifugation).…”

“…These oligomers dissociated to monomer on the SEC HPLC column due to the strong dilution. All these data indicated that the α-Syn oligomers formed during fibrillation are "transient" and either form stable fibrils or dissociate to monomers being diluted [45].…”