Eosin interaction of α-synuclein leading to protein self- oligomerization

Among various dyes including congo red, thioflavin S, thioflavin T, eosin, rhodamine 6G, and phenol red, the eosin was the only dye that induced self-oligomerization of α-synuclein in the presence of a chemical coupling reagent of N-(ethoxycarbonyl)-2-ethoxy-1,2-dihydroquinoline. To analyze chemical nature of the eosin interaction with α-synuclein, the phenomenon of self-oligomerization was further examined with eosin congeners such as ethyl eosin, eosin B, phloxine B, erythrosin B, and rose bengal. The followings are the conclusions we have reached. First of all, intactness of the benzoate moiety of eosin and the negative charge on the carboxylic group of the dye are important factors leading to the specific interaction with α-synuclein. Secondly, the localized negative charge on the xanthene moiety of eosin is another critical factor for the interaction. As far as substituting halides are concerned, bromides and iodides on the xanthene moiety of the dyes do not make any difference on the α-synuclein interaction because both eosin and erythrosin B have induced the common phenomenon of self-oligomerization. The binding curve between eosin and α-synuclein was sigmoidal as the dye concentrations were increased. A double reciprocal plot of the saturation curve showed that the maximum number of eosin binding sites on α-synuclein was 1.85 with a dissociation constant of 390 μM. The dye binding to the protein appeared to occur via a positive cooperativity. The eosin binding site(s) was suggested to be located predominantly on the NAC region and partly related to the acidic C-terminus of α-synuclein. It has been, therefore, expected that this information might be useful to develop α- synuclein interactive molecules, which could provide eventual preventive or possible therapeutic means against various α-synuclein related disorders including Parkinson's disease. (C) 2000 Elsevier Science B.V.