Polyphenols such as resveratrol, myricetin and quercetin, have been reported for their capability to inhibit formation of Alzheimer's disease (AD) amyloid-β fibrils (fAβ) or disassociate preformed fAβ, which is considered to be one promising disease-modifying therapy for AD. In these studies, quantification of fAβ formation and disassociation is widely completed by using thioflavin T (ThT). ThT is a fluorescent dye that interacts with the cross β-sheet conformation of fAβ, not Aβ monomers, dimmers or oligomers, and gives a characteristic shifted fluorescence emission and excitation.
In this work, a new group of polyphenols, including apigenin, luteolin, 3',4'-dihydroxyflavone and 5,7,3',4',5'-pentahydroxyflavone, was investigated for their ability to induce fAβ disassociation. A sharp drop of ThT fluorescence was observed upon addition of polyphenols to fAβ. The fast rate at which this decrease of ThT fluorescence occurred was not consistent with the stable structure of fAβ. Circular dichroism (CD) and transmission electron microscope (TEM) imaging of fAβ following polyphenol addition provided evidence that fAβ is not disassociated by these polyphenols. Alternative possible causes for the decrease in ThT fluorescence, including spectral interference and binding competition between polyphenol and ThT for fAβ, were thus considered. Internal filter effects can attenuate fluorescence by strong light absorption of the polyphenol at the excitation and emission wavelengths for ThT. When ThT fluorescence values were corrected for internal filtration, this effect was found to contribute only a small portion of the fluorescence decrease. To examine the possibility that the polyphenols may alter ThT-fAβ interactions, Lineweaver-Burke plots were constructed for different polyphenol concentrations. These plots indicate that polyphenols may likely bind noncompetitively to fAβ to disrupt ThT fluorescence. Thus, these polyphenols do not disassociate fAβ, but do bind fAβ and may thus influence the ability of fAβ to induce cellular damage.
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