amyloid-beta-peptides and myricetin

The mechanism by which flavonoids prevent formation of amyloid-β (Aβ) fibrils, as well as how they associate with non-fibrillar Aβ is still unclear. Fresh, un-oxidized myricetin exhibited excitation and emission fluorescence maxima at 481 and 531 nm, respectively. Introduction of either Aβ(1-42) or Aβ(25-40) resulted in a fluorescence decrease, when measured at 481 nm, suggesting formation of a myricetin-Aβ complex. Circular dichroism (CD) and ultraviolet resonance Raman (UVRR) studies indicate that the association of myricetin with the Aβ peptide or its hydrophobic fragment, Aβ(25-40), leads to subtle changes in each peptide's conformation. Aβ(25-40) formed amyloid fibrils at a similar rate, when compared to the full-length peptide, Aβ(1-42), using thioflavin T (ThT) fluorescence. Studies also indicated that myricetin was equally effective at preventing the formation of both Aβ(1-42) and Aβ(25-40) fibrils. Although ThT assays indicated that Aβ(1-16) did not form amyloid fibrils, CD studies of the hydrophilic fragment, Aβ(1-16), suggest possible interactions between myricetin and aromatic side chains. UVRR studies of the full-length peptide and Aβ(1-16) showed increases in the intensity of the aromatic modes upon introduction of myricetin. Our findings suggest that myricetin interacts with soluble Aβ via two mechanisms, association with the hydrophobic C-terminal region and interactions with the aromatic side chains.

Topics: Alzheimer Disease; Amyloid beta-Peptides; Benzothiazoles; Circular Dichroism; Flavonoids; Humans; Peptide Fragments; Protein Binding; Protein Structure, Secondary; Spectrum Analysis, Raman; Thiazoles

Cerebral deposition of amyloid beta-peptide (Abeta) in the brain is an invariant feature of Alzheimer's disease (AD). A consistent protective effect of wine consumption on AD has been documented by epidemiological studies. In the present study, we used fluorescence spectroscopy with thioflavin T and electron microscopy to examine the effects of wine-related polyphenols (myricetin, morin, quercetin, kaempferol (+)-catechin and (-)-epicatechin) on the formation, extension, and destabilization of beta-amyloid fibrils (fAbeta) at pH 7.5 at 37 degrees C in vitro. All examined polyphenols dose-dependently inhibited formation of fAbeta from fresh Abeta(1-40) and Abeta(1-42), as well as their extension. Moreover, these polyphenols dose-dependently destabilized preformed fAbetas. The overall activity of the molecules examined was in the order of: myricetin = morin = quercetin > kaempferol > (+)-catechin = (-)-epicatechin. The effective concentrations (EC50) of myricetin, morin and quercetin for the formation, extension and destabilization of fAbetas were in the order of 0.1-1 micro m. In cell culture experiments, myricetin-treated fAbeta were suggested to be less toxic than intact fAbeta, as demonstrated by 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide assay. Although the mechanisms by which these polyphenols inhibit fAbeta formation from Abeta, and destabilize pre-formed fAbetain vitro are still unclear, polyphenols could be a key molecule for the development of preventives and therapeutics for AD.

Topics: Alzheimer Disease; Amyloid beta-Peptides; Benzothiazoles; Cell Line; Cell Survival; Dose-Response Relationship, Drug; Flavonoids; Humans; Kidney; Kinetics; Microscopy, Electron; Peptide Fragments; Phenols; Polymers; Quercetin; Spectrometry, Fluorescence; Tetrazolium Salts; Thiazoles