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The observation of the co-deposition of metals and amyloid-β 42 (Aβ 42) in brain tissue in Alzheimer's disease prompted myriad investigations into the role played by metals in the precipitation of this peptide. Copper is bound by monomeric Aβ 42 and upon precipitation of the copper-peptide complex thereby prevents Aβ 42 from adopting a β-sheet secondary structure. Copper is also bound by β-sheet conformers of Aβ 42, and herein we have investigated how this interaction affects the conformation of the precipitated peptide. Copper significantly reduced the thioflavin T fluorescence of aged, fibrillar Aβ 42 with, for example, a 20-fold excess of the metal resulting in a ca 90% reduction in thioflavin T fluorescence. Transmission electron microscopy showed that copper significantly reduced the quantities of amyloid fibrils while Congo red staining and polarized light demonstrated a copper-induced abolition of apple …