Quantifying amyloid beta-peptide (Abeta) aggregation using the Congo red-Abeta (CR-abeta) spectrophotometric assay

Congo red (CR) is a histologic dye that binds to many amyloid proteins because of their extensive beta-sheet structure. The absorbance spectrum of the dye changes upon binding to amyloid. This spectral change has previously been exploited to develop a method to study the interaction of CR with fibrillar beta-sheet insulin fibrils, a model amyloid protein. The amyloid beta-peptide (Abeta) is an amyloid protein which is deposited in the brains of Alzheimer's disease victims. Abeta is toxic to neurons in vitro in a manner that is highly dependent on the assembly of this peptide into beta-sheet fibrils. The CR-insulin assay has been applied as a means of studying the aggregation of Abeta, despite the fact that the CR-insulin procedure was never adequately developed for this purpose. In this study, we modify our original CR-insulin assay specifically for the purpose of quantifying Abeta aggregation and discuss the reasons why application of the CR-insulin method is not valid for this purpose. The CR-Abeta method is equally simple and retains the advantages of speed and lack of necessity for specialized instrumentation or expensive/radioactive reagents. Furthermore, this method can directly provide quantitation of aggregated Abeta in absolute terms (i.e., microg/ml).