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

Congo red (CR) is a histologic dye that binds to many amyloid proteins beca use of their extensive beta-sheet structure, The absorbance spectrum of the dye changes upon binding to amyloid. This spectral change has previously b een exploited to develop a method to study the interaction of CR with fibri llar beta-sheet insulin fibrils, a model amyloid protein. The amyloid beta- peptide (A beta) is an amyloid protein which is deposited in the brains of Alzheimer's disease victims, A beta is toxic to neurons in vitro in a manne r 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 a ggregation of A beta, despite the fact that the CR-insulin procedure was ne ver adequately developed for this purpose. In this study, we modify our ori ginal CR-insulin assay specifically for the purpose of quantifying A beta a ggregation and discuss the reasons why application of the CR-insulin method is not valid for this purpose. The CR-A beta method is equally simple and retains the advantages of speed and lack of necessity for specialized instr umentation or expensive/radioactive reagents. Furthermore, this method can directly provide quantitation of aggregated A beta in absolute terms (i.e., mu g/ml). (C) 1999 Academic Press.