Competition of Congo red and thioflavin S binding to amyloid sites in Alzheimer's diseased tissue

Though the strong affinity of Congo red and thioflavin S for amyloid deposits has been known for a long time, the binding mechanism of these biological dyes to amyloid sites is still not understood. In the present work, the competition of these two dyes for binding in Alzheimer amyloid is investigated. Analyses by optical microscopy and microspectrofluorimetry were performed on several series of stained diseased brain sections. Microspectrofluorimetric analyses showed that bound Congo red and bound thioflavin S have characteristic spectra in which the fluorescence maxima are distinct of those for the free dyes. Both, optical microscopic analyses and fluorescence measurements of stained brain sections indicated that Congo red and thioflavin S compete with each other at the same interacting site on diseased cerebral tissue. The staining efficiency of the replacing dye seems to be a maximum for a concentration of about 1%. (C) 1995 John Wiley & Sons, Inc.