Purification, ultrastructure, and chemical analysis of Alzheimer disease amyloid plaque core protein.

Isolation of Alzheimer disease amyloid plaque core protein (APCP) was carried out by repetitive NaDodSO4/EDTA/sucrose extractions and by Ficoll-400 density-gradient centrifugations. The enriched APCP-Ficoll interface was labeled with the fluorochrome thioflavin T and separated from the contaminating lipofuscin by fluorescence-activated cell sorting. Electron microscopy demonstrated that APCP is made of two different kinds of filaments measuring 5.5-6 nm and 10-12 nm, respectively, and of variable length. Purified APCP and lipofuscin were chemically modified by performic acid oxidation. The amino acid composition of APCP revealed a high content of glycine and valine (30%) and 1% cysteine. By contrast, the protein moiety of the copurified lipofuscin contained 16% cysteine. The amino acid composition of APCP did not resemble that of any known protein.