A 25,000 molecular weight protein constituent of human amyloid fibrils related to amyloid protein AA

Amyloid fibrils from a patient with diffuse amyloid disease are dissociated in 6 m guanidine hydrochloride and fractionated by gel chromatography. Two major components are separated on Sepharose 6B. Both proteins are characterized by chromatography, immunodiffusion, discontinuous gel electrophoresis, amino acid tryptic peptide mapping and amino acid sequence analysis. The smaller of the two components is typical of the known protein AA by size (8400 daltons), amino acid composition and a 30-residue N-terminal sequence. The larger of the components (25,000 daltons) undergoes electrophoresis as a single band and appears unaffected by thiol reduction. It differs from protein AA in amino acid content and by its tryptic peptide map, although it contains an N-terminal amino acid sequence identical to protein AA when carried to 20 residues. Treatment of this larger component by mild acid hydrolysis results in the release of the 8400-dalton protein AA. Fractionation after guanidine hydrochloride treatment of this particular amyloid fibril preparation is compared to the fractionation of a typical secondary amyloid preparation that contains only protein AA as the major component. The origin and relationship of the 8,400- and 25,000-dalton protein components is discussed.