Amyloid Fibril Protein AA: Purification and Properties of the Antigenically Related Serum Component As Determined by Solid Phase Radioimmunoassay

The isolation by gel filtration of a serum component (SAA), antigenically related to the major filbrillar amyloid protein (AA), associated with "secondary" amyloidosis, has been monitored by a solid phase radioimmunoassay for the AA protein to detect cross-reacting serum fractions. Evidence is presented that not all cross-reacting antigenic determinants are accessible in native SAA, since additional determinants are revealed during the isolation procedure. The native structure of SAA appears to be quite labile. SAA from freshly collected serum has a m.w. of 180,000 and co-chromatographs with IgG. However, species of higher m.w. are observed after storage of serum at 4 degrees C or upon chromatography of serum in ammonium bicarbonate. Denatured SAA has a tendency to aggregate under strong dissociating conditions. A 12.500 m.w. antigenic species (SAAL) was detected upon guanidine-HCl denaturation of SAA, by earlier studies employing double immunodiffusion. However, evidence is presented here that the major part of the antigenic acitivity after guanidine-HCl treatment was of m.w. greater than 12,500, but was unreactive in double immunodiffusion. Formic acid treatment of cross-reacting serum fractions does result in virtually complete dissociation of SAA to SAAL, however. Furthermore, Formic acid-dissociated SAAL is of comparable immunoreactivity with AA, on a molar basis, unlike SAAL obtained from SAA by guanidine-HCl denaturation.