Isolation and Characterization of a x Amyloid Fibril Protein

The fibril in primary amyloidosis (AL) is composed of a monoclonal light chain or portions thereof. No unique primary structure has been identified that predisposes certain light chains to form amyloid fibrils. Currently, classification of amyloidosis is based on the biochemistry of the amyloid fibril. We determined the NH₂‐terminal sequence of an amyloid fibril and found it to be of the xI immunoglobulin subgroup. No structural alterations were detected to account for the conversion of the light‐chain fragment to an amyloid fibril. Antiserum produced to the fibril protein did not react in immunodiffusion with purified LEP or MAG antigens, which are xI proteins. This antiserum may be directed to antigenic sites unique to the immunizing protein and is unable to recognize homologous proteins, rendering it unsuitable for immunochemical identification of amyloid deposits of light‐chain origin. PAG represents the 10th reported variable xI amyloid fibril protein subjeeted to partial sequence analysis. Antisera that recognize antigenic determinants present in all members of an immunoglobulin subgroup need further development.