AA-amyloidosis. Tissue component-specific association of various protein AA subspecies and evidence of a fourth SAA gene product.

Protein AA, the major repetitive protein subunit present in fibrils deposited in AA-amyloidosis, is an N-terminal cleavage product of a 104-amino acid precursor, serum amyloid A (SAA). Protein AA subspecies varying between 45 and 94 amino acids in length have been described. In this study it is shown that the different protein AA subspecies are not evenly distributed in amyloid deposits and that in single patients, certain subspecies of protein AA are deposited in specific tissue component sites. Thus larger protein AA subspecies occur in lower concentration in amyloid in the glomeruli compared to other sites and are especially found in amyloid in vessel walls. Three different SAA forms have been predicted from genomic and complementary DNA studies. The existence of a fourth type has been suspected from amino acid sequence studies of purified SAA. Protein AA derived from this fourth type of SAA is now shown to be present in amyloid fibrils in one of the patients studied in this paper.