Brief Definitive Report AMYLOID-RELATED SERUM PROTEIN SAA IN ENDOTOXIN-INDUCED AMYLOIDOSIS OF THE MINK

Amyloid fibrils deposited in secondary amyloidosis contain a unique protein (AA) as the major constituent (1-3). An antigenically related serum protein (SAA) (4, 5), present in normal sera in low concentration (6), is elevated in amyloidosis and in diseases which predispose to amyloidosis, such as rheumatoid arthritis, leprosy, and tuberculosis (4-8). The finding that SAA has a monomeric form about 50% larger than AA (9, 10) is consistent with the suggestion that SAA represents a circulating precursor of the fibril protein (4, 5). However, direct evidence that SAA is converted into AA is lacking. To elucidate further the relationship between SAA and AA, we have studied the serum level of SAA during the development of amyloidosis. For this, we have used an animal model in which amyloidosis was induced in mink by repeated endotoxin injections (11). Endotoxin-induced amyloid fibrils in mink contain a protein equivalent to human AA, and the sera of some mink contain a related protein, mink SAA (12). A dramatic increase in the serum level of SAA occurred within 24 h of the first endotoxin injection. Subsequently, the concentration of SAA decreased to relatively low levels, but increased again during the early amyloidotic phase.