Serum amyloid A (SAA) is a precursor of amyloid A (AA), the fibrillar constituents in reactive amyloid deposits. In the murine model of reactive amyloidosis, an isotype of SAA (SAA2) forms fibrils, the other (SAA1) does not. We investigated the amyloidogenic properties of murine SAA isotypes using recombinant (r) proteins. rSAA2 was more soluble under neutral conditions and less soluble under the acidic conditions favorable for fibril formation than was rSAA1. When exposed to in vitro fibril-forming conditions
in vitro, both isotypes formed materials that bound congo red and exhibited amyloid-like structures on electron microscopy, suggesting that SAA1 may also have a fibril-forming potential. It is important to note that SAA2 exists predominantly in preamyloidotic organ extracts. This may result not from the resistancee of SAA2 to proteolysis, as judged by manner of digestion by lysosomal proteases, but from a factor like preferential distribution of SAA2 in the organs.
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