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Differential affinity of serum amyloid A1 isotypes for high-density lipoprotein

Serum amyloid A (SAA), a precursor of reactive amyloid deposits, is a multigene product. SAA1, predominant both as an amyloid precursor and in plasma, consists of three allelic variants (SAA1.1, SAA1.3, and SAA1.5). Several investigations have shown that the SAA1.3 allele is associated with susceptibility to AA-amyloidosis in Japanese, and the SAA1.5 allele is related with higher serum concentrations of SAA. However, these results have not been interpreted functionally. This study assessed the affinity of SAA isotypes for high-density lipoprotein (HDL), to which SAA binds in plasma. Using a surface plasmon resonance-based apparatus (BIAcore), the affinity between immobilized recombinant human SAAs and HDL was determined. The SAA concentration was measured in fractions after ultracentrifugation (d = 1.23) of sera from patients with rheumatoid arthritis, whose SAA1 genotypes were determined. In the BIAcore analysis, as the dissociation reaction under the conditions used was too rapid to fit the typical kinetic model, the steady-state affinity model was used. The affinity (kd) of SAA1.1, SAA1.3, and SAA1.5 for HDL was 1.4 × 10−5, 1.8 × 10−5, and 3.7 × 10−6, respectively. rSAA1.5 showed significantly (p < 0.05) stronger affinity than the other two. The fraction of lipid-free SAA in serum was significantly (p < 0.001) lower in the patients with larger numbers of the 1.5 allele at the SAA1 locus. These results suggest that the relatively high affinity of SAA1.5 may cause the high serum concentration and may be related to the low susceptibility to amyloidosis.