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Mapping of Antigenic Determinants of Purified, Lipid‐Free Human Serum Amyloid A Proteins

Serum amyloid A (SAA) is the major apolipoprotein of high‐density lipoproteins (HDL) present during the acute‐phase reaction. To map specific epitopes on purified, lipid‐free SAA, sequence‐specific antibodies raised against synthetic peptides corresponding to amino acid residues 1–17, 14–30, 27–44, 40–63, 59–72, 68–84, 79–94 and 89–104 of human SAA1 were studied. Using the indirect sandwich dissociation‐enhanced lanthanide fluorescence immunoassay, antibodies raised against epitopes comprising residues 1–17, 14–30, 40–63 and 79–94 failed to recognize the corresponding domains on isolated human SAA1/SAA2 or a mixture of both isoforms, indicating that these epitopes are masked, apparently because of specific folding and/or self‐aggregation (dimerization). The accessible antigenic determinants of isolated SAA are epitopes comprising residues 31–39, 64–78 and 95–104. The present findings indicate that: (i) the same epitopes are exposed, irrespective whether SAA is HDL‐associated or in its lipid‐free form and that (ii) monomeric and dimeric SAA co‐exist to a similar extent in the lipid‐free form, irrespective of whether conditions are non‐denaturating, denaturating, acidic or basic. From our studies it is proposed that isolated, purified SAA may serve as a reliable standard for quantification of HDL‐associated SAA and for mimicking the interaction of acute‐phase HDL particles with peripheral tissues in vitro.