Previous studies have indicated that a major antigenic site of human plasma high-density apolipoprotein A-II (apoA-II) is contained in the COOH-terminal portion (residues 27-77) of the protein. Furthermore, all of the immunoreactivity in high-density lipoproteins (HDL) was detected by a specific radioimmunoassay. The present study is an attempt to define more precisely the antigenic structure of apoA-II. Tryptic fragments and synthetic peptides of apoA-II were prepared corresponding to specific regions of the protein. Immunoreactivity was assessed by competitive inhibition of the binding of [125I]apoA-II to anti-apoA-II antisera. In the assays, tryptic fragments corresponding to residues 4-23 (the disulfide at Cys-6 was retained), residues 31-39, and residues 56-77 gave 28, 10, and 25% inhibition, respectively; a synthetic fragment representing residues 40-46 gave 10% inhibition. Further delineation of the antigenic reactive region(s) of the 56-77 fragment was achieved by chemical modification. After coupling glycine ethyl ester to Glu-59 and Glu-69 of the 56-77 tryptic fragment, the modified peptide did not form an immunoprecipitant line with anti-apoA-II. Since a synthetic peptide representing residues 60-77 gave an immunoprecipitate of complete identity to the 56-77 tryptic or synthetic fragments, we conclude that a major antigenic determinant is present between residues 60 and 77 and that Glu-69 is part of the determinant. Since previous studies have shown that the peptides corresponding to residues 1-26 and 56-77 do not bind phospholipid, the findings in this paper demonstrate the independence of such binding and the immunological reactions of apoA-II.
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