Characteristics of the epitope of protein-reactive anti-peptide antibodies.

The specificity of hen egg-white lysozyme (HEL)-reactive rabbit antibodies induced by the peptide loop I.II (sequences 57-107 containing Cys64-Cys80 and Cys76-Cys94) of HEL was clarified by analyzing their cross-reactions with various avian lysozymes and their reaction with synthetic peptides (sequences 59-82) in which alanine was substituted for the amino acid at certain positions. The Arg-68 residue of HEL plays a dominant role in the binding, while Gly-71, Ser-72, Arg-73, and Pro-79 also contribute to the binding of two anti-Ploop I.II antibodies (rabbit number 125 and 126). These residues, although remote in sequence, are grouped together in the crystal structure of HEL and may form an area of contact with the antibody. Contributions by Trp-63, Ile-78, and Asn-77 to the binding of the two antibodies to HEL were excluded. These results support the idea that the anti-Ploop I.II antibodies recognize a conformational type of epitope which is similar to that of native HEL. The immunogenicity of the reduced and alkylated form of Ploop I.II was also tested, but it failed to induce an HEL-reactive antibody.