Structure of Antigenic Determinants in the Amino‐Terminal Region of Bovine Fibrinogen Aα Chain

A radioimmunoassay was developed for peptide F‐CB1α from the amino end of bovine fibrinogen Aα chain, isolated after reduction and carboxymethylation of the multichain disulfide‐linked cyanogen bromide peptide F‐CB1. Seven out of twelve different rabbit antisera produced against fibrinogen, peptide F‐CB1 or Aα chain showed distinct binding to 125 I‐labelled F‐CB1α. Thrombin cleavage of F‐CB1α yielded two fragments: fibrinopeptide A (residues 1‐19) and the carboxy‐terminal fragment Th2 (residues 20‐54). Antisera could be classified into three groups, according to whether they recognized antigenic determinants on fibrinopeptide A, on peptide Th2 or as they showed diminished reactions with both fragments. Only little or no cross‐reaction was observed with the amino‐terminal cyanogen bromide peptides of Bβ and γ chain. Proteolytic fragments of fibrinopeptide A were isolated and tested for inhibitory activity with two antisera. One antiserum contained antibodies binding selectively to the amino‐terminal sequence (residues 4‐11) and did not cross‐react with human fibrinopeptide A. Another antiserum showed a specific binding restricted to the carboxy‐terminal sequence (residues 11‐18) and cross‐reacted completely with human fibrinopeptide A. These results correlate well with the primary structures of the two fibrinopeptides. The antigenic activity of the peptide fragment Th2 was localized on a 15‐residue tryptic peptide derived from the central portion of the sequence. These and further data indicate that at least six different antigenic determinants are present in peptide F‐CB1α.