Disulfide-linked cyanogen bromide peptides of bovine fibrinogen. II. Isolation and sequence analysis of the chain constituents from the amino terminal region.

Bovine fibrinogen was cleaved with CNBr and the peptide F-CB1 which originates from the amino end of the molecule was purified by chromatographic methods. After reduction and alkylation of F-CB1 three main polypeptide chains could be identified. They were derived from the A alpha chain (F-CB1 alpha), Bbeta chain (F-CB1 beta) and gamma chain (F-CB1 gamma) of fibrinogen and consisted of 54, 143 and 78 amino acid residues, respectively. Thrombin digestion released fibrinopeptide A from F-CB1 alpha and smaller amounts of the peptide Gly-Pro-Arg while fibrinopeptide B was released from F-CB1 beta. Fragment F-CB1 gamma was resistant to thrombin. The sequences of 35 amino acid residues in the portion of F-CB1 alpha adjacent to fibrinopeptide A and of 15 residues in F-CB1 beta beyond fibrinopeptide B were determined. They differ by six and two substitutions, respectively, when compared with the homologous part in human fibrinogen. However, the first 29 amino acid residues in bovine and human F-CB1 gamma were identical. The data show that the cysteine-containing regions and the portions to the right of the site of thrombin cleavage in fibrinogen show much less interspecies variability than that known for the fibrinopeptides.