Antigenic Determinants in the Disulfide Regions of Bovine Fibrinogen

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Abstract
Rabbit antibodies to bovine fibrinogen were used to study the antigenic activity of four cyanogen bromide peptides containing the disulfide regions of this molecule. In precipitation tests the highest activity was associated with the peptide F-CB3 which is exclusively derived from the alpha-chain. Reduction of the single disulfide bridge in peptide F-CB3 did not influence its serologic activity. Weaker reactions were observed with the N-terminal multichain peptide F-CB1. The antigenicity of peptide F-CB1 was not affected by removal of fibrinopeptides A and B but it was lost after reduction. The immunological activity of the multichain peptide F-CB2 was even less than that of peptide F-CB1 and the antigenic determinants were destroyed by reduction. A large fragment essentially composed of peptides F-CB1 and F-CB2 could be obtained by limited cyanogen bromide cleavage and showed considerably better immunological activity than peptides F-CB1 and F-CB2 together. Apparently no activity was associated with a mixture of small hydrophobic, disulfide-loop peptides tentatively called peptide F-CB4. The large loss of antigenic activity in cyanogen bromide digests of fibrinogen suggests that the disulfide-stabilized regions do not have an important role in maintaining conformational antigenic determinants of fibroinogen. Changes in noncovalently stabilized conformation requiring uncleaved chains is considered as a possible reason for the findings observed.