Conformation of the carboxy-terminal region of the Aalpha chain of fibrinogen as elucidated by immunochemical analyses

Abstract
The conformation of the carboxy-terminal aspects of the A.alpha. chain of human fibrinogen was assessed by immunochemically characterizing the A.alpha. 239-476 and A.alpha. 518-584 regions of the molecule. Two peptides, corresponding to these regions, were isolated from cyanogen bromide digests of the A.alpha. chain by molecular exclusion and high-performance liquid chromatography. Each peptide reacted with antibodies elicited by immunization with the A.alpha. chain and intact fibrinogen. A.alpha. 239-476 appears to be a relatively immunodominant region of the molecule. Competitive inhibition analyses confirmed the accessibility of these regions to antibody in native fibrinogen. Each peptide, however, contained 1 or more epitopes, which was occult in the native molecule. These occult epitopes were expressed by the intact A.alpha. chain and became accessible when fibrinogen was cleaved with plasmin. With plasmic degradation the epitopes expressed by fibrinogen and contained within these 2 peptide regions became significantly more reactive with antibody. This change occurred in concert with release of the A.alpha. 518-584 region from the core of the molecule but did not require the generation of free A.alpha. 239-476. Ultimately the epitopes within both regions were shed from the plasmin-resistant core of fibrinogen. Peptide epitopes were expressed in a similar manner by prolonged plasmic degradation of fibrinogen and fibrin with .alpha. chain cross-linking. These results are generally consistent with models depicting the carboxyl-terminal aspects of the A.alpha. chain as being surface-oriented but suggest a systemic ordering of structure when these regions are integrated into the native molecule. Plasmic cleavage significantly relaxes the conformational restraints on the organization within this region.