A unique proteolytic fragment of human fibrinogen containing the A alpha COOH-terminal domain of the native molecule.

The COOH-terminal portion of the A alpha chain of human fibrinogen is highly susceptible to proteolytic degradation. This property has prevented isolation of the COOH-terminal domain of fibrinogen for the direct investigation of its functional characteristics. Human fibrinogen was degraded with hementin, a fibrinogen-olytic protease from the posterior salivary glands of the leech, Haementeria ghilianii. Two initial fragments, Yhem1 and Dhem1, produced by cleavage through the three polypeptide chains in the connector region, were characterized and shown to retain the entire A alpha COOH-terminal domain. Late cleavages by hementin occurred in the A alpha chain COOH-terminal region to produce fragments Yhem and Dhem with shorter A alpha chain remnants. Fragments Dhem were isolated from an intermediate hementin digest of fibrinogen using anion-exchange chromatography. Fragment Dhem1 was separated further from Dhem fragments with shorter alpha chain remnants by affinity chromatography on immobilized plasma fibronectin. Fragment Dhem1 represents a unique proteolytic fragment of fibrinogen containing an intact A alpha chain COOH-terminal region. NH2-terminal sequence analysis of isolated chains from fragment Dhem1 located hementin cleavage sites in the connector region to A alpha Asn102-Asn103, B beta Lys130-Gln131, and gamma Pro76-Asn77. The specific interaction of fragment Dhem1 with immobilized fibronectin indicated that the binding site probably was located within the COOH-terminal 111 amino acids of the A alpha chain. The overall pattern of fibrinogen cleavage by hementin is similar to that of plasmin, yet hementin cleaves preferably in the coiled-coil connector, sparing the A alpha COOH-terminal domain.