Fragment X components (Mr 225,000 to 333,000) were distinguished on sodium dodecyl sulfate polyacrylamide gels. Western blotting with monoclonal antibodies to Aα-chain segments demonstrated that the Aα-chains of fibrinogen and the largest fragment X components (Mr 285,000-340,000) contained both Aα 259-276 and Aα 540-554. Fragment X components of Mr 270.000-285,000 contained Aα 259-276 but lacked Aα 540-554, whereas the smallest fragment X components (Mr 225.000-270,000) contained neither Aα 540-554 nor Aα 259-276. Studies of the small peptides generated during fragment X formation complemented the studies of the large molecules, by demonstrating peptides containing both Aα 259-276 and Aα 540-554 (Mr 41,600-41,800 and Mr 38,700-38,900), peptides containing Aα 540-554 but not Aα 259-276 (Mr 20,500-21,000 and Mr 17,300-17,500) and peptides containing only Aα 259-276 (Mr 23,600-24,000 and Mr 20,500-21,000). Cleavage of Bβ 1-42 from the amino terminal ends of the Bβ-chains, measured with a specific radioimmunoassay, was linear until 1.6 moles per mole of fibrinogen had been relased, and coincided with loss of the central and carboxy terminal Aα-chain regions, i. e. Aα 259-276 and Aα 540-554. Based on present and previously reported data, a model is proposed for the evolution of the heterogenous group of fragment X derivatives from fibrinogen with the simultaneous release of small peptides. Features of this model include
1. asymmetric cleavage of the fibrinogen dimer and
2. proteolyses of several different bonds occurring simultaneously but at distinct rates.