Evidence for four different polymerization sites involved in human fibrin formation.

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Abstract
The mechanism of association and the organization of human fibrin were studied by using affinity chromatography. Insolubilized fibrinogen, fibrin monomer and crosslinked fibrin were used to localize the binding sites on fibrinogen and fibrin derivatives. Four different polymerization sites were distinguished. A binding site (a), available without thrombin action, was present on the fibrinogen fragment D domain. The complementary (A) was inoperative in fibrinogen and required thrombin for activation; it was located on the fibrinogen NH2-terminal domain. A 3rd polymerization site (b) appeared to be formed by the alignment of the fragment D domains on 2 fibrin monomer molecules upon polymerization; this site functioned without thrombin mediation and the alignment was stabilized by the factor XIIIa-catalyzed crosslink bonds. The b site was complementary to another thrombin-activated site (B) on the fibrinogen NH2-terminal domain. The 2 thrombin activation sites, A and B, were distinguishable, although they were located in the same fibrinogen domain.