Four Sets of Complementary Binding Sites Involved in Fibrin Polymerization.

Activation of fibrinogen by thrombin releases fibrinopeptides A and B, followed by the formation of an ordered fibrin polymer, probably due to the association of complementary binding sites. The present investigation focused on defining areas of fibrin monomer molecule which are active during polymerization by identifying binding regions on plasmic degradation products of fibrinogen or fibrin. Studies were done in soluble and in solid phase systems. In the first, species were mixed in solution and monitored for complex formation on Tris-glycine gels; in the latter, affinity chromatography on insolublized fibrinogen, fibrin monomer or cross linked fibrin was done. Thrombin-treated fibrinogen, Fragments X, Y, or NDSK bound to Fragment DD in solution as well as to all three insolublized species. Therefore one type of polymerization site is present and available on the Fragment D region of fibrinogen. The complementary site is a thrombin activated region on th e NH2-terminal domain of fibrinogen. Fragments E1 and E2 from crosslinked fibrin complexed with Fragment DD but not with fibrinogen, Fragments X, Y, or D. Fragment E1 and E2 also bound to insolublized crosslinked fibrin but not to insolublized fibrinogen or fiorin monomer. The data indicate that Fragment DD contains a binding site generated in the aligned Fragment D moieties which is inoperative in a single Fragment D. This site is complementary to the fourth site, a thrombin-activated region on the NH2-terminal domain of fibrinogen localized to Fragments E1 and E2. It is concluded that the first two binding sites are active on fibrin monomer molecules but the other two sites become operative only after the formation of fibrin oligomers.