Thrombin Cleavage Enhances Exposure of a Heparin Binding Domain in the N - Terminus of the Fibrin p Chain

Thrombin (Ila)-cleavage of fibrinogen (FBG) t o form polymerized fibrin promotes ndothelial cell spreading, proliferation, and von Willebrand factor release, requiring the exposure of the p15-42 domain. Studies reported here indicate that Ilacleavage of fibrinopeptide B enhances exposure of a heparin binding domain at the /315-42 neo-N-terminus of fibrin. Crossed immunoelectrophoresis showed heparin-induced mobility shifts indicative of complexing with FBG and with N-terminal CNBr fragments of FBG (NDSK) and of fibrin (IlaNDSK), but no evidence of heparin complexing with FBG lacking Bp1-42 or with FBG fragments D and E was seen. Elution from heparin-agarose with a linear gradient of NaCl showed that bound portions of both intact FBG and D fragments eluted below physiologic salt concentrations, whereas E3 fragments lacking BPI-53 did not bind. NDSK bound with higher affinity than did intact FBG, whereas binding of Ila-NDSK was maximal in this system. Binding of fibrin(ogen1 t o heparin agarose was saturable as well as inhibitable in a dose-dependent manner with both FBG and heparin. Scatchard analysis indicated a single class of binding site, with dissociation constants (kd) of 0.3 pmol/L for