The Polymerization Of Fibrin And The Clearance Of Fibrinopeptide B By Thrombin

Thrombin cleaves fibrinogen to release fibrinopeptide A (FPA) and B (FPB) to form fibrin I and II respectively. Initially FPA is cleaved more rapidly than FPB but later the FPB cleavage rate increases and is similar to that of FPA. Studies were made at 1.65 μM fibrinogen concentration, 0.02U thrombin/ml, pH 7.4, 0.15M NaCl, 37°. The initial FPB cleavage rate was 7 pmol/min and then increased to 70 pmol/min. The accelerated rate of cleavage was associated with fibrin polymerization as indicated by light scattering and absorbance measurements. Fibrin polymerization was inhibited by the synthetic tetrapeptide Gly-Pro-Arg-Pro. At a concentration of tetrapeptide of 23mM (1400 fold molar ratio to fibrinogen) no detectable polymerization occurred over 180 min. At this tetrapeptide concentration the FPA cleavage rate was unaltered from that occurring without tetrapeptide but the rate of FPB cleavage did not show the acceleration normally associated with polymerization although virtually all the FPB was cleaved. The cleavage rates of FPB from fibrin I polymer and monomer were then compared. Fibrin I polymer was prepared by treating fibrinogen with reptilase and fibrin I monomer by similar treatment in the presence of excess tetrapeptide. The cleavage rate from monomer was similar to the initial slow cleavage rate when thrombin was added to fibrinogen. The cleavage rate from polymer was 25 fold more rapid than that from monomer. These data indicate that thrombin cleaves FPB slowly but completely from fibrin I monomer and at least 25-fold more rapidly from polymer. The data do not establish that thrombin cleaves FPB from fibrinogen.