Thrombin functions during tissue factor-induced blood coagulation

Tissue factor-induced blood coagulation was studied in 20 individuals, for varying periods of time during 54 months, in contact pathway-inhibited whole blood at 37degreesC and evaluated in terms of the activation of various substrates. After quenching over time with inhibitors, the soluble phases were analyzed for thrombin-antithrombin III (TAT) complex formation, prothrombin fragments, platelet activation (osteonectin release), factor Va generation, fibrinopeptide (FP) A and FPB release, and factor XIII activation. TAT complex formation, for 35 experiments, showed an initiation phase (up to 4.6+/-0.6 minutes) in which thrombin was generated at an average rate of 0.93+/-0.3 nM/min catalyzed by about 1.3 pM prothrombinase yielding approximately 26 nM thrombin. During a subsequent propagation phase, thrombin was generated at a rate of 83.9+/-3.8 nM/min by about 120 pM prothrombinase, reaching ultimate levels of 851+/-53 nM. Clot time, determined subjectively, occurred at 4.7+/-0.2 minutes and correlated with the inception of the propagation phase. The thrombin concentrations associated with the transitions to rapid product formation are 510+/-180 pM for platelet activation (1.9+/-0.2 minutes), 840+/-280 pM for factor XIII activation and factor Va generation (2.2+/-0.6 minutes), 1.3+/-0.4 nM for FPA release (2.5+/-0.7 minutes), 1.7+/-0.5 nM for FPB release and prethrombin 2 (2.8+/-0.8 minutes), 7.0+/-2.2 nM for thrombin B chain (3.6+/-0.2 minutes), and 26+/-6.2 nM for the propagation phase of TAT formation (4.6+/-0.6 minutes). These results illustrate that the initial activation of thrombin substrates occurs during the initiation phase at less than 2 nM thrombin (0.2%). Most thrombin (96%) is formed well after clotting occurs.