Thrombin functions during tissue factor–induced blood coagulation

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Abstract
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 37°C 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.