Regulation of Tissue Factor Initiated Thrombin Generation by the Stoichiometric Inhibitors Tissue Factor Pathway Inhibitor, Antithrombin-III, and Heparin Cofactor-II *

The effects of the stoichiometric inhibitors tissue factor pathway inhibitor (TFPI), antithrombin-III (AT-III) and heparin cofactor-II (HC-II) on thrombin generation were evaluated in a reaction system composed of coagulation factors VIIa, X, IX, VIII, and V and prothrombin initiated by tissue factor (TF) and phospholipids. Initiation of the reaction in the absence of inhibitors resulted in explosive thrombin generation for factor VIIa.TF concentrations varying from 100 to 0.25 pM with the lag time or initiation phase of thrombin generation increasing from 0 to 180 s with decreasing factor VIIa.TF concentrations. During the propagation phase, prothrombin is quantitatively activated to 1.4 micro;M alpha-thrombin. At normal plasma concentration (2.5 nM) full-length recombinant TFPI prolonged the initiation phase of thrombin generation 2-fold, and the rate of thrombin generation in the propagation phase of the reaction was 25-50% that of the uninhibited reaction when the reaction was initiated with 1.25-20 pM factor VIIa.TF. Inhibition of the reaction by TFPI is associated with a delay in factor V activation. In the presence of TFPI no explosive thrombin generation was observed when factor VIII was omitted from reactions initiated by factor VIIa.TF concentrations </=20 pM. This indicates that in the presence of TFPI the factor IXa.factor VIIIa pathway becomes essential at low factor VIIa.TF concentrations. In the reconstituted system, AT-III (3.4 micro;M) did not prolong the initiation phase of thrombin generation when the reaction was initiated with 1.25 pM factor VIIa.TF, nor did AT-III delay factor V activation. The rate of thrombin formation in the presence of AT-III was reduced to 30% that of the uninhibited reaction, and the alpha-thrombin formed was rapidly inhibited subsequent to its generation. The addition of HC-II alone at its physiological concentration (1.38 micro;M) to the procoagulant mixture did not alter the rate or extent of thrombin generation. Subsequently, the thrombin formed was slowly inhibited by HC-II. The slow inactivation of thrombin by HC-II does not contribute to thrombin inhibition in the presence of AT-III. In contrast, the combination of physiological levels of AT-III and TFPI inhibited explosive thrombin generation initiated by 1.25 pM factor VIIa.TF completely. The absence of prothrombin consumption indicated that the combination of TFPI and AT-III is able to prevent the formation of prothrombinase activity at low factor VIIa.TF concentrations. The data indicate that TFPI potentiates the action of AT-III by decreasing the rate of formation and thus the amount of catalyst formed in the reaction, enabling AT-III to effectively scavenge the limited traces of factor IXa and factor Xa formed in the presence of TFPI. The initiation of thrombin generation by increasing factor VIIa.TF concentrations in the presence of physiological concentrations of TFPI and AT-III showed dramatic changes in the maximal rates of thrombin generation over small changes in initiator concentration. These data demonstrate that significant thrombin generation becomes a "threshold-limited" event with regard to the initiating factor VIIa.TF concentration in the presence of TFPI and AT-III.