What is all that thrombin for?

The hemostatic process initiated by the exposure of tissue factor to blood is a threshold limited reaction which occurs in two distinct phases. During an initiation phase, small amounts of factor (F)Xa, FIXa and thrombin are generated. The latter activates the procofactors FV and FVIII to the activated cofactors which together with their companion serine proteases form the intrinsic FX activator (FVIIIa-FIXa) and prothrombinase (FVa-FXa) which generate the bulk of FXa and thrombin during a propagation phase. The clotting process (fibrin formation) occurs at the inception of the propagation phase when only 5-10 nM thrombin has been produced. Consequently, the vast majority (greater than 95%) of thrombin is produced after clotting during the propagation phase of thrombin generation. The blood of individuals with either hemophilia A or hemophilia B has no ability to generate the intrinsic FXase, and hence is unable to support the propagation phase of the reaction. Since clot based assays conclude before the propagation phase they are not sensitive to hemophilia A and B. The inception and magnitude of the propagation phase of thrombin generation is influenced by genetic polymorphisms associated with thrombotic and hemorrhagic disease, by the natural abundance of pro- and anticoagulants in healthy individuals and by pharmacologic interventions which influence thrombotic pathology. Therefore, it is our suspicion that the performance of the entire process of thrombin generation from initiation through propagation and termination phases of the reaction are relevant with respect to both hemorrhagic and thrombotic pathology.

Pl(A2) polymorphism of beta(3) integrins is associated with enhanced thrombin generation and impaired antithrombotic action of aspirin at the site of microvascular injury

BACKGROUND

Mechanisms by which the Pl(A2) (Leu33Pro) polymorphism of beta(3) integrins could lead to an increased risk for coronary events are unclear. This study was designed to examine the effect of this polymorphism on blood coagulation.

METHODS AND RESULTS

In normal subjects (12 with Pl(A1A1), 9 with Pl(A1A2), and 3 with Pl(A2A2)), we evaluated the activation of prothrombin, factor V, and factor XIII and fibrinogen removal by quantitative immunoblotting; thrombin-antithrombin III complex generation using ELISA; and levels of fibrinopeptide A and B by high-performance liquid chromatography in blood collected every 30 seconds at sites of standardized microvascular injury before and after 7 days of aspirin ingestion (75 mg/d). Compared with the Pl(A1A1) subjects, the Pl(A2) carriers exhibited higher maximum rates of thrombin B-chain generation (by 31.6%; P=0.005), thrombin-antithrombin III complex generation (by 30.7%; P=0.003), fibrinogen consumption (by 31.3%; P=0.002), prothrombin consumption (by 26.1%; P=0.011), and activation of factor V (by 14.1%; P=0.033) and factor XIII (by 27.0%; P=0.012). In the Pl(A1A1) homozygotes, aspirin ingestion resulted in reductions in the velocity of thrombin B-chain formation (by 32.1%; P=0.007), prothrombin consumption (by 30.4%; P=0.018), factor Va generation (by 28.9%; P=0.014), fibrinogen removal (by 41.2%; P=0.001), and factor XIII activation (by 22.6%; P=0.026). In the Pl(A2) carriers, aspirin did not alter the velocity of all these processes. After aspirin ingestion, fibrinopeptide A and B concentrations in the last 30-second interval were significantly reduced, but only in the Pl(A1A1) subjects.

CONCLUSIONS

The presence of the Pl(A2) allele is associated with enhanced thrombin formation and an impaired antithrombotic action of aspirin, which might favor coronary thrombosis in the Pl(A2) carriers.

Blood coagulation at the site of microvascular injury: effects of low-dose aspirin

The sequence of coagulant reactions in vivo following vascular injury is poorly characterized. Using quantitative immunoassays, the time courses were evaluated for activation of prothrombin, factor (F)V, FXIII, fibrinogen (Fbg) cleavage, and FVa inactivation in bleeding-time blood collected at 30-second intervals from 12 healthy subjects both before and after aspirin ingestion. Prothrombin decreased at a maximum rate of 14.2 +/- 0.6 nM per second to 10% of initial values at the end of bleeding. Significant amounts of alpha-thrombin B chain appeared rapidly at 90 seconds of bleeding and increased at a maximum rate of 0.224 +/- 0.03 nM per second to a peak value of 38 nM. Kinetics of prethrombin 2 generation was almost identical. Prothrombinase concentration reached a peak value of 22 pM at 150 seconds and then decreased to 9 pM at the end of bleeding. Prothrombin fragment 1.2 (F1.2) was produced explosively (0.673 +/- 0.05 nM per second), whereas thrombin-antithrombin III (TAT) complexes were generated at a much slower rate (0.11 +/- 0.008 nM per second; P =.002). FVa light chain was detectable 30 seconds later than the heavy chain (150 seconds) and was produced at a slightly slower rate (0.027 +/- 0.001 nM per second) when compared with the heavy chain (0.032 +/- 0.002 nM per second; P =.041). The 30 000 fragment (residues 307-506) of FVa heavy chain produced by activated protein C appeared as early as at 90 seconds and increased with time. Fbg was removed from the blood shed with a high rate of 0.047 +/- 0.02 microM/s and became undetectable at approximately 180 seconds of bleeding. The velocity of FXIII activation correlated with thrombin B-chain formation. A 7-day aspirin administration (75 mg/d) resulted in significant reductions in maximum rates of (1) prothrombin removal (by 29%; P =.008); generation of alpha-thrombin B-chain (by 27.2%; P =.022), and prethrombin 2 (by 26%; P =.014); formation of F1.2 (by 31.4%; P =.009) and TAT (by 30.3%; P = 0.013); (2) release of FVa heavy chain (by 25%; P =.003) and FVa light chain (by 29.6%; P =.007); (3) Fbg depletion from solution (by 30.5%; P =.002); and (4) FXIII activation (by 28.6%; P =.003). Total amounts of the proteins studied, collected at every interval, also significantly decreased following aspirin ingestion. These results indicate that low-dose aspirin impairs thrombin generation and reactions catalyzed by this enzyme at the site of the injury.