Page 2141 - Hematology_ Basic Principles and Practice ( PDFDrive )

1902 Part XII Hemostasis and Thrombosis

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and procoagulant proteins are activated, the procoagulant elements surface–binding sites), and the cofactor (factor Va) are activated
319
necessary for the full procoagulant response are generated, and a rapidly to produce a surplus that is ready for action. The coagula-
306
preliminary fibrin network is formed. Although this process is tion mechanism can become sensitive to factor V or platelets when
inefficient, this initial thrombin is essential for the acceleration of the confronted with congenital deficiencies, thrombocytopenia, platelet
process by serving as the activator of platelets through cleavage of pathology, or pharmacologic interventions. 322,323
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protease–activated receptors (PAR1 and PAR4) and the activation The initial factor Xa is generated via the tissue factor–factor VIIa
129
of the procofactors factor V and factor VIII. Thrombin also acti- complex during the initiation phase. Additional factor Xa is then
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vates factor XI to factor XIa, initiating the accessory pathway that generated by the intrinsic tenase complex (factor IXa–factor VIIIa–
2+
enhances factor IX activation. 308 membrane–Ca ). Initially, the concentration of the factor VIIa–tissue
In analyses of tissue factor–induced activation of the coagulation factor complex is higher than the concentration of the factor VIIIa–
process in whole blood, the initial period of thrombin generation factor IXa complex, which requires activation and assembly. As time
(based on levels of thrombin–antithrombin complexes) illustrates progresses, the contribution of the intrinsic tenase complex to factor
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that most catalyst formation occurs before fibrin clot formation (see Xa generation exceeds that of the extrinsic tenase. The intrinsic
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Fig. 126.14). The small amount of thrombin that is generated tenase complex is kinetically more efficient and activates factor X at
during the initiation phase is from the extrinsic factor tenase and is a 50–100-fold higher rate than the extrinsic tenase complex. 299,325,326
311
310
309
able to activate platelets, factor XIII, factor V, and factor The burst of factor Xa that is generated overcomes the levels of factor
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VIII , and release some fibrinopeptide A and fibrinopeptide B Xa inhibitors, such as TFPI, and achieves maximal prothrombinase
from fibrinogen to form fibrin. Less than 2% of the final thrombin activity and propagation of the procoagulant response. 162,315 The bulk
produced is required to achieve the activation of these catalysts pro- of thrombin (≈95%) is formed during the propagation phase after
duced in blood to form the initial clot. However, activation of the fibrin clot formation. 305,327 Without the intrinsic tenase complex
catalysts is essential to generate the bulk of thrombin (≈95%) that is being formed, as occurs in hemophilia A or B, factor Xa is not gener-
formed during the propagation phase of the reaction. The aggregated ated in levels sufficient to produce the propagation phase of thrombin
platelets and fibrin resulting from thrombin formation are the prin- generation. 317,328
cipal components of the initial vascular plug formation. It has recently Because the presentation of a clot in a low tissue factor model
been shown that a fraction of red blood cells (≤1%) provide phos- depends on the generation of 10–30 nM thrombin, 305,327 at high
phatidyl serine–associated membrane receptors that can support the tissue factor concentrations, tissue factor–factor VIIa generates factor
procoagulant complexes. These red blood cells are responsible for Xa rapidly and masks the contribution of the factor VIIIa–factor IXa
approximately 30% of all thrombin formation. 313 complex in clot end point assays. This is the case for the PT 266,267 in
After cofactor factor VIIIa is formed, it combines on activated which the concentration of the initiator, thromboplastin (tissue factor
platelets with the serine protease factor IXa that was generated by the and phospholipid), is chosen to produce a clot time of 11–15 seconds.
tissue factor–factor VIIa complex to form the intrinsic tenase complex This corresponds to a tissue factor concentration over 20 nM. In our
(see Fig. 126.13). This complex is the major activator of factor X; it whole-blood studies, a concentration of 5 pM tissue factor is used,
is 50-fold more efficient than factor VIIa–tissue factor in catalyzing which produces a clot time in the range of 5 minutes. 305,327 Therefore
factor X activation. 299,314 The extrinsic tenase complex is under the in hemophilia A, the PT does not reflect a change in clot time in this
control of TFPI. 315,316 In the absence of factor VIII (hemophilia A) well-established hemorrhagic disease. The major defect occurs after
or factor IX (hemophilia B), the intrinsic tenase complex cannot be clot time, during the propagation phase of thrombin generation,
assembled; thus no amplification of factor Xa generation occurs. This which is dramatically decreased. 317,321,329
is the principal defect observed in hemophilia 317,318 : initial production
of factor Xa by the tissue factor–factor VIIa complex is inadequate
to efficiently stem blood flow. Termination
Factor Xa combines with factor Va on the activated platelet
membrane surfaces at specific receptor sites to form the prothrombi- When blood flow has ceased because of the formation of a fibrin–
nase complex; the principal generator of thrombin (see Fig. platelet “dam,” the overwhelming concentration of inhibitors present
126.13). 319,320 This process serves as a major amplification loop of in blood, including TFPI and antithrombin, heparin cofactor II,
blood coagulation. The factor IXa and factor Xa constituent of α 2 -macroglobulin, α 1 -antitrypsin, and protein C inhibitor, can “catch
prothrombinase and the intrinsic tenase complex are protected from up” and inhibit the various reactants as they dissociate from their
inhibition by antithrombin and other plasma inhibitors when in the respective complexes (see Fig. 126.13). 150,315,330–332
complexed form. In the intact vasculature surrounding the growing thrombus,
procoagulant enzymes and cofactors escaping the wound site are
rapidly quenched under normal circumstances by the stoichiometric
Propagation and dynamic inhibitory systems of blood in cooperation with ele-
ments of the vascular endothelium. The free serine proteases (throm-
When a sufficient stimulus is provided to overcome the antagonist– bin, factor IXa, and factor Xa) of the coagulation system in the plasma
inhibitor threshold, the accumulating mass of activated platelets will environment are rapidly inhibited by the surplus of antithrombin
support increasing intrinsic tenase and prothrombinase complex molecules. The reaction is accelerated by the interaction of anti-
formation on their surfaces through specific platelet receptors, and thrombin with heparan sulfate proteoglycans presented constitutively
the local inhibitor concentrations are overwhelmed (see Fig. 126.13). on the surface of vascular endothelial cells. 333
These platelet-bound catalysts execute the propagation phase of Any thrombin escaping from the wound site may bind resident
the reaction, during which massive amounts of thrombin are pro- thrombomodulin molecules constitutively expressed by vascular
321
duced. This phase of thrombin generation continues independent endothelial cells. Thrombomodulin-bound thrombin is converted
of the initially presented tissue factor as long as there is a continu- from a procoagulant enzyme to an anticoagulant enzyme. 334,335
ous supply of blood to deliver new plasma procoagulant reactants, The thrombin–thrombomodulin complex (protein Case) activates
platelets, and fibrinogen to the site of perforation in the vascular protein C, which in turn downregulates the intrinsic factor tenase and
endothelium. prothrombinase procoagulants by cleaving factor VIIIa and factor
Essential to the formation of the prothrombinase complex is the Va, respectively. The rates of APC inactivation of factors Va and
generation of factor Xa. Factor Xa is a unique regulatory enzyme in VIIIa are enhanced by protein S. TAFI is also activated by protein
that it is formed through both the intrinsic tenase and the extrinsic Case and serves to delay clot lysis (see reviews listed in the Refer-
tenase complexes. Under normal conditions, the concentration of ences 53,336 ). Cleavage of factor Va by APC and inhibition of thrombin
factor Xa is the rate-limiting component of the prothrombinase generation also reduces thrombin–thrombomodulin–mediated TAFI
complex. The other components of the complex, platelets (membrane activation. 337