BLOOD COAGULATION CASCADE
Blood coagulates due to the transformation of soluble fibrinogen into insoluble fibrin by the enzyme thrombin. Several circulating proteins interact in a cascading series of limited proteolytic reac-tions (Figure 34–2). At each step, a clotting factor zymogen undergoes limited proteolysis and becomes an active protease (eg, factor VII is converted to factor VIIa). Each protease factor activates the next clotting factor in the sequence, culminating inthe formation of thrombin (factor IIa). Several of these factors are targets for drug therapy (Table 34–1).
Thrombin has a central role in hemostasis and has many func-tions. In clotting, thrombin proteolytically cleaves small peptides from fibrinogen, allowing fibrinogen to polymerize and form a fibrin clot. Thrombin also activates many upstream clotting fac-tors, leading to more thrombin generation, and activates factor XIII, a transaminase that cross-links the fibrin polymer and stabi-lizes the clot. Thrombin is a potent platelet activator and mitogen. Thrombin also exerts anticoagulant effects by activating the protein C pathway, which attenuates the clotting response (Figure 34–2). It should therefore be apparent that the response to vascular injury is a complex and precisely modulated process that ensures that under normal circumstances, repair of vascular injury occurs with-out thrombosis and downstream ischemia; that is, the response is proportionate and reversible. Eventually vascular remodeling and repair occur with reversion to the quiescent resting anticoagulant endothelial cell phenotype.
The main initiator of blood coagulation in vivo is the tissue factor (TF)-factor VIIa pathway (Figure 34–2). Tissue factor is a trans-membrane protein ubiquitously expressed outside the vasculature, but not normally expressed in an active form within vessels.
The exposure of TF on damaged endothelium or to blood that has extravasated into tissue binds TF to factor VIIa. This complex, in turn, activates factors X and IX. Factor Xa along with factor Va forms the prothrombinase complex on activated cell surfaces, which catalyzes the conversion of prothrombin (factor II) to thrombin (factor IIa). Thrombin, in turn, activates upstream clotting factors, primarily factors V, VIII, and XI, resulting in amplification of thrombin generation. The TF-factor VIIa-catalyzed activation of factor Xa is regulated by tissue factor pathway inhibitor (TFPI).
Thus after initial activation of factor X to Xa by TF-VIIa, further propagation of the clot is by feedback amplifica-tion of thrombin through the intrinsic pathway factors VIII and IX (this provides an explanation of why patients with deficiency of factor VIII or IX—hemophilia A and hemophilia B, respectively— have a severe bleeding disorder).
It is also important to note that the coagulation mechanism in vivo does not occur in solution, but is localized to activated cellsurfaces expressing anionic phospholipids such as phosphatidylser-ine, and is mediated by Ca2+ bridging between the anionic phos-pholipids and γ-carboxyglutamic acid residues of the clotting factors. This is the basis for using calcium chelators such as ethyl-enediamine tetraacetic acid (EDTA) or citrate to prevent blood from clotting in a test tube.
Antithrombin (AT) is an endogenous anticoagulant and amember of the serine protease inhibitor (serpin) family; it inacti-vates the serine proteases IIa, IXa, Xa, XIa, and XIIa. The endog-enous anticoagulants protein C and protein S attenuate the blood clotting cascade by proteolysis of the two cofactors Va and VIIIa. From an evolutionary standpoint, it is of interest that factors V and VIII have an identical overall domain structure and considerable homology, consistent with a common ancestor gene; likewise the serine proteases are descendants of a trypsin-like common ancestor. Thus, the TF–VIIa initiating complex, serine proteases, and cofac-tors each have their own lineage-specific attenuation mechanism (Figure 34–2). Defects in natural anticoagulants result in an increased risk of venous thrombosis. The most common defect in the natural anticoagulant system is a mutation in factor V (factor V Leiden), which results in resistance to inactivation by the protein C, protein S mechanism.
Fibrinolysis refers to the process of fibrin digestion by the fibrin-specific protease, plasmin. The fibrinolytic system is similar to the coagulation system in that the precursor form of the serine pro-tease plasmin circulates in an inactive form as plasminogen. In response to injury, endothelial cells synthesize and release tissue plasminogen activator (t-PA), which converts plasminogen to plasmin (Figure 34–3). Plasmin remodels the thrombus and limits its extension by proteolytic digestion of fibrin.
Both plasminogen and plasmin have specialized protein domains (kringles) that bind to exposed lysines on the fibrin clot and impart clot specificity to the fibrinolytic process. It should be noted that this clot specificity is only observed at physiologic levels of t-PA. At the pharmacologic levels of t-PA used in thrombolytic therapy, clot specificity is lost and a systemic lytic state is created, with attendant increase in bleeding risk. As in the coagulation cascade, there are negative regulators of fibrinolysis: endothelial cells synthesize and release plasminogen activator inhibitor (PAI), which inhibits t-PA; in addition α2 antiplasmin circulates in the blood at high concentrations and under physiologic conditions will rapidly inactivate any plasmin that is not clot-bound. However, this regulatory system is overwhelmed by therapeutic doses of plasminogen activators.
If the coagulation and fibrinolytic systems are pathologically activated, the hemostatic system may careen out of control, lead-ing to generalized intravascular clotting and bleeding. This process is called disseminated intravascular coagulation (DIC) and may follow massive tissue injury, advanced cancers, obstetric emergen-cies such as abruptio placentae or retained products of conception, or bacterial sepsis. The treatment of DIC is to control the underly-ing disease process; if this is not possible, DIC is often fatal.
Regulation of the fibrinolytic system is useful in therapeutics. Increased fibrinolysis is effective therapy for thrombotic disease. Tissue plasminogen activator, urokinase, and streptokinase allactivate the fibrinolytic system (Figure 34–3). Conversely, decreased fibrinolysis protects clots from lysis and reduces the bleeding of hemostatic failure. Aminocaproic acid is a clinically useful inhibitor of fibrinolysis. Heparin and the oral anticoagulant drugs do not affect the fibrinolytic mechanism.