Medical Physiology, 3rd Edition

Hemostasis and Fibrinolysis

We have already noted that two essential requirements of the circulatory system are that the blood must be a liquid and that it must not leak through the walls of the blood vessels. Meeting these two requirements is the job of the fibrinolytic and hemostatic machinery. Blood is normally in a liquid state inside blood vessels because it does not come into contact with negatively charged surfaces (e.g., the collagen beneath endothelial cells) that activate an intrinsic coagulation pathway, nor does it contact tissue factors (e.g., released from damaged tissue) that activate an extrinsic pathway. Furthermore, thrombolytic pathways keep the coagulation pathways in check. Indeed, plasma contains proteins that can be converted to proteases that digest fibrin and thereby lyse blood clots.

Hemostasis (from the Greek hemos [blood] + stasis [standing]), or the prevention of hemorrhage, can be achieved by four methods: (1) vasoconstriction, (2) increased tissue pressure, (3) formation of a platelet plug in the case of capillary bleeding, and (4) coagulation or clot formation.

Vasoconstriction contributes to hemostasis because it raises the critical closing pressure—as we discuss on page 454—and thus collapses vessels that have an intravascular pressure below the critical closing pressure. Vessel constriction is also promoted by chemical byproducts of platelet plug formation and of coagulation. For example, activated platelets release the vasoconstrictors thromboxane A2 (TXA2; see p. 64) and serotonin (5-HT; see p. 315). Moreover, thrombin, a major product of the clotting machinery, triggers the endothelium to generate endothelin 1 (ET-1; see p. 480), the most powerful physiological vasoconstrictor.

Increased tissue pressure contributes to hemostasis because it decreases transmural pressure (see p. 414), which is the difference between intravascular pressure and tissue pressure. Transmural pressure is the main determinant of blood vessel radius. Given the fourth-power relationship between flow and blood vessel radius (see Equation 17-9), an increase in tissue pressure that causes radius to decrease by a factor of 2 would diminish flow by a factor of 16. We all take advantage of this principle when we press a finger against a small cut to stop the bleeding. A tourniquet increases extravascular pressure and can thus halt an arterial hemorrhage in a limb. Finally, surgeons routinely make use of this principle when applying hemostatic clamps to close off “bleeders.”

In the next two sections, we discuss the third and fourth methods of hemostasis, platelet plug formation and coagulation.

Platelets can plug holes in small vessels

In a highly controlled fashion, platelets plug small breaches in the vascular endothelium. Plug formation is a process that includes adhesion, activation, and aggregation.

Adhesion

Platelets do not adhere to themselves, to other blood cells, or to endothelial membranes. One preventive factor may be the negative surface charge on both platelets and endothelial cells. In the case of endothelial cells, the negative surface charge reflects the presence of proteoglycans, mainly heparan sulfate. Platelet adhesion occurs in response to an increase in the shearing force (see p. 415) at the surface of platelets or endothelial cells and in response to vessel injury or humoral signals.

Platelet adhesion—the binding of platelets to themselves or to other components—is mediated by platelet receptors, which are glycoproteins in the platelet membrane. These platelet receptors are integral membrane proteins belonging to a class of matrix receptors known as integrins (see p. 17). They are usually heterodimers linked by disulfide bonds. One ligand naturally present in the blood plasma is von Willebrand factor (vWF),imageN18-5 a glycoprotein made by endothelial cells and megakaryocytes. vWF is found in Weibel-Palade bodies of the endothelial cells and in α granules of platelets. High shear, certain cytokines, and hypoxia all trigger the release of vWF from endothelial cells. vWF binds to the platelet receptor known as glycoprotein Ib/Ia (Gp Ib/Ia), which is a dimer of Gp Ib linked to Gp Ia.

A breach of the endothelium exposes platelet receptors to ligands that are components of the subendothelial matrix. These ligands include collagen, which binds to Gp Ia/IIa, and fibronectin and laminin (see p. 17), both of which bind to Gp Ic/IIa.

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Von Willebrand Disease

Contributed by Emile Boulpaep, Walter Boron

Von Willebrand disease is named for the Finnish internist Erik Adolf von Willebrand (1870–1949). In 1925 von Willebrand saw a 5-year-old patient from the Åland Islands in the Sea of Bothnia between Finland and Sweden. Four of her siblings had died from bleeding at an early age and both her mother and father came from families with histories of bleeding. Von Willebrand went to the Åland Islands and found that 23 of 66 family members had bleeding problems. In his report of the family in 1926, von Willebrand concluded that this was an unknown form of hemophilia. He called it pseudohemophilia. It is also known as vascular hemophilia and angiohemophilia.

Activation

The binding of these ligands—or of certain other agents (e.g., thrombin) that we will discuss below—triggers a conformational change in the platelet receptors that initiates an intracellular signaling cascade, which leads to an exocytotic event known as the release reaction or platelet activation. The signal-transduction cascade involves the activation of phospholipase C (see p. 58) and an influx of Ca2+. Activated platelets exocytose the contents of their dense storage granules, which include ATP, ADP, serotonin, and Ca2+. Activated platelets also exocytose the contents of their α granules, which contain several proteins, including a host of growth factors and three hemostatic factors: vWF (see above) and two clotting factors that we will discuss below, clotting factor V and fibrinogen. Activated platelets also use cyclooxygenase (see pp. 62–64) to initiate the breakdown of arachidonic acid to thromboxane A2, which they release. Platelet activation is also associated with marked cytoskeletal and morphological changes as the platelet extends first a broad lamellipodium and then many finger-like filopodia.

Aggregation

Signaling molecules released by activated platelets amplify the platelet activation response. ADP (which binds to P2Y12 receptors on platelets), serotonin, and thromboxane A2 all activate additional platelets, and this recruitment promotes platelet aggregation. Aspirin, an inhibitor of cyclooxygenase, inhibits clotting by reducing the release of thromboxane A2. Another antiplatelet agent—clopidogrel—acts by inhibiting the P2Y12 receptors on the platelet surface. As noted before, vWF released by activated platelets binds to the platelet receptor Gp Ib/Ia, thereby activating even more platelets and forming molecular bridges between platelets. Platelet activation also induces a conformational change in Gp IIb/IIIa, another platelet receptor, endowing it with the capacity to bind fibrinogen. Thus, as a result of the conformational change in Gp IIb/IIIa, the fibrinogen that is always present in blood forms bridges between platelets and participates in the formation of a platelet plug. As we will see below, when cleaved by thrombin, fibrinogen also plays a critical role in clotting.

A controlled cascade of proteolysis creates a blood clot

blood clot is a semisolid mass composed of both platelets and fibrin and—entrapped in the mesh of fibrin—erythrocytes, leukocytes, and serum. A thrombus is also a blood clot, but the term is usually reserved for an intravascular clot. Thus, the blood clot formed at the site of a skin wound would usually not be called a thrombus. The relative composition of thrombi varies with the site of thrombosis (i.e., thrombus formation). A higher proportion of platelets is present in clots of the arterial circulation, whereas a higher proportion of fibrin is present in clots of the venous circulation.

Platelet plug formation and blood clotting are related but distinct events that may occur in parallel or in the absence of one other. As we will see below, activated platelets can release small amounts of some of the factors (e.g., Ca2+) that play a role in blood clotting. Conversely, as we have already noted, some clotting factors (e.g., thrombin and fibrinogen) play a role in platelet plug formation. Thus, molecular crosstalk between the machinery involved in platelet plug formation and clot formation helps coordinate hemostasis.

The cardiovascular system normally maintains a precarious balance between two pathological states. On the one hand, inadequate clotting would lead to the leakage of blood from the vascular system and, ultimately, to hypovolemia. On the other hand, overactive clotting would lead to thrombosis and, ultimately, to cessation of blood flow. The cardiovascular system achieves this balance between an antithrombotic (anticoagulant) and a prothrombotic (procoagulant) state by a variety of components of the vascular wall and blood. Promoting an antithrombotic state is a normal layer of endothelial cells, which line all luminal surfaces of the vascular system. Promoting a prothrombotic state are events associated with vascular damage: (1) the failure of endothelial cells to produce the proper antithrombotic factors, and (2) the physical removal or injury of endothelial cells, which permits the blood to come into contact with thrombogenic factors that lie beneath the endothelium. Also promoting a prothrombotic state is the activation of platelets by any of the ligands that bind to platelet receptors, as discussed above. For instance, as platelets flow past artificial mechanical heart valves, the shearing forces can activate the platelets.

According to the classical view, two distinct sequences can precipitate coagulation: the intrinsic pathway and the extrinsic pathway. It is the intrinsic pathway that becomes activated when blood comes into contact with a negatively charged surface—in the laboratory we can mimic this process by putting blood into a glass test tube. The extrinsic pathway is activated when blood comes in contact with material from damaged cell membranes. In both cases, the precipitating event triggers a chain reaction that converts precursors into activated factors, which in turn catalyze the conversion of other precursors into other activated factors, and so on. Most of these “precursors” are zymogens that give rise to “activated factors” that are serine proteases. Thus, controlled proteolysis plays a central role in amplifying the clotting signals. However, the cascades do not occur in the fluid phase of the blood, where the concentration of each of these factors is low. In the case of the intrinsic pathway, the chain reaction occurs mainly at the membrane of activated platelets. In the case of the extrinsic pathway, the reactions occur mainly at a “tissue factor” that is membrane bound. Both pathways converge on a common pathway that culminates in generation of thrombin and, ultimately, “stable” fibrin. Table 18-4 provides the names, synonyms, and properties of the procoagulant and anticoagulant factors in various parts of the clotting scheme.

TABLE 18-4

Procoagulant and Anticoagulant Factors

NAME

ALTERNATE NAMES

PROPERTIES

Procoagulant Factors

Factor I

Fibrinogen

Plasma globulin

Factor Ia

Fibrin

 

Factor II

Prothrombin

Plasma α2-globulin
Synthesis in liver requires vitamin K*

Factor IIa

Thrombin

Serine protease

Factor III (cofactor)

Tissue factor
Tissue thromboplastin

Integral membrane glycoprotein; member of type II cytokine receptor family
Receptor for factor VIIa
Must be present in a phospholipid membrane for procoagulant activity

Factor IV

Ca2+

 

Factor V

Labile factor
Proaccelerin
Accelerator globulin

Plasma protein synthesized by liver and stored in platelets
Single-chain protein

Factor Va (cofactor)

 

Heterodimer held together by a single Ca2+ ion
Highly homologous to factor VIIIa

Factor VII

Stable factor
Serum prothrombin conversion accelerator (SPCA)
Proconvertin

Plasma protein
Synthesis in liver requires vitamin K*

Factor VIIa

 

Serine protease

Factor VIII

Antihemophilic factor (AHF)
Factor VIII procoagulant component (FVIII:C)

Plasma protein with phospholipid-binding domain

Factor VIIIa (cofactor)

 

Highly homologous to factor Va

Factor IX

Christmas factor
Plasma thromboplastin component (PTC)

Plasma protein
Synthesis in liver requires vitamin K*

Factor IXa

 

Protease
Disulfide-linked heterodimer

Factor X

Stuart factor

Plasma glycoprotein
Synthesis in liver requires vitamin K*

Factor Xa

 

Protease

Factor XI

Plasma thromboplastin antecedent (PTA)

Plasma protein produced by megakaryocytes and stored in platelets

Factor XIa

 

Protease
Disulfide-linked homodimer

Factor XII

Hageman factor (HAF)

Plasma glycoprotein

Factor XIIa

 

Protease

Factor XIII

Fibrin-stabilizing factor (FSF)

Plasma protein stored in platelets

Factor XIIIa

 

Transglutaminase
Tetramer of two A chains and two B chains

High-molecular-weight kininogen

HMWK
Fitzgerald factor

Plasma protein stored in platelets
Kallikrein clips bradykinin from HMWK

Plasma prekallikrein

Fletcher factor
Plasma kallikrein precursor

Plasma protein

Plasma kallikrein

 

Serine protease
Kallikrein clips bradykinin from HMWK

von Willebrand factor

vWF

Plasma glycoprotein made by endothelial cells and megakaryocytes
Stabilizes factor VIIIa
Promotes platelet adhesion and aggregation

Anticoagulant Factors

Tissue factor pathway inhibitor

TFPI

Protease inhibitor produced by endothelial cells
GPI linked to cell membrane

Antithrombin III

AT III

Plasma protein
Serine protease inhibitor, member of serpin family
Inhibits factor Xa and thrombin, and probably also factors XIIa, XIa, and IXa
Heparan and heparin enhance the inhibitory action

Thrombomodulin (cofactor)

 

Glycosaminoglycan on surface of endothelial cell
Binds thrombin and promotes activation of protein C

Protein C

Anticoagulant protein C
Autoprothrombin IIA

Plasma protein
Synthesis in liver requires vitamin K*

Protein Ca

Activated protein C

Serine protease
Disulfide-linked heterodimer

Protein S (cofactor)

 

Plasma protein
Synthesis in liver requires vitamin K*
Cofactor for protein C

*See page 970 for a discussion of vitamin K.

The proteins of the coagulation cascade have a distinct domain structure, including a signal peptide, a propeptide, an epidermal growth factor (EGF)–like domain, a kringle domain, and a catalytic domain, whereas some other domains are variable among these proteins. The signal peptide domain is required for the translocation of the polypeptide into the endoplasmic reticulum, where the signal peptide is cleaved. The propeptide or γ-carboxyglutamic acid–rich domain (Gla domain) is rich in glutamic acid residues that undergo γ-carboxylation under the influence of the γ-carboxylase that requires vitamin K. The presence of these γ-carboxyglutamic acid residues is required for Ca2+ binding. The EGF-like domain may appear multiple times and play a role in forming protein complexes. The kringle domain is loop structure created by several disulfide bonds that also play a role in forming protein complexes and attaching the protease to its target. The catalytic domain confers the serine protease function to the coagulation proteins and is homologous to trypsin, chymotrypsin, and other serine proteases.

Intrinsic Pathway (Surface Contact Activation)

The left branch of Figure 18-12 shows the intrinsic pathway, a cascade of protease reactions initiated by factors that are all present within blood. When in contact with a negatively charged surface such as glass or the membrane of an activated platelet, a plasma protein called factor XII (Hageman factor) can become factor XIIa—the suffix a indicates that this is the activated form of factor XII. A molecule called high-molecular-weight kininogen (HMWK), a product of platelets that may in fact be attached to the platelet membrane, helps anchor factor XII to the charged surface and thus serves as a cofactor. However, this HMWK-assisted conversion of factor XII to factor XIIa is limited in speed. Once a small amount of factor XIIa accumulates, this protease converts prekallikrein to kallikrein, with HMWK as an anchor. In turn, the newly produced kallikrein accelerates the conversion of factor XII to factor XIIa—an example of positive feedback. On pages 553–554, we see another example of an interaction between kallikreins and kininogens (e.g., HMWK), in which the proteolytic activity of kallikreins on kininogens leads to the release of small vasodilatory peptides called kinins.

image

FIGURE 18-12 Coagulation cascade, showing only the procoagulant factors. TF, tissue factor.

In addition to amplifying its own generation by forming kallikrein, factor XIIa (together with HMWK) proteolytically cleaves factor XI to factor XIa. In turn, factor XIa (also bound to the charged surface by HMWK) proteolytically cleaves factor IX (Christmas factor) to factor IXa, which is a protease. Factor IXa and two downstream products of the cascade—factors Xa and, most important, thrombin—proteolytically cleave factor VIII to factor VIIIa, a cofactor in the next reaction. Finally, factors IXa and VIIIa, together with Ca2+ (which may come largely from activated platelets) and negatively charged phospholipids, form a trimolecular complex called tenase. Tenase then converts factor X (Stuart factor) to factor Xa, yet another protease.

Extrinsic Pathway (Tissue Factor Activation)

The right branch of Figure 18-12 shows the extrinsic pathway, a cascade of protease reactions initiated by factors that are outside the vascular system. Nonvascular cells constitutively express an integral membrane protein called tissue factor (tissue thromboplastin, or factor III), which is a receptor for a plasma protein called factor VII. When an injury to the endothelium allows factor VII to come into contact with tissue factor, the tissue factor nonproteolytically activates factor VII to factor VIIa. Subsequently, tissue factor, factor VIIa, and Ca2+ form a trimolecular complex analogous to tenase. Like tenase, the trimolecular complex of [tissue factor + factor VIIa + Ca2+] proteolytically cleaves the proenzyme factor X to factor Xa. An interesting feature is that when factor X binds to the trimolecular complex, factor VIIa undergoes a conformational change that prevents it from dissociating from tissue factor.

Regardless of whether factor Xa arises by the intrinsic or extrinsic pathway, the cascade proceeds along the common pathway.

Common Pathway

Factor Xa from either the intrinsic or extrinsic pathway is the first protease of the common pathway (center of Fig. 18-12). Reminiscent of the conversion of factor VIII to the cofactor VIIIa in the intrinsic pathway, the downstream product thrombin clips factor V to form the cofactor Va. Factor V is highly homologous to factor VIII, and in both cases the proteolytic activation clips a single protein into two peptides that remain attached to one another. Factors Xa and Va, together with Ca2+ and phospholipids, form yet another trimolecular complex called prothrombinase. Prothrombinase acts on a plasma protein called prothrombin to form thrombin.

Thrombin is the central protease of the coagulation cascade, responsible for three major kinds of actions:

1. Activation of downstream components in the clotting cascade. The main action of thrombin is to catalyze the proteolysis of fibrinogen (see p. 429) by cleaving the Aα chain, releasing fibrinopeptide A, and cleaving the Bβ chain, releasing fibrinopeptide B. The release of the fibrinopeptide results in the formation of fibrin monomers that are still soluble. Fibrin monomers now composed of α, β, and γ chains then spontaneously polymerize to form a gel of fibrin polymers that traps blood cells. Thrombin also activates factor XIII to factor XIIIa, which mediates the covalent cross-linking of the α and γ chains of fibrin polymers to form a mesh called stable fibrin that is even less soluble than fibrin.

2. Positive feedback at several upstream levels of the cascade. Thrombin can catalyze the formation of new thrombin from prothrombin and can also catalyze the formation of the cofactors Va and VIIIa.

3. Paracrine actions that influence hemostasis. First, thrombin causes endothelial cells to release nitric oxide, prostaglandin I2 (PGI2), ADP, vWF, and tissue plasminogen activator (see below). Second, thrombin can activate platelets through PAR-1, a protease-activated receptor that belongs to the family of G protein–coupled receptors (see pp. 51–52). Thus, thrombin is a key part of the molecular crosstalk introduced above between platelet activation and blood clotting, both of which are required for optimal clot formation. On the one hand, thrombin is a strong catalyst for platelet activation, and on the other hand, activated platelets offer the optimal surface for the intrinsic pathway leading to additional thrombin generation.

Coagulation as a Connected Diagram

The concept of independent intrinsic and extrinsic branches converging on a common pathway is becoming obsolete. In such a “branching tree” (see p. 572), multiple branches converge to form larger downstream branches, eventually converging on a single “trunk”—with no crosstalk between branches. However, coagulation is best conceptualized as a “connected diagram” (see p. 572) in which the branches may interconnect in both the upstream and downstream directions. One example of interconnections is thrombin's multiple actions just discussed. Another example is the trimolecular complex of [tissue factor + factor VIIa + Ca2+] of the extrinsic pathway, which activates factors IX and XI of the intrinsic pathway. In the other direction, factors IXa and Xa of the intrinsic pathway can activate factor VII of the extrinsic pathway. Thus, the intrinsic pathway and extrinsic pathway are strongly interconnected to form a network.

What parts of this network are most important for coagulation in vivo? Clinical evidence suggests that coagulation depends largely on the extrinsic pathway. Although tissue factor is normally absent from intravascular cells, inflammation can trigger peripheral blood monocytes and endothelial cells to express tissue factor, which increases the risk of coagulation. Indeed, during sepsis, the tissue factor produced by circulating monocytes initiates intravascular thrombosis. imageN18-6

N18-6

Hemophilias

Contributed by Emile Boulpaep

Deficiencies of certain factors affect coagulation. Hemophilia A (the most common form of hemophilia) is caused by a deficiency in factor VIII:C. The gene for factor VIII is located on the X chromosome, which explains why females are carriers of hemophilia A, whereas males present with the disease.

Hemophilia B or Christmas disease is caused by a deficiency in factor IX. The very mild Hemophilia C is caused by a deficiency in factor XI.

A genetic deficiency of a particular factor does not always result in a clotting defect, however. For instance, factor V as a component of the prothrombinase complex is a procoagulant factor. However, factor V also participates in the inactivation of activated factor VIII (FVIIIa). As a result mutations in factor V gene may produce either hemorrhagic or thrombotic phenotypes. The most common thrombophilic mutation is the factor V Leiden mutation in which factor V is resistant to degradation by activated protein C. The mutation is associated with increased risk of venous thromboembolism.

Anticoagulants keep the clotting network in check

Thus far, our discussion has focused on the coagulation cascade and attendant positive feedback. Just as important are the mechanisms that prevent hemostasis from running out of control. Endothelial cells are the main sources of the agents that help maintain normal blood fluidity. These agents are of two general types, paracrine factors and anticoagulant factors.

Paracrine Factors

Endothelial cells generate prostacyclin (PGI2; see p. 64), which promotes vasodilation (see Table 20-8) and thus blood flow and also inhibits platelet activation and thus clotting. Stimulated by thrombin, endothelial cells also produce nitric oxide (see pp. 66–67). Through cGMP, nitric oxide inhibits platelet adhesion and aggregation.

Anticoagulant Factors

As summarized in Figure 18-13, endothelial cells also generate anticoagulant factors that interfere with the clotting cascade that generates fibrin. Table 18-4 lists these factors.

1. Tissue factor pathway inhibitor (TFPI). TFPI is a plasma protein that binds to the trimolecular complex [tissue factor + factor VIIa + Ca2+] in the extrinsic pathway and blocks the protease activity of factor VIIa. TFPI is glycosylphosphatidylinositol (GPI) linked (see p. 32) to the endothelial cell membrane, where it maintains an antithrombotic surface.

2. Antithrombin III (AT III). AT III binds to and inhibits factor Xa and thrombin. The sulfated glycosaminoglycans (see p. 39heparan sulfate and heparin enhance the binding of AT III to factor Xa or to thrombin, thus inhibiting coagulation. Heparan sulfate is present on the external surface of most cells, including endothelial surfaces. Mast cells and basophils release heparin.

3. Thrombomodulin. A glycosaminoglycan product of endothelial cells, thrombomodulin can form a complex with thrombin, thereby removing thrombin from the circulation and inhibiting coagulation. In addition, thrombomodulin also binds protein C.

4. Protein C. After protein C binds to the thrombomodulin portion of the thrombin-thrombomodulin complex, the thrombin activates protein C. Activated protein C (Ca) is a protease. Together with its cofactor protein S, activated protein C inactivates the cofactors Va and VIIIa, thus inhibiting coagulation.

5. Protein S. This is the cofactor of protein C and is thus an anticoagulant.

image

FIGURE 18-13 Abbreviated version of the coagulation cascade, showing the anticoagulant factors. The anticoagulant pathways are indicated in red. TF, tissue factor.

Finally, clearance of activated clotting factors by the Kupffer cells of the liver also keeps hemostasis under control.

Fibrinolysis breaks up clots

As noted on page 429, cross-linked stable fibrin traps RBCs and WBCs as well as platelets in a freshly formed thrombus. Through the interaction of actin and myosin in the platelets, the clot shrinks to a plug and thereby expels serum. After plug formation, fibrinolysis—the breakdown of stable fibrin—breaks up the clot in a more general process known as thrombolysis. As shown in Figure 18-14, the process of fibrinolysis begins with the conversion of plasminogen to plasmin, catalyzed by one of two activators: tissue-type plasminogen activator or urokinase-type plasminogen activator. Table 18-5 summarizes the properties of fibrinolytic factors.

image

FIGURE 18-14 Fibrinolytic cascade.

TABLE 18-5

Fibrinolytic Factors

NAME

ALTERNATE NAME

PROPERTIES

Tissue-type plasminogen activator

t-PA

Serine protease that catalyzes hydrolysis of plasminogen at the junction between the N-terminal heavy chain and C-terminal light chain
N terminus contains two loop structures called kringles

Urokinase-type plasminogen activator

u-PA

Serine protease

Urokinase-type plasminogen activator receptor

u-PAR

Binds to and required for the activity of u-PA

Plasminogen

 

Single-chain plasma glycoprotein with large N-terminal and small C-terminal domain.
N terminus contains five kringles

Plasmin

Fibrinolysin

Serine protease

Plasminogen activator inhibitor 1

PAI-1

Serpin (serine protease inhibitor)
In plasma and platelets
Forms 1 : 1 complex with t-PA in blood

Plasminogen activator inhibitor 2

PAI-2

Serpin (serine protease inhibitor)
Detected only in pregnancy

α2-antiplasmin

α2-AP

Serpin (serine protease inhibitor)
Forms 1 : 1 complex with plasmin in blood

The source of tissue plasminogen activator (t-PA), a serine protease, is endothelial cells. t-PA consists of a single peptide chain with two kringles at the N-terminal portion of the molecule and a protease motif in the C-terminal portion. Kringles are loop structures created by three disulfide bonds and serve to anchor the molecule to its substrate. t-PA converts the plasma zymogen plasminogen to the active fibrinolytic protease plasmin. The presence of fibrin greatly accelerates the conversion of plasminogen to plasmin.

Besides t-PA, the other plasminogen activator, urokinase-type plasminogen activator (u-PA), is present in plasma either as a single-chain protein or as the two-chain product of a proteolytic cleavage. Like t-PA, u-PA converts plasminogen to the active protease plasmin. However, this proteolysis requires that u-PA attach to a receptor on the cell surface called urokinase plasminogen activator receptor (u-PAR).

Plasminogen, mainly made by the liver, is a large, single-chain glycoprotein that is composed of an N-terminal heavy chain (A chain) and a C-terminal light chain (B chain). The N-terminal heavy chain contains five kringles, and the C-terminal light chain contains the protease domain. t-PA cleaves plasminogen at the junction between the heavy and light chains, yielding plasmin. However, the two chains in plasmin remain connected by disulfide bonds.

Plasmin is a serine protease that can break down both fibrin and fibrinogen. The five kringles of the heavy chain of plasminogen are still present in plasmin. These anchors attach to lysine residues on fibrin, holding the protease portion of the molecule in place to promote hydrolysis. Plasmin proteolytically cleaves stable fibrin to fibrin breakdown products. Plasmin can also cleave t-PA between the kringle and protease motifs of t-PA. The C terminus of single-chain t-PA nonetheless retains its protease activity.

The cardiovascular system regulates fibrinolysis at several levels, using both enhancing and inhibitory mechanisms. Catecholamines and bradykinin increase the levels of circulating t-PA. Two serine protease inhibitors (serpins) reduce the activity of the plasminogen activators: plasminogen activator inhibitor 1 (PAI-1) and plasminogen activator inhibitor 2 (PAI-2). PAI-1 complexes with and inhibits both single-chain and two-chain t-PA as well as u-PA. PAI-1 is produced mainly by endothelial cells. PAI-2 mainly inhibits u-PA. PAI-2 is important in pregnancy because it is produced by the placenta and may contribute to increased risk of thrombosis in pregnancy.

It is of interest that activated protein C, which inhibits coagulation as shown in Figure 18-13, also inhibits PAI-1 and PAI-2, thereby facilitating fibrinolysis. Only one serpin targets plasmin, α2-antiplasmin (α2-AP) made by liver, kidney, and other tissues. When plasmin is not bound to fibrin (i.e., when the plasmin is in free solution), α2-AP complexes with and thereby readily inactivates plasmin. However, when plasmin is attached to lysine residues on fibrin, the inhibition by α2-AP is greatly reduced. In other words, the very presence of a clot (i.e., fibrin) promotes the breakdown of the clot (i.e., fibrinolysis).