The fibrinolytic system is a critical regulatory network of proteins that provide balance to the hemostatic mechanism by ensuring maintenance of vascular patency by opposing the procoagulant forces of the coagulation cascade proteins. The key fibrinolytic enzyme in blood is plasmin, a serine protease that circulates as the inactive zymogen, plasminogen. Although plasminogen expression has been identified in a broad range of tissues, including the adrenal glands, brain, heart, intestine, kidney, lung, spleen, testis, thymus, and uterus, plasminogen is primarily synthesized in the liver. Plasminogen is a 92-kD, 791–amino acid glycoprotein whose plasma concentration in normal adults is 200 mg/L.1–3 The concentration of plasminogen in premature infants and newborns, respectively, is 75% and 50% lower than in adults.⁴ The half-life of plasminogen is 2.2 days, primarily because of catabolic degradation rather than consumption via conversion to plasmin. Although plasmin/plasminogen is primarily known for its function in the dissolution of fibrin thrombi, it also plays a key role in digestion of proteins critical for cellular migration, angiogenesis, and wound healing. Homozygous deficiency of plasminogen is associated with ligneous conjunctivitis, hydrocephalus, and abnormalities in wound healing, defects that can be corrected with plasminogen replacement therapy. Despite its role in thrombolysis, plasminogen deficiency or dysfunction has not been associated with an increased risk for thrombosis in affected patients.^1,5

The plasminogen molecular structure consists of an N-terminal activation peptide followed by five disulfide-bonded kringle domains, an activation loop, and the serine protease active site domain at the C-terminus.

For more information on the molecular structure and function of plasminogen, including a structural diagram, please visit the website www.expertconsult.com.

The N-terminal activation peptide is so named because cleavage of it (usually promoted by plasmin) from full-length Glu1-plasminogen results in Lys78-plasminogen that exists in a relaxed conformation that is more easily activated by plasminogen activators. The five kringle domains consist of 80–amino acid loops held together by three disulfide bridges that allow plasminogen to bind to fibrin/fibrinogen, bacterial proteins, cell surfaces, endogenous inhibitors such as α₂-antiplasmin, and antifibrinolytic medications such as ε-aminocaproic acid and tranexamic acid. Homologous structures are found in tissue plasminogen activator (tPA), urokinase plasminogen activator (uPA), prothrombin, lipoprotein (a), factor XIII, and hepatocyte growth factor. Binding of fibrinogen converts plasminogen to a more relaxed conformation that facilitates activation of it by plasminogen activators and mediates the concentration of reactants (plasminogen, plasminogen activator, and fibrin) to form a ternary complex that increases the efficiency of fibrinolysis. Adjacent to the kringle domains is the activation or C (cleavage) loop, a 39–amino acid structure that contains the Arg561-Val562 sequence, which is cleaved by plasminogen activators to generate the active enzyme plasmin. The C-terminal end of plasminogen contains the serine protease active site that cleaves target substrate proteins such as fibrin.1,2,5–8

Conversion of plasminogen to plasmin by plasminogen activators is mediated by cleavage of the Arg561-Val562 bond in the C loop of plasminogen (Fig. e21-1). This event creates a two-chain structure composed of a heavy (or A) chain of 561 amino acids that contains the amino-terminal kringle domains and a light (B) chain of 230 amino acids that contains the carboxy-terminal active site. Plasminogen is also digested by plasmin into a truncated form of plasmin known as Lys78-plasminogen, which lacks the N-terminal 77 amino acids that are present in the full-length form of plasminogen known as Glu1-plasminogen (see Fig. e21-1). Both Glu1- and Lys78-plasminogen can be converted by plasminogen activators to plasmin; however, Lys78-plasminogen appears to be activated at a higher rate than Glu1-plasminogen in the absence of fibrin.^1,2,6,8

The carboxy-terminal active site of plasmin is formed by a catalytic triad of three amino acid residues, His602, Asp645, and Ser740, and it shares considerable homology with other serine proteases, including trypsin, thrombin, and factor Xa. As a serine protease, it cleaves peptide bonds containing lysine or arginine on the carboxy side of the molecule. The principal physiologic substrate for plasmin degradation is fibrin/fibrinogen, although plasmin has also been demonstrated to degrade other coagulation factors, including factors Va and VIIIa, as well as adhesive proteins such as von Willebrand factor, the platelet membrane receptors glycoprotein IIb/IIIa and glycoprotein lb, and basement membrane proteins such as laminin, fibronectin, and thrombospondin. Cleavage of fibrin by plasmin generates a series of cross-linked fragments, including D-dimers, DD/E complexes, and combinations of D, X, and Y fragments that are measurable in plasma (Fig. e21-2). These fragments can impede fibrin polymerization and, in conjunction with the degradation of fibrinogen factors Va and VIIIa and platelet glycoprotein receptors, contribute to the antithrombotic properties of plasmin.^1,2,6–8

A Model of Regulated Fibrinolysis

The fibrinolytic system plays a key role in balancing the procoagulant forces of the coagulation cascade to ensure that vascular patency is not sacrificed to maintain vascular continuity (Fig. 21-1). Vascular injury exposes tissue factor to initiate activation of the coagulation cascade through the extrinsic pathway of coagulation. Subsequent activation of the intrinsic pathway leads to amplification of the coagulation response and ultimately hemostatic plug formation. In addition to the negative regulatory response of endogenous anticoagulant proteins such as the protein C/protein S complex and antithrombin, the fibrinolytic system is also activated to dampen and focus clot formation strictly at the site of vascular injury. tPA is released from the surrounding undamaged endothelium in response to thrombin and venous stasis. tPA activates plasminogen poorly in solution but efficiently activates fibrin-bound plasminogen to form plasmin, which initiates clot dissolution. Circulating PAI-1 and α₂-antiplasmin efficiently inactivate tPA and plasmin, respectively, in solution, thereby ensuring that systemic activation of fibrinolysis does not occur. Conversely, covalent cross-linking of fibrin monomers and incorporation of α₂-antiplasmin into the substance of the clot ensure that clot lysis does not proceed too rapidly before vascular wall repair has progressed sufficiently to prevent delayed rebleeding. Inactivation of fibrin-bound plasmin is greatly slowed in comparison to plasmin free in solution.

This mechanism serves to concentrate activation of plasmin on the surface of the clot. Initial fibrinolysis exposes free lysine residues that serve as high-affinity binding sites for plasminogen and increase the sensitivity of plasminogen to activation by tPA. This mechanism serves to accelerate fibrinolysis as it progresses and is negatively regulated by TAFI, which as a carboxypeptidase prunes away these free lysine residues. The interaction of these components ensures focused self-limited clot formation and remodeling that preserves vascular integrity and patency (see Fig. 21-1).