Figure 19.20 • Scheme of blood coagulation and fibrinolysis. (*, a vitamin K-dependent factor; f, inhibition by heparin and antithrombin III.)

blood. It is activated when blood comes into contact with a damaged vessel wall or a foreign substance. Each of the plasma coagulation factors (Table 19.6), with the exception of factor III (tissue thromboplastin), circulates as an inactive proenzyme. Except for fibrinogen, which precipitates as fibrin, these factors are usually activated by enzymatic removal of a small peptide in the cascade of reactions that make up the clotting sequence (Fig. 19.20). The extrinsic clotting system refers to the mechanism by which thrombin is generated in plasma after the addition of tissue extracts. When various tissues, such as brain or lung (containing thromboplastin), are added to blood, a complex between thromboplastin and factor VII in the presence of calcium ions activates factor X, bypassing the time-consuming steps of the intrinsic pathway that form factor X.

The intrinsic and extrinsic pathways interact in vivo. Small amounts of thrombin formed early after stimulation of the extrinsic pathway accelerate clotting by the intrinsic pathway by activating factor VIII. Thrombin also speeds up the clotting rate by activating factor V, located in the common pathway. Thrombin then converts the soluble protein fibrinogen into a soluble fibrin gel by acting on Gly-Arg bonds to remove small fibrinopeptides from the N-terminus, enabling the remaining fibrinogen molecule to polymerize. It also activates factor XIII, which stabilizes the fibrin gel in the presence of calcium by cross-linking between the chains of the fibrin monomer through intermolecular y-glutamyl-lysine bridges to form an insoluble mass.

Anticoagulant Mechanisms

In the milieu of biochemicals being formed to facilitate the clotting of blood, the coagulation cascade in vivo is controlled by a balance of inhibitors in the plasma to prevent all of the blood in the body from solidifying. Thrombin plays a pivotal role in blood coagulation. It cleaves fibrinogen, a reaction that initiates formation of the fibrin gel, which constitutes the framework of the blood clot. As mentioned previously, it activates the cofactors factor V and factor VIII to accelerate the coagulation process. Intact endothelial cells express a receptor, thrombomodulin, for thrombin. When thrombin is bound to thrombomodulin, it does not have coagulant activity, which thus prevents clot formation beyond damaged areas and onto intact endothelium. In this bound state, however, thrombin does activate protein C, which then inactivates two cofactors and impedes blood clotting. Thrombin also activates factor XIII, leading to cross-linking of the fibrin gel. The activity of thrombin is regulated by its inactivation by plasma protein inhibitors: ^-proteinase inhibitor, a2-macroglobulin, antithrombin (antithrombin III), and heparin cofactor II. These belong to a family of proteins called serpins, an acronym for serine protease inhibitors.

Antithrombin III, an a2-globin, neutralizes thrombin and the serine proteases in the coagulation cascade—Xa, IXa, XIa, and XIIa. Although antithrombin III is a slow-acting inhibitor, it becomes a rapid-acting inhibitor of thrombin in the presence of heparin. Heparin is a naturally occurring anticoagulant that requires antithrombin III (discussed previously) for its biological property of preventing blood clot formation. It binds at the lysine site of the antithrombin III molecule, causing a change in the conformation of antithrombin III and increasing its anticoagulant properties. Heparin can then dissociate from antithrombin III to bind to another antithrombin III molecule. An additional system, which controls unwanted coagulation, involves protein C, a vitamin K-dependent zymogen in the plasma. Protein C is converted to a serine protease when thrombin and factor Xa, formed in the blood in the coagulation cascade, interact with thrombomodulin. The now-activated protein C inhibits factors V and VIII and, in so doing, blocks further production of thrombin. Protein C also enhances fibrinolysis by causing release of the tissue plasminogen activator.

The biosynthesis of prothrombin (factor II) depends on an adequate supply of vitamin K. A deficiency of vitamin K results in the formation of a defective prothrombin molecule. The defective prothrombin is antigenically similar to normal prothrombin but has reduced calcium-binding ability and no biological activity. In the presence of calcium ions, normal prothrombin adheres to the surface of phospho-lipid vesicles and greatly increases the activity of the clotting mechanism. The defect in the abnormal prothrombin is in the NH2-terminal portion, in which the second carboxyl residue has not been added to the y-carbon atom of some glutamic acid residues on the prothrombin molecule to form y-carboxyglutamic acid.73 Administration of vitamin K antagonists decreases synthesis of a biologically active pro-thrombin molecule and increases the clotting time of blood in humans.74

Vitamin K is critical to the formation of clotting factors VII, IX, and X. These factors are glycoproteins that have y-carboxyglutamic acid residues at the N-terminal end of the peptide chain. The enzyme involved in forming an active prothrombin is a vitamin K-dependent carboxylase located in the microsomal fraction of liver cells. It has been suggested that vitamin K drives the carboxylase reaction by abstracting a proton from the relatively unreactive methylene carbon of the glutamyl residue, forming a 2,3-epoxide. Oral

Figure 19.21 • Mechanism of action of vitamin K and sites of action of warfarin.

anticoagulants interfere with the y-carboxylation of glutamic acid residues by preventing the reduction of vitamin K to its hydroquinone form (Fig. 19.21).

Hemophilia A, a blood disease characterized by a deficiency of coagulation factor VIII, is the most common inherited blood coagulation disorder. Treatment of this disease over the past 25 years has depended on the concentration of the antihemophilic factor (factor VIII) by cryoprecipi-tation and immunoaffinity chromatography separation technology. The impact of this therapy has been diminished by the presence of viruses that cause the acquired immunodeficiency syndrome (AIDS) and other less tragic viral diseases in humans. Recombinant antihemophilic factor preparations have been produced since 1989 with use of mammalian cells genetically altered to secrete human factor VIII. Kogenate and Helixate are recombinant preparations, obtained from genetically altering baby hamster kidney cells that contain high concentrations of factor VIII. Recombinant factor VIIa, an active factor in the extrinsic pathway, now in phase III clinical trials (Novo Seven), has been used to treat patients with hemophilia A factor VII deficiency. Hemophilia B, another genetic blood disorder, which constitutes about 20% of hemophilia cases, is caused by a deficiency of factor IX and has been treated from cryoprecipitated fractions obtained from plasma. Monoclonal antibody technology has produced an essentially pure, carrier-free preparation of native factor IX (Mononine). Recombinant technology has solved the problem of limited supply and viral contamination of these critical blood factors.

Blood Degradation

Some patients suffer from a rare, potentially life-threatening disease of the blood characterized by hemolytic anemia and thrombosis, which is known as paroxysmal nocturnal hemoglobinuria (PNH). One symptom of the disease includes red urine, which results from the breakdown of RBC products (hemoglobin and hemosiderin) in the urine. An inconsistent, but potentially life-threatening, complication of PNH is the development of venous thrombosis.

Platelet Aggregation and Inhibitors

Blood platelets play a pivotal role in hemostasis and thrombus formation. Actually, they have two roles in the cessation of bleeding: a hemostatic function, in which platelets, through their mass, cause physical occlusion of openings in blood vessels, and a thromboplastic function, in which the chemical constituents of the platelets take part in the blood coagulation mechanism. The circulatory system is self-sealing because of the clotting properties of blood. The pathological formation of clots within the circulatory system, however, creates a potentially serious clinical situation that must be dealt with through the use of anticoagulants.

Platelets do not adhere to intact endothelial cells. They do become affixed to subendothelial tissues, which have been exposed by injury, to cause hemostasis. Platelets bind to collagen in the vessel wall and trigger other platelets to adhere to them. This adhesiveness is accompanied by a change in shape of the platelets and may be caused by mobilization of calcium bound to the platelet membrane. The growth of the platelet mass depends on the ADP released by the first few adhering cells and enhances the aggregation process. A secondary phase (phase II) immediately follows, with additional platelet aggregation. In this secondary phase, the platelets undergo a secretory process during which enzymes such as cathepsin and acid hydrolases, along with fibrinogen, are


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