Antithrombin

Antithrombin, already mentioned in the context of heparin, is the most abundantly occurring natural inhibitor of coagulation. It is a single-chain 432 amino acid glycoprotein displaying four oligosaccharide side chains and an approximate molecular mass of 58 kDa. It is present in plasma at concentrations of 150 pig ml 1 and is a potent inhibitor of thrombin (factor Ha), as well as of factors IXa and Xa. It inhibits thrombin by binding directly to it in a 1 1 stoichiometric complex. 

Plasma-derived antithrombin concentrates have been used medically since the 1980s for the treatment of hereditary and acquired antithrombin deficiency. Hereditary (genetic) deficiency is characterized by the presence of little/no native antithrombin activity in plasma and results in an increased risk of inappropriate blood clot/emboli formation. Acquired antithrombin deficiency can be induced by drugs (e.g. heparin and oestrogens), liver disease (decreased antithrombin 

Heparin has been used clinically for decades to prevent and treat thromboembolic disease and is isolated on an industrial scale from animal tissues, in particular pig intestinal mucosa. The conesponding physiological blood anticoagulant is presumably not heparin, but an HS species that is located on the surface of vascular endothelial cells. Both heparin and HS contain a specific pentasaccharide sequence that binds and activates the plasma proteinase inhibitor AT. This pentasaccharide sequence (4) is present only in a subfraction of heparin and HS preparations. It displays a characteristic structural feature, namely a 3-0-sulfated GlcN residue, that is only rarely seen in other portions of heparin/HS chains. 

but also major rearrangement of the hehcal domains of the molecule, clo- 

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Affinity chromatography is used in the preparation of more highly purified Factor IX concentrates (53—55) as well as in the preparation of products such as antithrombin III [9000-94-6] (56,57). Heparin [9005-49-6], a sulfated polysaccharide (58), is the ligand used most commonly in these appHcations because it possesses specific binding sites for a number of plasma proteins (59,60). 

Factor VIII, immunoglobulin, and albumin are all held as protein precipitates, the first as cryoprecipitate and the others as the Cohn fractions FI + II + III (or FII + III) and FIV + V (or FV), respectively (Table 7, Fig. 2). Similarly, Fractions FIVj + FIV can provide an intermediate product for the preparation of antithrombin III and a-1-proteinase inhibitor. This abiUty to reduce plasma to a number of compact, stable, intermediate products, together with the bacteriacidal properties of cold-ethanol, are the principal reasons these methods are stiU used industrially. 

IV, a-l-proteinase inhibitor antithrombin III IgM ceruloplasmin a- and P-globutins a-tipoprotein albumin 5-10... 

Alpha-1-proteinase inhibitor and antithrombin III are used to treat people with hereditary deficiencies of these proteins. Both can be recovered from Cohn Fraction IV (Table 7) using ion-exchange chromatography (52) and affinity chromatography (197), respectively. Some manufacturers recover antithrombin III directiy from the plasma stream by affinity adsorption (56,198,199). 

The flexible loop region in the active form of antithrombin (Figure 6.23a) is in the same general position as in ovalbumin but the first few residues form a short sixth p strand in p sheet A inserted between strands pS and pis. Furthermore there is no a helix in the loop which is extended outside the main body of the molecule, ready to be inserted into the active site of thrombin. 

In viw PAI and antithrombin are stabilized in their active forms by binding to vitronectin and heparin, respectively. These two serpins seem to have evolved what Max Perutz has called "a spring-loaded safety catch" mechanism that makes them revert to their latent, stable, inactive form unless the catch is kept in a loaded position by another molecule. Only when the safety catch is in the loaded position is the flexible loop of these serpins exposed and ready for action otherwise it snaps back and is buried inside the protein. This remarkable biological control mechanism is achieved by the flexibility that is inherent in protein structures. 

Schreuder, H.A., et al. The intact and cleaved human antithrombin III complex as a model for serpin-proteinase interactions. Nature (Struct. Biol.)... 

Anion-exchangers 145, 147 Anti-Rh sera 171 Antibodies, antitetanus 144 Antitetanus antibodies 144 Antithrombin III 145 Assembly systems, molecular 52... 

There are several additional plasmin inhibitors in the blood, e.g., a2-macroglobulin, ai-proteinase inhibitor, antithrombin, but their role in the control of fibrinolysis is questionable, because their action on plasmin is eliminated by fibrin. 

Heparin inhibits the formation of fibrin clots, inhibits the conversion of fibrinogen to fibrin, and inactivates several of the factors necessary for the clotting of blood. Heparin cannot be taken orally because it is inactivated by gastric acid in the stomach therefore, it must be given by injection. Heparin has no effect on clots that have already formed and aids only in preventing the formation of new blood clots (thrombi). The LMWHs act to inhibit clotting reactions by binding to antithrombin HI, which inhibits the synthesis of factor Xa and the formation of thrombin. 

Heparin is an important anticoagulant. It binds with factors IX and XI, but its most important interaction is with plasma antithrombin III (discussed in Chapter 51). Heparin can also bind specifically to lipoprotein lipase present in capillary walls, causing a release of this enzyme into the circulation. 

Four naturally occurring thrombin inhibitors exist in normal plasma. The most important is antithrombin III (often called simply antithrombin), which contributes approximately 75% of the antithrombin activity. Antithrombin III can also inhibit the activities of factors IXa, Xa, XIa, Xlla, and Vila complexed with tissue factor. a2-Macroglobulin contributes most of the remainder of the antithrombin activity, with heparin cofactor II and aj-antitrypsin acting as minor inhibitors under physiologic conditions. 

The endogenous activity of antithrombin III is gready potentiated by the presence of acidic proteoglycans such as heparin (Chapter 48). These bind to a specific cationic site of antithrombin III, inducing a conformational change and promoting its binding to... 

Anticoagulant combines with antithrombin III to inhibit thrombin... 

An important namral inhibitor of coagulation is antithrombin III genetic deficiency of this protein can result in thrombosis. 

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