Mechanisms for the Anticoagulant Effects of Synthetic Antithrombins
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The important roles of thrombin in the development and propagation of thrombosis are well recognized. In addition to being the enzyme for clotting fibrinogen (the major protein component of blood clots), thrombin accelerates its own generation by activating factor V, factor VIII, factor XI and platelets. It accelerates the stabilization of clots by activating factor XIII to factor XIIIa, the enzyme which crosslinks fibrin. There are probably two major pathways for regulating the availability of thrombin in vivo: inactivation of thrombin (by antithrombin III/vessel wall heparan sulfate and perhaps by other endogenous antithrombins) and the inactivation of factor Va and factor VIIIa by activated protein C. Factor Va and factor VIIIa accelerate the production of thrombin. However, when thrombin becomes bound to fibrin (in clots or possibly on cell surfaces), the ability of antithrombin III/heparin to inactivate thrombin is then reduced significantly. Impairment by fibrin of thrombin inhibition by antithrombin III may account in part for the inability of unfractionated heparin to prevent post-operative deep vein thrombosis in up to 20% of patients who undergo major elective orthopaedic surgery, and may also explain the need for oral anticoagulants after unfractionated and low molecular weight heparins are used to initiate the treatment of established deep vein thrombi. The ineffectiveness of the antithrombin III/heparin pathway for inhibiting thrombin under some circumstances has been a contributory factor for the development, evaluation and identification of other inhibitors of thrombin which are more able than antithrombin III/heparin to inactivate thrombin when the enzyme is bound to fibrin. The focus of this review is to detail how these synthetic agents, by directly or indirectly inactivating thrombin, can also effectively inhibit prothrombin activation in vitro.