Quantitative Investigation into the Mechanisms of Coagulation and Fibrinolysis
Cook, P. Michael
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Coagulation and fibrinolysis are essential processes that ensure a rapid, localized response to vascular damage and subsequent removal once the damage is repaired. These two processes are extensively regulated by various feedback mechanisms and inhibitors. In blood, clotting appears to be an all or none event. Such a phenomenon is governed by thresholding behaviour. Thresholding occurs when a precursor generates a response that can either feedback or be inhibited. Prothrombin activation under conditions in which both of these mechanisms are present was determined to experience thresholding. Thrombin generation initiated by factor Xa in the presence of factor V and antithrombin was fit to a model of prothrombin activation. This model showed that factor Va generation before complete factor Xa inhibition is essential for thrombin generation. Upon factor Va formation, factor Xa becomes highly protected from inhibition, and as a result activates prothrombin more efficiently. These data provided mechanistic detail into the thresholding behavior of coagulation. Thrombin activatable fibrinolysis inhibitor (TAFIa) suppresses fibrinolysis by removing carboxyl-terminal lysine residues exposed by plasmin (Pn) on fibrin. These cleavages result in reduced plasminogen (Pg) activation and increased Pn inhibition by antiplasmin (AP). The effects of TAFIa on tissue type Pg activator (tPA) inhibition by Pg activator inhibitor type 1 (PAI-1) on fibrin were quantified. High molecular weight fibrin degradation products (HMW-FDPs), a surrogate for Pn-modified fibrin, decreased the rate constant for tPA inhibition 3-fold. In the presence of Pg, HMW-FDPs decreased the inhibition rate constant 30-fold. TAFIa treatment of the HMW-FDPs removed the protection associated with Pg, but not HMW-FDPs. TAFIa also abolished the protection of Pn from AP associated with Pn-modified fibrin in clots formed with Glu-Pg, but did not do so in clots formed with Lys-Pg. These data showed that once Lys-Pg and Lys-Pn are generated, the ability of TAFIa to prolong lysis is greatly reduced. These data identified new mechanisms in which TAFIa can attenuate fibrinolysis.