Does an enzyme other than thrombin contribute to unexpected changes in the levels of the different forms of thrombin activatable fibrinolysis inhibitor in patients with hemophilia A, hemophilia B and von Willebrand disease?
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abstract
Pro-thrombin activatable fibrinolysis inhibitor (pro-TAFI), also called plasma procarboxypeptidase B or U, is one of the modulators of fibrinolysis in blood. Pro-TAFI is activated by thrombin/thrombomodulin complex or by plasmin to a carboxypeptidase B-like enzyme (TAFI) of 35.8 kD molecular weight. TAFI spontaneously becomes inactive as a result of a temperature-dependent conformational change in the protein (TAFIi). In this study, pro-TAFI, total TAFI antigen and TAFI-TAFIi antigen levels were measured in 32 patients with hemophilia A, 4 patients with hemophilia B, 21 patients with von Willebrand disease (VWD) and 13 healthy controls. A statistically significant decrease in pro-TAFI was found in all groups (10.72+/-4.57 mg/L (p<0.001); 8.00+/-2.35 mg/L (p<0.01) and 8.98+/-2.33 mg/L (p <0.001) for hemophilia A, hemophilia B and VWD, respectively) compared to controls (17.85+4.61 mg/L). A statistically significant increase in TAFI-TAFIi antigen was found in hemophilia A (1.05+/-1.01 mg/L) (p<0.05) and in VWD patients (0.96+/-1.01 mg/L) (p<0.05) compared to controls (0.55+/-0.36 mg/L). There was no difference in total TAFI antigen levels between any group of patients and the controls. Neither did pro-TAFI nor TAFI-TAFIi levels differ within the group of hemophilia A patients in relation to severity (mild, moderate and severe) or among the VWD patients in relation to subtype (type 1, type 2A and type 3). These findings indicate an increased conversion of pro-TAFI to TAFI and/or TAFIi in patients with bleeding disorders. As thrombin generation is seriously impaired in these patients and almost absent in hemophilia A and B and in type 3 VWD, it is possible that plasmin mediates pro-TAFI activation in these patients. Enhanced fibrinolysis via generation of plasmin has previously been reported in hemophilia and VWD. Activation of pro-TAFI by plasmin may be a feedback mechanism that counterbalances increased fibrinolysis in patients with bleeding disorders. The relationship between the TAFI activation pathway and bleeding complications associated with hemophilia A, hemophilia B and VWD requires further investigation.
