TAFI, OR PLASMA PROCARBOXYPEPTIDASE-B, COUPLES THE COAGULATION AND FIBRINOLYTIC CASCADES THROUGH THE THROMBIN-THROMBOMODULIN COMPLEX

TAFI (thrombin-activatable fibrinolysis inhibitor) is a recently disco vered plasma protein that can be activated by thrombin-catalyzed prote olysis to a carboxypeptidase B-like enzyme that inhibits fibrinolysis, This work shows that the thrombin-thrombomodulin complex, rather than free thrombin, is the most likely physiologic activator, Thrombomodul in increases the catalytic efficiency of the reaction by a factor of 1 250, an effect expressed almost exclusively through an increase in k(c at). The kinetics of the reaction conform to a model whereby thrombin can interact with either TAFI (K-m = 1.0 mu M) or thrombomodulin (K-d = 8.6 nM), and either binary complex so formed can then interact with the third component to form the ternary thrombin-thrombomodulin-TAFI c omplex from which activated TAFI is produced with kcat = 1.2 s(-1). Th is work also shows that activated TAFI down-regulates tPA-induced fibr inolysis half maximally at a concentration of 1.0 nM in a system of pu rified components. This concentration of TAFI is about 2% of the level of the zymogen in plasma, which indicates that ample activated TAFI c ould be generated to very significantly modulate fibrinolysis in vivo. Therefore, TAFI in vitro and possibly in vivo defines an explicit mol ecular connection between the coagulation and fibrinolytic cascades, s uch that expression of activity in the former down-regulates the activ ity of the latter.