title: Prothrombinase is protected from inactivation by tissue factor pathway inhibitor: competition between prothrombin and inhibitor.
abstract: The inhibition of prothrombinase by tissue factor pathway inhibitor (TFPI) has been studied in the presence and absence of prothrombin. The rate constant of association of prothrombinase with full-length TFPI was 2.1x10(7) M-1.s-1 and 0.05x10(7) M-1.s-1 for the reaction with C-terminus truncated TFPI (TFPI1-161). The rate constant of dissociation was 0.65x10(-4) s-1 in both cases. The rate constant of inhibition of prothrombinase by TFPI1-161 was similar to that of solution-phase factor Xa. In contrast, phospholipids and factor Va enhanced the association rate of the reaction between factor Xa and full-length TFPI by approx. 20-fold. Although TFPI, and in particular the full-length variant of the molecule, is a potent inhibitor of prothrombinase (overall inhibition constant of 3 pM), we also found that prothrombin competed very effectively with TFPI for the active site of factor Xa in the prothrombinase complex. A 50% reduction of the rate constant of inhibition was measured in the presence of 4 nM prothrombin, i.e. 0.2% of the plasma concentration of prothrombin. The physiological significance of TFPI as an inhibitor of prothrombinase activity is thus questionable.
authors.affiliation: Department of Biochemistry, Maastricht University, PO Box 616, 6200 MD Maastricht, The Netherlands.
authors.forename: J
authors.initials: J
authors.lastname: Franssen
authors.forename: I
authors.initials: I
authors.lastname: Salemink
authors.forename: G M
authors.initials: GM
authors.lastname: Willems
authors.forename: T C
authors.initials: TC
authors.lastname: Wun
authors.forename: H C
authors.initials: HC
authors.lastname: Hemker
authors.forename: T
authors.initials: T
authors.lastname: Lindhout
mesh_terms: D000818:Animals; D001667:Binding, Competitive; D002417:Cattle; D065427:Factor Xa Inhibitors; D006801:Humans; D007700:Kinetics; D008074:Lipoproteins / Q000378:metabolism*; D010743:Phospholipids / Q000378:metabolism; D011516:Prothrombin / Q000378:metabolism*; D015842:Serine Proteinase Inhibitors / Q000494:pharmacology*; D013925:Thromboplastin / Q000037:antagonists & inhibitors / Q000378:metabolism*
chemical_list: D065427:Factor Xa Inhibitors; D008074:Lipoproteins; D010743:Phospholipids; D015842:Serine Proteinase Inhibitors; C051928:lipoprotein-associated coagulation inhibitor; D011516:Prothrombin; D013925:Thromboplastin