J. Franssen et al., PROTHROMBINASE IS PROTECTED FROM INACTIVATION BY TISSUE FACTOR PATHWAY INHIBITOR - COMPETITION BETWEEN PROTHROMBIN AND INHIBITOR, Biochemical journal, 323, 1997, pp. 33-37
The inhibition of prothrombinase by tissue factor pathway inhibitor (T
FPI) has been studied in the presence and absence of prothrombin, The
rate constant of association of prothrombinase with full-length TFPI w
as 2.1 x 10(7) M(-1). s(-1) and 0.05 x 10(7) M(-1). s(-1) for the reac
tion with C-terminus truncated TFPI (TFPI1-161). The rate constant of
dissociation was 0.65 x 10(-4) s(-1) in both cases. The rate constant
of inhibition of prothrombinase by TFPI1-161 was similar to that of so
lution-phase factor Xa. In contrast, phospholipids and factor Va enhan
ced the association rate of the reaction between factor Xa and full-le
ngth TFPI by approx. 20-fold. Although TFPI, and in particular the ful
l-length variant of the molecule, is a potent inhibitor of prothrombin
ase (overall inhibition constant of 3 pM), we also found that prothrom
bin 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 sig
nificance of TFPI as an inhibitor of prothrombinase activity is thus q
uestionable.