FACTOR XA CLEAVAGE OF TISSUE FACTOR PATHWAY INHIBITOR IS ASSOCIATED WITH LOSS OF ANTICOAGULANT ACTIVITY

Tissue factor : factor VIIa induced activation of blood coagulation is inhibited by the complex between factor Xa and tissue factor pathway inhibitor (factor Xa : TFPI). We recently reported that phospholipid-b ound factor Xa reduces the high binding affinity of factor Xa : TFPI f or negatively charged phospholipids by a partial degradation of TFPI ( 17). The present study was undertaken to elucidate the factor Xa cleav age sites in TFPI and to delineate the consequences of this proteolysi s with respect to the inhibitory activity of factor Xa : TFPI. We foun d that phospholipid-bound factor Xa cleaves in TFPI the peptide bonds between Lys(86)-Thr(87) and Arg(199)-Ala(200). Interestingly, Arg(199) is the P1 residue of the third Kunitz-type protease inhibitor domain. The fast cleavage of the Arg(199)-Ala(200) bond results in a 50-70% r eduction of the anticoagulant activity of factor Xa : TFPI, as determi ned with a dilute tissue factor assay, but is not associated with a di minished inhibitory activity of factor Xa : TFPI towards TF : factor V IIa catalyzed activation of factor X. On the other hand, the slower cl eavage of the Lys(86)-Thr(87) peptide bond was associated with both a diminished anticoagulant and anti-TF: factor VIIa activity. Dissociati on of factor Xa from the cleaved TFPI was not observed. These data pro vide evidence for a dual role of factor Xa since it is the essential c ofactor in the TFPI-controlled regulation of TF-dependent coagulation as well as a catalyst of the inactivation of TFPI.