Tissue factor pathway inhibitor-alpha inhibits prothrombinase during the initiation of blood coagulation

Tissue factor (TF) pathway inhibitor (TFPI) is a well-characterized activated factor X (FXa)-dependent inhibitor of TF-initiated coagulation produced in two alternatively spliced isoforms, TFPI alpha and TFPI beta. The TFPI alpha C terminus has a basic sequence nearly identical to a portion of the factor V (FV) B domain necessary for maintaining FV in an inactive conformation via interaction with an acidic region of the B domain. We demonstrate rapid inhibition of prothrombinase by TFPI alpha mediated through a high-affinity exosite interaction between the basic region of TFPI alpha and the FV acidic region, which is retained in FXa-activated FVa and platelet FVa. This inhibitory activity is not mediated by TFPI beta and is lost upon removal of the acidic region of FVa by thrombin. The data identify a previously undescribed, isoform-specific anticoagulant function for TFPI alpha and are a unique description of physiologically relevant inhibition of prothrombinase. These findings, combined with previous descriptions of differential expression patterns of TFPI alpha and TFPI beta in platelets and endothelial cells, suggest that the TFPI isoforms may act through distinct mechanisms to inhibit the initial stages of intra-vascular coagulation, with TFPI beta acting to dampen TF expressed on the surface of vascular cells, whereas TFPI alpha dampens the initial prothrombinase formed on the activated platelet surface.