Blood coagulation can be initiated when factor VII or VIIa, a plasma protease, binds to its essential cofactor, tissue factor (TF), and proteolytically activates factors IX and X, triggering a cascade of events which eventually leads to the formation of thrombin and a fibrin clot. Plasma contains a lipoprotein-associated coagulation inhibitor (LACI) which inhibits activated factor X (Xa) directly and, in a Xa-dependent way, inhibits VII(a)/TF activity, presumably by forming a quaternary Xa/LACI/VII(a)/TF complex. Sequence analysis of complementary DNA clones has shown that LACI contains three tandemly repeated Kunitz-type serine protease inhibitory domains. To investigate the relationship between these Kunitz structures and LACI function, we have used site-directed mutagenesis to produce altered forms of LACI in which the residue at the active-site cleft of each Kunitz domain has been individually changed. The second Kunitz domain is required for efficient binding and inhibition of Xa, and both Kunitz domains 1 and 2 are required for the inhibition of VIIa/TF activity; but alteration of the active-site residue of the third Kunitz domain has no significant effect on either function. We propose that in the putative inhibitory complex, Kunitz domain 1 is bound to the active site of VII(a)/TF and that Kunitz domain 2 is bound to Xa's active site.

^Jlood coagulation proceeds through a series of reactions in which plasma zymogens of serine enzymes are sequentially activated by limited proteolytic cleavage. Mechanistically, the initiation of coagulation has been separated into two pathways, "extrinsic" and "intrinsic", that converge at the activation of factor X with subsequent generation of thrombin proceeding through a single, "common", pathway (MacFarlane, 1964; Davie & Ratnoff, 1964) (Figure 1). In the extrinsic pathway, factor VIIa bound to its cofactor, tissue factor (TF), can activate factor X directly. In the intrinsic pathway, exposure

An inhibitor of tissue factor-induced coagulation was rediscovered in the 1980’s and subsequently named tissue factor pathway inhibitor (TFPI). Three isoforms of TFPI are transcribed through alternative mRNA splicing: TFPIα, which contains an acidic aminoterminus followed by three tandem Kunitz-type protease inhibitor domains and a basic carboxyterminus; TFPIβ, in which the Kunitz-3 and carboxyterminus of TFPIα are replaced with a different carboxyterminus containing a glycosyl phosphatidyl inositol (GPI) anchor; and TFPIδ, which is truncated following the Kunitz-2 domain. The microvascular endothelium is thought to be the principal source of TFPI and TFPIα is the predominant isoform expressed in humans. TFPIα, apparently attached to the surface of the endothelium in an indirect GPI-anchor-dependent fashion, represents the greatest in vivo reservoir of TFPI. The Kunitz-2 domain of TFPI is responsible for factor Xa inhibition and the Kunitz-1 domain is responsible for factor Xa-dependent inhibition of the factor VIIa/tissue factor catalytic complex. The anticoagulant activity of TFPI in one-stage coagulation assays is due mainly to its inhibition of factor Xa through a process that is enhanced by protein S and dependent upon the Kunitz-3 and carboxyterminal domains of full-length TFPIα. Carboxyterminal truncated forms of TFPI as well as TFPIα in plasma, however, inhibit factor VIIa/tissue factor in two-stage assay systems. Studies in gene-disrupted mice demonstrate the physiological importance of TFPI.

Stimulation with thrombin or the calcium ionophore, A23187 caused human platelets to release a coagulation inhibitor similar to the Lipoprotein Associated Coagulation Inhibitor (LACI). This was documented functionally, with clotting assays measuring tissue factor inhibition and factor Xa inhibition, as well as immunologically, in a competitive immunoassay. The total amount of LACI released by 3 x 10(8) platelets after two hours stimulation was 7% to 8% of the amount found in 1 mL of serum. Half of the LACI was released by five minutes. The LACI was present in the platelet supernatant and was not associated with the platelet membrane or shed vesicles. The tissue factor and factor Xa inhibitory activities that were released were neutralized by preincubating the platelet supernatants with specific rabbit polyclonal anti-LACI IgG. On Western blot, platelet LACI appeared to run as a doublet with a molecular weight (mol wt) 45,000 to 47,000. Blood samples obtained from the site of a wound (template bleeding time) demonstrated a progressive increase in LACI concentration. A cDNA probe, derived from endothelial cell LACI cDNA, hybridized selectively to 4.0 and 1.4 kb transcripts in a preparation of platelet mRNA.