HEMOSTASIS, THROMBOSIS, AND VASCULAR BIOLOGY Characterization of the protein Z–dependent protease inhibitor

(ZPI) is a 72-kd member of the serpin superfamily of proteinase inhibitors that produces rapid inhibition of factor Xa in the presence of protein Z (PZ), procoagu-lant phospholipids, and Ca11 (t1/2 less than 10 seconds). The rate of factor Xa inhibition by ZPI is reduced more than 1000-fold in the absence of PZ. The factor Xa–ZPI complex is not stable to sodium dodecyl sulfate–polyacrylamide gel elec-trophoresis, but is detectable by alkaline– polyacrylamide gel electrophoresis. The combination of PZ and ZPI dramatically delays the initiation and reduces the ulti-mate rate of thrombin generation in mix-tures containing prothrombin, factor V, phospholipids, and Ca11. In similar mix-tures containing factor Va, however, PZ and ZPI do not inhibit thrombin genera-tion. Thus, the major effect of PZ and ZPI is to dampen the coagulation response prior to the formation of the prothrombi-nase complex. Besides factor Xa, ZPI also inhibits factor XIa in the absence of PZ, phospholipids, and Ca11. Heparin (0.2 U/mL) enhances the rate (t1/2 5 25 sec-onds vs 50 seconds) and the extent (99 % vs 93 % at 30 minutes) of factor XIa inhibition by ZPI. During its inhibitory interaction with factor Xa and factor XIa, ZPI is proteolytically cleaved with the release of a 4.2-kd peptide. The N-terminal amino acid sequence of this peptide (SMPPVIKVDRPF) establishes Y387 as the P1 residue at the reactive center of ZPI. ZPI activity is consumed during the in vitro coagulation of plasma through a proteolytic process that in-volves the actions of factor Xa wit