INHIBITORY PROPERTIES OF A NOVEL HUMAN KUNITZ-TYPE PROTEASE INHIBITORHOMOLOGOUS TO TISSUE FACTOR PATHWAY INHIBITOR

In a previous report, we described the molecular cloning, expression, and partial characterization of a second human tissue factor pathway i nhibitor (TFPI), which we designated as TFPI-2 [Sprecher, C. A., et al . (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 3353-3357]. Recombinant TFP I-2 inhibited the amidolytic activity of trypsin as well as that of fa ctor VIIa in complex with tissue factor. TFPI-2 recently has been show n to be identical to placental protein 5 (PP5), a glycoprotein origina lly isolated from placenta that exhibits serine protease inhibitory ac tivity. In the present study, we have examined TFPI-2/PP5 for its abil ity to inhibit a number of serine proteases involved in blood coagulat ion and fibrinolysis, inasmuch as TFPI-2/PP5 prolonged the coagulation time of human plasma induced by either tissue factor or contact activ ation in a dose-dependent manner. In addition to its ability to inhibi t the amidolytic and proteolytic activities of the factor Wa-tissue fa ctor complex, TFPI-2/PP5 strongly inhibited the amidolytic activities of human factor XIa, human plasma kallikrein, and human plasmin with K -i values of 15, 25, and 3 nM. respectively. TFPI-2/PP5 was also a wea k inhibitor of the activation of factor X by a complex of human factor IXa and poly(lysine) with an apparent K-i of 410 nM. Heparin markedly enhanced the ability of TFPI-2/PP5 to inhibit factor VIIa-tissue fact or both in the solution phase and on cell surfaces. In addition. hepar in augmented the inhibition of human factor Xa amidolytic activity at relatively high levels (10-100 nM) of TFPI-2/PP5. No significant inhib ition of glandular kallikrein, urinary plasminogen activator, tissue p lasminogen activator, human activated protein C, human factor Xa, huma n thrombin, or leukocyte elastase was observed when these proteases we re incubated with TFPI-2 in the absence of heparin.