Activation of Human Factor VII in Plasma and in Purified Systems

Abstract

A B S T R A C T Factor VII can be activated, to a molecule giving shorter clotting times with tissue factor, by incubating plasma with kaolin or by clotting plasma. The mechanisms of activation differ. With kaolin, activated Factor XII (XIIa) was the apparent principal activator. Thus, Factor VII was not activated in Factor XII-deficient plasma, was partially activated in prekallikrein and high-molecular weight kininogen (HMW kininogen)-deficient plasmas, but was activated in other deficient plasmas. After clotting, activated Factor IX (IXa) was the apparent principal activator. Thus, Factor VII was not activated in Factor XII-, HMW kininogen-, XI-, and IX-deficient plasmas, but was activated in Factor VIII-, X-, and V-deficient plasmas. In further studies, purified small-fragment Factor XIIa (,8-XIIa), kallikrein, and Factor IXa were added to partially purified Factor VII and to plasma.

Effect of recombinant factor VIIa on the hemostatic defect in dogs with hemophilia A, hemophilia B, and von Willebrand disease.

“…The two hemophilic diseases are due to a deficiency of different proteins that act in a different manner in the intrinsic coagulation pathway. The tl2 of injected rF.VIIa in these experiments was in the 2-to 3-hr range, similar to that observed in inhibitor hemophilia A patients given clinical factor IX concentrates (32). The administration of rF.VIIa in excess seems to induce normal hemostasis in the hemophilic dogs, bypassing the activation of factor IXa and perhaps activation of factor VII as well.…”

Binding of human factor VII and VIIa to monocytes.

“…In fact, radioactively labeled bovine Factor VII has been used as a probe to localize tissue factor in tissue slices (34). Since the binding of Factor VIIa to the monocyte surface reported here was directly related to the velocity of subsequent Factor X activation, in all probability (essentially by definition), the Factor A number of proteases have been shown to activate Factor VII in purified, and in vitro plasma systems (38)(39)(40)(41)(42). The kinetically most efficient of these appears to be Factor Xa, at least when purified reagents are used (38).…”

Residue Met156 Contributes to the Labile Enzyme Conformation of Coagulation Factor VIIa

“…VIIa circulates in the blood plasma as zymogen as well as the cleaved two-chain enzyme (2), but proteolytic function only ensues upon binding to its cell surface receptor and catalytic cofactor tissue factor (TF). TF has two distinct effects that regulate proteolysis by VIIa.…”

“…In the absence of cofactor, VIIa has only very low catalytic activity toward small peptidyl substrate mimetics and TF stimulates the amidolytic of VIIa up to 100-fold (1). However, macromolecular substrate factor X scissile bond catalysis is enhanced Ͼ1000-fold (6), indicating that cofactorinduced conformational changes may influence extended macromolecular substrate recognition regions in addition to the S1-S3 subsite 2 that is probed by the small substrates. The low catalytic activity of free VIIa results from a zymogen-like conformation of the enzyme.…”