The FVII activating protease cleaves single-chain plasminogen activators.

A serine protease isolated from plasma sharing structural characteristics with a hepatocyte growth factor activator-like protease has been demonstrated recently to activate FVII. Accordingly, it was named 'FVII activator'. Until now an impact of this protease on the fibrinolytic system has not been reported. We islolated the protease from cryo-poor plasma by subsequent ion exchange chromatography and adsorption to immobilized heparin and/or aprotinin. Incubation of single-chain plasminogen activators (sc-PAs) with the FVII activator revealed significant activation of urokinase sc-PA (scu-PA) and tissue sc-PA (sct-PA) in vitro. It was enhanced in the presence of calcium and heparin. Compared to kallikrein, a more efficient activation of scu-PA was observed, whereas sct-PA appeared to be a poorer substrate for the FVI activator. At low protease concentrations and in the presence of heparin the scu-PA activation was comparable to plasmin. Employing recalcified whole blood thrombelastography, the lysis of initially formed fibrin was observed after addition of a combination of scu-PA and the FVII activator, whereas the scu-PA alone had a negligible effect at the concentration used. The study results as presented demonstrate that the FVII activator is a potent activator of sc-PAs in vitro. Whether it plays a physiological role in fibrinolysis deserves further investigation. Its comparatively high affinity to heparin assumes a function in cell surface or matrix events.

A protease isolated from human plasma activating factor VII independent of tissue factor.

An activity detected in a prothrombin complex concentrate, termed 'thrombin-like' due to its amidolytic properties, was recently reported by another working group. This serine-protease revealed partial structural homology with a 'hepatocyte growth factor activator'. An impact of this protease on coagulation has not yet been described. The protease was isolated from plasma fractions by ion exchange chromatography and adsorption to immobilized heparin and/or aprotinin. Clotting tests including the FVIIa-rTF assay were performed employing coagulometry. A monoclonal antibody-derived F(ab')2 to FVIII was used to investigate the FVIII bypassing activity (FEIBA). The identity of the-protease with the so-called 'thrombin-like' protease was supported by sequencing of the amino-termini. Its amidolytic activity was significantly enhanced in the presence of calcium and/or heparin. Incubation with purified FVII revealed the generation of FVIIa, but was prevented by pre-incubation of the protease with aprotinin. In contrast, purified FV and FVIII were inactivated. Studying coagulation parameters, clotting times like plasma recalcification times and the prothrombin times were found to be shortened by addition of the protease. Employing a FVIII-inhibitory F(ab')2 and enhancing clotting times significantly, FEIBA of the protease was found. We demonstrated that the isolated protease activates FVII independent of tissue factor. Net acceleration of coagulation was found in several global clotting assays resulting in an in vitro FEIBA. The physiological relevance of these findings deserves further investigation.