Factor VIIa/tissue factor generates a form of factor V with unchanged specific activity, resistance to activation by thrombin, and increased sensitivity to activated protein C.

Factor VIIa, in complex with tissue factor (TF), is the serine protease responsible for initiating the clotting cascade. This enzyme complex (TF/VIIa) has extremely restricted substrate specificity, recognizing only three previously known macromolecular substrates (serine protease zymogens, factors VII, IX, and X). In this study, we found that TF/VIIa was able to cleave multiple peptide bonds in the coagulation cofactor, factor V. SDS-PAGE analysis and sequencing indicated the factor V was cleaved at Arg679, Arg709, Arg1018, and Arg1192, resulting in a molecule with a truncated heavy chain and an extended light chain. This product (FVTF/VIIa) had essentially unchanged activity in clotting assays when compared to the starting material. TF reconstituted into phosphatidylcholine vesicles was ineffective as a cofactor for the factor VIIa cleavage of factor V. However, incorporation of phosphatidylethanolamine in the vesicles had little effect over the presence of 20% phosphatidylserine. FVTF/VIIa was as sensitive to inactivation by activated protein C (APC) as thrombin activated factor V as measured in clotting assays or by the appearance of the expected heavy chain cleavage products. The FVTF/VIIa could be further cleaved by thrombin to release the normal light chain, albeit at a significantly slower rate than native factor V, to yield a fully functional product. These studies thus reveal an additional substrate for the TF/VIIa complex. They also indicate a new potential regulatory pathway of the coagulation cascade, i.e., the production of a form of factor V that can be destroyed by APC without the requirement for full activation of the cofactor precursor.