The conversion of bovine Factor V (FV) to Factor Va (FVa) by thrombin appears to proceed in two steps as shown below. FV(300,000) → thrombin Factor V intermediate (FVi) 2 sub-units; 220,000 & 100,000)→FVa (2 subunits; FVa-H.C., 100,000 & FVa-L.C., 73,000)+activation peptide/s. The 220,000 Mol. wt. subunit of FVi gives rise to the FVa-L.C. EDTA dissociates both FVi and FVa into subunits, Mn2+ facilitates reassociation. Even following dissociation with EDTA, 1 mole of Ca2+ remains bound to each of the FVa subunits.Support of the above model of FV activation comes from an immunological characterization of the FV. Antibodies to FV cross react with FVa, FVi and both subunits of Fva. Antibodies to FVa cross react with FVi both subunits of FVi and both subunits of FVa. Antibodies to the FVa-L.C. cross react with FV, FVj and FVa, but do not with the FVa-H.L. Antibodies to the FVa-H.C. cross react with FV, the FVi, FVa but not with the FVa-L. C.A protease from Russell’s viper venom activated FV by making a single proteolytic cleavage. This results in the formation of two subunits of Mol. wt., 250,000 & 73,000. The 73,000 Mol. wt. subunit is functionally and immunologically identical to the FVa-L.C. the 250,000 Mol. wt. subunit contains the antigenic determinants to the FVa-H.C. and can be converted into this subunit by incubation with thrombin. The appearance of two disdinct high Mol. wt. activation products from FV; one containing the FVa-L.C.; the other containing the FVa-H.C., is compatible with placing the activation peptide region between the two subunits of Fva.
Identification and characterization of a phospholipid-binding site of bovine factor Va.
