Immunochemical Studies of Factor V of Bovine Platelets

Abstract

Immunochemical techniques were used to investigate the molecular properties of bovine platelet factor V. An antiserum prepared against purified factor V rapidly inactivated platelet factor V, indicating that platelet factor V is antigenically related to plasma factor V. A reaction of identity between factor V in plasma and purified factor V was documented by immunodiffusion against an antibody to platelet factor V. When platelet factor V was extracted with Triton X-100 in the presence of protease inhibitors and subjected to immunoelectrophoressis against the antiserum to plasma factor V, a single antigenic component migrating toward the anode was observed. In the absence of protease inhibitors, following release by collagen or solubilization, platelet factor V appeared close to the origin, suggesting proteolytic alteration. Platelet factor V released ty collagen or extracted with Triton X-100 was activated by thrombin 7.4-fold and 4.5-fold, respectively, compared to a 17-fold activation of factor V in plasma under identical conditions. The ability of thrombin to activate platelet factor V and the close correspondence of factor V activity and antigen released by collagen indicates that the molecule is largely in the unactivated form after release. A single component of MW 270,000 was seen when platelet factor V released by collagen was immunoprecipitated and subjected to dodecylsulfate gel electrophoresis. Factor V coagulant assays and immunoelectrophoresis of subcellular fractions showed that platelet factor V is localized primarily in the .alpha.-granules. Platelet factor V appears to be similar to, if not identical with, plasma factor V by the criteria of immunologic identity, similar electrophoretic mobility and virtually identical MW.