Molecular changes associated with proteolysis of bovine factor V by thrombin

Native factor V contains two major polypeptide chains, h and 1. The molecular weights determined by gel electrophoresis in the presence of sodium dodecylsulfate and dithiothreitol (125 000 and 73 000) are in reasonable agreement with those obtained by gel filtration in 5 M guanidine-HC1 (125000 and 64000). Exposure of factor V to thrombin results in cleavage of the heavier chain to an altered form with a molecular weight of 87000. The other fragment of this proteolytic reaction appears to be a carbohydrate-rich piece, which migrates abnormally slowly on gel electrophoresis conducted under denaturing and reducing conditions. Both proteolytic cleavage products remain associated with the light chain during the purification of factor V. The 87000-Mr fragment is present in samples of factor V which are isolated by immunoprecipitation of blood obtained from a single animal by venous catheter. This finding suggests that some proteolysis may occur in vivo. In contrast, the molecular weight of the light chain is unaltered after thrombin proteolysis of either purified factor V or thrombin-treated plasma.