Coagulation factor V exists uncomplexed in bovine plasma

Bovine coagulation factor V has been examined immunochemically to ascertain whether the coagulant polypeptide (h) with M_r = 290 000–330 000 is complexed in plasma with a second immunochemically distinct polypeptide (I₂) of M_r = 400 000. Antiserum containing antibodies to h and l₂ detects the l₂ polypeptide eluting earlier than the h chain on gel filtration of plasma with either added calcium or EDTA, consistent with the behavior of a higher molecular weight noninteracting species. An immobilized monospecific antibody to l₂ removes only the l₂ polypeptide from a purified factor V preparation containing both h and l₂. Moreover, while a monospecific antibody to the h chain was able to precipitate purified radioactively labelled h chain alone or mixed with plasma, the l₂ antibody was unable to precipitate radioactively labelled h chain even after attempted recombination of the h chain with l₂ present in plasma. These studies indicate that the l₂ polypeptide is not complexed to the h chain in a purified system or in plasma and reinforce the conclusion that factor V is a single polypeptide chain uncomplexed in plasma.