Insensitivity of factor V and factor Va to diisopropylfluorophosphate and antithrombin III

Bovine plasma Factor V and Factor Va, the latter prepared by thrombin or venom activator action on Factor V, are not inactivated by diisopropylfluorophosphate or antithrombin III nor do they form identifiable complexes with either of these reagents. On the basis of these data, it is concluded that bovine Factor V and Factor Va are not serine proteases.