Kinetic Properties and Molecular Size of Thrombin-activated Factor V

Native bovine factor V exists in three forms of differing molecular size, the oligomeric interconvertible forms A and C and a phospholipid-containing complex designated form L. Thrombin increases the activity of form A to a greater extent than form L and does not alter the activity of form C. Thrombin-activated form A and form C function as potent inhibitors of thrombin activation of form A but cannot inhibit the action of thrombin on other substrates as α-N-toluene-sulfonyl-l-arginine methyl ester and fibrinogen. No change in the molecular size of any of these three forms was detected after exposure to thrombin. Factor V derived from serum by endogenous thrombin action appeared to have a smaller Stokes radius than that of form A, the predominant form in plasma. However, the serum factor V associates to a form similar in size to form A after storage in 50% glycerol. These results suggest the formation of a complex between these inhibitor forms of factor V and activated factor V (form A), suggesting that both substrate and product inhibition occur.