Factor V: An Activator of Factor Xa in the Absence of Prothrombin

S ummary . Purified factor V accelerates the rate of conversion of prothrombin to thrombin by activated factor X (factor Xa) in the presence of phospholipid and calcium. Factor V potentiates the arginine esterase activity of factor Xa more than three‐fold in the presence of phospholipid and calcium but in the absence of prothrombin. The increase of factor Xa esterase activity as a function of factor‐V concentration demonstrated signoidal kinetics and a linear Hill plot, suggesting cooperative binding of factor V to multiple sites on the phospholipid micellar surface. An increase in both K m and V m of factor Xa for tosyl l ‐arginine methyl ester occurs in the presence of factor V, suggesting that factor V enhances the dissociation of the factor‐Xa substrate complex. In addition, factor V alters the catalytic efficiency of factor Xa toward various substituted basic amino acid esters. Based on these observations, a model is suggested in which factor V interacts with factor Xa on the surface of a phospholipid micelle to alter the affinity of factor Xa for certain susceptible bonds on the prothrombin molecule.