Kinetics of human factor VII activation

In this study the activation of human factor VII by a variety of potential activators in the presence and absence of mixed phospholipid vesicles [25% phssphatidylserine (PS), 75% phosphatidylcholine (PC)] is evaluated. At the plasma concentration of factor VII, 10 nM, the activation rate of the zymogen by 0.05 nM factor Xa is anionic phospholipid (PCPS) dependent and achieves a maximum value of 18 pM/s at 5-20 mu M PCPS; further increases in the levels of PCPS decrease the activation rate of factor VII. The maximum activation rate of factor VII (10 nM) by the factor VIIa-tissue factor complex (0.1 nM), 0.76 pM/s, is achieved at 200 mu M PCPS. No detectable activation of 10 nM factor VII is observed under similar conditions when either thrombin (0.1 nM) or factor IXa (0.1 nM) is used as an activator. Factor VIIa (10 nM) and factor XIa(1 nM) are not observed to activate factor VII at detectable rates. The observed Michaelis-Menten constants (K-M) for factor VII activation in the presence of PCPS at optimal concentrations vary from 1.2 mu M for factor Xa to 3.2 mu M for the factor VIIa-tissue factor complex. The highest catalytic constant (k(cat)) value (15.2 s(-1)) is observed for factor Xa-PCPS. The factor VIIa-tissue factor complex, factor IXa, and thrombin k(cat) values are 1.4, 0.32, and 0.061 s(-1), respectively. Tissue factor does not increase the factor VII activation rate by factor Xa, factor IXa, or thrombin. Factor VIIIa in the presence of PCPS has no effect on factor VII activation by factor IXa. In contrast, factor Va decreases the factor VII activation rate by factor Xa, reaching saturation at concentrations consistent with complete prothrombinase complex formation. The formed prothrombinase complex activates factor VII at approximately 30% the rate of factor Xa bound to phospholipids. These data allow us to conclude that the predominant physiological factor VII activator is, most likely, membrane-bound factor Xa.