Kinetics of Factor X Activation by Factor IXa the Effects of Phospholipid and Factor VIIIa
The kinetics of factor X activation by activated clotting factor IXa was measured either in the presence or absence of phospholipid, Ca-ions and factor VIIIa. This study was carried out with purified bovine clotting factors. The rate of factor Xa formation was measured using the chromogenic substrate S 2222. Both the Km for factor X and the Vmax of factor Xa formation were determined for different compositions of the factor X activating complex in order to get more insight in the role of phospholipid, Ca2+ and factor VIII in factor X activation. With factor IXa alone, the activation of factor X is a very inefficient process. The presence of phospholipids predominantly lowers the Km for factor X while in contrast factor VIIIa mainly increases the Vmax of factor Xa formation. The implications of these findings for the mechanism of, factor X activation in the intrinsic pathway will be discussed.