Activation of 125I-factor IX and 125I-factor X: effect of tissue factor and factor VII, factor Xa and thrombin.

Activation of Factor IX and Factor X was studied by adding 125I-Factor IX or 125I-Factor X to reaction mixtures and quantitating cleavage products by reduced sodium dodecylsulfate gel electrophoresis. Thrombin failed to activate Factors IX or X; Factor Xa produced insignificant amounts of cleavage products of both factors. In contrast, the reaction product of tissue factor and Factor VII cleaved large amounts of both Factor IX and Factor X in purified systems and in plasma. In incubation mixtures of plasma containing added 125I-Factor IX or 125I-Factor X, tissue factor and Ca2+ ions, the percentage of total radioactivity in the heavy chain peak of 125I-IXa and the heavy chain peak of 125I-Xa increased at a similar rate. When the tissue factor was diluted, similar curves were obtained for percent cleavage of 125I-Factor IX and percent cleavage of 125I-Factor X plotted against tissue factor concentration. These findings support the hypothesis that activation of Factor IX by the tissue factor-Factor VII reaction produce represents a physiologically significant step in normal haemostasis.