Extrinsic Activation of Human Blood Coagulation Factors IX and X

Summary

We studied activation of human coagulation factors IX and X by factor VIIa in the presence of calcium ions, phospholipid (phosphatidylserine/phosphatidylcholine, 50/50, mol/mol) and purified tissue factor apoprotein. Activation of factor IX and factor X was found to occur without a measurable lag-phase and hence initial rates of factor IXa and factor Xa formation could be determined. Like previously observed for the activation of factor X, the activation of factor IX was saturable with respect to factor VIIa, tissue factor apoprotein and phospholipid. The results suggested that in the presence of a Ca²⁺ ions the same ternary complex of factor VIIa-tissue factor apoprolein-phospholipid is responsible for the activation of factor IX and factor X. Roth the apparent K_m of 22 nM-factor IX and the apparent K_cat of 28 min⁻¹ were about 3-fold lower than the coiicsponding parameters of factor X activation by this complex. Hence, the catalytic efficiency (K_cat/K_m) of factor IX and factor X activation was about equal. However, the two substrates inhibited the activation of each other by competition for the same catalytic sites. The apparent K_inh of factor IX for inhibition of extrinsic factor X activation is 30 nM. The apparent K_inh of factor X for inhibition of extrinsic factor IX activation is 116 nM. From these kinetic data it was calculated that at plasma concentration of factors IX and X, the rate of extrinsic factor IX activation would be half the rate of factor X activation. These relative rates of extrinsic factor IX and factor X activation in combination with previously reported kinetic data on the activation of factor X by factor IXa in the presence of factor VIIIa provide support for the concept that at low levels of tissue factor, factor IXa formation might play an important role in the extiinsic pathway of coagulation in vivo.

• References

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