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Updated: May 3, 2026
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Measurement of Factor V Activity in Human Plasma Using a Microplate Coagulation Assay
Published on: September 9, 2012
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Role of human factor VIII in factor X activation
The Journal of Clinical Investigation
|April 1, 1982
Summary
Unactivated Factor VIII requires activation by Factor Xa feedback, showing a lag in Factor X activation. Pre-activated Factor VIII (with thrombin) bypasses this lag, enabling rapid Factor X activation and influencing coagulation kinetics.
Area of Science:
- Biochemistry
- Hematology
- Molecular Biology
Background:
- Factor VIII is a crucial cofactor in the coagulation cascade.
- Its precise role in Factor X activation kinetics requires further elucidation.
Purpose of the Study:
- To investigate the cofactor function of human Factor VIII in Factor X activation.
- To determine the activation requirements and kinetic effects of Factor VIII.
Main Methods:
- Initial-rate assays using radiolabeled Factor X.
- Employing purified human factors (IXa, VIII, X) with Ca2+, phospholipids, or platelets.
- Utilizing specific inhibitors for Factor Xa and thrombin.
Main Results:
- Unactivated Factor VIII exhibited a lag phase in Factor X activation, dependent on Factor Xa feedback.
- Pre-activation of Factor VIII by thrombin abolished the lag phase.
- Activated Factor VIII significantly increased Vmax and, for human Factor X, decreased Km, aligning with plasma concentrations.
Conclusions:
- Factor VIII requires activation, likely by Factor Xa feedback, to initiate Factor X activation.
- Pre-activated Factor VIII enables rapid Factor X activation, independent of reactant order or phospholipid source.
- Kinetic changes induced by Factor VIII suggest plasma Factor X concentration significantly impacts in vivo coagulation rates.
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