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Flow dependence of factor X activation by tissue factor-factor VIIa
P B Contino1, H A Andree, Y Nemerson
1Department of Medicine, Mount Sinai School of Medicine of the City University of New York, N.Y.
Summary
Blood coagulation begins with Tissue Factor (TF). Studies show factor X activation by TF-VIIa is transport-limited, not enzyme-limited, under continuous flow conditions.
Area of Science:
- Biochemistry
- Hemostasis
- Biophysical Chemistry
Background:
- Blood coagulation initiates upon exposure to Tissue Factor (TF).
- The TF-VIIa complex plays a crucial role in activating factor X.
- Understanding flow dynamics is key to hemostasis research.
Purpose of the Study:
- To investigate the flow-dependent kinetics of factor X activation by the TF-VIIa complex.
- To compare enzyme kinetics on vesicle surfaces versus macroscopic surfaces.
- To elucidate the rate-limiting steps in factor X activation under flow.
Main Methods:
- Utilized a continuous flow reactor to study TF-VIIa activity.
- Measured factor X activation rates at varying enzyme densities and flow rates.
- Determined kinetic parameters, including Km values.
Main Results:
- Observed a significantly lower apparent Km for factor X with TF-VIIa in vesicles (16 nM) compared to macroscopic surfaces (> 600 nM).
- Factor Xa production rate showed insensitivity to enzyme density within a tested range (17-70 fmoles TF-VIIa/cm2).
- Factor X activation rate exhibited a linear relationship with the cube root of the flow rate.
Conclusions:
- Factor X activation by the TF-VIIa complex under continuous flow is primarily limited by the transport of factor X across a boundary layer.
- Findings contrast with classical Michaelis-Menten kinetics, highlighting the importance of mass transport in biological systems.
- This research provides insights into the biophysics of blood coagulation initiation.