Figure 2. Transport limitations in FXa and thrombin generation in tubular bioreactors.

Purified solutions of zymogens (FX and prothrombin) are perfused through tubular bioreactors that consists of a capillary tube coated with lipid bilayers (PC:PS, 75:25) containing enzyme complexes (TF:FVIIa, FVa:FXa). The effluent is collected and the concentration of enzyme (FXa, thrombin) is measured as function of wall shear rate. A. Human FXa generation on 2.2 fmol/cm² TF:FVIIa crosses over from a transport-limited regime to a reaction-limited regime at a wall shear rate of 500 s⁻¹ owing to a reduction in the boundary layer thickness.³⁵ This transition is characterize by an in increase in FXa flux with shear rate up to 500 s⁻¹ (inset). B. Bovine thrombin generation on 1.7 fmol/cm² FVa:FXa is reaction-limited over the entire range of shear rates. Reduction in thrombin concentration with increasing shear rate is due solely to dilution by flow.³⁴ Note that the thrombin flux is constant over entire range of shear rates (inset). C. Double log plot of FXa and thrombin concentrations exiting the reactor as a function of flow velocity for isolated experiments with FX and thrombin. The slope of −1.2 for thrombin is indicative of a reaction-limited regime. The slope of −0.68 for FXa is indicative of a transport-limited regime (Eq. 5).³⁵ D. Double log plot for simultaneous generation of FXa and thrombin in presence of both TF:FVIIa and FVa:FXa. The slope of −0.8 shows that the competition of FX for binding sites with prothrombin causes a transition in FXa production to a regime where transport rates and reaction rates are on the same order-of-magnitude.³⁵