TRANSPORT RATE LIMITED CATALYSIS ON MACROSCOPIC SURFACES - THE ACTIVATION OF FACTOR-X IN A CONTINUOUS-FLOW ENZYME REACTOR

Blood coagulation is initiated on cells which present a macroscopic su rface to the flowing blood stream. We have used a continuous flow enzy me reactor to model this system and to investigate the effects of shea r rate and mass transport on the activation of factor X by the complex of the transmembrane protein, tissue factor, and the serine protease, factor VIIa. This initial step of blood coagulation was found to be h alf-maximal at very low enzyme densities (0.03-0.06%) on the wall of t he capillaries. In agreement with hydrodynamic theory, the apparent K( m) in the flow reactor was correlated with the cube root of the wall s hear rate. These data indicate that at high tissue factor densities (> 0.6%) the activation of 150 nM factor X is controlled by the flux of X toward the surface, which is controlled by wall shear rate and substr ate concentration. The appearance of the product, Xa, in the effluent was delayed to 8-12 min, which was caused by high-affinity binding of Xa to the phospholipid. This delay was considerably shortened by embed ding tissue factor into PC or by coating the PS/PC surface with the ph ospholipid binding protein, annexin V. At low tissue factor densities, annexin V inhibited X activation by 45%, while no inhibition was obse rved at high densities. We demonstrate that when the reaction is limit ed by substrate flux, addition of further enzyme does not increase rea ction rates. This contrasts with classical three-dimensional catalysis in which the initial velocity is ordinarily linear with the enzyme co ncentration.