Inhibition of tissue factor-factor VIIa-catalyzed factor X activation by factor Xa-tissue factor pathway inhibitor: A rotating disc study on the effect of phospholipid membrane composition

The physiological inhibitor of tissue factor (TF) factor Wa (FVIIa), full-length tissue factor pathway inhibitor (TFPIFL) in complex with factor Xa (FXa), has a high affinity for anionic phospholipid membranes, The role of anionic phospholipids in the inhibition of TF.FVIIa-catalyzed FX activation was investigated. FXa generation at a rotating disc coated with TF embedded in a membrane composed of pure phosphatidylcholine (TF PC) or 25% phosphatidylserine and 75% phosphatidylcholine (TF.PSPC) was measured in the presence of preformed complexes of FXa.TFPIFL or FXa.TFPI1-161 (TFPI lacking the third Kunitz domain and C terminus). At TF.PC, FXa.TFPIFL and FXa.TFPI1-161 showed similar rate constants of inhibition (0.07 x 10(8) M-1 s(-1) and 0.1 x 10s RI-1 S-1, respectively). With phosphatidylserine present, the rate constant of inhibition for FXa.TFPIFL increased 3-fold compared with a 9-fold increase in the rate constant for FXa.TFPI1-161. Incubation of TF.PSPC with FXa TFPI,, in the absence of FVIIa followed by depletion of solution FXa.TFPI1-161 showed that FXa.TFPIFL remained bound at the membrane and pursued its inhibitory activity. This was not observed with FXa.TFPI1-161 or at TF.PC membranes. These data suggest that the membrane-bound pool of FXa. TFPIa may be of physiological importance in an on-site regulation of TF.FVIIa activity.