Inhibition of tissue factor-factor VIIa-catalyzed factor X activation by factor Xa-tissue factor pathway inhibitor. A...

The physiological inhibitor of tissue factor (TF).factor VIIa (FVIIa), full-length tissue factor pathway inhibitor (TFPI(FL)) 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.TFPI(FL) or FXa.TFPI(1-161) (TFPI lacking the third Kunitz domain and C terminus). At TF.PC, FXa.TFPI(FL) and FXa.TFPI(1-161) showed similar rate constants of inhibition (0.07 x 10(8) M(-1) s(-1) and 0.1 x 10(8) M(-1) s(-1), respectively). With phosphatidylserine present, the rate constant of inhibition for FXa.TFPI(FL) increased 3-fold compared with a 9-fold increase in the rate constant for FXa. TFPI(1-161). Incubation of TF.PSPC with FXa.TFPI(FL) in the absence of FVIIa followed by depletion of solution FXa.TFPI(FL) showed that FXa.TFPI(FL) remained bound at the membrane and pursued its inhibitory activity. This was not observed with FXa.TFPI(1-161) or at TF.PC membranes. These data suggest that the membrane-bound pool of FXa.TFPI(FL) may be of physiological importance in an on-site regulation of TF.FVIIa activity.