Summary
Factor VIIa (F. VIIa)/tissue factor (TF) function was examined using purified human
TF reconstituted into mixed phospholipid vesicles and TF expressed on cultured human
umbilical vein endothelial cells (HUVEC) treated with thrombin. In reaction mixtures
containing either type of TF, F. VIIa, 10 nM, either 3H-factor X or 3H-factor IX, 88 nM, and Ca2+, 5 mM, F. VIIa/TF activated factor X (F. X) several fold faster than it activated
factor IX (F. IX). Adding heparin, 1 U/ml, increased rates of activation of both substrates
and F. X remained the preferred substrate. Adding plasma at concentrations of 5%;or
above inhibited factor VIIa/TF catalytic activity. Inhibition was shown to require
F. Xa as a cofactor, was prevented by antibodies to extrinsic pathway inhibitor (EPI),
and was reversible by decalcification. Thus, with factor VIIa/TF formed with both
types of TF, EPI appeared responsible for inhibition induced by plasma. Our data indicate
that functional properties of factor VIIa/TF as delineated in reaction mixtures made
with purified TF reconstituted into mixed phospholipid vesicles also hold for factor
VIIa/TF activity on the surface of cultured HUVEC.
Keywords
Tissue factor - Extrinsic pathway inhibitor - Heparin - Factor X and factor IX - Endothelial
cells
