Abstract
Single chain factor VII is converted by limited proteolysis to its activated form, factor VIIa, by a number of blood coagulation proteases including factor IXa and factor Xa. We have determined the relative rate of human factor VII activation by human factors IXa and Xa in two different systems: one containing Ca++ and human bladder carcinoma (J82) cells, and the other containing Ca++ and mixed brain phospholipids. The rate of factor VII activation was determined by a one stage coagulation assay, and proteolytic cleavage of factor VII was assessed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting techniques. On a molar basis, factor Xa was sixfold more efficient than factor IXa beta in activating factor VII when the activation reaction occurs on J82 cell surfaces. In contrast, when incubation takes place in a suspension of mixed phospholipids, factor Xa was 18-fold more efficient in activating factor VII than factor IXa beta. In addition, factor IXa alpha activated factor VII at a rate approximately one-half that observed using factor IXa beta. In the absence of cells or phospholipids, no activation of factor VII by either factors IXa or Xa was observed. The addition of stoichiometric amounts of either recombinant human factor VIII (des B-domain) or plasma-derived factor VIIIa failed to augment the rate of factor VII activation by either factors IXa alpha or IXa beta. Likewise, purified human factor Va failed to influence the rate of factor VII activation by factor Xa in either system. Collectively, our studies reveal that J82 cells possess procoagulant phospholipid capable of readily supporting the activation of factor VII by either factors IXa beta or Xa. Our data also demonstrate that the relative ability of factor IXa beta and Xa to activate factor VII is significantly different when these reactions occur on tumor cell surfaces as compared with suspensions of mixed phospholipids.
