TISSUE FACTOR-DEPENDENT AUTOACTIVATION OF HUMAN BLOOD-COAGULATION FACTOR-VII

We recently showed that single-chain zymogen factor VII is converted to two-chain factor VIIa in an autocatalytic manner following complex formation with either cell-surface or solution-phase relipidated tissue factor apoprotein (Nakagaki, T., Foster, D. C., Berkner, K. L., and Kisiel, W. (1991) Biochemistry 30, 10819-10824). We have now performed a detailed kinetic analysis of the autoactivation of human plasma factor VII in the presence of relipidated recombinant tissue factor apoprotein and calcium. Incubation of factor VII with equimolar amounts of relipidated tissue factor apoprotein resulted in the formation of factor VIIa amidolytic activity coincident with the conversion of factor VII to factor VIIa. The time course for the generation of factor VIIa amidolytic activity in this system was sigmoidal, characterized by an initial lag phase followed by a rapid linear phase until activation was complete. The duration of the lag phase was decreased by the addition of exogenous recombinant factor VIIa. Relipidated tissue factor apoprotein was essential for factor VII autoactivation. No factor VII activation was observed following complex formation between factor VII and a recombinant soluble tissue factor apoprotein construct consisting of the amino-terminal extracellular domain in the presence or absence of phospholipids. Kinetic analyses revealed that factor VII activation in the presence of relipidated tissue factor apoprotein can be defined by a second-order reaction mechanism in which factor VII is activated by factor VIIa with an apparent second-order rate constant of 7.2 x 10(3) M-1 s-1. Benzamidine inhibited factor VII autoactivation with an apparent K(i) of 1.8 mM, which is identical to the apparent K(i) for the inhibition of factor VIIa amidolytic activity by this active site competitive inhibitor. Our data are consistent with a factor VII autoactivation mechanism in which trace amounts of factor VIIa rapidly activate tissue factor-bound factor VII by limited proteolysis.