Complex-dependent inhibition of factor VIIa by antithrombin III and heparin

The regulation of the factor VIIa-tissue factor complex is essential for control of the hemostatic response. However, the role of the inhibitor antithrombin III in the regulation of factor VIIa has remained in question. The inhibition of factor VIIa activity by antithrombin III and heparin in the presence and absence of tissue factor was evaluated using the fluorescent substrate m-LGR-nds. Our data show that the activity of recombinant human factor VIIa is inhibited by antithrombin III in the presence of heparin at a rate of 1.7 x 10(2) M-1 s-1. In the presence of tissue factor, the rate constant for this reaction increases to 5.6 x 10(3) M-1 s-1. A 1:1 stoichiometric complex between factor VIIa and antithrombin III, with an apparent molecular weight of 110,000, was detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A heterogeneous mixture of factor VIIa products with molecular weights between 50,000 and 80,000, most likely representing proteolytically degraded factor VIIa-antithrombin III complexes, was also observed.