INCORPORATION OF AN ACTIVE-SITE INHIBITOR IN FACTOR VIIA ALTERS THE AFFINITY FOR TISSUE FACTOR

Recent studies showed that the administration of active site-inhibited factor VIIa blocked factor VIIa/tissue factor-induced fibrin and thro mbus formation in ex vivo and in vivo model systems. These studies sug gest that inactivated factor VIIa competes efficiently with plasma fac tor VII(a) for a limited number of tissue factor sites. In the present study, we compared the interactions of factor VIIa and active site in hibited factor VIIa with tissue factor. Competition studies of factor VIIa and active site-inhibited factor VIIa in a factor X activation as say showed that the affinity of the latter for relipidated tissue fact or was 5-fold higher than that of factor VIIa. Radioligand binding stu dies with a human bladder carcinoma cell line (J82) and surface plasmo n resonance studies using soluble tissue factor demonstrated a faster association and a slower dissociation for the active site-inhibited fa ctor VIIa. Studies of equilibrium binding to cell surface tissue facto r showed that the affinity of active site-inhibited VIIa was 5-fold hi gher than that of factor VIIa to non-functional tissue factor sites, w hereas both inactivated factor VIIa and factor VIIa bound to functiona l tissue factor sites with the same high affinity. Comparison of the C D spectra of factor VIIa and active site-inactivated factor VIIa revea led structural differences in the protease domain. The potential physi ological implications of these findings are discussed.