A sulfated rabbit endothelial cell glycoprotein that inhibits factor VIIa/tissue factor is functionally and immunologically identical to rabbit extrinsic pathway inhibitor (EPI)

Colburn and Buonassisi (In Vitro Cell Dev. Biol. 24, 1133–1136, 1988) have isolated a single chain sulfated glycoprotein inhibitor of factor VIIa/tissue factor-catalyzed activation of factor X from conditioned media of an established rabbit endothelial cell line. We report herein that their endothelial cell-derived inhibitor and extrinsic pathway inhibitor (EPI) isolated from rabbit plasma have identical functional properties with respect to their interactions with factor Xa and with factor VIIa/ tissue factor. In addition, the endothelial cell inhibitor and rabbit plasma EPI migrate with the same apparent molecular weights on non-reduced SDS-PAGE and contain similar amounts of N-linked carbohydrate. Like the endothelial cell inhibitor the EPI of rabbit plasma exists as a single chain molecule. Furthermore, the endothelial cell inhibitor is recognized and neutralized by a polyclonal antibody raised against rabbit plasma EPI. We therefore conclude that cultured rabbit endothelial cells produce an inhibitor of factor VIIa/tissue factor activity that is functionally and immunologically identical to rabbit plasma EPI.