Two independent binding sites on monolayers of human endothelial cells are responsible for interaction with coagulation factor VII and factor VIIa

The interaction of radiolabeled factor VII (FVII) and factor VIIa (FVlIa) with endotoxin-stimulated endothelial cells (EC), known to express tissue factor (TF), and unstimulated EC was studied. FVII FVIIa binding to EC-monolayers was saturable within 4.5-6 h, reversible, temperature and calcium dependent on both, endotoxin-stimulated and on unstimulated EC. Upon 2 h of incubation on EC, FVII was partially converted to FVIIa in the absence of protease inhibitors. The affinity of this binding was K(d) = 45.4 ± 18.7 nM with a calculated number of binding sites B(max) = 3.75 ± 0.31 x10⁶ molecules/cell. In addition to unlabeled FVII and FVlIa, other vitamin K-dependent proteins reduced binding of [¹²⁵I]-FVII/FVIIa to about 60-70%, and this type of common binding site for vitamin K-dependent proteins revealed a K(d) = 32.2 ± 5.6 nM and a B(max) = 3.03 ± 0.14 x10⁶ molecules/ cell. Moreover, in the presence of 1 μM prothrombin to suppress common binding sites, only on endotoxin-stimulated EC additional inhibition of FVII FVlIa binding was achieved by anti-TF antibodies. The characteristics of the FVII/FVIIa-TF interaction with a K(d) = 17.2 ± 5.2 nM and a B(max) = 342,000 ± 1,100 binding sites/cell revealed a similar saturation kinetics in radioligand binding and in functional factor X activation within 90-120 min. These data indicate the presence of at least two independent binding sites for FVII/FVIIa on stimulated EC of which about 10% are TF specific. The existence of binding sites common for vitamin K-dependent proteins on both types of EC may improve the availability of FVII/FVIIa once EC become stimulated and express TF on their surface.