Interaction of factor VIII with phospholipids: role of composition and negative charge

Radiolabelled human anti-FVIII:C antibody was affinity-purified according to its ability to bind to factor VIII-phospholipid (FVIII-PL) complexes, yielding a fraction directed against the phospholipid binding-site (PL-site antibody). This antibody was used as a specific probe for FVIII binding to PL vesicles containing a variety of natural and synthetic PLs. Of purified PLs tested for FVIII binding, phosphatidyl serine (PS) and phosphatidic acid (PA) were highly active, phosphatidyl inositol (PI) much less so, and both phosphatidyl ethanolamine (PE) and phosphatidyl choline (PC) inactive: the apparent dissociation constant (Kd app) for FVIII binding to PS:PC vesicles showed a strong dependence on PS content. Free-flow electrophoresis of vesicles confirmed FVIII binding to PS:PC required both net negative charge and specific head-group: neither PS vesicles given a positive charge with stearylamine nor PC vesicles made negative with dicetyl phosphate bound FVIII. It is concluded that the negative charge required for FVIII binding must be presented on the phospholipid surface in the correct orientation: phosphatidyl serine supplies this charge in coagulant-active PL preparations.