title: Specificity of phosphatidylserine-containing membrane binding sites for factor VIII. Studies with model membranes supported by glass microspheres (lipospheres).
abstract: Factor VIII functions in an enzyme complex upon the activated platelet membrane where phosphatidylserine exposure correlates with expression of receptors for factor VIII. To evaluate the specificity of phosphatidylserine-containing membrane binding sites for factor VIII, we have developed a novel membrane model in which phospholipid bilayers are supported by glass microspheres (lipospheres). The binding of fluorescein-labeled factor VIII to lipospheres with membranes of 15% phosphatidylserine was equivalent to binding to phospholipid vesicles (KD = 4.8 nM). Purified von Willebrand factor (vWf), a carrier protein for factor VIII, decreased membrane binding of factor VIII with a Ki of 10 micrograms/ml. Likewise, normal plasma decreased bound factor VIII by more than 90% whereas plasma lacking vWf decreased the binding of factor VIII by only 20%. Proteolytic activation of factor VIII by thrombin, which releases factor VIII from vWf, increased liposphere binding in the presence of vWf and in the presence of normal plasma. Although factor V is homologous to factor VIII and binds to lipospheres with the same affinity, purified factor V was not an efficient competitor for the membrane binding sites of factor VIII. These results indicate that phosphatidylserine-containing membrane sites have sufficient specificity to select thrombin-activated factor VIII from the range of phospholipid-binding proteins in plasma.
authors.affiliation: Department of Medicine, Brockton-West Roxbury Veterans Administration Medical Center, West Roxbury, Massachusetts 02132.
authors.forename: G E
authors.initials: GE
authors.lastname: Gilbert
authors.forename: D
authors.initials: D
authors.lastname: Drinkwater
authors.forename: S
authors.initials: S
authors.lastname: Barter
authors.forename: S B
authors.initials: SB
authors.lastname: Clouse
mesh_terms: D001665:Binding Sites; D005165:Factor V / Q000378:metabolism; D005169:Factor VIII / Q000378:metabolism*; D005898:Glass; D006801:Humans; D007700:Kinetics; D008027:Light; D008081:Liposomes; D008433:Mathematics; D008563:Membrane Lipids / Q000378:metabolism*; D008856:Microscopy, Fluorescence; D008863:Microspheres; D008954:Models, Biological; D010718:Phosphatidylserines / Q000378:metabolism*; D012542:Scattering, Radiation
chemical_list: D008081:Liposomes; D008563:Membrane Lipids; D010718:Phosphatidylserines; D005165:Factor V; D005169:Factor VIII