Role of divalency in the high-affinity binding of anticardiolipin antibody-beta(2)-glycoprotein I complexes to lipid membranes

beta(2)-Glycoprotein I (beta(2)GPI) is an essential cofactor for the binding to lipids of anticardiolipin antibodies (ACA), isolated from patients with anti-phospholipid syndrome. We used ellipsometry to study the binding of beta(2)GPI and the beta(2)GPI-mediated binding of ACA to planar membranes composed of phosphatidylcholine (PC) and 5-20 mol % phosphatidylserine (PS). No binding of beta(2)GPI was observed to neutral (PC) membranes. Maximal binding of beta(2)GPI was 3.2-3.6 pmol . cm(-2). Affinity decreased strongly with decreasing PS content; increasing the NaCl and CaCl2 concentrations also led to a decrease in affinity. At physiologic conditions (10 mol % PS, 120 mM NaCl, and 3 mM CaCl2), a K-d Of 14 mu M was observed. Binding constants were insensitive to the chemical composition of the negatively charged phospholipid headgroup. ACA (1.25-10 mu g . mL(-1)) caused a 30-40-fold enhancement of beta(2)GPI binding to PS/PC membranes (20 mol % PS), resulting in the binding of about 2 pmol . cm(-2) divalent ACA-(beta(2)GPI)(2) complexes at 100 nM beta(2)GPI. In the absence of beta(2)GPI, binding of ACA was negligible. Ad- and desorption kinetics of ACA-beta(2)GPI complexes indicate that the initial monovalent association of ACA to membrane-bound beta(2)GPI is rapidly followed by formation of divalent ACA-(P2GPI)2 complexes. Experiments with monovalent Fab(1) fragments of ACA showed no appreciable effect on the beta(2)GPI binding to lipid, substantiating the notion that divalent interactions are essential for the high-affinity binding of ACA-beta(2)GPI. The anticoagulant effect of ACA is rationalized by the observation that binding of ACA-beta(2)GPI complexes to the PSPC membrane severely restricts the adsorption of blood coagulation factor Xa.