Binding of β2-glycoprotein I to platelets: Effect of adenylate cyclase activity

β2-glycoprotein I was trace-labelled with 3H by reductive methylation. It was shown to bind specifically to washed human platelets. No binding was measured to erythrocytes and only insignificantly small amounts were bound to lymphocytes. Bound and free [3H]β2-glycoprotein I were measured after separation from the suspending medium by centrifugation. Maximum binding was obtained within 45 min and was proportional to the number of platelets in the incubation mixture. Equilibrium binding was established with reequilibration after dilution. Analysis of β2-glycoprotein I as a function of β2-glycoprotein I concentration indicates that platelets bind 6 × 105 β2-glycoprotein I molecules per platelet at saturation with an apparent dissociation constant of 559 nM. In the presence of Ca2+ a significantly lower amount of β2-glycoprotein I is bound with an apparent dissociation constant of 334 nM. Platelets treated with formalin bind more β2-glycoprotein I than do untreated platelets but the dissociation constant is unaffected by the formalin treatment. Neuraminidase treatment of the platelets has no effect on the binding. The presence of other serum proteins in binding assays performed at 25°C has no effect on the binding. The binding of β2-glycoprotein I to the platelets can be correlated with inhibition of adenylate cyclase activity.