Effects of Receptor Mobility and Association on Thrombin Binding in Platelets

We have studied the effects on thrombin binding of factors influencing membrane fluidity and receptor association. Platelets fixed with glutaraldehyde were able to bind thrombin but quantitative data could not be obtained due to agglutination. Increases in membrane microviscosity by incubation with cholesterol-rich liposomes resulted in an increase in ability to bind thrombin at the high affinity sites but no change in low affinity binding, hile decreases in membrane microviscosity were without effect. Lectins were also used to link the carbohydrate portions of glycocalicin, the putative receptor, without affecting access of thrombin to the receptor in the peptide “tail”. Con A, WGA and Lens culinaris (40 ug/ml) caused a 10-fold increase (Kd 4 nM to 0.4 nM) in the affinity of binding to formalinized platelets but a 75% decrease (2400 to 600 sites) in the amount of thrombin bound in the high affinity range while peanut agglutinin and Ricinus communis lectin had the same affinity as the control but with a doubling of the number of sites (2400 to 4200) in the latter case: low affinity binding could not be measured because of platelet agglutination in the presence of the lectins at high thrombin concentration. These results suggest that factors affecting membrane mobility and receptor cross-linklng can modulate the ability of platelets to bind thrombin.