Two N-acetylgalactosamine-reactive lectins, Helix pomatia (HPA) and Dolichos biflorus (DBA), were used to study the energies involved in cell-cell interactions through the specific binding of these lectins to their membrane receptors on genotype AO human erythrocytes (red blood cells) (RBCs). The energy required to dissociate a unit of aggregated membrane area (gamma d) of two RBCs bridged by lectin molecules was determined from the shear force needed to dissociate two-cell aggregates in a flow channel. When HPA were used as bridging molecules, gamma d (0.4 X 10(-4) to 3.8 X 10(-4) dyn/cm) was proportional to the density (D = 175 to 1,060 molecules/micron 2) of HPA molecules bound on the RBC membrane. A similar gamma d/D ratio was also obtained for DBA. These results indicate that the number of lectin molecules bound on the interface plays an important role in determining the energy required for cell-cell dissociation. The aggregation energy per unit membrane area (gamma a) in lectin-induced aggregates was calculated from the degree of encapsulation of a lectin-bound, heat-sphered human RBC by a normal discoid RBC. A minimum of approximately 1,800 HPA molecules/micron 2 on the spheres was required to form stable aggregates with the RBC. By using spheres having a surface HPA density of 1,830 to 2,540 molecules/micron 2, or 1.1-1.5 X 10(12) combining sites/cm2, the gamma a value for HPA-induced aggregation was found to be 2.2 X 10(-3) dyn/cm. This higher value of gamma a than gamma d has been explained on the basis of several differences in aggregation and disaggregation processes. The gamma a value for DBA-induced aggregation was not obtainable by the sphere encapsulation method because of the relative low D values. A comparison of the present results with the published value of the free energy change of 5 kcal/mol for the interactions of HPA and DBA with their ligands suggests that only a small fraction of the lectin molecules bound to RBC surface participate in the bridging of adjacent cells
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