Analysis of the rate-limiting step in a ligand-cell receptor interaction: the immunoglobulin E system.

Theory predicts that the kinetics of simple interactions between a ligand and a receptor bound on the surface of a cell will be affected by the occupancy of receptors on the same cell. In a diffusion-limited reaction the effect will be on the rate of dissociation but not on the rate of association until the cell is virtually saturated with ligand. If the rate of reaction is not diffusion limited, then the opposite holds; i.e., the forward velocities will be proportional to the concentration of vacant receptors, but the reverse reactions will not be. We examined the kinetics of reaction between immunoglobulin E (IgE) and its receptor and clearly demonstrated that the reaction is not diffusion controlled. The substantial (congruent to 30-fold) increase in the forward rate constant observed for the reaction of IgE with solubilized receptors as opposed to cell-bound receptors is therefore not an artifact of calculation. Since the reverse rate constants show little difference, we postulate that the presence of other surface components (rather than conformational differences in the receptor) affects the reaction with the cells. As an aid to the analysis, the theory has been extended so that not only the rate constants but also the entire course of the reaction of ligand with cell receptors can be predicted for diffusion-limited vs. non-diffusion-limited interactions.