Interpretation and analysis of receptor binding experiments which yield non-linear Scatchard plots and binding constants dependent upon receptor concentration.

Receptor-binding assays with radiolabelled ligands are widely used to evaluate the biological activity of drugs and hormones. The affinity, usually expressed as the dissociation constant (Kd), and the capacity (Bmax) of the receptor preparation for various ligands are determined in order to compare quantitatively various agonists and antagonists. Experiments with the same ligand and receptor, however, often yield rather disparate values for these binding parameters. One obvious reason for variability in Kd is that straight lines are fitted to data that are clearly curved. Another and more serious reason is that a ligand's apparent dissociation constant decreases when the receptor preparation is diluted and so experiments done at different receptor concentrations do not give identical results. We demonstrate that both of these observations, i.e. the effect of receptor concentration and the curvature of Scatchard plots, can be explained by the presence of a competitive inhibitor in the receptor preparation, a possibility which is not normally considered in the analysis and interpretation of receptor binding assays. We show that the apparent Kd obtained by the conventional one- or two-site analysis may be several orders of magnitude larger than the true dissociation constant and the affinity is therefore seriously underestimated. Application of a model, which assumes that an inhibitor is present in the receptor preparation, will improve the quantitative determination of Kd and Bmax significantly. As a simple alternative method we explain how the apparent binding parameters obtained by the conventional method should be interpreted and how they can be used to estimate the true affinity, provided sufficiently low concentration data are available.