Apparent competitive inhibition of radioligand binding to receptors: experimental and theoretical considerations in the analysis of equilibrium binding data.

Radioligand binding and displacement experiments are often interpreted in terms of simple competition between two ligands for occupancy of a single binding site on a receptor. Given our current understanding of the complexities of receptor structure and function, it is probable that more complex interactions occur in many cases. By analysis of a hypothetical two-site receptor model, we show that apparent competitive inhibition can arise in several ways, depending on the specificities of the two sites and the interactions between them. We show that binding experiments can in some cases be used to rule out certain models from among a group of apparently plausible ones, provided that experimental criteria are met which permit a meaningful statistical comparison of models to be made. Ideally, these should include: i) an independent study of ligand and inhibitor binding in the absence of each other; ii) carrying out saturation binding and displacement experiments over as wide a range of ligand and inhibitor concentrations as possible; iii) computerized curve-fitting and statistical analysis as a tool for model-testing. While practical limitations may restrict the attainment of such goals, a thorough study of the equilibrium binding properties of a particular receptor system provides the foundation for the design of more definitive experiments at the molecular level, upon which the proof of any binding model ultimately must rest.