A Method for Dissecting Two Site Receptor Interactions in Ligand Binding Studies

A general model has been developed describing the relationship between the measured (IC50) and absolute affinities (KI), observed in radioligand binding studies when two ligands, one radioactive, interact with two receptors or binding sites. The model shows the dependence of the IC50's upon the concentration of radioligand for any combinations of the absolute affinities of the radioligand (Kd's) and the displacing ligand (KI's). By constraining the affinities of the two ligands for the sites, five special cases of the general model can be described that model all possible 'selectivities' the ligands may have for the sites. The properties of these five cases can be exploited experimentally to probe the nature of the ligand/site interactions by the simple expedient of constructing a number of displacement curves at different radioligand concentrations. The method has been tested experimentally in three situations where two ligand/two site interactions occur, and is shown to be a useful technique to qualitatively examine the underlying binding reactions.