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

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Abstract
A general model was developed describing the relationship between the measured (IC50 [median inhibitory concentration]) and absolute affinities (KI), observed in radioligand binding studies [rat brain membranes] when 2 ligands [spiperone and dihydroalprenolol], 1 radioactive, interactive with 2 receptors or binding sites. The model shows the dependence of the IC50 upon the concentration of radioligand for any combinations of the absolute affinities of the radioligand Kd and the displacing ligand KI. By constraining the affinities of the 2 ligands for the sites, 5 special cases of the general model can be described that model all possible selectives the ligands may have for the sites. The properties of these 5 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 was tested experimentally in 3 situations where 2 ligand/2 site interactions occur, and is a useful technique to qualitatively examine the underlying binding reactions.