Displacement analysis of binding inhomogeneities in crude extracts of receptors

Guidelines are given to distinguish different kinds of binding inhomogeneities of non-radiolabeled ligands in crude extracts of receptors if an appropriate 'binding analogue' of the displacers is available in radiolabeled form. Three minimal models for the simplest types of binding inhomogeneities are analysed theoretically. These models include a cooperative system (with two interacting sites on the same receptor molecule) and two non-cooperative systems (one of them with a single-site receptor having two conformational states in equilibrium and the other with two single-site receptors independent of each other). In certain cases one can distinguish these systems experimentally. Furthermore, if a group of displacers is already classified according to the above models, then dissociation constants can be determined. The quantitative comparison of these displacers on the basis of their dissociation constants is more appropriate (e.g. in Quantitative Structure Activity Relationship studies) than on the basis of their ID50 and Ki values or Hill coefficients, which is often done.