Analysis of competition binding assays: assessment of the range of validity of a commonly invoked assumption.

A common assumption invoked in the analysis of competition binding assays is that the fractional saturation of sites with the unlabeled ligand is given by 1-(the concentration of bound labeled ligand in the presence of unlabeled ligand)/(the concentration of bound labeled ligand in the absence of unlabeled ligand). This assumption is critically evaluated in the context of several binding models: (a) binding of univalent ligands to multiple classes of equivalent and independent sites, with and without nonspecific binding; (b) cooperative binding of univalent ligands; and (c) binding of multivalent ligands to a single class of univalent acceptors. We show that the conventional assumption is only valid when the labeled ligand is mainly in the free form, occupies a small fraction of the total sites and binds univalently to all sites in an equivalent and independent manner, and when the unlabeled ligand forms 1:1 complexes with the acceptor sites. When these conditions are satisfied, the conventional assumption is valid even if the unlabeled ligand binds to nonequivalent sites or exhibits cooperativity. Finally, we apply the theory derived for case (a) above to the binding of fluoresceinated epidermal growth factor to A431 cells and demonstrate that the analysis of data obtained from both conventional and competition assays provides information which is difficult, if not impossible, to obtain from either assay alone.