Effect of a contaminating competitive ligand on ligand-binding curves. Inverse protein concentration dependence.

A theoretical binding model is considered which provides an explanation for the inverse protein concentration dependence observed for a variety of ligands. The model describes the inhibition of binding caused by a highly bound contaminant. The complete binding equation is derived and examined in terms of form, limits, and protein dependence. Furthermore, several approximate relations are derived which are useful for obtaining initial estimates of the model parameters and for a qualitative test of the applicability of the model. It is found that the binding curve may show a characteristic plateau at a saturation equal to the uncontaminated fraction of the protein and that the free ligand concentration at half saturation depends linearly on protein concentration. The practical implications of the present findings are discussed based on an analysis of simulated as well as experimental data.