Contamination by a competitive ligand as an explanation for the inverse dependence of HABA binding parameters upon the protein concentration.

There is evidence that the apparent association constant (K) and/or the number of binding sites (n) are inversely dependent upon protein concentration for a number and variety of ligands with no obvious structure-activity relationships. A model recently shown to explain this effect with an inorganic ligand has now been applied to 2-(4'-hydroxybenzeneazo) benzoic acid (HABA) and also explains the inverse protein dependence of the binding of this compound to human albumin. The model explains this inverse dependence on the basis of a highly bound contaminant which competes with the ligand for the high affinity site. HABA was found to interact with human albumin at three or more sites, the high affinity site which was about 95% contaminated had an association constant of 2 x 10(5)/M, an order of magnitude higher than that found previously when the effect of a contaminant was not considered. The association constant of the competitive contaminant was estimated to be about 5 x 10(6)/M. Since the model accounts for the phenomenon in terms of a property of the protein, rather than of the ligand, it could provide a general explanation for this effect with other ligand-acceptor combinations including a wide variety of drug and hormone receptor preparations.