Graphical analysis of competitive binding of comparable concentrations of ligand, inhibitor and protein. Ligand binding to serum albumin.

In contrast to analysis for competitive binding in enzyme kinetics, no linear plot for analysing competitive binding of two ligands to a protein, where the concentrations of the three reactants are comparable, seems to exist. In the present communication it is shown that in this situation a linear plot can be obtained by the use of the simple equation VA/VB = KA/KB X [Af]/[Bf], where VA and VB are the average number of moles of ligand A and ligand B bound per mole of protein, respectively; [Af] and [Bf] are the concentrations of free ligand A and free ligand B, respectively; and KA and KB are the corresponding association constants. The plot is commented on both theoretically and experimentally using ligand binding to human serum albumin as an example.