Protein interaction with immobilized ligands: quantitative analyses of equilibrium partition data and comparison with...

Quantitative or analytical affinity chromatography has been successful primarily for the analysis of biologically determined macromolecular affinity relationships. Quantitative approaches are also needed to better characterize simpler, chemically defined immobilized ligands with potential for selective interaction with specific, predetermined protein surface groups. Protein interaction with immobilized metal is a rather selective and versatile, high-affinity adsorption technique for which there is little quantitative information. Using model protein interactions with immobilized Cu2+ ions, we have compared analytical frontal affinity chromatographic methods to a simple, nonchromatographic protocol for the rapid determination of quantitative affinity relationships. Values obtained for the equilibrium dissociation constant (Kd) and binding capacity (Lt) characterizing the interaction of lysozyme with immobilized Cu2+ were quite similar by frontal analysis (Kd = 37-42 X 10(-6) M; Lt = 6.8-7.4 X 10(-6) mol protein/ml gel) and by equilibrium binding analyses (Kd = 33 +/- 4.7 X 10(-6) M; Lt = 5.8-6.1 X 10(-6) mol protein/ml gel; 14 determinations). The interaction of ovalbumin with immobilized Cu2+ was characterized by an affinity (Kd = 4.2-4.8 X 10(-6) M) and capacity (Lt = 1.5-2.1 X 10(-6) mol protein/ml gel) which were also the same regardless of the method for affinity analysis. These values indicate that the total protein bound at saturation corresponds to as much as 17% of the total immobilized Cu2+ ions (approximately 40 X 10(-6) mol/ml gel). Thus, depending on the fraction of total immobilized Cu2+ available for interaction with a given protein (e.g., lysozyme), the number of individual immobilized ligands actively participating as well as those rendered unavailable upon individual protein binding events may be greater than 1. Linear Scatchard plots obtained for both lysozyme and ovalbumin (purified) suggest the presence of only a single type of immobilized Cu2+-protein interaction operative under the experimental conditions employed. However, Scatchard analyses of data obtained by the nonchromatographic equilibrium binding method also demonstrated the ability to simultaneously resolve the contribution of two components whose presence was predicted by frontal chromatography. Our results support the validity and utility of equilibrium binding data analyzed according to the equations outlined by Scatchard and others as an alternative to analytical chromatographic methods.