Sequential and competitive adsorption of bovine serum albumin and β-lactoglobulin, and their resistance to exchange with α-lactalbumin and β-casein

Abstract

The sequential and competitive adsorption of bovine serum albumin (BSA) and β-lactoglobulin at silanized silica surfaces were investigated using protein radiolabeling and in situ ellipsometry. Simultaneous adsorption of 14C-labeled BSA and β-lactoglobulin revealed an initial dominance of the surface by β-lactoglobulin, with subsequent replacement by BSA. In sequential adsorption experiments, where the surface was contacted with 14C-labeled BSA followed by contact with protein-free buffer and addition of β-lactoglobulin, most of the adsorbed BSA was not removed. The effect of adding α-lactalbumin, β-casein and β-lactoglobulin to adsorbed BSA, and the effect of adding α-lactalbumin, β-casein and BSA to adsorbed β-lactoglobulin, were investigated using in situ ellipsometry. β-Casein was the most effective eluting agent, while α-lactalbumin was the least effective. The abilities of α-lactalbumin and β-casein to exchange with adsorbed BSA and β-lactoglobulin were evaluated with reference to a simple kinetic model for protein adsorption and exchange. The model is based on a mechanism allowing for reversible adsorption followed by either a surface-induced conformational change yielding an irreversibly adsorbed form, or exchange with a dissimilar protein from the solution. Differences in exchange behavior were generally found to be explainable with reference to the rate constant for surface-induced conversion of the adsorbed protein as opposed to that describing exchange of adsorbed protein with a dissimilar protein. This suggests that under these experimental condition exchange was mediated by a protein-surface interaction, i.e. a displacement mechanism, where protein-protein associations were of minor significance.