SEQUENTIAL AND COMPETITIVE ADSORPTION OF BOVINE SERUM-ALBUMIN AND BETA-LACTOGLOBULIN, AND THEIR RESISTANCE TO EXCHANGE WITH ALPHA-LACTALBUMIN AND BETA-CASEIN

The sequential and competitive adsorption of bovine serum albumin (BSA ) and beta-lactoglobulin at silanized silica surfaces were investigate d using protein radiolabeling and in situ ellipsometry. Simultaneous a dsorption of C-14-labeled BSA and beta-lactoglobulin revealed an initi al dominance of the surface by beta-lactoglobulin, with subsequent rep lacement by BSA. In sequential adsorption experiments, where the surfa ce was contacted with C-14-labeled BSA followed by contact with protei n-free buffer and addition of beta-lactoglobulin, most of the adsorbed BSA was not removed. The effect of adding alpha-lactalbumin, beta-cas ein and beta-lactoglobulin to adsorbed BSA, and the effect of adding a lpha-lactalbumin, beta-casein and BSA to adsorbed beta-lactoglobulin, were investigated using in situ ellipsometry. beta-Casein was the most effective eluting agent, while alpha-lactalbumin was the least effect ive. The abilities of alpha-lactalbumin and beta-casein to exchange wi th adsorbed BSA and beta-lactoglobulin were evaluated with reference t o a simple kinetic model for protein adsorption and exchange. The mode l is based on a mechanism allowing for reversible adsorption followed by either a surface-induced conformational change yielding an irrevers ibly adsorbed form, or exchange with a dissimilar protein from the sol ution. Differences in exchange behavior were generally found to be exp lainable with reference to the rate constant for surface-induced conve rsion of the adsorbed protein as opposed to that describing exchange o f adsorbed protein with a dissimilar protein. This suggests that under these experimental conditions exchange was mediated by a protein-surf ace interaction, i.e. a displacement mechanism, where protein-protein associations were of minor significance. (C) 1998 Elsevier Science Ltd . All rights reserved.