Proteolysis of milk proteins lactosylated in model systems

Five different milk proteins (alpha-casein, beta-casein, kappa-casein, beta-lactoglobulin, and lactoferrin) and a peptide substrate were applied as substrates for the investigation of how lactosylation affected proteolysis by different proteases. After a lactosylation period of 4 days in aqueous solution, at 65 degrees C and pH 6.8 in a protein: lactose ratio of 1000 the proteins were enzymatically hydrolyzed by the three milk relevant proteases plasmin, cathepsin D, and chymosin. Lactosylation of all substrates affected hydrolysis by plasmin negatively, with the largest effect on the globular proteins. This could be explained by modification of lysine residues, being the preferred cleavage site for plasmin, but also the residue generally preferred for lactosylation. Lactosylation of the caseins and of beta-lactoglobulin did not affect subsequent cleavage by cathepsin D and chymosin significantly, but for beta-lactoglobulin, both the secondary as well as the tertiary structure were affected by lactosylation. In contrast, decreased hydrolysis by cathepsin D and chymosin was observed for lactoferrin after lactosylation. Decreased hydrolysis may be caused by a more compact tertiary structure induced by lactosylation of lactoferrin, as indicated by fluorescence spectroscopy measurements.