Physiological phosphatidylcholine protects bovine β-lactoglobulin from simulated gastrointestinal proteolysis

We have investigated the effect of phosphatidylcholine (PC) on the resistance of bovine beta-lactoglobulin (beta-Lg) to simulated in vitro gastrointestinal proteolysis. Whilst addition of PC did not affect the resistance of beta-Lg to gastric pepsinolysis, it protected the protein from subsequent degradation under duodenal conditions. The effect was dependent on the ratio of PC to beta-Lg, 16% of the protein remaining intact in the presence of an equimolar ratio of PC/protein, which increased to 62% when a 60-fold molar excess of PC was included. PC also altered the pattern of digestion products observed by SDS-PAGE. Thermal denaturation of beta-Lg abolished this effect showing that it was dependent on the native folded structure of the protein. Since neither of the beta-Lg ligands retinol or palmitate exerted a protective effect, it is unlikely that PC is mediating its effect by occupying the central calyx. An alternative explanation may be that the lipids bind to a secondary fatty acid binding site in beta-Lg, thus blocking the action of proteases for steric reasons. These data indicate how biomolecular interactions between proteins and lipids may alter patterns of proteolysis and need to be taken into consideration in any in vitro model of digestion.