β-lactoglobulin/linoleate complexes: binding properties and biological functions

Food structure can have a profound influence on delivering health benefits. Bioaccessibility of nutrients can be affected by their interaction with food components. The dairy protein β-lactoglobulin (βlg) is known to bind hydrophobic ligands such as fatty acids (FA). However, this protein is highly sensitive to the process conditions used in the dairy industry. Therefore βlg is often present in non-native or aggregated form in processed food. This structural change may modify the protein affinity for FA and the biological properties of the FA/protein complexes. The aim of this thesis was to investigate the interaction of bovine βlg in different structural forms (native, covalent dimer and nanoparticles) with linoleate (C18:2, cis,cis-9,12-octadecadienoic acid) and conjugated linoleic acids (CLA, C18:2), and the impact of those complexes on their biological activity in vitro. Two different sets of binding sites were determined for the interaction between linoleate and βlg, regardless of its state of aggregation, using intrinsic fluorescence spectroscopy and isothermal titration calorimetry. By increasing the level of βlg aggregation, the linoleate/βlg stoichiometry increased but the association constants remained similar for both sets of binding sites. In the presence of linoleate, the native protein was more sensitive to gastric in vitro digestion, due to the increased level of denaturation/aggregation of βlg. Transport of linoleate in Caco-2 cells was decreased in presence of the native βlg as observed by confocal microscopy and a monolayer that mimics the intestinal barrier. Cytotoxicity of linoleate on Caco-2 cells was reduced when the FA was bound to βlg compared to free FA. CLA, which is less water soluble than linoleate, is more cytotoxic when complexed by βlg than in its free form. Therefore, it is proposed that βlg can act as a molecular carrier and alter the bioaccessibility of FA depending on their solubility.