Mass spectrometry for the analysis of protein lactosylation in milk products

Milk and dairy products are usually submitted to thermal procedures in order to preserve microbiological safety and to prolong their shelf-life; these actions may eventually induce structural modifications of several components. In particular, milk proteins undergo a non-enzymatic glycation, known as the Maillard reaction, in which reducing sugars (such as lactose) mainly react with the ?-amino group of Lys residues, leading to the formation of the Amadori product lactulosyl-lysine. Protein lactosylation leads to a decrease of milk nutritional value as it reduces the bioavailability of the essential amino acid lysine and to a potential increase of the specific allergen activity, due to the formation of new hapten-like antigens. These aspects are of particular importance in products intended for infant diet, such as milk powders and infant milk formulas; accordingly, a detailed structural characterization of the extent and site-specificity of milk protein lactosylation is mandatory. Analytical methodologies based on mass spectrometry and more recently proteomics have been thoroughly applied to the structural analysis of milk proteins and their modified forms, including glycation products. This review highlights the contribution of mass spectrometry to the extensive investigation of lactosylation events in milk proteins, with a particular focus on the most up-to date technological improvements.