Identification of a beta-lactoglobulin lactosylation site.

Thermal treatment of milk leads to non-enzymatic glycosylation of proteins through Maillard reaction. Free NH2 groups of basic amino acids react with the reducing carbonyl group of lactose forming the so-called Amadori products. Electrospray mass spectrometry analysis shows that L-lactoglobulin (L-LG), the major whey protein, undergoes lactosylation under industrial thermal treatment. In order to investigate the specificity of reactive sites for lactose binding the analysis of trypsin hydrolysates of L-LG isolated from different industrial milks was performed. Results demonstrate that Lys-100 is a preferential lactosylation site of L-LG during industrial milk treatment. These results were confirmed by an analysis of the three-dimensional model of the protein which showed that Lys-100 had the highest solvent accessibility and proximity to another amino group making Lys-100 the best candidate to lactosylation. Lys-47, previously identified by other authors, showed a good proximity to another Lys residue, but an intermediate level of exposition to solvent.