Correlation between lactosylation and denaturation of major whey proteins: an investigation by liquid chromatography–electrospray ionization mass spectrometry

The Maillard-reaction-induced lactosylation of the major whey proteins, α-lactalbumin (α-La) and β-lactoglobulins (β-Lg) A and B, occurring upon heating at 70, 80 and 90 °C for 1 to 5 h in the presence of lactose excess, was studied by HPLC coupled to electrospray ionization single and tandem mass spectrometry (HPLC-ESI-MS, MS/MS). The presence of significant amounts of mono and bi-lactosylated forms of the three proteins and their increase with heating temperature and time were assessed from MS data. Evidences for a concomitant, significant denaturation, involving partial tertiary structure unfolding, were also obtained in the case of β-lactoglobulins. A subsequent ESI-MS and MS/MS investigation on the tryptic digests of heated protein solutions exhibiting high percentages of mono and bi-lactosylated forms provided information on lactosylation sites. In particular, the latter were identified both on tryptic and on aspecific peptides, whose unusual relevance (compared to similar studies) was found to be due mainly to heat-induced protein degradation, occurring before protein digestion with trypsin. Among lactosylation sites identified only on tryptic peptides, i.e., those reasonably related to intact protein lactosylation, two lysines residues were found for α-La, both located in accessible regions of its tertiary structure. In the case of β-Lg, besides three sites common to variants A and B (leucine 1, lysines 70, and 75), lysine 69 was found to be lactosylated only in variant B. Its proximity to a critical region of β-Lg tertiary structure suggests that the difference between the two variants could be ascribed to a different evolution of their conformation upon heating. Figure 3D-charts comparing the peak area percent distribution for unmodified, mono- and bi-lactosylated forms of α-La, β-LgA and β-LgB under different temperature/time conditions. As shown by the legend, colour intensification corresponds to a temperature increase.