Evaluating the extent of protein damage in dairy products: simultaneous determination of early and advanced glycation-induced lysine modifications

An isotope dilution liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed to determine lysine (Lys), N(epsilon)-fructosyllysine (FL), N epsilon-carboxymethyllysine (CML), and pyrraline (Pyr) in dairy products. The presented approach entails protein cleavage via enzymatic digestion to liberate the aforementioned compounds, which were then quantified using a stable isotope dilution assay. LC-MS/MS analysis was performed by positive electrospray ionization recording two transition reactions per analyte in selected reaction monitoring mode. The CML and Lys values obtained with enzymatic digestion were compared to those acquired with acid hydrolysis HCl (6 mol/L), and the two proteolysis methods yielded comparable quantifications. Allowing for the fact that the investigated compounds are formed during different stages of the glycation process, the method is able to reveal the progress of protein glycation in dairy products.