Characteristics of glycation and glycation sites of lysozyme by matrix-assisted laser desorption/ionization time of flight/timeof-flight mass spectrometry and liquid chromatography-electrospray ionization tandem mass spectrometry

Protein glycation with reducing sugars through the Maillard reaction is regarded as one of the most important reactions in food chemistry. Amadori rearrangement products (ARPs) are produced at the initial stage of the Maillard reaction and then advanced glycation products may be formed. We report here that using matrix-assisted laser desorption/ionization mass spectrometry with time-of-flight detection (MALDI-TOF-MS) and electrospray ionization mass spectrometry (ESI-MS) to monitor the glycation process in lysozyme and the d-glucose model system. MALDI-TOF-MS displayed a heterogeneous distribution of glycation via a total mass shift in spectra. However electrospray ionization mass spectrometry (ESI-MS) data showed that a total of four molecules of glucose reacted with lysozyme at an increase in molecular weight by a 162 Da unit. Further, we identified the glycation sites of lysozyme by using MALDI-TOF/TOF-MS and liquidchromatography (LC)-ESI-MS/MS. Besides the two glycation sites of Lys₁ and Lys₉₇ identified by MALDI-TOF/TOF-MS, the other two glycation sites of Lys₁₃ and Lys₁₁₆ were characterized unambiguously by LC-ESI-MS/MS. Both MALDI-TOF/TOF-MS and LC-ESI-MS/MS provided confidence in the study of the glycation by restricting the number of possible residues through (un)modified ions. The study is useful to monitor and characterize glycation in protein systems based on both MALDI-TOF-MS and ESI-MS. Comparatively, LC-ESI-MS/MS provides more fragments with better recovery for the identification of glycation than MALDI-TOF/TOF-MS.