Direct Observation of Protein Glycosylation by Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry

Nonenzymatic glycosylation products of several peptides and lysozyme were observed directly by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) operating in a linear mode, and the number of glucose units added was determined from the observed mass. Oxidation products of the peptides and the glycosylated peptides were also observed. Lysine residues were readily glycosylated; however, a nonapeptide without lysine residue also showed addition of up to four glucose units, suggesting that other amino acid residues can also be glycosylated. The MALDI-TOF-MS technique was useful for comparing reaction products from the glycosylation of different peptides as well as for demonstrating inhibition of glycosylation by N-α-acetyl-l-lysine. Keywords: MALDI-TOF-MS; nonenzymatic glycosylation; Maillard reaction; glucagon; lysozyme