Duplex Stable Isotope Labeling (DuSIL) for Simultaneous Quantitation and Distinction of Sialylated and Neutral N-Glycans by MALDI-MS

Quantitative N-glycomics provides an effective tool for detecting glycosylation changes in cancer and other diseases. However, the lability of sialic acid and its lower ionization efficiency compared with that of neutral glycans make their analysis by mass spectrometry complicated and prevents the simultaneous quantitation and distinction of sialylated and neutral N-glycans by MS. To address this problem, we developed a novel approach duplex stable isotope labeling (DuSIL), to relatively quantify neutral and sialylated glycans concurrently by MALDI-MS. The duplex labeling strategy includes isotopic methylamidation labeling on the sialic acids and amino acid reductive amination on the reducing ends of N-glycans. Using this method, the labeled N-glycans showed doublet peaks with (6 + 3*N) Da mass difference for relative quantitation and discrimination of the number of sialic acids (N). The DuSIL strategy is of high labeling efficiency, high reproducibility (CV < 20%), and good linearity (R-2 > 0.99) within 2 orders of magnitude of dynamic range. The strategy is successfully applied to measure N-glycan changes of IgG from human serum with colorectal cancer, demonstrating its potential in relative quantitation of the N-glycome in clinical samples.