Quantitative N-glycoproteomics using stable isotopic diethyl labeling.

Protein N-glycosylation plays an essential role on cancers and other pathological processes. Its structural and functional studies rely on complete qualitative and quantitative information. Thus, it is important to quantify differentially expressed intact N-glycopeptides (DEGPs) at their molecular level. Here we report our application of stable isotopic diethyl labeling (SIDE) of amino groups for relative quantitation of intact N-glycopeptides. The amino groups from both the N-terminal and lysine residues of N-glycopeptides were diethylated with XHXHO (X = C or C) and NaBHCN. A linear quantitation dynamic range up to 50-fold was obtained with R = 0.9985 with intact N-glycopeptides from standard glycoprotein ribonuclease B (RNase B). In proof-of-principle comparative N-glycoproteomics study of aberrant N-glycosylation of gastric cancer tissues vs. adjacent tissues using SIDE, 644 DEGPs (≥1.5-fold change, p < 0.05, p was calculated using t-test) were discovered. With its accuracy and big dynamic range, SIDE can be applied to quantitative study of any aberrant glycosylation at the intact glycopeptide level.