The N-glycosylation of proteins is one of the most important post-translational modifications relevant to various biological functions. The identification and quantification of N-glycoproteins in liquid chromatography-mass spectrometry (LC-MS) is challenging because of their low analytical sensitivity and selectivity. This is due to their microheterogeneity and the difficulty of synthesizing N-glycopeptides as an internal standard. Parallel reaction monitoring (PRM) is widely used in targeted LC-MS. The key advantage of LC-PRM is that it can identify N-glycopeptides using tandem mass spectrometry (MS/MS) fragmentation, even without an internal standard. We investigated the feasibility of analyzing N-glycoproteins using multiplex immunoprecipitation to improve sensitivity and selectivity. We targeted N-glycoproteins [α-fetoprotein (AFP), vitronectin (VTN), and α-1-antichymotrypsin (AACT)] that are abnormally glycosylated in hepatocellular carcinoma (HCC). Their tryptic N-glycopeptides were selected to determine the percentages of fucosylated N-glycopeptides using Y ions, which include glycopeptide fragments with amino acid sequences. Finally, we confirmed that the area under the receiver operating characteristic curve (AUC = 0.944) for the combination of AFP and VTN increased more so than for a single glycopeptide (AUC = 0.889 for AFP and 0.792 for VTN) with respect to discriminating between HCC and cirrhosis serum. This study shows that an LC-PRM method using multiplex N-glycoproteins immunoprecipitated from serum could be applied to develop and verify cancer biomarkers. Graphical abstract.
Keywords: Hepatocellular carcinoma; Immunoprecipitation; Internal standard; N-Glycopeptide; Parallel reaction monitoring.