Sialic acid linkage-specific permethylation for improved profiling of protein glycosylation by MALDI-TOF MS

•Linkage-specific sialic acid permethylation (SSAP) shows high specificity to derivatize α2,3- and α2,6-sialic acids.•SSAP combined with MALDI-TOF MS is suitable to analyze both Neu5Ac- and Neu5Gc-glycan isomers.•SSAP characterizes by MS sensitivity enhancement (>10-fold) and distinct MS/MS spectra to the derivatized glycans.•SSAP is the simplest and speediest method for isomeric, sensitive and robust analysis of sialylated glycans.•SSAP applies to in-depth profiling of N-glycans derived from IgGs and human serum associated with rheumatoid arthritis.

Protein glycosylation mediates a wide range of cellular processes, affecting development and disease in mammals. Deciphering the “glycocodes” requires rapid, sensitive and in-depth characterization of diverse glycan structures derived from biological samples. In this study, we described a two-step derivatization strategy termed linkage-specific sialic acid permethylation (SSAP) consisting of dimethylamination and permethylation for the improved profiling of glycosylation by matrix-assisted laser desorption/ionization (MALDI) time-of-fight (TOF) mass spectrometry (MS). High linkage-specificity (∼99%) of SSAP to both the two most common forms of sialic acid, N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc), permitted direct discrimination of α2,3- and α2,6-linked sialic acids in MALDI-TOF MS. The enhanced intensity (>10-fold) and increased detection limit (>10-fold) of derivatized glycans were valued for sensitive glycomics. Moreover, the good compatibility and reaction efficiency of the two steps of SSAP allowed rapid sample preparation (<2 h), benefiting robust analysis of glycans in a high-throughput manner. The SSAP strategy was further applied to investigate the protein glycosylation of human serum associated with rheumatoid arthritis (RA). It was demonstrated that the relative abundances of individual glycans were different in RA negative and RA positive samples, and meanwhile the RA patient/control ratios of both α2,3- and α2,6-sialylated glycans tended to elevate accompanied with the increase of sialylation. Those findings of the glycosylation changes occurred in human serum protein may contribute to the diagnosis of RA. Herein, SSAP derivatization combined with MALDI-TOF MS exhibits unique advantages for glycomic analysis and shows potential in glycosylation profiling of therapeutic proteins and clinical glycan biomarker discovery.

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