Quantitative derivatization of sialic acids for the detection of sialoglycans by MALDI MS

Recently, glycans have been recognized as valuable biomarkers for various disease states. In particular, sialoglycans, which have sialic acids at their terminal end, are likely to have relevance to diseases such as cancer and inflammation. Mass spectrometry (MS) has become an indispensable tool for biomarker discovery. However, matrix-assisted laser desorption ionization (MALDI) MS of sialoglycans normally causes loss of sialic acid. Methylesterification or amidation of carboxyl functionality in sialic acid has been reported to suppress the loss of sialic acids. We found that the modifications of alpha 2,3-linked sialic acids proceed less efficiently than those at alpha 2,6-linkages. Furthermore, the modifications of the alpha 2,3-linked sialic acids are incomplete. This variability in the extent of derivatization presents a major problem in terms of glycan biomarker discovery using MALDI MS. In this study, we developed a novel amidation using acetohydrazide which can completely modify both types of linkages of sialoglycans. With the use of this method, we demonstrate MS profiling of N-linked glycans released from a bovine fetuin which is rich in alpha 2,3-linked sialic acids.