Simplifying the Exoglycosidase Digestion/MALDI-MS Procedures for Sequencing N-Linked Carbohydrate Side Chains

Exoglycosidase digestion coupled with matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is an effective technique for sequencing the N-linked carbohydrate side chains of a glycoprotein. However, the buffers currently used in the enzymatic procedures are detrimental to MALDI-MS, and thus desalting is required before the digestion products can be analyzed. We demonstrate that a 25 mM ammonium acetate solution adjusted to the proper pH can replace the normal exoglycosidase digestion buffers. The use of these ammonium acetate solutions permits direct MALDI-MS analysis of the digestion mixture without desalting. More importantly, we show that many of the commonly used exoglycosidases retain both their activity and their specificity under these conditions.