Identifying transglutaminase reaction products via mass spectrometry as exemplified by the MUC2 mucin - Pitfalls and traps

In order to demonstrate transglutaminase activity in biological samples immunological as well as glutamine- and amine-donor based assays are commonly used. However, the identification of the transglutaminase reaction product, i. e. the isopeptide cross-linked peptides/proteins or the deamidated protein/peptide are often neglected. This article describes a workflow for the detection of the products of transglutaminase-catalyzed reactions. In particular, possible pitfalls and traps that can arise during the mass spectrometry-based identification of isopeptide cross-links are addressed and characterised on actual samples.