MALDI-TOF mass spectrometry analysis of tryptic peptides generated from dansylcadaverin- and PGGQQIV-labeled trappin-2, elafin and SLPI.

Each labeled inhibitor was hydrolysed with trypsin, the resulting peptides reduced with DTT and alkylated with iodoacetamide before MALDI-TOF analysis. Putative glutamine and lysine residues targeted by TGase were identified by the altered mass of corresponding tryptic peptides bearing DsC on Gln residues or the PGGQQIV peptide (TGS1) on Lys residues. The release of an ammonia molecule that accompanies the formation of an isopeptide bond by TGase indicated that the expected increase in mass of DsC-labeled Gln is +318 Da (335-17) and that of TGS1-labeled Lys is +681.38 Da (698.38-17). The theoretical masses of unsubstituted tryptic peptides were calculated with PeptideMass software (http://www.expasy.ch/tools/peptide-mass.html).*theoretical mass calculated assuming carbamidomethyl-Cys (carboxyamido 57.0214 g/mol).¶trappin-2 numbering.