Proteolytic O-18 labeling for comparative proteomics: Evaluation of endoprotease Glu-C as the catalytic agent

Recently, proteolytic O-18 labeling has been demonstrated as a promising strategy for comparative proteomic studies (Yao, X.; Freas, A.; Ramirez, J.; Demirev, P. A.; Fenselau, C. Anal. Chem. 2001, 73, 2836-42). In this approach, protein mixtures are digested in parallel in (H2O)-O-16 and (H2O)-O-18 and the ratios of isotopically distinct peptide products are measured by mass spectrometry. In the initial report from this laboratory, trypsin was shown to catalyze incorporation of two O-18 atoms into the carboxyl terminus of each new peptide formed by cleavage of the adenovirus proteome. In the present study, a second enzyme, endoprotease Glu-C, is evaluated as an agent for cleavage and labeling. Proteolytic O-18 labeling by Glu-C is shown to occur readily with phosphorylated and glycosylated proteins and with cysteine-alkylated and disulfide-linked proteins. A sequential double-labeling strategy is used to characterize N-linked glycopeptides. Labeled and unlabeled peptide pairs are found to coelute chromatographically, and measurements of isotope ratios by nanospray and capillary LC-MS are found to be accurate and precise.