Protease- and acid-catalyzed labeling workflows employing (18)O-enriched water.

Stable isotopes are essential tools in biological mass spectrometry. Historically, (18)O-stable isotopes have been extensively used to study the catalytic mechanisms of proteolytic enzymes(1-3). With the advent of mass spectrometry-based proteomics, the enzymatically-catalyzed incorporation of (18)O-atoms from stable isotopically enriched water has become a popular method to quantitatively compare protein expression levels (reviewed by Fenselau and Yao(4), Miyagi and Rao(5) and Ye et al.(6)). (18)O-labeling constitutes a simple and low-cost alternative to chemical (e.g. iTRAQ, ICAT) and metabolic (e.g. SILAC) labeling techniques(7). Depending on the protease utilized, (18)O-labeling can result in the incorporation of up to two (18)O-atoms in the C-terminal carboxyl group of the cleavage product(3). The labeling reaction can be subdivided into two independent processes, the peptide bond cleavage and the carboxyl oxygen exchange reaction(8). In our PALeO (protease-assisted labeling employing (18)O-enriched water) adaptation of enzymatic (18)O-labeling, we utilized 50% (18)O-enriched water to yield distinctive isotope signatures. In combination with high-resolution matrix-assisted laser desorption ionization time-of-flight tandem mass spectrometry (MALDI-TOF/TOF MS/MS), the characteristic isotope envelopes can be used to identify cleavage products with a high level of specificity. We previously have used the PALeO-methodology to detect and characterize endogenous proteases(9) and monitor proteolytic reactions(10-11). Since PALeO encodes the very essence of the proteolytic cleavage reaction, the experimental setup is simple and biochemical enrichment steps of cleavage products can be circumvented. The PALeO-method can easily be extended to (i) time course experiments that monitor the dynamics of proteolytic cleavage reactions and (ii) the analysis of proteolysis in complex biological samples that represent physiological conditions. PALeO-TimeCourse experiments help identifying rate-limiting processing steps and reaction intermediates in complex proteolytic pathway reactions. Furthermore, the PALeO-reaction allows us to identify proteolytic enzymes such as the serine protease trypsin that is capable to rebind its cleavage products and catalyze the incorporation of a second (18)O-atom. Such "double-labeling" enzymes can be used for postdigestion (18)O-labeling, in which peptides are exclusively labeled by the carboxyl oxygen exchange reaction. Our third strategy extends labeling employing (18)O-enriched water beyond enzymes and uses acidic pH conditions to introduce (18)O-stable isotope signatures into peptides.