Assessing Enzyme Activities Using Stable Isotope Labeling and Mass Spectrometry*□S

Activity-based protein profiling has emerged as a valuable technology for labeling, enriching, and assessing protein activities from complex mixtures. This is primarily accom-plished via a two-step identification and quantification process. Here we show a highly quantitative and stream-lined method, termed catch-and-release activity profiling of enzymes (CAPE), which reduces this procedure to a single step. Furthermore the CAPE approach has the ability to detect small quantitative changes that may have been missed by alternative mass spectrometry-based techniques. Molecular & Cellular Proteomics 6: 1771–1777, 2007. Mass spectrometry and stable isotopes have recently sur-faced as fundamental tools for large scale proteomics analy-ses. Among these techniques are ICAT (1) and stable isotop