We established a novel technique for differential activity-based gel electrophoresis (DABGE) of lipolytic enzymes from two different biological samples. For this purpose, a set of three fluorescent suicide inhibitors was developed. These probes possess the same substrate analogous structures but carry different cyanine dyes (Cy2b, Cy3, and Cy5) as reporter fluorophores. For comparison of enzyme profiles, two samples are individually labeled with a different probe followed by mixing, gel electrophoresis, fluorescence imaging, and identification of the tagged proteins by MS/MS. Protocols for quantitative determination of active enzymes were developed on the basis of lipolytic proteomes that had been admixed with defined amounts of known lipases and esterases. A detailed analysis of the fluorescence intensities showed that the found enzyme ratios very closely reflected the relative amounts of the labeled enzymes that were used for spiking. The DABGE method was used to compare the lipolytic proteomes of brown and white adipose tissue showing specific enzyme patterns of both samples. This study represents the first application of this technology for comparative analysis of lipases and esterases. Further applications of this technique can be expected to provide entirely new information on lipid enzymology in health and disease with high precision.-Morak, M., H. Schmidinger, P. Krempl, G. Rechberger, M. Kollroser, R. Birner-Gruenberger, and A. Hermetter. Differential activity-based gel electrophoresis for comparative analysis of lipolytic and esterolytic activities. In the postgenomic era, researchers are now challenging the proteome with methods like two-dimensional gel electrophoresis or multidimensional chromatography followed by mass spectrometry and various other methods for abundance-based proteome profiling. However, the amount of proteins present at a certain state of the cell might not correlate with the enzyme activities responsible for the metabolic fluxes, cell management, and signal transduction. Therefore, elucidation of changes in protein activity is the ultimate goal of functional proteomics.Methods have been developed for specific detection of enzymes on the basis of their catalytic activities. For this purpose, activity recognition probes (ARPs) (1) or activitybased probes are used. Basically, an ARP is a molecule consisting of i) a recognition site targeting a certain enzyme species, ii) a properly positioned reactive site that forms a covalent bond with the target, and iii) a tag for visualization and/or isolation of the covalently bound target (2-4). Many reactive groups have been developed so far for identifying different types of enzyme activity, i.e., fluorophosphonates for serine hydrolases (5), p-nitrophenyl organophosphonates for lipases and esterases (3, 6), epoxides for cysteine proteases (7), and sulfonate esters for various enzyme classes (8, 9). Two-dimensional (2D) gel electrophoresis is a well-established technique for simultaneous separation and display of hundreds to thousan...