A rationally designed MALDI-TOF MS probe for rapid quantitative analysis of carboxylesterase activity

The rapid and selective detection of biomarkers for the diagnosis of related diseases still remains a significant challenge due to the complexity of biosamples. Carboxylesterase (CES) is one of the most abundant serine hydrolases in the liver, playing an important role in the metabolism of xenobiotic compounds. In particular, the upregulation of CES is closely associated with hepatocellular carcinoma (HCC), which is a potential biomarker to improve the diagnostic accuracy. Herein, based on the "substrate-hydrolysis enzymatic reaction" strategy, we developed a matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) probe, named QF, for the quantitative analysis of CES activity. The QF contained an amide group serving as the enzymatic recognition moiety, and a quinoline tag was introduced to improve the ionization efficiency of both the substrate and its hydrolysis product. Therefore, the CES activity could be directly quantified by the relative intensity ratio of the product and substrate via MALDI-TOF MS without any additional internal standard. Owing to its operating simplicity, high-throughput capacity and quantification accuracy, the proposed method was successfully applied for CES activity quantification, inhibitor screening, and CES activity detection in the cell lysates. This study provides a tool for the rapid detection of CES activity and CES drug development.