A New Fluorometric Turn-on Assay for Carboxylesterase and Inhibitor Screening Based on Aggregation Induced Emission Behavior of Tetraphenylethylene Molecules

It is known that carboxylesterase (CaE) are a group of isoenzymes commonly distributed in mammalian organs, and they can catalyze the hydrolysis of carboxyl ester. As a result, they play an important role in detoxification of narcotics or chemical toxin clearance. Moreover, they serve as important drug candidates for protein-based therapeutics or drug targets for chemotherapeutic prodrug activation. It is reported recently that human plasma carboxylesterase can be a novel biomarker candidate for hepatocellular carcinoma. Therefore, establishing a reliable fluorescent system for detecting carboxylesterase is of great importance in terms of biochemical studies as well as clinical applications. Herein, we report a new fluorometric turn-on assay for carboxylesterase activity and inhibitor screening with compound 1 by utilizing the aggregation-induced emission (AIE) feature of tetraphenylethylene (TPE) molecules. The sensing mechanism is illustrated in Figure 1 and explains as follows: (i) the pyridinium moiety may render compound 1 water-soluble. As a result it is anticipated that compound 1 is weakly emissive in aqueous solutions according to previous studies; (ii) the incubation of carboxylesterase with compound 1 can result in cleaving the carboxylic ester bond, followed by hydrolysis and 1,6-elimination of p-quinonemethide to yield the p-pyridine substituted TPE (TPE-Py). TPE-Py is not soluble in aqueous solutions, thus aggregation will occur and turn on the fluorescence of TPE moiety based on the ATE feature of TPE compounds. In this way, compound 1 can be employed for the fluorescence turn-on assay for carboxylesterase activity. The results reveal that the buffer solution of compound 1 emitted very weakly. However, the green fluorescence emission was switched on after addition of carboxylesterase. Carboxylesterase at concentrations as low as 5.67 X 10(-5) U/mL can be assayed with compound 1. Further results clearly indicate that compound 1 can be utilized not only for carboxylesterase activity assay but also for the corresponding inhibitor screening. More importantly, this probe can be applied for detection of carboxylesterases in living cells.