Directionally Modified Fluorophores for Super-Resolution Imaging of Target Enzymes: A Case Study with Carboxylesterases

In the need for improving the labeling quality of super-resolution imaging, multifarious fluorescent labeling strategies have sprang up. Among them, a small molecule inhibitor-probe (SMI-probe) shows its advancement in fine mapping due to its smaller size and its specific binding to a specific site. Herein, we report a novel protocol of mechanism-guided directional modification of fluorophores into fluorescent inhibitors for enzyme targeting, which could half the size of the SMI-probe. To confirm the feasibility of the strategy, carboxylesterase (hCE) inhibitors are designed and developed. Among the constructed molecule candidates, NIC-4 inhibited both isoforms of hCE1 and hCE2, with IC₅₀ values of 4.56 and 4.11 μM. The CE-targeting specificity of NIC-4 was confirmed by colocalizing with an immunofluorescent probe in fixed-cell confocal imaging. Moreover, NIC-4 was used in live-cell super-resolution microscopy, which indicates dotlike structures instead of the larger staining with the immunofluorescent probe. Moreover, it enables the real-time tracking of dynamic flow of carboxylesterases in live cells.