A Versatile Linker for Probes Targeting Hydrolases via In Situ labeling

Hydrolases are important molecules that are involved in a wide range of biological functions and their activities are tightly regulated in healthy or diseased states. Detecting or imaging the activities of hydrolases, therefore, can reveal underlying molecular mechanisms in the context of cells to organisms, and their correlation with different physiological conditions can therefore be used in diagnosis. Due to the nature of hydrolases, substrate-based probes can be activated in their catalytic cycles, and cleavage of covalent bonds frees reporter moieties. For test-tube type bulk detection, spatial resolution is not a measure of importance, but for cell- or organism-based detection or imaging, spatial resolution is a key factor for probe sensitivity that influences signal-to-background ratio. One strategy to improve spatial resolution of the probes is to form a covalent linkage between the reporter moiety and intracellular proteins upon probe activation by the enzyme. In this work, we developed a generalizable linker chemistry that would allow in situ labeling of various imaging moieties via quinone methide species. To do so, we synthesized probes containing a monofluoromethyl or a difluoromethyl groups for {beta}-galactosidase activation, while using fluorescein as a fluorescent reporter. The labeling efficacy of these two probes was evaluated in vitro. The probe bearing a monofluormethyl group exhibited superior labeling efficiency in imaging {beta}-galactosidase activity in living cells. This study provides a versatile linker for applying quinone methide chemistry in the development of hydrolase-targeting probes involving in situ labeling.