Leucine aminopeptidase
(LAP), one of the important proteolytic enzymes, is intertwined with
the progress of many pathological disorders as a well-defined biomarker.
To explore fluorescent aminopeptidase probe for quantitative detection
of LAP distribution and dynamic changes, herein we report a LAP-targeting
near-infrared (NIR) fluorescent probe (DCM–Leu) for ratiometric
quantitative trapping of LAP activity in different kinds of living
cells. DCM–Leu is composed of a NIR-emitting fluorophore (DCM)
as a reporter and l-leucine as a triggered moiety, which
are linked together by an amide bond specific for LAP cleavage. High
contrast on the ratiometric NIR fluorescence signal can be achieved
in response to LAP activity, thus enabling quantification of endogenous
LAP with “build-in calibration” as well as minimal background
interference. Its ratiometric NIR signal can be blocked in a dose-dependent
manner by bestatin, an LAP inhibitor, indicating that the alteration
of endogenous LAP activity results in these obviously fluorescent
signal responses. It is worth noting that DCM–Leu features
striking characteristics such as a large Stokes shift (∼205
nm), superior selectivity, and strong photostability responding to
LAP. Impressively, not only did we successfully exemplify DCM–Leu
in situ ratiometric trapping and quantification of endogenous LAP
activity in various types of living cells, but also, with the aid
of three-dimensional confocal imaging, the intracellular LAP distribution
is clearly observed from different perspectives for the first time,
owing to the high signal-to-noise of ratiometric NIR fluorescent response.
Collectively, these results demonstrate preclinical potential value
of DCM–Leu serving as a useful NIR fluorescent probe for early
detection of LAP-associated disease and screening inhibitor
