Precipitated Fluorophore-Based Molecular Probe for Imaging of Aminopeptidase N in Living Cells and Tumors.

Aminopeptidase N (APN) is capable of cleaving N-terminal amino acids from peptides with alanine in the N-terminal position and plays a key role in the growth, migration, and metastasis of cancer. However, reliable information is hard to be obtained with the current APN-responsive molecular probes because the released fluorophores are cytoplasmic soluble and thus rapidly depart from the enzymatic reaction sites and spread out all over the cytoplasm. Here, we report a precipitated fluorophore, , which is completely insoluble in water and shows strong yellow solid emission when excited with a 405 nm laser. Owing to the controllable solid fluorescence of by the protection-deprotection of phenolic hydroxyl, we further utilized to design an APN-responsive fluorogenic probe () for the imaging of intracellular APN. Importantly, can not only perform imaging of APN in different organelles (, lysosomes, mitochondria, endoplasmic reticula, and so forth) but also display a stable and indiffusible fluorescent signal for reliable mapping of the distribution of APN in living cells. In addition, through real-time imaging of APN in 4T1 tumors, we found that the fluorescent signal with high fidelity generated by could remain constant even after 12 h, which further confirmed its diffusion-resistant ability and long-term reliable imaging ability. We believe that the precipitated fluorophore may have great potential for long-term imaging.