Hydrogen sulfide (H₂S) has been identified as the third important gaseous transmitter after nitric oxide (NO) and carbon monoxide (CO). It plays important roles in both normal physiology conditions and the process/progress of several diseases. Selective detection of tissue-specific H₂S represents an goal to aid cell biology research and disease diagnosis.

Based on the previous construction of chemical probes (Chem. Commun. 2014, 50:11735; ACS Appl. Mater. Interfaces 2014, 6:19600; Analyst 2013, 138:7087), researchers from Shanghai Institute of Materia Medica of Chinese Academy of Sciences, and from East China University of Science and Technology have developed a galactosyl azidonaphthalimide probe for the selective fluorogenic imaging of hepatocellular H₂S, which sets a basis for the target-specific imaging of H₂S in the liver, the main organ that produces this gaseous transmitter. 

The desired probe was synthesized by a click-coupling of alkynyl naphthalimide with azido galactoside. The presence of the azide weakens the fluorescence of naphthalimide by an intramolecular charge transfer (ICT) process. A subsequent reduction by H₂S produces aminonaphthalimide with an enhanced fluorescence. A kinetic investigation determined that DT-Galactose (DT-Gal) showed rapid fluorogenic response to H₂S, and good linearity was observed by plotting the fluorescence intensity of the probe as a function of increasing H₂S. The limit of detection of DT-Gal for H₂S was determined to be 0.78 µM. 

The addition of the probe to the cell lines pre-treated with H₂S leads to an evident fluorescence enhancement for the human hepatoma cell line, Hep-G2 with over-expressed asialoglycoprotein receptor (ASGPR), but only a slight fluorescence increase for other cancer cells without the receptor expression such as human colon cancer HCT116, cervix cancer HeLa and lung cancer A549. In contrast, co-incubation of DT-OH, a known H₂S probe that lacks the galactosyl targeting agent, with the cells led to slight, unselective fluorescence enhancement.  

The data from both the ASGPR knock-down and the free galactose competition proves the assumption that the strong fluorogenic signal selectively produced by the probe in Hep-G2 cells is facilitated by ASGPR-mediated endocytosis. 

This research work was accomplished by graduate students and colleagues in CHEN Guorong’s and LI Jia’s groups. The study has been published in Chem Commun 2015, 51:3653. 

Figure: Selective fluorogenic imaging of hepatocellular H2S by a galactosyl azidonaphthalimide probe (Image by ZANG Yi's group)