Synthesis and Characterization of a Series of Highly Fluorogenic Substrates for Glutathione Transferases, a General Strategy

Glutathione transferases (GSTs) are used in biotechnology applications as fusion partners for facile purification and are also overexpressed in certain tumors. Consequently, there is a need for sensitive detection of the enzymes. Here we describe a general strategy for the synthesis and characterization of novel fluorogenic substrates for GSTs. The substrates were synthesized by introducing an electrophilic sulfonamide linkage to fluorescent molecules containing an amino group [e.g., 2,4-dinitrobenzenesulfonamide (DNs) derivatives of coumarin, cresyl violet, and rhodamine]. The derivatives were essentially nonfluorescent, and upon GST catalyzed cleavage of the dinitrobenzenesulfonamide, free fluorophore is released (and 1-glutathionyl-2,4-dinitrobenzene + SO2). All the coumarin-, cresyl violet- and rhodamine-based fluorogenic probes turned out to be good substrates for most GSTs, especially for GSTA1–1, in terms of strong fluorescence increases (71–1200-fold), high k cat/K m values (104–107 M–1 s–1) and significant rate enhancements (106–109-fold). The substrates were successfully applied to quantitate very low levels of GST activity in cell extracts and DNs-cresyl violet was also successfully applied to the imaging of microsomal MGST1 activity in living cells. The cresyl violet stained cells retained their fluorescence after fixation, which is a very useful property. In summary, we describe a general and versatile strategy to generate fluorogenic GST substrates, some of them providing the most sensitive assays so far described for GSTs.