Real-Time Visualization of Cysteine Metabolism in Living Cells with Ratiometric Fluorescence Probes

Sulfite from cysteine metabolism in living cells plays a crucial role in improving the water solubility of metabolic xenobiotics for their easier excretion in urine or bile. However, an imbalance of sulfite in vivo would lead to oxidative stress or age-related diseases, and an effective strategy for real-time imaging of cysteine metabolism in living cells is still lacking due to its low metabolite concentration and rapid reaction kinetics. Herein, a cyanine moiety based ratiometric fluorescence probe was developed for highly selective and sensitive detection of sulfite in aqueous solution and living cells. The free probe exhibited an orange emission color, and the fluorescence color would gradually change to blue once sulfite anions selectively reacted with the unsaturated carbon double bonds in the probe molecule. This ratiometric fluorescence manner endowed the probe excellent sensitivity with a detection limit of 0.78 nM, which was then explored to image the kinetic process of sulfite release in hepatic BRL cells after incubating with an excess amount of cysteine. This strategy opens new opportunities for revealing thiol-containing species metabolism and even quantitatively tracking their distributions in live cells or organelles.