Msr-Ratio (Msr-green) (Msr-green) is a ratiometric fluorescent probe specifically designed for the detection of methionine sulfoxide reductase (MSR) activity. It has an excitation wavelength of 375 nm and an emission wavelength of 550 nm. This probe offers the ability to monitor the enzyme activity both in vitro and in live cells, making it a valuable tool for studying MSR function.

Methionine sulfoxide reductase converts Msr-Ratio into its sulfide variant, which exhibits advantageous characteristics, including an almost 400-fold increase in fluorescence, a rapid response (<30 min), a considerable Stokes shift (120 nm), and green emission (550 nm) [1]. In live HL60 cells, Msr-Ratio introduces a minimal background signal. Furthermore, as the incubation period of Msr-Ratio (10 μM) within the cells extends from 0 to 8 hours, there is a consistent enhancement in the fluorescence signal [1].