Msr-Ratio is converted to its corresponding sulfide form by methionine sulfoxide reductase. Msr-Ratio displays favorable properties such as a nearly 400-fold fluorescence change, fast response rate (<30 min), large Stokes shift (120 nm), and green emission (550 nm).
There is a very weak background signal in live HL60 cells after the addition of Msr-Ratio. As the Msr-Ratio (10 μM) incubation time increased in cells from 0 to 8 h, the fluorescence signal constantly strengthened.