Msr-blue, a first turn-on fluorescent probe for methionine sulfoxide reductase, exhibits a more than 100-fold fluorescence increment and is used to monitor enzyme activity in live cells (λex=340 nm, λem=440 nm) [1].

Msr-blue emits blue fluorescence after activation by methionine sulfoxide reductase A (Msr A). Msr-blue responds to Msr A in both a time- and dose-dependent manner, with more than a 100-fold increase in emission observed. It is converted to its corresponding sulfide (15′) under catalysis by either purified Msr A or a cell lysate [1]. In a 6-OHDA-treated PC12 cellular model of Parkinson's disease (PD), Msr-blue was employed to probe Msrs function. Using Msr-blue, a decline in Msr activity in the PD model was disclosed for the first time [1].