Considerable evidence indicates that methionine sulfoxide (MetO) reductase A (MsrA) plays an important role in cytoprotection against oxidative stress and serves as a potential drug target. To screen for MsrA regulators, a rapid and specific assay to monitor MsrA activity is required. Most of current assays for MsrA activity are based on the reduction of radioactive substrates such as [3H]-N-acetyl-MetO or fluorescent derivatives such as dimethylaminoazo-benzenesulfonyl-MetO. However, these assays require extraction procedures and special instruments. Here, we developed a specific colorimetric microplate assay for testing MsrA activity quickly, which was based on the fact that MsrA can catalyze the reduction of methyl sulfoxides and simultaneously oxidize dithiothreitol (DTT), whose color can be produced by reacting with Ellman's reagent (dithio-bis-nitrobenzoic acid, DTNB). The corresponding absorbance change at 412 nm was recorded with a micro plate reader as the reaction proceeded. This method to monitor MsrA activity is easy to handle. Our findings may serve as a rapid method for the characterization of recombinant enzyme and for the screening of enzyme inhibitors, pharmacological activators, gene expression regulators and novel substrates. (C) 2013 Elsevier Inc. All rights reserved.