Stereospecific EMMA for methionine sulfoxide reductase enzymes Methionine sulfoxide [Met(O)] which may be generated via oxidation by reactive oxygen species (ROS) is reduced by methionine sulfoxide reductase (Msr) enzymes in a stereospecific manner. The present assay allows the in-capillary incubation of recombinant human Msr enzymes followed by separation and analysis of the Met(O) diastereomers as well as the product methionine.