The Glutathione (GSH) Assay utilizes an enzymatic method which uses Ellman’s reagent (DTNB) and glutathione reductase (GR) to measure total glutathione (GSH) in biological samples. GR is used to catalyze the reduction of glutathione disulfide (GSSG) to GSH so that total GSH can be measured. DTNB reacts with GSH to form a yellow product that can be measured spectrophotometrically at an absorbance of 412 nM.