GPx catalyzes the reduction of hydroperoxides, including H2O2, using reduced glutathione and thereby functions to protect the cell from oxidative damage. Cayman’s Glutathione Peroxidase Assay measures GPx activity indirectly by a coupled reaction with GR. Oxidized GSSG, produced upon reduction of an organic hydroperoxide by GPx, is recycled to its reduced state by GR and NADPH. The oxidation of NADPH to NADP+ is accompanied by a decrease in absorbance at 340 nm. The rate of decrease in the absorbance at 340 nm is directly proportional to the GPx activity in the sample. Cayman’s GPx Assay can be used to measure all of the glutathione-dependent peroxidases in plasma, erythrocyte lysates, tissue homogenates, and cell lysates.