The Glutathione Peroxidase (GPx) Activity Assay determines the activity of GPx by measuring its ability to convert reduced glutathione (GSH) to oxidized glutathione (GSSG) in the presence of H2O2. The GSSG that is produced by GPx is quantified using the enzyme glutathione reductase (GR) which interacts with GSSG. GR reduces GSSG to GSH with NADPH as a cofactor, which converts the NADPH to its oxidized form, NADP+. Because NADPH can be measured spectrophotometrically at an absorbance of 340 nm, the depletion of NADPH levels in samples can be used to determine GPx activity. Since GPx activity causes NADPH to be consumed, elevated GPx activity is observed as a decrease in absorbance at 340 nm.