The PromoKine Glutathione Peroxidase Activity Assay Kit measures glutathione dependent peroxidase activity through a coupled reaction with glutathione reductase (GR) in different sample materials (e.g. plasma, erythrocyte lysates, tissue homogenates, cell lysates) with a high detection sensitivity. In this assay, glutathione peroxidase (GPx) reduces cumene hydroperoxide, and oxidizes GSH to GSSG. The generated GSSG is reduced to GSH with consumption of NADPH by GR. The decrease of NADPH is proportional to GPx activity and can be easily measured by absorbance at 340 nm.