Glutathione Peroxidase (EC 1.11.1.9) family of enzymes play important roles in the protection of organisms from oxidative damage. GPX converts reduced glutathione (GSH) to oxidized glutathione (GSSG) while reducing lipid hydroperoxides to their corresponding alcohols or free hydrogen peroxide to water. Several isozymes have been found in different cellular locations and with different substrate specificity. Low levels of GPX have been correlated with free radical related disorders. Glutathione Peroxidase Microplate Assay Kit is a sensitive assay for determining Glutathione Peroxidase activity in various samples. GPX reduces Hydroperoxide while oxidizing GSH to GSSG. GSH may react with DTNB. The decrease of GSH is proportional to GPX activity.