Protects the hemoglobin in erythrocytes from oxidative breakdown. In platelets, plays a crucial role of glutathione peroxidase in the arachidonic acid metabolism. 2 glutathione + H2O2 = glutathione disulfide + 2 H2O (12S)-hydroperoxy-(5Z,8Z,10E,14Z)-eicosatetraenoate + 2 glutathione = (12S)-hydroxy-(5Z,8Z,10E,14Z)-eicosatetraenoate + glutathione disulfide + H2O Homotetramer. Interacts with MIEN1 (By similarity). During periods of oxidative stress, Sec-52 may react with a superoxide radical, irreversibly lose hydroselenide and be converted to dehydroalanine. Belongs to the glutathione peroxidase family.