Protects cells and enzymes from oxidative damage, by catalyzing the reduction of hydrogen peroxide, lipid peroxides and organic hydroperoxide, by glutathione. 2 glutathione + H2O2 = glutathione disulfide + 2 H2O 2 glutathione + tert-butyl hydroperoxide = glutathione disulfide + H2O + tert-butanol Homotetramer. Secreted in plasma. Belongs to the glutathione peroxidase family.