The increased activity of GRx can be explained as occurring to avoid the toxic effect caused by increased levels of GSSG (oxidized glutathione) in PKU patients. Background: Alterations in antioxidant systems (AOS) have been related to multiple inborn errors of metabolism. Since many of the antioxidants come from the diet, patients with phenylketonuria (PKU) are susceptible to different dietary deficiencies of antioxidant vitamins and trace elements. It is important to identify alterations of this system in order that they can be corrected with supplements.