Which of the following statements concerning glutathione is true?

Glutathione is a tripeptide, 7- glutamylcysteinylglycine, present in high concentration in all cells. The 'y-glutamyl cycle is used by some cells to transpo amino acids across the cell membrane. A membrane-associated enzyme, y-glutamyl transpeptidase forms peptide bonds between the glutamyl moiety of glutathione and the amino acid to be transpoed into the cell. The glutamyl-aniino acid is split off from the cysteinylglycine pa of the molecule as both are brought into the cell. The transpoed amino acid is then split from the glutamyl moiety, depositing the free amino acid on the cytosolic side of the membrane. 5-Oxyproline is produced from the glutamyl moiety, which has to be replaced or resynthesized, as does the glutathione. This process uses 3 adenosine triphosphates (ATPs), but is very active in some tissues, such as the kidney. Glutathione reductase is the enzyme used to protect the red cell from oxidative stress and to reduce methemoglobin. Glutathione peroxidase is the selenium containing enzyme used to scavenge free radicals and peroxides. Glutathione is present in all cells in relatively large amounts, attesting to its impoance in metabolism. Glutathione S-transferase is a liver enzyme used in detoxification reactions. It forms a glutathione complex with the compound to be detoxified. There is no such enzyme as glutathione transaminase.