N-Acetyl-D-cysteine has antioxidant activities and scavenges ROS through the reaction with its thiol group, but cannot enter the glutathione metabolic pathway.In Vitro: N-Acetyl-D-cysteine (20 mM; 1 hour pretreatment; 12 hours) does not increase intracellular GSH levels, but GSH monoester does. D-NAC can not enhance hypoxic apoptosis. This demonstrate that GSH rather than D-NAC or NAC is responsible for enhancing hypoxic apoptosis.