PPA also causes a decrease in glutathione and Glutathione peroxidase activity. Mammalian cells are protected by presence of catalase, glutathione, peroxidases, and superoxide dismutase. Selenium enzymes are involved in the small reducing molecules glutathione and thioredoxin. L-cysteine is a precursor to the biologic antioxidant glutathione. Selenium enzymes are involved in utilization of the small reducing molecules glutathione and thioredoxin. These reactions are catalysed by transferase enzymes such as glutathione S-transferases. It reduces glutathione via glutathione reductase, which converts reactive HO into HO by glutathione peroxidase. It is a cofactor and acts on glutathione peroxidase. Some pathways affected may concern glutathione and glucose metabolism. There are at least four different glutathione peroxidase isozymes in animals. "N"-Acetylcysteine (NAC) is a precursor to glutathione, an antioxidant. Other enzymes using glutathione as a substrate are glutaredoxins. However, the endogenous scavenger compound glutathione naturally protects from toxic electrophiles in the body. NAPQI decreases the liver's glutathione and directly damages cells in the liver.