* Dietary Supplement * Antioxidant and Liver Support N-Acetyl cysteine is a more stable form of the sulfur amino acid L-cysteine, and is a powerful antioxidant. It is an excellent precursor of glutathione, another major antioxidant. Glutathione is also the precursor, along with selenium, of glutathione peroxidase, one of the most important antioxidant enzymes in the body.