N-Acetyl Cysteine from Source Naturals is a stable form of the sulfur amino acid L-cysteine. It's a powerful natural antioxidant amino acid that is a precursor to glutathione. Glutathione is activated by the selenium-containing glutathione peroxidase, one of the most important antioxidant enzymes in the body. N-acetyl cysteine may be more helpful than some other glutathione supplements, which may break down during digestion and lose potency.