N-Acetyl Cysteine 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 a highly valued component in your body s natural defense mechanism. Glutathione is activated by the selenium-containing glutathione peroxidase one of the most important antioxidant enzymes in the body. N-acetyl cysteine is better than glutathione supplements which may break down during digestion and lose potency.