Glutathione is a naturally occurring biologically active, sulfur-containing tripetide, composed of the amino acids L-cysteine, L-glutamic acid, and glycine. It is a potent antioxidant and works in the liver to protect the body from harmful substances. It also functions as a precursor to glutathione peroxidase, a key antioxidant enzyme which is a major protector against lipid peroxidation (fats turning rancid).