Life Extension L-cysteine is a conditionally essential amino acid, one of only three sulfur-containing amino acids. It plays a role in the sulfation cycle, acting as a sulfur donor in phase II detoxification and as a methyl donor in the conversion of homocysteine to methionine. Cysteine also helps synthesize glutathione, the body’s most important intracellular antioxidant and a vital detoxifier.