L-Cysteine-d 3 , 15 N is the deuterium and 15 N-labeled L-Cysteine. L-Cysteine is a conditionally essential amino acid, which acts as a precursor for biologically active molecules such as hydrogen sulphide (H2S), glutathione and taurine. L-Cysteine suppresses ghrelin and reduces appetite in rodents and humans.