l‐Cysteine metabolism and its nutritional implications

l ‐Cysteine is a nutritionally semiessential amino acid and is present mainly in the form of l ‐cystine in the extracellular space. With the help of a transport system, extracellular l ‐cystine crosses the plasma membrane and is reduced to l ‐cysteine within cells by thioredoxin and reduced glutathione (GSH). Intracellular l ‐cysteine plays an important role in cellular homeostasis as a precursor for protein synthesis, and for production of GSH, hydrogen sulfide (H 2 S), and taurine. l ‐Cysteine‐dependent synthesis of GSH has been investigated in many pathological conditions, while the pathway for l ‐cysteine metabolism to form H 2 S has received little attention with regard to prevention and treatment of disease in humans. The main objective of this review is to highlight the metabolic pathways of l ‐cysteine catabolism to GSH, H 2 S, and taurine, with special emphasis on therapeutic and nutritional use of l ‐cysteine to improve the health and well‐being of animals and humans.