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 (H2S), and taurine. l-Cysteine-dependent synthesis of GSH has been investigated in many pathological conditions, while the pathway for l-cysteine metabolism to form H2S 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, H2S, and taurine, with special emphasis on therapeutic and nutritional use of l-cysteine to improve the health and well-being of animals and humans.