Cysteine is an amino acid, a building block of proteins used throughout the body. When taken as a supplement, it is usually in the form of N-acetyl-L-cysteine (NAC). The body makes this into cysteine and glutathione, a powerful antioxidant.

Benefits

Cysteine is a sulfur-containing amino acid with diverse roles in protein function and oxidative metabolism. Cysteines in proteins can generate disulfide bonds essential for protein folding. Cysteine availability also impacts iron-sulfur cluster-containing proteins, which are involved in oxidative phosphorylation, heme biosynthesis, and central carbon metabolism. In addition to its proteinogenic role, cysteine contributes to synthesizing intracellular metabolites such as taurine, coenzyme A, and glutathione (GSH), a tripeptide antioxidant. Among these, glutathione synthesis is the most studied function of cysteine metabolism and its levels strongly depend on cysteine availability.

Side effects

Using a single amino acid supplement may lead to a negative nitrogen balance. This can lessen how well your metabolism works. It can make your kidneys work harder. In children, single amino acid supplements may cause growth problems.

Synthesis

Cysteine is synthesized from methionine in four steps - the first two steps are part of the methionine cycle and the second two are part of the transsulfuration pathway. As described above in the methionine cycle, methionine gives rise to SAM, from which homocysteine is subsequently synthesized (Fig. 1). Homocysteine is coupled to serine in the first reaction of the transsulfuration pathway by the enzyme cystathionine-β-synthase (CBS) to form cystathionine. In the next reaction, cystathionine is cleaved by the enzyme cystathion-γ-lyase (CGL) into cysteine, 2-oxobutanoate and ammonia.

Dietary sources

Your body makes cysteine from methionine, an essential amino acid. Cysteine is also found in most high-protein foods, including:

Ricotta

Cottage cheese

Yogurt

Pork

Sausage meat

Chicken

Turkey

Duck

Lunch meats

Wheat germ

Granola

Oat flakes