Cysteine is a nonessential sulfur-containing amino acid. It is obtained from the diet and is also endogenously made from cystathionine. Dietary cysteine sources include poultry, eggs, beef, and whole grains. [L]
Cysteine is an important glutathione component. Recent studies provide convincing data to support the view that cysteine is the limiting amino acid for glutathione synthesis in humans. [L] This conversion requires the enzyme glutathione synthetase (GSS). Cysteine can alternatively be converted to taurine (another amino acid) and the organic acid pyruvate, which are used in the mitochondrial citric acid cycle and/or excreted in the urine. [L]
Cysteine metabolism can assist in the production of either glutathione or taurine. Lower plasma cysteine levels favor its utilization in glutathione formation during oxidative stress, given the importance of glutathione. Conversely, high levels of cysteine in the absence of oxidative stress favor its metabolism towards pyruvate and taurine. [L]
References:
Larsson SC, Hakansson N, Wolk A. Dietary cysteine and other amino acids and stroke incidence in women. Stroke. 2015;46(4):922-926. [L]
Lyons J, Rauh-Pfeiffer A, Yu Y, et al. Blood glutathione synthesis rates in healthy adults receiving a sulfur amino acid-free diet. Pro Nat Acad Sci. 2000;97(10):5071-5076. [L]
Stipanuk MH, Ueki I. Dealing with methionine/homocysteine sulfur: cysteine metabolism to taurine and inorganic sulfur. J Inher Metab Dis. 2011;34(1):17-32. [L]
Stipanuk MH, Coloso RM, Garcia RAG, Banks MF. Cysteine Concentration Regulates Cysteine Metabolism to Glutathione, Sulfate and Taurine in Rat Hepatocytes. J Nutr. 1992;122(3):420-427. [L]
Blom HJ, Smulders Y. Overview of homocysteine and folate metabolism. With special references to cardiovascular disease and neural tube defects. J Inher Metab Dis. 2011;34(1):75-81. [L]
DeStefano Vea. Linkage disequilibrium at the cystathionine beta-synthase (CBS) locus and the association between genetic variation at the CBS locus and plasma levels of homocysteine. Ann Human Genet. 1998;62(6):481-490. [L]
Obeid R. The metabolic burden of methyl donor deficiency with focus on the betaine homocysteine methyltransferase pathway. Nutrients. 2013;5(9):3481-3495. [L]
Stabler S, et al. Elevation of serum cystathionine levels in patients with cobalamin and folate deficiency. Blood. 1993;81(12):3404-3413 [L]
Cyst(e)ine
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