Cysteine is a nonessential sulfur-containing amino acid. It is obtained from the diet and is also endogenously made from the intermediate amino acid cystathionine.
Dietary cysteine sources include poultry, eggs, beef, and whole grains.
This amino acid should not be confused with the oxidized derivative of cysteine called cystine. Cystine is formed by combining two cysteine molecules within a redox reaction.
The urinary FMV amino acid test reports cysteine and cystine separately.
The plasma amino acid test combines both cysteine and cystine as one biomarker called "Cyst(e)ine".
Cysteine is an important component of glutathione. Recent studies provide some data to support the view that cysteine may be a limiting amino acid for glutathione synthesis in humans.
This synthesis 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.
When cysteine levels are low, this favors their 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.
Low dietary protein intake, GI malabsorption, and maldigestion may all contribute to lower amino acid levels. Because vitamin B6 is a cofactor in several steps within the transsulfuration pathways, deficiency may contribute to lower cysteine by inhibiting or slowing the enzyme that converts cystathionine to cysteine.
A diet high in cysteine-rich proteins can elevate cysteine levels. As with all sulfur-containing amino acids, the enzyme sulfite oxidase catabolizes the amino acid into sulfite for excretion. An important cofactor for this enzyme is molybdenum. With that, insufficient molybdenum can contribute to elevated cysteine.
Homocysteine is pulled into the transsulfuration pathway via the enzyme cystathionine-beta-synthase (CBS) to become cysteine, with cystathionine formation as an intermediate step. Cysteine levels may be elevated due to a CBS SNP which results in an upregulation of the enzyme and more cystathionine and cysteine production.
Zinc is an important cofactor downstream from cysteine in transsulfuration. Because of this, cysteine elevations can also be seen in zinc insufficiency.
Vitamin B12 may also be a cofactor in the peripheral utilization of cysteine; therefore functional deficiencies of vitamin B12 can contribute to higher levels.
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