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 -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.
References:
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Cysteine
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