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 a sulfur-containing, conditionally-essential amino acid. The sulfur group (thiol or sulfhydryl group) in cysteine accounts for most of cysteine’s functions. Cysteine can be oxidized with itself to form cystine.

Cysteine has six major functions:

1) incorporation into amino acid sequences of proteins, where cysteine promotes protein structure by sulfhydryl bonding;

2) ratelimiting precursor for glutathione synthesis;

3) precursor for taurine (used in bile formation and nerve function);

4) source of sulfate for connective tissue synthesis;

5) source of pyruvate for energy or glucose production;  

6) neurotransmitter. As a component of glutathione, cysteine functions include being a powerful antioxidant, detoxification agent, component of some prostaglandins, and an amino acid transporter across membranes. Formation of cysteine from homocysteine is one pathway to reduce homocysteine levels.

Cysteine (FMV Urine) is part of a group of markers that indicates the balance and sufficiency of the methylation/sulfation pathway substrates and co-factors.

Common food sources:

Soybeans, beef, lamb, sunflower seeds, chicken, oats

Physiological Function:

→ Cysteine has antioxidant properties itself, but is also a precursor molecule to glutathione production, the master antioxidant.

→ Cysteine is also an important source of sulfide for iron-sulfide metabolism.

→ Cysteine will bind metals easily to its thiol group, such as iron, nickel, copper, zinc, and heavy metals such as mercury and lead, which may confer some chelation benefits.

→ Cysteine counteracts acetaldehyde effects from consumption of alcohol and can reduce hangovers.

The marker Cysticercosis (Taenia solium) refers to a diagnostic test used to detect the presence of antibodies against Taenia solium, the parasitic tapeworm responsible for cysticercosis. Cysticercosis is an infection that occurs when a person ingests eggs from the tapeworm, which are typically spread through contaminated food, water, or contact with infected feces. Once inside the body, the eggs hatch into larvae and can migrate to various tissues, including muscles, eyes, and the central nervous system, forming cysts. This can lead to a range of symptoms, depending on the location and number of cysts.

Cystine is formed from the oxidation of cysteine, or from the degradation of glutathione oxidation products. It is two cysteines linked together with a disulfide bond.

The urine FMV amino acid test reports cysteine and cystine separately. The plasma amino acid test combines both cysteine and cystine as one biomarker.

Cystine is the oxidized disulfide form of cysteine (Cys) and is the predominant form of cysteine in the blood due to its greater relative stability. Cystine is derived from dietary protein and, end formed endogenously from cysteine.

Cystine is formed from the oxidation of cysteine, or from the degradation of glutathione oxidation products. It is two cysteines linked together with a disulfide bond.

Cystine is formed from the oxidation of cysteine, or from the degradation of glutathione oxidation products. It is two cysteines linked together with a disulfide bond.

The urine FMV amino acid test reports cysteine and cystine separately. The plasma amino acid test combines both cysteine and cystine as one biomarker.

Cystine is formed from the oxidation of cysteine, or from the degradation of glutathione oxidation products. It is two cysteines linked together with a disulfide bond.

Cystine is rate limiting for glutathione production. Cystine is the oxidized form of cysteine.

→ Low cystine may be reflective of reduced glutathione levels and has also been noted in those with celiac disease and lower BMD.

→ Cysteine can be imported into cells either directly or as cystine, within the cell, cystine is immediately reduced to cysteine.

→ Higher plasma cystine has been associated with older age, female, higher BMI, lower GFR, diabetes mellitus, metabolic syndrome, hypertension, lower total cholesterol levels, statin use, lower ejection fraction, and higher hsCRP.

→ Higher urine cystine may be indicative of impaired amino acid reabsorption defects and has been associated with recurrent cystine kidney stones.

→ Cystine from foods sources is considered nutritionally equivalent to cysteine (egg, beef, and whole grains, fish, lentils, and oatmeal).

Cystine is formed from the oxidation of cysteine, or from the degradation of glutathione oxidation products. It is two cysteines linked together with a disulfide bond.

The urine FMV amino acid test reports cysteine and cystine separately. The plasma amino acid test combines both cysteine and cystine as one biomarker.

Cystine is the oxidized disulfide form of cysteine (Cys) and is the predominant form of cysteine in the blood due to its greater relative stability. Cystine is derived from dietary protein and, end formed endogenously from cysteine.

Cystine is formed from the oxidation of cysteine, or from the degradation of glutathione oxidation products. It is two cysteines linked together with a disulfide bond.

The urine FMV amino acid test reports cysteine and cystine separately. The plasma amino acid test combines both cysteine and cystine as one biomarker.

Cystine is the oxidized disulfide form of cysteine (Cys) and is the predominant form of cysteine in the blood due to its greater relative stability. Cystine is derived from dietary protein and, end formed endogenously from cysteine.

Cystine is rate limiting for glutathione production. Cystine is the oxidized form of cysteine.

Cystine is formed from the oxidation of cysteine, or from the degradation of glutathione oxidation products. It is two cysteines linked together with a disulfide bond.