Unit Converter
Cysteine (Cys)
(Sulfur-Containing Amino Acid – Protein Synthesis, Antioxidant Function & Metabolic Disorders)
Synonyms
- Cysteine
- L-Cysteine
- Cys
- 2-Amino-3-mercaptopropionic acid
- Sulfur amino acid (SAA)
Units of Measurement
- µmol/L
- mg/L
- mg/dL
- mg/100 mL
- mg%
- µg/mL
1 mg/L = 0.1 mg/dL
1 µg/mL = 1 mg/L
mg/dL = mg% = mg/100 mL
Description
Cysteine is a non-essential, sulfur-containing amino acid synthesized from methionine via the transsulfuration pathway. It contains a thiol (–SH) group that plays a vital role in:
- Protein structure (disulfide bonds)
- Antioxidant defense (precursor of glutathione)
- Detoxification
- Metabolism of sulfur compounds
Measurement of cysteine levels is clinically relevant in evaluating:
- Amino acid metabolism disorders
- Nutritional status
- Oxidative stress
- Renal dysfunction
- Homocysteine-related metabolic diseases
Physiological Role
1. Protein Structure
Cysteine forms disulfide bonds (Cys–Cys) critical for:
- Enzyme folding
- Insulin structure
- Keratin (hair, skin, nails)
2. Glutathione Synthesis
Cysteine is the rate-limiting precursor of glutathione (GSH), the body’s major antioxidant.
3. Detoxification
Cysteine participates in:
- Xenobiotic metabolism
- Heavy-metal chelation
- Acetaminophen detoxification (N-acetylcysteine antidote)
4. Metabolism
Connected with:
- Methionine–homocysteine cycle
- Taurine synthesis
- Sulfate production
Clinical Significance
Elevated Cysteine (Hypercysteinemia)
Usually associated with homocystinuria and disordered sulfur metabolism.
Causes
- Homocystinuria (CBS deficiency)
- Elevated homocysteine + cysteine abnormalities
- Elevated homocysteine + cysteine abnormalities
- Transsulfuration defects
- Renal insufficiency
- Reduced clearance
- Reduced clearance
- Vitamin deficiencies
- B6, B12, folate
- B6, B12, folate
- High methionine intake
Clinical Features
- Vascular thrombosis
- Developmental delay
- Skeletal abnormalities
- Eye lens dislocation (in homocystinuria)
Low Cysteine (Hypocysteinemia)
Causes
- Malnutrition
- Protein malabsorption
- Cystinuria / cystine stone disease
- Chronic illness with oxidative stress
- Deficient methylation cycle
Symptoms
- Poor antioxidant status
- Weak immunity
- Hair/skin abnormalities (keratin defects)
Reference Intervals
(Tietz 8E + LC-MS/MS Amino Acid Panels + Mayo + ARUP)
Typical fasting plasma cysteine range:
Adults
- 200 – 350 µmol/L
Children
- Slightly lower (150–300 µmol/L)
Interpretive Notes
- Levels depend on protein intake & renal clearance
- Fasting sample required for accuracy
Diagnostic Uses
1. Inborn Errors of Metabolism
Key for:
- Homocystinuria
- Cystathionine β-synthase (CBS) deficiency
- Cystinuria
- Sulfur amino acid pathway disorders
2. Nutritional Assessment
Reflects protein adequacy.
Low in malnutrition, chronic illness.
3. Oxidative Stress & GSH Deficiency
Cysteine depletion → low glutathione.
Measured in:
- Critical illness
- Sepsis
- Liver disease
4. Renal Disease
High cysteine due to decreased clearance.
5. Research Marker
Used in studies of:
- Aging
- Metabolic syndrome
- Inflammation
Analytical Notes
- Fasting sample is mandatory
- Plasma preferred (EDTA or heparin)
- Must be deproteinized quickly to avoid oxidation
- LC-MS/MS is gold standard
- Hemolysis affects results
- Avoid air exposure (thiol oxidation)
Clinical Pearls
- Cysteine is tightly linked to homocysteine, methionine, and glutathione - interpret together.
- Elevated cysteine alone is less clinically important; interpret with total homocysteine for metabolic disease.
- Low cysteine → low glutathione → increased oxidative stress.
- In cystinuria, cysteine is low in plasma but high in urine → kidney stones.
Interesting Fact
Cysteine’s thiol (-SH) group allows it to form strong disulfide bonds, making it crucial in the structural rigidity of hair and nails - hence why cysteine supplementation is found in hair-growth products.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Amino Acids.
- Mayo Clinic Laboratories - Plasma Amino Acids.
- ARUP Consult - Inborn Errors of Metabolism.
- IFCC Amino Acid Quantification Guidelines.
- MedlinePlus / NIH - Amino Acid Disorders.