Unit Converter
Selenocysteine (Sec)
(The 21st Genetically Encoded Amino Acid - Essential Component of Antioxidant Selenoproteins)
Synonyms
- Selenocysteine
- Sec
- Se-Cys
- 21st amino acid
- UGA-encoded amino acid
- Selenium-containing cysteine
Units of Measurement
(For research reference - not routinely measured clinically)
- µmol/L
- mg/L
- mg/dL
- mg/100 mL
- mg%
- µg/mL
Key Unit Conversions
Molecular Weight of Selenocysteine ≈ 168.1 g/mol
µmol/L ↔ mg/L
1 µmol/L=0.1681 mg/L1\ \text{µmol/L} = 0.1681\ \text{mg/L}1 µmol/L=0.1681 mg/L 1 mg/L=5.95 µmol/L1\ \text{mg/L} = 5.95\ \text{µmol/L}1 mg/L=5.95 µmol/L
mg/dL → mg/L
1 mg/dL=10 mg/L1\ \text{mg/dL} = 10\ \text{mg/L}1 mg/dL=10 mg/L
mg% = mg/dL
µg/mL → mg/L
1 µg/mL=1 mg/L1\ \text{µg/mL} = 1\ \text{mg/L}1 µg/mL=1 mg/L
Description
Selenocysteine (Sec) is a naturally occurring amino acid incorporated into proteins by recoding the UGA “stop codon” into a Sec codon using a special tRNA and SECIS element.
Selenocysteine is found in over 25 human selenoproteins, including:
- Glutathione peroxidases (GPx)
- Thioredoxin reductases (TrxR)
- Iodothyronine deiodinases (T3/T4 metabolism)
- Selenoprotein P (SePP1)
- Selenoprotein W, N, K, M
These proteins are essential for antioxidant defense, thyroid hormone regulation, and immune function.
⚠️ Clinical labs do NOT measure selenocysteine directly, because Sec exists only inside proteins and is not present freely in plasma.
Instead, clinicians measure:
- Serum Selenium (Se)
- Selenoprotein P
- Glutathione peroxidase activity
Physiological Role
1. Antioxidant Defense
Sec is the active catalytic residue in:
- Glutathione peroxidases
- Thioredoxin reductases
This protects cells from peroxides and oxidative stress.
2. Thyroid Hormone Metabolism
Deiodinase enzymes (requiring Sec) convert:
- T4 → T3
- rT3 → T2
3. Immune Function
Supports T-cell activation and reduces inflammation.
4. Protein Folding & Redox Control
Regulates intracellular reduction–oxidation pathways.
5. Neuroprotection
Selenoproteins reduce neuronal oxidative damage.
Clinical Significance
HIGH SELENOCYSTEINE
(Not directly measurable; inferred from high selenium status)
Occurs in:
- Excess selenium supplementation
- Selenosis
- Industrial selenium exposure
Symptoms of Selenium Excess
- Hair loss
- Brittle nails
- Garlic-like breath
- Neuropathy
- GI upset
- Dermatitis
LOW SELENOCYSTEINE
Seen in:
1. Selenium Deficiency
→ Directly reduces all selenocysteine-based protein synthesis.
2. Malabsorption
- Celiac disease
- Gastric bypass
- Chronic diarrhea
3. Critical Illness / Sepsis
High oxidative stress → low selenoprotein levels.
4. Keshan Disease
Due to severe selenium deficiency.
5. Poor Thyroid Conversion
Low Sec → low T3 due to impaired deiodinases.
Symptoms
- Muscle weakness
- Cardiomyopathy
- Hypothyroidism (low T3)
- Fatigue
- Increased susceptibility to infections
Reference Intervals
⚠️ There is no clinical reference interval for “Selenocysteine”
because Sec is never measured free in blood.
Laboratory testing relies on:
Serum Selenium
- 70 – 150 µg/L (normal)
Selenoprotein P
- 2.5 – 6.0 mg/L (functional indicator)
Glutathione Peroxidase Activity
Used for antioxidant capacity assessment.
Diagnostic Uses
Although Sec itself is not a clinical test, its biology is central to:
1. Thyroid Disorders
Due to Sec-dependent deiodinase enzymes.
2. Selenium Deficiency
Measured using selenium + SePP1.
3. Critical Care
Low selenoproteins predict mortality.
4. Neuromuscular Disorders
Oxidative stress pathology.
5. Cardiomyopathy
6. Redox / Antioxidant Research
Sec is studied using advanced mass spectrometry.
Analytical Notes
- Sec is unstable as a free amino acid
- Only measured after acid hydrolysis of proteins
- Requires LC–MS/MS or advanced proteomics
- Not part of clinical amino acid panels
- Not used in routine diagnostics anywhere globally
Clinical Pearls
- Selenocysteine is the only amino acid inserted by recoding a stop codon.
- All critical selenium-dependent enzymes require Sec.
- Selenium status, not Sec itself, determines function.
- Low Sec-dependent enzymes → oxidative damage and poor T3 production.
- Selenium deficiency is more common in chronic illness and malabsorption states.
Interesting Fact
Selenocysteine is sometimes called the “21st amino acid” and is more chemically reactive than cysteine, making it a highly efficient catalytic residue in antioxidant enzymes.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Trace Elements & Selenoproteins
- NIH Office of Dietary Supplements - Selenium
- WHO Trace Element Guidelines
- Mayo Clinic Laboratories - Selenium Testing
- ARUP Consult - Selenium & Selenoprotein P
- Biochemistry Texts - Selenocysteine Encoding Mechanism (SECIS elements)