Unit Converter
Methionine (Met)
(Essential Sulfur-Containing Amino Acid - Key for Methylation, SAM Formation & Inborn Errors of Metabolism)
Synonyms
- Methionine
- L-Methionine
- Met
- 2-Amino-4-(methylthio)butanoic acid
- Sulfur amino acid
Units of Measurement
- µmol/L
- mg/L
- mg/dL
- mg/100 mL
- mg%
- µg/mL
Unit Conversions
Molecular Weight of Methionine = 149.21 g/mol
µmol/L ↔ mg/L
1 µmol/L=0.149 mg/L1\, \text{µmol/L} = 0.149\, \text{mg/L}1µmol/L=0.149mg/L 1 mg/L=6.70 µmol/L1\, \text{mg/L} = 6.70\, \text{µmol/L}1mg/L=6.70µmol/L
mg/dL → mg/L
1 mg/dL=10 mg/L1\, \text{mg/dL} = 10\, \text{mg/L}1mg/dL=10mg/L
mg% = mg/dL
µg/mL → mg/L
1 µg/mL=1 mg/L1\, \text{µg/mL} = 1\, \text{mg/L}1µg/mL=1mg/L
Description
Methionine is an essential amino acid that must be obtained from the diet.
It plays a central role in:
- Methylation reactions
- Formation of S-adenosylmethionine (SAM) – the universal methyl donor
- Synthesis of cysteine, glutathione & taurine
- Transsulfuration pathway
- Protein synthesis & one-carbon metabolism
In clinical labs, methionine is measured as part of plasma amino-acid profiling, especially when evaluating metabolic disorders in newborns and children.
Physiological Role
1. SAM Cycle
Methionine → SAM → SAH → Homocysteine → Methionine
Regulates:
- DNA methylation
- Neurotransmitter synthesis
- Phospholipid metabolism
2. Transsulfuration
Methionine is a precursor to:
- Cysteine
- Glutathione (major antioxidant)
- Taurine
3. Growth & Protein Synthesis
Needed for normal hepatic and muscle protein formation.
Clinical Significance
HIGH METHIONINE
Causes:
1. Genetic Disorders
- MAT I/III deficiency (Methionine adenosyltransferase deficiency)
- Cystathionine β-synthase (CBS) deficiency (homocystinuria)
- Glycine N-methyltransferase deficiency (GNMT deficiency)
- S-adenosylhomocysteine hydrolase deficiency
These disorders present in infancy or childhood.
2. Liver Disease
- Acute or chronic hepatitis
- Cirrhosis
- Impaired transamination
3. Parenteral Nutrition
High amino-acid infusion.
4. Premature Infants
Immature metabolic pathways → transient elevation.
5. Vitamin B6, B12 or Folate Deficiency
Impaired remethylation or transsulfuration.
6. Methionine Loading
After oral load for metabolic stress tests.
Symptoms of High Methionine
- Lethargy
- Developmental delay
- Liver dysfunction
- Odor of “cabbage-like breath” (in severe hypermethioninemia)
- Possible neurological issues depending on enzyme defect
LOW METHIONINE
Causes
- Malnutrition / protein deficiency
- Malabsorption
- Chronic liver disease
- Severe illness / sepsis
- Homocystinuria with loss into alternate pathways
- SAM cycle disorders
- Diets deficient in sulfur amino acids
Symptoms
Usually subtle; may relate to:
- Poor growth
- Low muscle mass
- Low glutathione (poor antioxidant capacity)
- Fatigue
Reference Intervals
(Tietz 8E + Mayo + ARUP + ACMG metabolic reference ranges)
Plasma Methionine
| Age Group | Reference Interval |
| Adults | 10 – 40 µmol/L |
| Children | 12 – 45 µmol/L |
| Newborns | 15 – 70 µmol/L (higher due to immature pathways) |
Clinical Significance Thresholds
- > 80 µmol/L → hypermethioninemia; evaluate liver/metabolic disorders
- > 200–300 µmol/L → consider MAT I/III deficiency
- > 500 µmol/L → severe metabolic disease (CBS or SAH hydrolase deficiency)
Diagnostic Uses
1. Inborn Errors of Methionine Metabolism
- MAT I/III deficiency
- Homocystinuria (CBS deficiency)
- GNMT deficiency
- SAH hydrolase deficiency
2. Liver Function Evaluation
Methionine elevation is sensitive to hepatic dysfunction.
3. Nutritional Status Assessment
Especially in TPN patients.
4. Investigation of Neurological Disorders
Some metabolic defects cause progressive neurological disease.
5. Monitoring Therapy
- Pyridoxine (B6) for CBS deficiency
- Methionine restriction diets
- SAM cycle interventions
Analytical Notes
- Fasting sample recommended
- Avoid hemolysis (releases intracellular amino acids)
- Plasma should be deproteinized or frozen promptly
- Measured by HPLC or LC–MS/MS
- Newborn screening detects methionine elevation via tandem MS/MS
Clinical Pearls
- Elevated methionine in a newborn is a red flag for homocystinuria or MAT deficiency.
- Always interpret methionine together with homocysteine, cysteine, SAM/SAH if available.
- In liver disease, methionine rises earlier than AST/ALT abnormalities.
- Severe hypermethioninemia may produce a “cabbage odor” due to volatile sulfur compounds.
- Methionine restriction diet + B6 therapy can normalize methionine in some conditions.
Interesting Fact
Methionine is the first amino acid inserted during protein synthesis in all eukaryotic and prokaryotic systems (start codon AUG → Met).
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Amino Acids
- ACMG Guidelines for Metabolic Disorders & Newborn Screening
- Mayo Clinic Laboratories - Plasma Amino Acids
- ARUP Consult - Amino Acid Interpretation
- NIH / MedlinePlus - Methionine
- NEJM - Hyperhomocysteinemia & Sulfur Amino Acid Disorders