Unit Converter
Leucine (Leu)
(Essential Branched-Chain Amino Acid – Key Marker in MSUD, Metabolic Disorders & Nutritional Assessment)
Synonyms
- Leucine
- L-Leucine
- Leu
- Branched-chain amino acid (BCAA)
- Plasma leucine
Units of Measurement
- µmol/L
- mg/L
- mg/dL
- mg/100 mL
- mg%
- µg/mL
Molecular Weight
Leucine ≈ 131.17 g/mol
(Same as isoleucine; structural isomer)
Key Unit Conversions
1 mg/L = 7.63 µmol/L
1 mg/dL = 76.3 µmol/L
1 µg/mL = 1 mg/L
1 µmol/L = 0.131 mg/L
mg% = mg/dL
Description
Leucine is an essential branched-chain amino acid (BCAA) along with:
- Isoleucine
- Valine
It plays major roles in:
- Protein synthesis
- Muscle metabolism
- Insulin signaling
- Energy production (ketogenic & glucogenic)
- Regulation of mTOR pathway
Leucine is crucial in neonatal and pediatric metabolic evaluations, especially in detecting inborn errors like Maple Syrup Urine Disease (MSUD).
Physiological Role
1. Muscle Protein Synthesis
Leucine directly activates mTORC1, stimulating muscle growth.
2. Energy Metabolism
Leucine is:
- Ketogenic → produces acetyl-CoA & acetoacetate
- Glucogenic → via intermediates feeding into TCA cycle
3. Insulin Secretion
Acts as a signal nutrient to augment insulin release.
4. Brain Function
BCAAs compete with aromatic amino acids for BBB transport, influencing:
- Serotonin synthesis
- Dopaminergic balance
5. Growth & Development
Essential in infants, children & pregnancy.
Clinical Significance
HIGH Leucine
(Most important: hallmark of MSUD)
1. Maple Syrup Urine Disease
Caused by deficiency of branched-chain α-ketoacid dehydrogenase (BCKD).
Findings:
- Leucine highly elevated (often >1000 µmol/L)
- Isoleucine ↑
- Valine ↑
- Alloisoleucine present (diagnostic marker)
Clinical features:
- Sweet-smelling urine
- Poor feeding, vomiting
- Lethargy, seizures
- Progressive neurologic deterioration
2. Liver Disease
Decreased breakdown → elevated leucine.
3. High Protein Intake / TPN
Leucine-containing supplements and formulations.
4. Inborn Errors of Metabolism
BCAA transport or enzymatic defects.
5. Severe Catabolic Stress
Trauma, burns, sepsis may raise levels transiently.
LOW Leucine
Conditions:
- Malnutrition
- Poor oral intake
- Malabsorption (e.g., celiac disease)
- Chronic liver disease
- Long-term parenteral nutrition without balanced amino acids
- Hypercatabolic states (critical illness)
Effects:
- Muscle wasting
- Impaired protein synthesis
- Growth restriction (children)
- Immune dysfunction
Reference Intervals
(Tietz 8E + Mayo + ARUP + ACMG)
Plasma Leucine
- Adults: 60 – 170 µmol/L
- Children: 60 – 180 µmol/L
- Infants: 80 – 200 µmol/L
- Newborns: 90 – 250 µmol/L
Pathologic Levels in MSUD
- > 300 µmol/L: abnormal
- > 500 µmol/L: strongly indicative
- > 1000 µmol/L: diagnostic in the right clinical context
- Presence of alloisoleucine confirms diagnosis.
Diagnostic Uses
1. Evaluation for MSUD
Leucine is the most severely elevated amino acid in MSUD.
2. Plasma Amino Acid Profile
Screening for:
- Inborn errors of metabolism
- Hepatic dysfunction
- Nutritional deficiencies
3. Monitoring Therapy in MSUD
Follow-up on dietary restriction & metabolic control.
4. Nutritional Assessment
Part of essential amino acid status.
5. Critical Illness Monitoring
Leucine depletion reflects protein catabolism.
Analytical Notes
- Fasting sample recommended
- Plasma (heparin) ideal; serum acceptable
- Must be deproteinized promptly for LC-MS/MS
- Hemolysis can alter amino acid concentrations
- Evaluate together with valine, isoleucine, alloisoleucine
Clinical Pearls
- Leucine is the primary toxic metabolite in MSUD causing neurological injury.
- Alloisoleucine is pathognomonic for MSUD-even if leucine is only moderately elevated.
- Neonates with failure to thrive + ketonuria + neurologic signs → check leucine urgently.
- High leucine can cause cerebral edema in severe MSUD crises.
- BCAA deficiencies occur in critically ill and malnourished patients.
Interesting Fact
Leucine is one of the strongest natural activators of mTOR, the master regulator of cell growth, muscle hypertrophy, and protein synthesis-hence its central role in sports nutrition.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Amino Acids
- ACMG Guidelines - Maple Syrup Urine Disease
- Mayo Clinic Laboratories - Plasma Amino Acid Profile
- ARUP Consult - Inborn Errors of Metabolism
- MedlinePlus / NIH - Amino Acid Testing