Unit Converter
Histidine (His)
(Essential Amino Acid – Key in Growth, Histamine Synthesis & Inborn Errors of Metabolism)
Synonyms
- Histidine
- L-Histidine
- His
- Essential amino acid H
- Plasma histidine
Units of Measurement
- µmol/L
- mg/L
- mg/dL
- mg/100 mL
- mg%
- µg/mL
Key Conversions
(Molecular Weight ≈ 155.15 g/mol)
1 mg/L = 6.45 µmol/L
1 mg/dL = 64.5 µmol/L
1 µg/mL = 1 mg/L
mg/dL = mg% = mg/100 mL
1 µmol/L = 0.155 mg/L
Description
Histidine is an essential amino acid, required especially during:
- Infancy
- Rapid growth
- Tissue repair
- Immune activity
It is a precursor for:
- Histamine (immune + allergic responses)
- Carnosine (muscle buffer)
- Hemoglobin structure (heme-binding via imidazole ring)
Plasma histidine measurement is used in amino acid profiling, nutritional assessment, and inborn error of metabolism workups.
Physiological Role
1. Precursor for Histamine
Via histidine decarboxylase
Histamine involved in:
- Immune response
- Gastric acid secretion
- Neurotransmission
- Allergic reactions
2. Growth & Development
Essential for:
- Infant nutrition
- Protein synthesis
- RBC function (heme coordination via histidine residues)
3. Antioxidant & pH Buffering
Carnosine (His + β-alanine) buffers muscle pH.
4. Metal Binding
Helps regulate:
- Copper
- Zinc
Important in metalloprotein function.
5. Myelin & CNS Support
Histidine metabolites influence CNS development in infants.
Clinical Significance
High Histidine (Hyperhistidinemia)
Most relevant for Inborn Errors of Metabolism (IEM).
1. Histidinemia (HAL gene deficiency)
Rare metabolic disorder caused by:
- Histidase (histidine ammonia-lyase) deficiency
Findings:
- Elevated histidine
- Mild/no symptoms
- Sometimes developmental delay
- Usually benign & detected in newborn screening
2. Liver Disease
Reduced catabolism → elevated histidine.
3. High-Protein Intake / TPN
Infants on total parenteral nutrition (TPN) may show elevated levels.
4. Renal Insufficiency
Reduced amino acid clearance.
5. Chronic Inflammation
Histidine levels may rise in some inflammatory states.
Low Histidine (Hypohistidinemia)
Causes:
- Malnutrition
- Kwashiorkor
- Chronic illness / cancer cachexia
- Hemodialysis (loss of histidine)
- Pregnancy (increased consumption)
- Genetic disorders of amino acid transport
- Zinc deficiency (histidine transport linked)
Clinical consequences:
- Poor growth
- Weak immunity
- Anemia (due to impaired heme binding)
Reference Intervals
(Tietz 8E + Mayo + ARUP + ACMG metabolic data)
Plasma Histidine
- Adults: 60 – 120 µmol/L
- Newborns: 70 – 150 µmol/L
- Children: 60 – 130 µmol/L
Critical Values
- >300 µmol/L → strongly suggest histidinemia
- <40 µmol/L → possible malnutrition or metabolic stress
Diagnostic Uses
1. Inborn Errors of Metabolism
Primary utility is in:
- Histidinemia (HAL deficiency)
- Aminoacidopathies
Often measured with: - Phenylalanine
- Tyrosine
- Methionine
- Other amino acids
2. Nutritional Assessment
Histidine depletion associated with:
- Protein malnutrition
- Chronic disease
- Elderly frailty
- Long-term dialysis
3. Neonatal Screening (in some programs)
Histidinemia detection.
4. Liver Disease Evaluation
Part of amino acid pattern seen in hepatic failure.
5. TPN Monitoring
Histidine levels help adjust amino-acid mixtures.
Analytical Notes
- Use fasting plasma
- Sample must be deproteinized promptly
- Transport on ice
- Serum not recommended (platelet effects)
- Measured via HPLC or tandem mass spectrometry
- Extremely stable if frozen
Clinical Pearls
- Histidinemia is usually benign, unlike many aminoacidopathies.
- Histidine is essential for infants, even though adults can synthesize it.
- Low histidine may worsen anemia, as histidine stabilizes heme on hemoglobin.
- Histidine deficiency can reflect inadequate intake in vegans, elderly, or dialysis patients.
- Elevated histidine with normal ammonia usually means benign histidinemia, not a urea cycle disorder.
Interesting Fact
Histidine contains an imidazole ring, allowing it to function as a pH buffer, which is why it’s abundant in hemoglobin and muscle tissue.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Amino Acids.
- ACMG Guidelines - Aminoacidopathies.
- Mayo Clinic Laboratories - Plasma Amino Acids.
- ARUP Consult - Metabolic Disorders.
- NIH / NLM - Histidine Metabolism.
- MedlinePlus / NIH - Amino Acid Test.