Unit Converter
Tryptophan (Trp)
(Essential Aromatic Amino Acid - Precursor of Serotonin, Melatonin & Niacin)
Synonyms
- L-Tryptophan
- Trp
- α-Amino-β-(indole-3-yl)propionic acid
- Essential amino acid
- Indole amino acid
Units of Measurement
- µmol/L
- mg/L
- mg/dL
- mg/100 mL
- mg%
- µg/mL
Unit Conversions
Molecular Weight of Tryptophan = 204.23 g/mol
µmol/L → mg/L
1 µmol/L=0.204 mg/L1\ \text{µmol/L} = 0.204\ \text{mg/L}1 µmol/L=0.204 mg/L 1 mg/L=4.90 µmol/L1\ \text{mg/L} = 4.90\ \text{µmol/L}1 mg/L=4.90 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
µg/mL ↔ mg/L
1 µg/mL=1 mg/L1\ \text{µg/mL} = 1\ \text{mg/L}1 µg/mL=1 mg/L
mg/100 mL = mg% = mg/dL
Description
Tryptophan is an essential aromatic amino acid, obtained exclusively through diet.
It plays key biochemical roles:
- Serotonin precursor (via 5-hydroxytryptophan)
- Melatonin precursor (pineal gland)
- Niacin (Vitamin B3) synthesis via kynurenine pathway
- Protein building block
- Immune regulation through kynurenine metabolites
Tryptophan metabolism is tightly linked to:
- Mood regulation
- Sleep cycles
- Inflammation
- Gut–brain axis
Physiological Role
1. Neurotransmitter precursor
- Serotonin (mood, appetite, sleep)
- Melatonin (circadian rhythm)
2. NAD⁺ / Niacin synthesis
Conversion to kynurenine → quinolinic acid → NAD⁺.
3. Immune–Metabolic Regulation
Indoleamine 2,3-dioxygenase (IDO) activation during inflammation → ↓ tryptophan.
4. Growth & Protein Synthesis
Essential amino acid - must be supplied through diet.
Clinical Significance
ELEVATED TRYPTOPHAN
Less common. Seen in:
1. High Protein Intake / Supplementation
Dietary excess or bodybuilding supplements.
2. Hartnup Disease (paradoxical)
Defective renal & intestinal amino acid transport → variable plasma levels.
3. Liver Disease (mild elevation)
Reduced hepatic metabolism.
4. Serotonin-Producing Tumors
Try to consume tryptophan → sometimes low; however early cases may show variable levels.
5. Pregnancy
Due to increased binding to albumin and altered metabolism.
Clinical Note: High serotonin (5-HIAA elevated) does not require high tryptophan — most carcinoid patients show low tryptophan due to serotonin overproduction.
LOW TRYPTOPHAN
1. Malnutrition / Protein Deficiency
2. Chronic Inflammation
Cytokines activate IDO, shunting tryptophan → kynurenine.
Seen in:
- Tuberculosis
- Autoimmune disease
- Malignancy
- HIV/AIDS
- Sepsis
- COVID-related cytokine activation (IDO pathway)
3. Liver Disease
Reduced metabolism and transport.
4. Pellagra / Niacin Deficiency
Due to:
- Malnutrition
- Isoniazid therapy
- Carcinoid syndrome (tryptophan diverted to serotonin pathway)
5. Carcinoid Syndrome
Markedly ↓ tryptophan due to excessive serotonin synthesis.
6. Malabsorption
Celiac disease, Crohn’s, pancreatic insufficiency.
7. Rare Disorders
- Tryptophan 2,3-dioxygenase defects
- Tryptophan hydroxylase deficiency
Reference Intervals
(Tietz 8E + Mayo + ARUP, fasting sample recommended)
Plasma Tryptophan
- 40 – 90 µmol/L
(= 8 – 18 mg/L)
Urine Tryptophan
Useful mainly in metabolic disorders (Hartnup disease).
Diagnostic Uses
1. Nutritional Assessment
Low in protein-energy malnutrition.
2. Evaluation of Carcinoid Syndrome
Low tryptophan + high serotonin/5-HIAA.
3. Metabolic Disorders
- Hartnup disease
- Tryptophan metabolism defects
- Kynurenine pathway abnormalities
4. Liver Disease Assessment
Altered amino acid profile.
5. Psychiatric & Neurologic Research
Serotonin-related pathways:
- Depression
- Anxiety
- Sleep disorders
(Not standard diagnostic use.)
6. Niacin Deficiency / Pellagra
Low tryptophan due to impaired conversion.
Analytical Notes
- Fasting sample preferred - tryptophan is diet-sensitive.
- Avoid hemolysis; RBCs contain amino acids.
- Plasma separated quickly to prevent degradation.
- Quantified by LC-MS/MS or HPLC in amino-acid profiling.
- Tryptophan decreases with prolonged sample storage.
Clinical Pearls
- Low tryptophan is a strong inflammation marker via IDO activation.
- Carcinoid syndrome → low Trp + very high serotonin + high 5-HIAA.
- Tryptophan deficiency → low serotonin → insomnia, mood changes (research-based).
- High protein meals artificially raise tryptophan - fasting is essential.
- In pellagra, tryptophan and niacin deficiency coexist.
Interesting Fact
It takes 60 mg of tryptophan to produce 1 mg of niacin - explaining why diets low in tryptophan cause pellagra even if niacin itself is not deficient.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Amino Acids
- Rosenberg et al. - Disorders of Amino Acid Metabolism
- Mayo Clinic Laboratories - Amino Acid Profile
- ARUP Consult - Amino Acid Disorders
- MedlinePlus (NIH) -Amino Acid Tests
- WHO - Niacin Deficiency / Pellagra Guidelines