Tryptophan Metabolism
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Direct answer
Tryptophan, the amino acid with the largest side chain and the smallest daily requirement, splits into two major routes: the serotonergic pathway (tryptophan hydroxylase adding BH4 to make 5-hydroxytryptophan, decarboxylated to serotonin, then N-acetylated and O-methylated in the pineal to melatonin) and the kynurenine pathway, which degrades the bulk of dietary tryptophan through indoleamine 2,3-dioxygenase and ends in nicotinamide adenine dinucleotide. About 60 mg of tryptophan yields the niacin equivalent of 1 mg, which is why pure tryptophan deficiency mimics pellagra. Serotonin itself is degraded to 5-hydroxyindoleacetic acid (5-HIAA), the urinary marker of carcinoid syndrome.
What you must remember
- Tryptophan hydroxylase is rate-limiting for serotonin and is BH4-dependent; the second step uses aromatic amino acid decarboxylase with pyridoxal phosphate, the same enzyme that makes dopamine and histamine.
- Kynurenine pathway handles roughly 95% of tryptophan catabolism, starting with indoleamine 2,3-dioxygenase (IDO), which is induced by interferon-gamma — hence low tryptophan and high kynurenine in chronic infection and pregnancy-related immune tolerance.
- Kynurenic acid blocks NMDA and cholinergic receptors (neuroprotective) while quinolinic acid agonises NMDA receptors (neurotoxic); their balance is discussed in Huntington disease and AIDS dementia.
- Niacin equivalence: 60 mg tryptophan = 1 mg niacin — the conversion that fails in Hartnup disease and in vitamin B6 deficiency caused by isoniazid, producing pellagra-like dermatitis.
- Melatonin synthesis: serotonin is N-acetylated by N-acetyltransferase (the rate-limiting, night-activated step) then O-methylated by HIOMT; the pineal is wired from the suprachiasmatic nucleus, so light suppresses melatonin.
- Carcinoid syndrome: urinary 5-HIAA above roughly 25 mg per 24 hours (normal under about 8) supports the diagnosis; avoid banana, tomato and walnut for a day before collection, since they falsely raise it.
- Distribution of serotonin: about 90% gut enterochromaffin cells, 8-10% platelets (which take it up but cannot synthesise it), under 2% brain — SSRI-related diarrhoea is gut-receptor pharmacology.
A pellagra work-through from the ward
A 40-year-old on antitubercular therapy for three months returns with a photosensitive rash in a Casal's necklace distribution, loose stools and irritability. Isoniazid has depleted pyridoxal phosphate, and the pyridoxine-dependent steps at kynureninase stall, so tryptophan cannot be converted to niacin; add a marginal cereal-based diet and the niacin equivalent pool collapses. The fix is layered: pyridoxine 10-25 mg daily with INH continued, oral nicotinamide 100 mg three times a day for the deficit, and a protein-adequate diet so tryptophan substrate returns. Contrast the same rash in Hartnup disease — a neutral amino acid transport defect affecting renal and intestinal handling — where neutral amino acids flood the urine and tryptophan malabsorption in the colon yields indole metabolites; there the treatment adds nicotinamide plus a high-protein diet, and neomycin to cut colonic indole production in severe cases. Both conditions teach one sentence worth writing in any answer: pellagra is a disease of the tryptophan-to-niacin economy, not merely of dietary niacin.
High-yield viva angles
Viva examiners first test the enzyme pair: tryptophan hydroxylase for serotonin, tyrosine hydroxylase for catecholamines — both BH4-dependent, both rate-limiting, easily confused under pressure. The second angle is "why is carcinoid 5-HIAA high but serotonin in platelets irrelevant" — carcinoid tumours, especially ileal with liver metastases, overflow serotonin into blood where platelets sequester it and monoamine oxidase in liver, lung and brain degrades the rest to 5-HIAA. The third is Indian and practical: jowar (sorghum)-heavy diets with excess leucine antagonise the tryptophan-niacin conversion, cited in Indian nutrition texts as a reason pellagra appeared in Deccan populations eating little maize.
Frequently asked questions
Which is the rate-limiting enzyme of serotonin synthesis?
Tryptophan hydroxylase, which converts tryptophan to 5-hydroxytryptophan using tetrahydrobiopterin; decarboxylation to serotonin follows.
How much tryptophan equals one milligram of dietary niacin?
Approximately 60 mg of tryptophan provides 1 mg niacin equivalent, a conversion needing pyridoxine at the kynureninase step.
Why does Hartnup disease cause pellagra-like features?
The defective neutral amino acid transporter starves the body of absorbed tryptophan, cutting niacin equivalents, while unabsorbed tryptophan is converted by colonic bacteria to indoles.
What regulates melatonin secretion acutely?
N-acetyltransferase activity, driven by norepinephrine from sympathetic fibres in darkness and suppressed by light via the retinohypothalamic-suprachiasmatic pathway.
Which urinary marker confirms carcinoid syndrome?
Twenty-four-hour urinary 5-HIAA, typically well above 25 mg per day in classical carcinoid, collected after excluding dietary serotonin for a day.