Tryptophan is an essential amino acid obtained through diet that has several important metabolic functions in the body. It can be broken down through four main pathways: 1) the serotonin pathway where tryptophan is converted to serotonin, an important neurotransmitter; 2) the kynurenine pathway which clears excess tryptophan and provides precursors for other important molecules; 3) incorporation into proteins; and 4) bacterial degradation in the gut producing compounds like indole. Dysregulation of these pathways can lead to conditions like carcinoid syndrome or affect circadian rhythms and reproduction.

1. Tryptophan Metabolism
11.Dec.16

2. Tryptophan
- It’s an essential amino acid that is made from plant or animal
sources.
- Tryptophan is important to the functions of many organs in the body.
- Tryptophan is essential for the production of the B vitamin, niacin.
- It is used by the brain to produce serotonin, a necessary
neurotransmitter that transfers nerve impulses from one cell to
another. Consequently, tryptophan helps to depression and insomnia
and to stabilize moods

3. Metabolic Pathways
1 -Serotonin pathway.(1-3 %)
2 -kynurenine pathway.(90 %)
3 -Protein synthesis.
4 -Bacterial degradation. (4-6 %)
- Approximately 0.5% of ingested tryptophan is excreted unchanged in
urine.

4. 1- Serotonin Pathway
- Serotonin is one of the most important signaling molecules
within the Gut, where it plays a pivotal role in initiating secretory and
motor reflexes.
Tryptophan (Tryptophan hydroxylase ) → 5-hydroxytryptophan (AADC) → Serotonin
- Serotonin is produced by thr GIT Enterochromaffin cells &
stored in a large dense core vesicles and synaptic-like
microvesicles.

5. - Carcinoid Syndrome , It’s characterized by flushing, diarrhoea,
cramps, skin abnormalities, asthmatic wheezing and valvular heart
disease associated with elevated 5-HT levels , Due to the deficiency
of AADC (Aromatic amino acid decarboxylase).
- Melatonin has important effects in Circadian rhythms , sleep-
wake cycles and other phenomena showing circadian rhythm.
Consequently, it has been reported to have significant effects
on reproduction for some Animals.
- Serotonin (serotonin N-acetyltransferase) → N-acetylserotonin (Methylation) → Melatonin

6. 2- Kynurenine Pathway
- Important in Clearance of excess tryptophan & Regulation of plasma
tryptophan levels.
- Provides :
● Quinolinic acid -> immunoregulatory processes.
● Nicotinamide & Nicotinic acid -> precursors for niacin ( Vit. B3)
● Picolinic acid -> chelating agent.

7. 3- Protein synthesis , Tryptophan is a rate-limiting
amino acid in protein synthesis Due of its limited availability in
food.
4- Bacterial Degradation :
- Approximately 4–6% of tryptophan undergoes bacterial
degradation yielding indole, indican, and indole acid
derivates.
- The main bacterial breakdown product of tryptophan is
indole.