2. Tryptophan Metabolism & its associated disorders
Hartnup disease reflects impaired intestinal and renal transport of tryptophan and other neutral amino acids. defect limits tryptophan availability for niacin biosynthesis and accounts for the pellagra-like signs and symptoms
Dr. Ketki K Assistant Professor Dept of Biochemistry
HIMS Varanasi

3. Content Substances produced from Tryptophan
Major Catabolic Pathway of Tryptophan
Niacin Synthesis
Synthesis & degradation of Serotonin
Function of serotonin & effect of drug on Serotonin
Carcinoid tumor
Synthesis of Melatonin
Function of Melatonin
Hartnup’s disease

4. Tryptophan Aromatic aa Heterocyclic aa Indole ring Neutral Essential
Polar
Both glucogenic & ketogenic

5. Substances produced from tryptophan
4) Niacin/NAD+
5) Serotonin
6) Melatonin
7) Indolacetic acid
8) Indican
1) Alanine
2) Acetoacetyl Co A
3) Formyl Group

6. Major Catabolic Pathway of Tryptophan

7. Major catabolic pathway of Tryptophan
Step1 :
Tryptophan pyrrolase or oxygenase cleaves the five-membered ring of the indole nucleus to produce N - formyl-kynurenine.
Tryptophan pyrrolase is Hemoprotein
The enzyme is inducible by corticosteroids.

8. Total 11 carbon atoms of tryptophan are catabolized as
Formyl group (1C which enters the one carbon pool),
Alanine (3C, entering the glucose pathway)
Acetoacetate (4C, going to ketogenic pathway).
Tryptophan is both glucogenic & ketogenic.
The remaining 3 carbons are removed as 3 CO2 molecules.

9. Step 2:
Formamidase hydrolyses N-formyl-kynurenine, so Iiberates formate & kynurenine
Kynurenine formed in this reaction is a branch point with different fates

10. Step 3:
In the prominent pathway, kynurenine undergoes NADPH-dependent hydroxylation by kynurenine hydroxylase to give 3-OH-kynurenine
Kynurenine hydroxylase is inhibited by estrogen
Women are more susceptible to Pellagra

11. Step 4:
Kynureninase, a PLP - dependent enzyme acts on the 3-OH-kynurenine & splits off alanine (glucogenic pathway) & 3-hydroxy-anthranilic acid (which is used for NAD+ pathway)
Kynureninase is sensitive to vitamin B6 deficiency.
Due to the lack of PLP, kynureninase reaction is blocked & 3-OH-kynurenine/Kynurenine accumulates, which is diverted to form xanthurenic acid.

12. EIevated excretion of xanthurenic acid serves as an indication of B6 deficiency.
Administration of isoniazid, an antituberculosis drug-induces B6 deficiency & results in xanthurenic acid excretion in urine.
Defects in the activity of kynureninase (in B6 deficiency) cause reduced synthesis of NAD+ from tryptophan & manifestations of pellagra.

13. Step 5: 3-Hydroxyanthranilate is cleaved by an oxidase (Fe2+ dependent) to form an unstable intermediate, Aminocarboxy muconaldehyde
Step 6: Which undergoes decarboxylation to form amino muconate aldehyde
Step 7: Which undergoes deamination to form keto adipic acid
Step 8: Which undergoes decarboxylation to form Acetoacetyl CoA( Ketogenic)

14. Conversion of tryptophan to indole acetate
Tryptophan undergoes transamination & deamination to produce indolepyruvate
Which is decarboxylated to indoleacetate
Which is excreted in urine.

15. Conversion of tryptophan to indole acetate
Transdeamination
NH3
Indole 3-pyruvate
Decarboxylation
CO2
Indole acetate
Urine

17. Synthesis of niacin from tryptophan
QPRT

18. Nicotinic acid Pathway of Tryptophan
97% molecules of tryptophan: goes for major metabolic pathway, 3% : diverted at 3-OH-anthranilic acid to form NAD+
QPRT : rate limiting enzyme
3-OH-anthranilic acid to Quinolinic acid
Quinolinic acid undergo decarboxylation to form Niacin/nicotinic acid

19. Nicotinic acid to nicotinate mononucleotide(NMN) by the enzyme quinolinate phosphoribosyl transferase (QPRT) in presence of PRPP to PPi
From nicotinate mononucleotide to desamido NAD+ & NAD+ are synthesized.

20. Serotonin & melatonin synthesis

21. Serotonin pathway
Serotonin or 5-hydroxytryptamine (5HT) is a neurotransmitter, synthesized from tryptophan.
About 1% of the tryptophan is converted to serotonin.
The production of 5HT occurs in the target tissues.

22. Synthesis of serotonin
Site: Brain, mast cells, platelets, gastrointestinal tract mucosa & intestinal cells.
Step 1:
Tryptophan is first hydroxylated at 5th carbon by tryptophan hydroxylase to form 5-hydroxy tryptophan(5-HT)
It requires tetrahydrobiopterin as a cofactor.

23. Step 2:
5-Hydroxytryptophan is decarboxylated by PLP-dependent decarboxylase to give 5-hydroxytryptamine (serotonin)
Platelets contain high concentration of 5HT.

24. Degradation of serotonin
Monoamine oxidase (MAO) degrades serotonin to 5-hydroxyindoleacetic acid (5-HIAA) which is excreted in urine.
Small portion of serotonin is conjugated with sulfate or with glucuronic acid & excreted through urine.

25. Functions of serotonin
Neurotransmitter, Involved in the regulation of cerebral activity (excitation)
Powerful vasoconstrictor & results in smooth muscle contraction in bronchioles & arterioles
Controls the behavioural patterns, sleep,mood,appetite, blood pressure & body temperature
Necessary for the motility of GIT
Platelets (platelet aggregation)

26. Its deficiency causes depression (That means ,it acts as an anti depressant)
Level decreased in depressive psychosis
Carbohydrate rich meal induce sleep,protein rich meal causes alertness, explain:
Protein rich meal→ traffic jam of aa in brain → tryptophan ,bulkiest amino acid , taken up very slowly
Carbohydrate rich meal → increased insulin release → decrease amino acid in blood → tryptophan easily enters in brain → it induces sleep

27. Effect of drugs on serotonin
Mechanism of action
Use
iproniazid
inhibits MAO & elevates serotonin levels
psychic stimulant
Reserpine
increases the degradation of serotonin
depressant drug
LSD
(Lysergic acid diethylamide)
competes with serotonin
depressant
SSRIs(selective serotonin reuptake inhibitor)/sertraline
Increases serotonin in brain & synaptic site
antidepressant

28. Carcinoid tumor/ Malignant carcinoid syndrome
c/a Argentaffinomas (Tumor of argentaffin cells of gastrointestinal tract- mainly intestine & appendix) ↓
uncontrolled growth of these cells
↑ ↑ Serotonin
C/F: Flushing,sweating,intermittent
diarrhoea,fluctuating hypertension

29. In carcinoid syndrome, very high amount (up to 60%) of tryptophan is diverted for serotonin production ↓
impairs the synthesis of NAD+
Niacin deficiency
C/F: Pellagra
(dermatitis,diarrhoea,dementia,death)

30. Diagnosis
The excretion of 5-hydroxyindole acetatic acid in urine is tremendously elevated (upto 500 mg/day against normal <5 mg/day) in carcinoid tumor [of gut, bronchus, oat cell ca of lung]
The estimation of 5 HIAA in urine is used for the diagnosis of this disorder

31. Melatonin Melatonin is a hormone. Synthesized by the pineal gland.
Serotonin-produced from tryptophan-is acted upon by serotonin N-acetylase to give N-acetylserotonin.
Serotonin N-acetylase is a rate limiting enzyme.

32. N-acetylserotonin undergoes methylation, S-adenosylmethionine being the methyl group donor to produce melatonin
The synthesis & secretion of melatonin from pineal gland is controlled by light.

33. Functions of melatonin
Melatonin is involved in circadian rhythms or diurnal variations (24 hr cyclic process) of the body.
It plays a significant role in sleep & wake process.
Melatonin inhibits the production of melanocyte stimulating hormone (MSH) & adrenocorticotropic hormone (ACTH)
Melatonin also performs a neurotransmitter function

34. Hartnup’s disease It is an hereditary disorder/Autosomal recessive
Cause: due to an impairment in the absorption and/or transport of tryptophan & other neutral amino acids from the intestine, renal tubules & probably brain.
Leads to deficiency of niacin & serotonin
Symptoms – pellagra like symptoms ,dermatitis, ataxia, mental retardation.

35. Diagnosis:
1) Low plasma levels of tryptophan & other neutral amino acids & their elevated urinary excretion
(aminoaciduria)
2) Increased urinary output of indoleacetic acid & indolepyruvic acid: detected by Obermeyer test
(Major excretory products of tryptophan comes in urine)

36. Treatment:
High protein diet with supplementation of niacin
Minimum exposure to sunlight

37. Question:
Explain why tryptophan deficiency leads to niacin deficiency?
Explain how carbohydrate rich meal induces sleep?/why do a person feels sleepy after eating heavy meal(rice)?