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

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

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

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

Major Catabolic Pathway of Tryptophan

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.

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.

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

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

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.

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.

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)

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

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

Synthesis of niacin from tryptophan QPRT

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

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.

Serotonin & melatonin synthesis

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.

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.

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

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.

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)

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

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

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

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)

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

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.

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.

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

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.

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)

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

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)?