Phenylalanin: the pathway in the body

Phenylalanin is part of the pathway “Dopamine, noradrenaline, adrenaline”. This page shows the whole pathway; the station of Phenylalanin is highlighted.

Where this laboratory value sits: Phenylalanine — amino acid. Phenylalanine is an amino acid that the body cannot make itself. It comes from dietary protein and from the breakdown of the body's own proteins. Source 2

In brief

Dopamine, noradrenaline and adrenaline are catecholamines formed from the amino acids phenylalanine and tyrosine. They carry signals in the nervous system; adrenaline also acts as a hormone from the adrenal medulla.

The pathway step by step

1. Phenylalanine → Tyrosine PAH · BH4, Iron Phenylalanine hydroxylase (PAH) attaches an OH group to phenylalanine, forming tyrosine. The enzyme needs iron and the cofactor tetrahydrobiopterin (BH4). Source 2, 3, 1
2. Tyrosine → L-DOPA Tyrosine hydroxylase · BH4, Iron Tyrosine hydroxylase turns tyrosine into L-DOPA; it too needs iron and BH4. This is the slowest step of the pathway – here the cell regulates how much is formed. Source 1, 3
3. L-DOPA → Dopamine AADC · Vitamin B6 (PLP) The enzyme AADC (decarboxylase) removes a carboxyl group from L-DOPA, forming dopamine. It needs pyridoxal phosphate, the active form of vitamin B6. Source 4, 1
4. Dopamine → Noradrenaline Dopamine β-hydroxylase · Vitamin C, Copper In storage vesicles, dopamine β-hydroxylase attaches an OH group to dopamine. This copper-containing enzyme uses vitamin C (ascorbate) as an electron donor. Source 1, 5
5. Noradrenaline → Adrenaline PNMT · SAM In the adrenal medulla, PNMT transfers a methyl group from SAM (S-adenosylmethionine) to noradrenaline. The formation of this enzyme is regulated mainly by cortisol. Source 6, 1
6. DOPAC → Homovanillic acid COMT · SAM The enzyme COMT (catechol-O-methyltransferase) attaches a methyl group to DOPAC. This forms homovanillic acid, which is excreted in urine. Source 7
7. DHPG → MHPG COMT · SAM Outside the nerve cells, COMT attaches a methyl group to DHPG. This forms MHPG. Source 7
8. MHPG → Vanillylmandelic acid ADH, ALDH · NAD⁺ In the liver, alcohol and aldehyde dehydrogenases convert MHPG into vanillylmandelic acid. The metanephrines also lead into this substance via MAO. Source 7
9. Metanephrines → Vanillylmandelic acid MAO In the liver, alcohol and aldehyde dehydrogenases convert MHPG into vanillylmandelic acid. The metanephrines also lead into this substance via MAO. Source 7

Cofactors in this pathway

• Tetrahydrobiopterin (BH4) — Cofactor of phenylalanine and tyrosine hydroxylase when oxygen is inserted Source 3
• Iron — Sits in the active site of phenylalanine and tyrosine hydroxylase Source 1, 2In the ORY catalogue as a laboratory value: Eisen
• Vitamin B6 — As pyridoxal phosphate, cofactor of AADC, which converts L-DOPA into dopamine Source 4In the ORY catalogue as a laboratory value: Vitamin B6
• Copper — Metal at the centre of dopamine β-hydroxylase, which converts dopamine into noradrenaline Source 5In the ORY catalogue as a laboratory value: Kupfer (Cu)
• Vitamin C — Electron donor of dopamine β-hydroxylase inside the storage vesicle Source 5In the ORY catalogue as a laboratory value: Vitamin C (Ascorbinsäure)
• SAM (from methionine) — Methyl group donor of PNMT and of COMT Source 6, 7In the ORY catalogue as a laboratory value: Methionin
• Magnesium — Ion in the active site of COMT when catecholamines are converted Source 9In the ORY catalogue as a laboratory value: Magnesium
• NAD⁺ — Accepts hydrogen at the aldehyde dehydrogenases of the breakdown route Source 7In the ORY catalogue as a laboratory value: NAD⁺ (Nicotinamidadenindinukleotid)

What acts on this pathway

• Carbidopa — Carbidopa inhibits the decarboxylase outside the brain, so less L-DOPA is converted to dopamine there. Carbidopa does not enter the brain. The product information describes this mechanism. Source 8

Sources

1. Daubner SC, Le T, Wang S. Tyrosine hydroxylase and regulation of dopamine synthesis. Arch Biochem Biophys 2011 · PubMed 21176768
2. Flydal MI, Martinez A. Phenylalanine hydroxylase: function, structure, and regulation. IUBMB Life 2013 · PubMed 23457044
3. Werner ER, Blau N, Thöny B. Tetrahydrobiopterin: biochemistry and pathophysiology. Biochem J 2011 · PubMed 21867484
4. Paiardini A, Giardina G, Rossignoli G et al. New Insights Emerging from Recent Investigations on Human Group II Pyridoxal 5'-Phosphate Decarboxylases. Curr Med Chem 2017 · PubMed 27881066
5. Prigge ST, Mains RE, Eipper BA et al. New insights into copper monooxygenases and peptide amidation: structure, mechanism and function. Cell Mol Life Sci 2000 · PubMed 11028916
6. Wong DL. Epinephrine biosynthesis: hormonal and neural control during stress. Cell Mol Neurobiol 2006 · PubMed 16645894
7. Eisenhofer G, Kopin IJ, Goldstein DS. Catecholamine metabolism: a contemporary view with implications for physiology and medicine. Pharmacol Rev 2004 · PubMed 15317907
8. US prescribing information Sinemet (Carbidopa/Levodopa, DailyMed), section Clinical Pharmacology, Pharmacodynamics · Prescribing information
9. Ma Z, Liu H, Wu B. Structure-based drug design of catechol-O-methyltransferase inhibitors for CNS disorders. Br J Clin Pharmacol 2014 · PubMed 23713800

Whole pathway: Dopamine, noradrenaline, adrenaline

Related pathways

• Vitamin B6 — l-dopa
• Carnitine — Eisen, Vitamin B6
• Estradiol — Eisen, Methionin
• Estrogen breakdown — Eisen, Methionin
• Spermidine — Eisen, Vitamin B6

As of 2026-09-16. Draft, written by Claude to schema v2; sources checked in PubMed; expert approval pending
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