Biosynthesis and Catabolism of Catecholamines

I. BIOSYNTHESIS

Precursor and Sites of Synthesis

• Adrenal medullary chromaffin cells (primarily EPI)
• Postganglionic sympathetic neurons (primarily NE)
• Specific CNS neurons (dopamine, NE, EPI)

Figure: Biosynthesis of catecholamines — Harper's Illustrated Biochemistry, 32nd Ed

Step-by-Step Pathway

Step 1: Tyrosine → L-DOPA

• End-product feedback inhibition: Free cytosolic catecholamines compete with TH for the BH₄ cofactor — this is the primary short-term regulatory mechanism
• Short-term activation: Depolarization activates protein kinases (PKA, PKC, CaM kinases) that phosphorylate TH, increasing its affinity for BH₄ and reducing sensitivity to end-product inhibition
• Long-term upregulation: Sustained sympathetic activity increases TH and dopamine-β-hydroxylase (DBH) mRNA via CREB phosphorylation and binding to CRE in the gene promoter, driving increased enzyme synthesis transported down the axon
• Competitive inhibition: α-Methyltyrosine blocks TH

Step 2: L-DOPA → Dopamine

Step 3: Dopamine → Norepinephrine

Step 4: Norepinephrine → Epinephrine

• Present in chromaffin cells of the adrenal medulla, organ of Zuckerkandl, and a subset of CNS neurons only
• Synthesis of PNMT is induced by glucocorticoids that reach the medulla via the intra-adrenal portal system, which provides a ~100-fold steroid concentration gradient over systemic blood
• Since PNMT is cytoplasmic, NE must exit its vesicles (via spontaneous leakage or post-synaptic reuptake) into the cytoplasm, be methylated to EPI, and then be transported back into vesicles for storage

Summary Table: Biosynthetic Enzymes

Storage and Release

• After synthesis, dopamine is transported from the cytosol into vesicles via VMAT2 (vesicular monoamine transporter 2) — an ATP-dependent secondary active transport driven by a proton (H⁺) gradient generated by vesicular ATPase (V-ATPase)
• Intravesicular catecholamine concentration is ~0.5 M (approximately 100× cytosolic), stored complexed with ATP and chromogranins
• An action potential triggers Ca²⁺ influx → vesicle fusion with the plasma membrane → exocytosis of catecholamines, ATP, chromogranins, and DBH into the extraneuronal space
• VMAT-1 is expressed mainly in extraneuronal chromaffin cells; VMAT-2 is the neuronal form

II. CATABOLISM (Inactivation and Degradation)

A. Reuptake (Primary mechanism for endogenous catecholamines)

• Uptake-1 (neuronal): Active, stereospecific, energy-requiring reuptake into the presynaptic terminal via NET (norepinephrine transporter) or DAT (dopamine transporter) — the predominant pathway for NE and dopamine at the synapse
  • Blocked by tricyclic antidepressants and cocaine → elevated synaptic catecholamines
• Uptake-2 (extraneuronal): Minor pathway — NE enters effector cells where it is metabolized by MAO and COMT
• Reuptake is less relevant for exogenous catecholamines (administered pharmacologically), whose inactivation is predominantly hepatic/renal metabolism — explaining why exogenous catecholamines have longer action than endogenous ones

B. Enzymatic Degradation (Two key enzymes)

Figure: Inactivation of catecholamines — Basic Medical Biochemistry, 6th Ed. MAO and COMT act in either order; the final product is vanillylmandelic acid (VMA).

1. Monoamine Oxidase (MAO)

• Location: Outer mitochondrial membrane of many cells, including the presynaptic terminal
• Reaction: Oxidative deamination — oxidizes the carbon bearing the amino group to an aldehyde, releasing NH₄⁺
• Two isoforms:
  • MAO-A: Preferentially deaminates NE and serotonin → targeted by MAO-A inhibitors (e.g., clorgyline, moclobemide)
  • MAO-B: Acts on a wide spectrum of phenylethylamines (including dopamine) → inhibited by selegiline (deprenyl), used in Parkinson's disease
• In the presynaptic terminal, MAO inactivates catecholamines that are not protected within storage vesicles — thus drugs that deplete vesicles (e.g., reserpine) indirectly increase MAO-mediated degradation

2. Catechol-O-Methyltransferase (COMT)

• Location: Cytoplasm of many cells — especially liver, kidney, erythrocytes, glial cells, and extraneuronal tissues; acts on catecholamines that have diffused away from the synapse
• Reaction: O-methylation — transfers a methyl group from SAM to one of the hydroxyl groups on the catechol ring (the 3-OH group), producing methoxyderivatives
  • Epinephrine → Metanephrine
  • Norepinephrine → Normetanephrine
  • Dopamine → 3-Methoxytyramine
• Because this reaction requires SAM, it is indirectly dependent on folate and vitamin B₁₂

C. Diffusion

• A small fraction of catecholamines escapes reuptake and metabolic degradation, diffuses into the circulation, and is metabolized by the liver and kidney (the predominant route for exogenous catecholamines)

Final Degradation Products

• MAO first → aldehyde intermediate → oxidation → DHMA → COMT → VMA
• COMT first → normetanephrine/metanephrine → MAO → aldehyde → oxidation → VMA
• Both routes converge on VMA

Dopamine-Specific Catabolism

• MAO → DOPAC (3,4-dihydroxyphenylacetic acid)
• COMT → 3-methoxytyramine
• Sequential MAO + COMT → Homovanillic acid (HVA)

III. CLINICAL CORRELATES

Biosynthesis and Catabolism of Catecholamines

Introduction

BIOSYNTHESIS (5 marks)

Precursor

Sites

• Adrenal medullary chromaffin cells (mainly EPI)
• Postganglionic sympathetic neurons (mainly NE)
• Specific CNS neurons (dopamine, NE, EPI)

Pathway (4 steps)

• Enzyme: Tyrosine hydroxylase (TH)
• Cofactor: Tetrahydrobiopterin (BH₄)
• Location: Cytosol
• This is the rate-limiting step
• TH is regulated by:
  • Feedback inhibition — free catecholamines compete for the BH₄ cofactor
  • Phosphorylation by PKA, PKC, CaM kinases upon depolarization → increased BH₄ affinity

• Enzyme: DOPA decarboxylase / AADC (aromatic amino acid decarboxylase)
• Cofactor: Pyridoxal phosphate (PLP, vitamin B₆)
• Location: Cytosol
• Dopaminergic neurons stop here

• Enzyme: Dopamine β-hydroxylase (DBH)
• Cofactors: Ascorbate (vitamin C), Cu²⁺
• Location: Inside storage vesicles
• Noradrenergic neurons stop here

• Enzyme: Phenylethanolamine-N-methyltransferase (PNMT)
• Cofactor: SAM (S-adenosylmethionine)
• Location: Cytoplasm of adrenal medullary cells
• PNMT is induced by glucocorticoids via the intra-adrenal portal system
• Occurs only in adrenal medulla, organ of Zuckerkandl, and select CNS neurons

Summary Table

CATABOLISM (5 marks)

1. Reuptake (Primary)

• Catecholamines are actively transported back into the presynaptic terminal (Uptake-1) via NET (norepinephrine transporter) or DAT (dopamine transporter)
• Blocked by tricyclic antidepressants and cocaine
• Minor extraneuronal uptake (Uptake-2) occurs into effector cells

2. Enzymatic Degradation — MAO and COMT

• Location: Outer mitochondrial membrane
• Reaction: Oxidative deamination → converts the amine to an aldehyde + NH₄⁺
• Two isoforms:
  • MAO-A — preferentially deaminates NE and serotonin
  • MAO-B — acts on dopamine and phenylethylamines
• Inactivates unprotected (extra-vesicular) cytosolic catecholamines

• Location: Cytoplasm of liver, kidney, erythrocytes, glia, extraneuronal cells
• Reaction: O-methylation using SAM — transfers a methyl group to the catechol ring
• NE → Normetanephrine; EPI → Metanephrine; Dopamine → 3-Methoxytyramine

3. Diffusion

• Small amounts escape into circulation and are metabolized by liver/kidney
• This is the primary route for exogenous catecholamines (explaining their longer pharmacological duration)

Final Urinary Metabolites

CLINICAL APPLICATIONS

Biosynthesis and Catabolism of Catecholamines

BIOSYNTHESIS

Key: TH is feedback-inhibited by catecholamines. PNMT is induced by glucocorticoids (adrenal medulla only).

CATABOLISM

• MAO (mitochondria) → oxidative deamination → removes NH₄⁺
• COMT (cytoplasm) → O-methylation using SAM

Final Products

3 Mechanisms of Termination

1. Reuptake into presynaptic terminal (main route for endogenous)
2. MAO + COMT degradation
3. Diffusion → hepatic/renal metabolism (main route for exogenous)

Clinical: ↑ Urine VMA + metanephrines → Pheochromocytoma; MAO inhibitors → antidepressants; COMT inhibitors (entacapone) → Parkinson's