Biosynthesis of catecholamines is a vital subject for B. Pharm students, linking pharmacology and biochemistry to explain how dopamine, norepinephrine and epinephrine are synthesized, stored and degraded. This concise introduction highlights key enzymes (tyrosine hydroxylase, aromatic L‑amino acid decarboxylase, dopamine β‑hydroxylase, phenylethanolamine N‑methyltransferase), essential cofactors (tetrahydrobiopterin, pyridoxal phosphate, vitamin C, SAM, copper), regulation, cellular compartments, and clinical implications including enzyme deficiencies and drug interactions. Mastery of rate‑limiting steps, transporters (VMAT, DAT, NET), and metabolic enzymes (MAO, COMT) helps interpret drug actions, therapeutic strategies and adverse effects. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. Which enzyme catalyzes the rate-limiting step in catecholamine biosynthesis?
- Tyrosine hydroxylase
- Aromatic L‑amino acid decarboxylase
- Dopamine β‑hydroxylase
- Phenylethanolamine N‑methyltransferase
Correct Answer: Tyrosine hydroxylase
Q2. What is the immediate product formed after tyrosine is hydroxylated by tyrosine hydroxylase?
- L‑DOPA (L‑3,4‑dihydroxyphenylalanine)
- Dopamine
- Norepinephrine
- Epinephrine
Correct Answer: L‑DOPA (L‑3,4‑dihydroxyphenylalanine)
Q3. Which cofactor is essential for tyrosine hydroxylase activity?
- Tetrahydrobiopterin (BH4)
- Pyridoxal phosphate (PLP)
- Ascorbic acid (vitamin C)
- S‑adenosylmethionine (SAM)
Correct Answer: Tetrahydrobiopterin (BH4)
Q4. Aromatic L‑amino acid decarboxylase (AADC) requires which cofactor?
- Pyridoxal phosphate (vitamin B6)
- Tetrahydrobiopterin (BH4)
- Ascorbic acid (vitamin C)
- S‑adenosylmethionine (SAM)
Correct Answer: Pyridoxal phosphate (vitamin B6)
Q5. Which enzyme converts dopamine to norepinephrine inside secretory vesicles?
- Dopamine β‑hydroxylase (DBH)
- Phenylethanolamine N‑methyltransferase (PNMT)
- Monoamine oxidase (MAO)
- Catechol‑O‑methyltransferase (COMT)
Correct Answer: Dopamine β‑hydroxylase (DBH)
Q6. Which cofactor is required by dopamine β‑hydroxylase?
- Ascorbic acid (vitamin C) and copper
- Pyridoxal phosphate (vitamin B6)
- Tetrahydrobiopterin (BH4)
- S‑adenosylmethionine (SAM)
Correct Answer: Ascorbic acid (vitamin C) and copper
Q7. Where does conversion of norepinephrine to epinephrine primarily occur?
- Adrenal medulla cytosol via PNMT
- Synaptic vesicles via DBH
- Mitochondrial matrix via MAO
- Liver microsomes via COMT
Correct Answer: Adrenal medulla cytosol via PNMT
Q8. Which methyl donor is used by phenylethanolamine N‑methyltransferase (PNMT)?
- S‑adenosylmethionine (SAM)
- Tetrahydrobiopterin (BH4)
- Pyridoxal phosphate (PLP)
- Glutathione (GSH)
Correct Answer: S‑adenosylmethionine (SAM)
Q9. Which hormone upregulates PNMT expression in adrenal medulla?
- Cortisol
- Insulin
- Glucagon
- Thyroxine
Correct Answer: Cortisol
Q10. Vesicular monoamine transporters (VMAT) are responsible for:
- Uptake of catecholamines into synaptic vesicles
- Degradation of catecholamines by MAO
- Methylation of catecholamines by COMT
- Transport of catecholamines across the blood–brain barrier
Correct Answer: Uptake of catecholamines into synaptic vesicles
Q11. Which drug blocks VMAT and depletes catecholamine stores?
- Reserpine
- Carbidopa
- Metyrosine
- Entacapone
Correct Answer: Reserpine
Q12. Which transporter is primarily responsible for dopamine reuptake into presynaptic neurons?
- Dopamine transporter (DAT)
- Norepinephrine transporter (NET)
- Serotonin transporter (SERT)
- PMAT (plasma membrane monoamine transporter)
Correct Answer: Dopamine transporter (DAT)
Q13. Which enzymes are mainly involved in catecholamine degradation?
- Monoamine oxidase (MAO) and catechol‑O‑methyltransferase (COMT)
- Tyrosine hydroxylase and AADC
- Phenylalanine hydroxylase and tyrosinase
- Alkaline phosphatase and acetylcholinesterase
Correct Answer: Monoamine oxidase (MAO) and catechol‑O‑methyltransferase (COMT)
Q14. Which MAO isoform is more selective for dopamine metabolism in the brain?
- MAO‑B
- MAO‑A
- Both MAO‑A and MAO‑B equally
- Neither; COMT is primary
Correct Answer: MAO‑B
Q15. Which drug class inhibits COMT and is used adjunctively in Parkinson’s disease?
- Entacapone and tolcapone
- MAO inhibitors like selegiline
- VMAT inhibitors like reserpine
- Alpha‑blockers like phenoxybenzamine
Correct Answer: Entacapone and tolcapone
Q16. A deficiency of aromatic L‑amino acid decarboxylase (AADC) leads to:
- Reduced synthesis of dopamine and serotonin with movement disorders
- Excess norepinephrine and hypertension
- Increased epinephrine secretion from adrenal medulla
- Hyperactivity of tyrosine hydroxylase
Correct Answer: Reduced synthesis of dopamine and serotonin with movement disorders
Q17. Metyrosine (α‑methyltyrosine) is used clinically to treat pheochromocytoma by inhibiting:
- Tyrosine hydroxylase
- Dopamine β‑hydroxylase
- Phenylethanolamine N‑methyltransferase
- Monoamine oxidase
Correct Answer: Tyrosine hydroxylase
Q18. Carbidopa enhances central L‑DOPA therapy by:
- Inhibiting peripheral AADC to prevent peripheral conversion of L‑DOPA
- Blocking VMAT to increase synaptic dopamine
- Inhibiting COMT to reduce L‑DOPA methylation
- Stimulating PNMT to increase epinephrine
Correct Answer: Inhibiting peripheral AADC to prevent peripheral conversion of L‑DOPA
Q19. Which metabolic product is a major urinary end‑product of catecholamine breakdown?
- Vanillylmandelic acid (VMA)
- Homovanillic acid (HVA)
- Serotonin sulfate
- 3,4‑Dihydroxyphenylalanine (DOPA)
Correct Answer: Vanillylmandelic acid (VMA)
Q20. Homovanillic acid (HVA) is primarily a metabolite of which catecholamine?
- Dopamine
- Norepinephrine
- Epinephrine
- Serotonin
Correct Answer: Dopamine
Q21. Which cellular location houses monoamine oxidase (MAO)?
- Outer mitochondrial membrane
- Endoplasmic reticulum
- Synaptic vesicle membrane
- Golgi apparatus
Correct Answer: Outer mitochondrial membrane
Q22. Which cofactor participates in regeneration of tetrahydrobiopterin (BH4) indirectly affecting catecholamine synthesis?
- NADPH via dihydropteridine reductase
- FAD via monoamine oxidase
- ATP via kinases
- SAM via methyl transfer
Correct Answer: NADPH via dihydropteridine reductase
Q23. Dopamine formed in the cytosol is transported into vesicles by which VMAT isoform primarily found in neurons?
- VMAT2
- VMAT1
- VMAT3
- DAT
Correct Answer: VMAT2
Q24. Which genetic deficiency results in inability to synthesize norepinephrine and epinephrine, causing orthostatic hypotension?
- Dopamine β‑hydroxylase deficiency
- Tyrosine hydroxylase deficiency
- Aromatic L‑amino acid decarboxylase deficiency
- Monoamine oxidase deficiency
Correct Answer: Dopamine β‑hydroxylase deficiency
Q25. Which statement best describes the feedback regulation of tyrosine hydroxylase?
- End‑product inhibition by catecholamines and activation by phosphorylation
- Permanently active enzyme not regulated by catecholamines
- Inhibited by SAM and activated by COMT
- Activated only when VMAT is inhibited
Correct Answer: End‑product inhibition by catecholamines and activation by phosphorylation
Q26. In peripheral sympathetic neurons, most norepinephrine is released into the synaptic cleft and then cleared predominantly by:
- Reuptake via NET (norepinephrine transporter)
- Diffusion into bloodstream and liver metabolism
- Degradation by COMT in synaptic cleft only
- Endocytosis via dopamine transporter (DAT)
Correct Answer: Reuptake via NET (norepinephrine transporter)
Q27. Which pharmacological inhibitor reduces peripheral catecholamine synthesis and is useful preoperatively for pheochromocytoma?
- Metyrosine
- Reserpine
- Selegiline
- Entacapone
Correct Answer: Metyrosine
Q28. Which enzyme primarily methylates catecholamines at catechol OH groups?
- Catechol‑O‑methyltransferase (COMT)
- Monoamine oxidase (MAO)
- Phenylethanolamine N‑methyltransferase (PNMT)
- Dopamine β‑hydroxylase (DBH)
Correct Answer: Catechol‑O‑methyltransferase (COMT)
Q29. Which laboratory measurement is most sensitive for diagnosing pheochromocytoma?
- Plasma free metanephrines
- Plasma dopamine levels
- Urinary serotonin levels
- Serum epinephrine only
Correct Answer: Plasma free metanephrines
Q30. Phenylalanine hydroxylase deficiency (PKU) can secondarily affect catecholamine synthesis by reducing:
- Tyrosine availability
- SAM levels
- VMAT activity
- MAO expression
Correct Answer: Tyrosine availability
Q31. Which drug is a selective MAO‑B inhibitor used to enhance dopamine levels in Parkinson’s disease?
- Selegiline
- Tranylcypromine
- Phenelzine
- Moclobemide
Correct Answer: Selegiline
Q32. Which enzyme resides in chromaffin granules and catalyzes the conversion of dopamine to norepinephrine?
- Dopamine β‑hydroxylase (DBH)
- Tyrosine hydroxylase
- Catechol‑O‑methyltransferase (COMT)
- Monoamine oxidase (MAO)
Correct Answer: Dopamine β‑hydroxylase (DBH)
Q33. Which cofactor deficiency would most directly reduce AADC activity and dopamine synthesis?
- Vitamin B6 (pyridoxal phosphate)
- Vitamin C (ascorbic acid)
- Folate
- Vitamin K
Correct Answer: Vitamin B6 (pyridoxal phosphate)
Q34. Which of the following increases catecholamine synthesis by increasing tyrosine hydroxylase phosphorylation?
- Activation of protein kinase A by cAMP
- Inhibition of adenylate cyclase
- Activation of COMT
- Blocking calcium influx
Correct Answer: Activation of protein kinase A by cAMP
Q35. Which pathological condition is characterized by excessive catecholamine secretion from adrenal medulla tumors?
- Pheochromocytoma
- Parkinson’s disease
- Myasthenia gravis
- Multiple sclerosis
Correct Answer: Pheochromocytoma
Q36. Where is the enzyme PNMT primarily localized within adrenal medullary cells?
- Cytosol of chromaffin cells
- Secretory vesicle lumen
- Outer mitochondrial membrane
- Plasma membrane
Correct Answer: Cytosol of chromaffin cells
Q37. Which molecule acts as an electron donor for DBH‑mediated hydroxylation of dopamine?
- Ascorbate (vitamin C)
- NADH
- NADPH
- FADH2
Correct Answer: Ascorbate (vitamin C)
Q38. A drug that selectively blocks NET would most likely cause:
- Increased synaptic norepinephrine and enhanced sympathetic activity
- Decreased norepinephrine release and bradycardia
- Increased VMAT function and storage
- Direct inhibition of COMT
Correct Answer: Increased synaptic norepinephrine and enhanced sympathetic activity
Q39. Which metabolite is produced when catecholamines are first acted upon by MAO?
- Aldehyde intermediates that are further oxidized to acids
- Direct methylated metanephrines
- Conjugated glucuronides immediately excreted
- Active catecholamine sulfates
Correct Answer: Aldehyde intermediates that are further oxidized to acids
Q40. Which organ has high COMT activity contributing to peripheral catecholamine metabolism?
- Liver
- Brain
- Heart only
- Skeletal muscle exclusively
Correct Answer: Liver
Q41. Which statement about VMAT1 and VMAT2 is correct?
- VMAT1 is predominant in neuroendocrine cells; VMAT2 is predominant in neurons
- VMAT2 is only in the adrenal medulla; VMAT1 is only in the brain
- Both isoforms are identical in tissue distribution
- VMAT1 transports serotonin only
Correct Answer: VMAT1 is predominant in neuroendocrine cells; VMAT2 is predominant in neurons
Q42. Which therapeutic approach increases central dopamine without peripheral conversion to dopamine?
- L‑DOPA plus carbidopa
- Reserpine monotherapy
- Systemic MAO‑A inhibitors only
- VMAT inhibitors without L‑DOPA
Correct Answer: L‑DOPA plus carbidopa
Q43. COMT inhibitors used in Parkinson’s disease primarily affect which compartment of L‑DOPA metabolism?
- Peripheral methylation of L‑DOPA and dopamine
- MAO‑mediated oxidation in brain mitochondria
- Vesicular storage of dopamine
- PNMT‑mediated methylation to epinephrine
Correct Answer: Peripheral methylation of L‑DOPA and dopamine
Q44. Which of the following increases dopamine synthesis by providing substrate for tyrosine hydroxylase?
- Increased plasma tyrosine from dietary intake
- Decreased SAM levels
- Inhibition of AADC
- Low tetrahydrobiopterin levels
Correct Answer: Increased plasma tyrosine from dietary intake
Q45. Which enzyme deficiency presents in infancy with oculogyric crises, hypotonia and autonomic dysfunction due to dopamine shortage?
- Aromatic L‑amino acid decarboxylase (AADC) deficiency
- Catechol‑O‑methyltransferase (COMT) deficiency
- Phenylethanolamine N‑methyltransferase (PNMT) deficiency
- Monoamine oxidase (MAO) deficiency
Correct Answer: Aromatic L‑amino acid decarboxylase (AADC) deficiency
Q46. Which factor stimulates catecholamine release from adrenal chromaffin cells?
- Splanchnic nerve stimulation releasing acetylcholine
- High circulating insulin levels only
- Direct COMT activation
- Peripheral MAO inhibition exclusively
Correct Answer: Splanchnic nerve stimulation releasing acetylcholine
Q47. Which drug would decrease peripheral norepinephrine synthesis by inhibiting DBH?
- Disulfiram
- Carbidopa
- Selegiline
- Entacapone
Correct Answer: Disulfiram
Q48. Which metabolic pathway converts catecholamine aldehydes to their corresponding acids?
- Aldehyde dehydrogenase (ALDH) oxidation
- Aromatic L‑amino acid decarboxylation
- Direct conjugation by UDP‑glucuronosyltransferase
- Mitochondrial β‑oxidation
Correct Answer: Aldehyde dehydrogenase (ALDH) oxidation
Q49. During sustained stress, increased cortisol leads to which effect on catecholamine profile?
- Upregulation of PNMT and increased epinephrine synthesis
- Inhibition of tyrosine hydroxylase and decreased catecholamines
- Increased COMT activity lowering epinephrine exclusively
- Direct activation of MAO‑B to degrade dopamine
Correct Answer: Upregulation of PNMT and increased epinephrine synthesis
Q50. Which clinical laboratory value is more specific for increased catecholamine turnover in the brain rather than peripheral secretion?
- Cerebrospinal fluid homovanillic acid (HVA)
- Plasma free epinephrine
- Urinary vanillylmandelic acid (VMA)
- Serum norepinephrine alone
Correct Answer: Cerebrospinal fluid homovanillic acid (HVA)
I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.
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