Catabolism of catecholamines MCQs With Answer provides B.Pharm students a focused, SEO-rich introduction to metabolic pathways that degrade dopamine, norepinephrine and epinephrine. This concise guide emphasizes key enzymes (MAO, COMT), cofactors (SAM), transporters (VMAT, NET, DAT), major metabolites (VMA, metanephrines, DOPAC, HVA), and clinical links such as pheochromocytoma testing and MAO/COMT inhibitor effects. Ideal for pharmacology and biochemistry revision, these questions deepen understanding of biochemical steps, tissue localization, drug interactions, and therapeutic implications. The content balances mechanistic detail with exam-style practice to build mastery for B.Pharm courses. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. What is the primary enzyme family responsible for oxidative deamination of catecholamines?
- Monoamine oxidase (MAO)
- Catechol-O-methyltransferase (COMT)
- Aldehyde dehydrogenase (ALDH)
- Monoamine transporter (MAT)
Correct Answer: Monoamine oxidase (MAO)
Q2. Which enzyme catalyzes methylation of catechol hydroxyl groups using S-adenosylmethionine (SAM)?
- Monoamine oxidase (MAO)
- Catechol-O-methyltransferase (COMT)
- Tyrosine hydroxylase
- Vesicular monoamine transporter (VMAT)
Correct Answer: Catechol-O-methyltransferase (COMT)
Q3. Dopamine is first metabolized by MAO to form which aldehyde intermediate?
- 3,4-Dihydroxyphenylacetaldehyde (DOPAL)
- 3-Methoxytyramine (3-MT)
- Vanillylmandelic acid (VMA)
- 3,4-Dihydroxyphenylacetic acid (DOPAC)
Correct Answer: 3,4-Dihydroxyphenylacetaldehyde (DOPAL)
Q4. The major urinary end product of norepinephrine and epinephrine metabolism is:
- Homovanillic acid (HVA)
- Vanillylmandelic acid (VMA)
- 3-Methoxytyramine (3-MT)
- Normetanephrine
Correct Answer: Vanillylmandelic acid (VMA)
Q5. Which enzyme converts aldehyde intermediates (e.g., DOPAL) to their corresponding acids (e.g., DOPAC)?
- Aldehyde dehydrogenase (ALDH)
- Aldehyde reductase
- COMT
- Phenylethanolamine N-methyltransferase (PNMT)
Correct Answer: Aldehyde dehydrogenase (ALDH)
Q6. COMT inhibitors such as entacapone are used clinically to:
- Increase peripheral methylation of catecholamines
- Prevent degradation of L-DOPA in Parkinson’s disease
- Block MAO-A selectively
- Enhance VMAT function
Correct Answer: Prevent degradation of L-DOPA in Parkinson’s disease
Q7. Which isoform of MAO is more selective for serotonin and norepinephrine?
- MAO-A
- MAO-B
- Both equally
- Neither; MAO-C is selective
Correct Answer: MAO-A
Q8. Selegiline is an irreversible inhibitor of which MAO isoform at typical therapeutic doses?
- MAO-A
- MAO-B
- Both MAO-A and MAO-B equally
- COMT
Correct Answer: MAO-B
Q9. In chromaffin cells of the adrenal medulla, epinephrine is primarily converted to metanephrine by which enzyme before release?
- Catechol-O-methyltransferase (COMT)
- Monoamine oxidase (MAO)
- Aldehyde dehydrogenase (ALDH)
- Phenylethanolamine N-methyltransferase (PNMT)
Correct Answer: Catechol-O-methyltransferase (COMT)
Q10. Which transporter is responsible for reuptake of norepinephrine into presynaptic terminals?
- Norepinephrine transporter (NET)
- Vesicular monoamine transporter (VMAT)
- Serotonin transporter (SERT)
- Monoamine oxidase (MAO)
Correct Answer: Norepinephrine transporter (NET)
Q11. Vesicular monoamine transporter (VMAT) function is inhibited by which drug, leading to catecholamine depletion?
- Reserpine
- Entacapone
- Selegiline
- Clonidine
Correct Answer: Reserpine
Q12. Which metabolite is primarily used in plasma testing for pheochromocytoma due to its stability?
- Plasma free metanephrines
- Plasma dopamine
- Urinary epinephrine
- Plasma MAO levels
Correct Answer: Plasma free metanephrines
Q13. COMT exists in which cellular forms?
- Soluble (S-COMT) and membrane-bound (MB-COMT)
- Nuclear and mitochondrial
- Extracellular and lysosomal
- Only as a single cytosolic form
Correct Answer: Soluble (S-COMT) and membrane-bound (MB-COMT)
Q14. Which cofactor provides the methyl group for COMT-catalyzed reactions?
- S-adenosylmethionine (SAM)
- NADH
- FAD
- ATP
Correct Answer: S-adenosylmethionine (SAM)
Q15. Homovanillic acid (HVA) is mainly a metabolite of which catecholamine?
- Dopamine
- Norepinephrine
- Epinephrine
- Serotonin
Correct Answer: Dopamine
Q16. In the brain, which enzyme contributes significantly to dopamine breakdown and is targeted in Parkinson’s therapy?
- COMT and MAO-B (both relevant)
- PNMT only
- ALDH only
- NET only
Correct Answer: COMT and MAO-B (both relevant)
Q17. Which drug class can cause a hypertensive “cheese reaction” by interacting with dietary tyramine?
- Irreversible MAO inhibitors
- COMT inhibitors
- Alpha-2 agonists
- VMAT enhancers
Correct Answer: Irreversible MAO inhibitors
Q18. Phenylethanolamine N-methyltransferase (PNMT) converts norepinephrine to epinephrine using which cofactor?
- S-adenosylmethionine (SAM)
- FAD
- NAD+
- Biotin
Correct Answer: S-adenosylmethionine (SAM)
Q19. Which organ is the major site for peripheral catecholamine metabolism?
- Liver
- Kidney only
- Adrenal medulla only
- Pancreas
Correct Answer: Liver
Q20. Which metabolite results from COMT acting on norepinephrine or epinephrine before MAO action in adrenal medulla?
- Metanephrine and normetanephrine
- DOPAC
- HVA
- Serotonin
Correct Answer: Metanephrine and normetanephrine
Q21. Which MAO isoform is more abundant in platelets and commonly measured indirectly in studies?
- MAO-B
- MAO-A
- MAO-C
- Both are absent in platelets
Correct Answer: MAO-B
Q22. 3,4-Dihydroxyphenylacetic acid (DOPAC) is mainly produced from dopamine by which sequence?
- Dopamine → MAO → DOPAL → ALDH → DOPAC
- Dopamine → COMT → DOPAC directly
- Dopamine → PNMT → DOPAC
- Dopamine → NET → DOPAC
Correct Answer: Dopamine → MAO → DOPAL → ALDH → DOPAC
Q23. Which analytical test is most sensitive for detecting catecholamine-secreting tumors?
- Plasma free metanephrines
- Serum epinephrine level
- Urinary creatinine only
- Platelet MAO activity
Correct Answer: Plasma free metanephrines
Q24. Tolcapone differs from entacapone because it:
- Crosses the blood–brain barrier and inhibits central COMT
- Is selective for MAO-A
- Enhances VMAT activity
- Irreversibly inhibits MAO-B
Correct Answer: Crosses the blood–brain barrier and inhibits central COMT
Q25. Which reaction is catalyzed by MAO on primary amine catecholamines?
- Oxidative deamination to form an aldehyde
- Methylation of hydroxyl groups
- Reduction to alcohols
- Conjugation with glucuronic acid
Correct Answer: Oxidative deamination to form an aldehyde
Q26. Which drug increases synaptic norepinephrine by blocking NET-mediated reuptake?
- Cocaine
- Entacapone
- Selegiline
- Tolcapone
Correct Answer: Cocaine
Q27. After deamination by MAO, the aldehyde product is typically converted to an acid by ALDH. Which acid is derived from dopamine?
- DOPAC (3,4-dihydroxyphenylacetic acid)
- VMA
- Normetanephrine
- Serotonin
Correct Answer: DOPAC (3,4-dihydroxyphenylacetic acid)
Q28. Which pharmacological effect results from COMT inhibition in Parkinson’s patients receiving L-DOPA?
- Prolonged L-DOPA plasma half-life and enhanced central availability
- Increased peripheral MAO activity
- Decreased dopamine synthesis
- Immediate hypertensive crisis
Correct Answer: Prolonged L-DOPA plasma half-life and enhanced central availability
Q29. Which metabolite is primarily measured in cerebrospinal fluid as a marker of central dopamine turnover?
- Homovanillic acid (HVA)
- Vanillylmandelic acid (VMA)
- Metanephrine
- Normetanephrine
Correct Answer: Homovanillic acid (HVA)
Q30. MAO inhibitors used as antidepressants increase levels of catecholamines by:
- Preventing oxidative deamination of monoamines
- Enhancing COMT activity
- Blocking NET transporters directly
- Stimulating VMAT uptake
Correct Answer: Preventing oxidative deamination of monoamines
Q31. Which enzyme predominates in converting dopamine to 3-methoxytyramine (3-MT) via methylation?
- COMT
- MAO
- ALDH
- PNMT
Correct Answer: COMT
Q32. Which statement is true regarding intracellular localization of COMT?
- Both soluble cytosolic and membrane-bound forms exist
- COMT is exclusively mitochondrial
- COMT is secreted extracellularly
- COMT is only present in synaptic vesicles
Correct Answer: Both soluble cytosolic and membrane-bound forms exist
Q33. A patient on MAO inhibitors ingests tyramine-rich foods. The hypertensive crisis occurs because:
- Tyramine is normally degraded by MAO; inhibition leads to excess NE release
- Tyramine directly blocks COMT
- Tyramine increases ALDH activity
- Tyramine inhibits NET causing catecholamine depletion
Correct Answer: Tyramine is normally degraded by MAO; inhibition leads to excess NE release
Q34. Which metabolite results when dopamine is methylated by COMT before MAO action?
- 3-Methoxytyramine (3-MT)
- Vanillylmandelic acid (VMA)
- Normetanephrine
- Serotonin
Correct Answer: 3-Methoxytyramine (3-MT)
Q35. Which drug is a selective MAO-A inhibitor used in depression and affects norepinephrine breakdown?
- Moclobemide
- Selegiline (at low doses)
- Tolcapone
- Entacapone
Correct Answer: Moclobemide
Q36. Which measurement is least useful for diagnosing catecholamine-secreting tumors?
- Platelet MAO activity
- Urinary VMA
- Plasma free metanephrines
- 24-hour urine catecholamines
Correct Answer: Platelet MAO activity
Q37. Which metabolic pathway predominates for synaptic termination of catecholamine action?
- Reuptake via transporters followed by intracellular metabolism
- Direct extracellular COMT degradation only
- Rapid renal excretion unchanged
- Conversion to peptides by peptidases
Correct Answer: Reuptake via transporters followed by intracellular metabolism
Q38. Which cofactor is required by MAO as a prosthetic group for catalytic activity?
- FAD (flavin adenine dinucleotide)
- NADPH
- S-adenosylmethionine (SAM)
- Biotin
Correct Answer: FAD (flavin adenine dinucleotide)
Q39. In peripheral tissues, COMT methylation often precedes MAO action in catecholamine catabolism inside which cellular compartment?
- Cytosol
- Mitochondrial matrix
- Lysosome
- Endoplasmic reticulum lumen
Correct Answer: Cytosol
Q40. Which condition can result from chronic MAO inhibition due to altered catecholamine catabolism?
- Orthostatic hypotension and hypertensive crises with dietary amines
- Excessive urinary glucose
- Increased insulin release
- Renal tubular acidosis
Correct Answer: Orthostatic hypotension and hypertensive crises with dietary amines
Q41. Which enzyme in the adrenal medulla converts norepinephrine to epinephrine?
- Phenylethanolamine N-methyltransferase (PNMT)
- COMT
- MAO
- ALDH
Correct Answer: Phenylethanolamine N-methyltransferase (PNMT)
Q42. Which urinary metabolite is increased in patients with dopamine-secreting tumors?
- Homovanillic acid (HVA)
- Vanillylmandelic acid (VMA)
- Normetanephrine
- Metanephrine
Correct Answer: Homovanillic acid (HVA)
Q43. NET inhibitors used therapeutically would be expected to:
- Increase synaptic norepinephrine concentrations
- Increase COMT activity
- Decrease catecholamine synthesis
- Enhance MAO-mediated breakdown
Correct Answer: Increase synaptic norepinephrine concentrations
Q44. Which statement about COMT’s substrate specificity is correct?
- COMT methylates catecholamines and other catecholic compounds
- COMT only acts on aliphatic amines
- COMT exclusively methylates peptides
- COMT requires ATP to function
Correct Answer: COMT methylates catecholamines and other catecholic compounds
Q45. Which drug used in Parkinson’s disease is a peripheral COMT inhibitor that does not cross the blood–brain barrier?
- Entacapone
- Tolcapone
- Selegiline
- Moclobemide
Correct Answer: Entacapone
Q46. Which metabolic fate of norepinephrine in the synaptic cleft requires direct action of extracellular enzymes?
- COMT acting on released catecholamines
- MAO acting extracellularly primarily
- ALDH acting extracellularly
- PNMT methylating in the synapse
Correct Answer: COMT acting on released catecholamines
Q47. Which of the following increases the formation of metanephrines within chromaffin cells prior to secretion?
- COMT activity in the cytosol of chromaffin cells
- Enhanced MAO activity on released catecholamines
- Increased VMAT-mediated uptake only
- Reduced SAM availability
Correct Answer: COMT activity in the cytosol of chromaffin cells
Q48. Which enzyme deficiency would most directly reduce conversion of DOPAL to DOPAC?
- Aldehyde dehydrogenase (ALDH) deficiency
- COMT deficiency
- PNMT deficiency
- NET deficiency
Correct Answer: Aldehyde dehydrogenase (ALDH) deficiency
Q49. Which pharmacologic action would likely raise urinary VMA levels acutely?
- Administration of exogenous epinephrine
- COMT inhibition
- MAO inhibition
- NET blockade only
Correct Answer: Administration of exogenous epinephrine
Q50. Which laboratory finding supports a diagnosis of pheochromocytoma most strongly?
- Markedly elevated plasma free metanephrines
- Low urinary VMA with normal metanephrines
- Reduced platelet MAO activity
- Elevated serum ALDH only
Correct Answer: Markedly elevated plasma free metanephrines
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