A 40-year-old woman has recurrent episodes of throbbing headache, drenching sweats, and palpitations with a blood pressure of 210/120 mmHg. Between episodes she is normotensive. Plasma free metanephrines are markedly elevated. Metanephrines are formed from catecholamines primarily by the action of which enzyme?
- A Catechol-O-methyltransferase (COMT) ✓
- B Dopamine beta-hydroxylase
- C Monoamine oxidase (MAO)
- D Tyrosine hydroxylase
Correct answer: A. Catechol-O-methyltransferase (COMT)
Explanation
COMT methylates the catechol hydroxyl groups of epinephrine and norepinephrine to form metanephrine and normetanephrine respectively. MAO then further deaminates these products toward VMA. Tumoral chromaffin cells metabolize catecholamines continuously rather than only during surges, which makes plasma free metanephrines the most sensitive screening test for pheochromocytoma. Tyrosine hydroxylase and dopamine beta-hydroxylase are synthetic enzymes upstream of the catecholamines themselves.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.