A patient with major depressive disorder fails to respond to two adequate SSRI trials. She is started on a medication that requires a tyramine-free diet to prevent hypertensive crises. Which enzyme does this drug inhibit?
- A Catechol-O-methyltransferase (COMT)
- B Monoamine oxidase A and B (MAO-A/B) irreversibly ✓
- C Aldehyde dehydrogenase
- D Dopamine beta-hydroxylase
Correct answer: B. Monoamine oxidase A and B (MAO-A/B) irreversibly
Explanation
Classic MAO inhibitors (phenelzine, tranylcypromine, isocarboxazid) irreversibly inhibit both MAO-A (which metabolises serotonin and norepinephrine) and MAO-B (which metabolises dopamine). This prevents tyramine degradation in the gut and liver, so dietary tyramine enters the systemic circulation and triggers massive norepinephrine release, causing potentially fatal hypertensive crises. COMT inhibitors (entacapone) are used in Parkinson's disease; aldehyde dehydrogenase is inhibited by disulfiram; dopamine beta-hydroxylase converts dopamine to norepinephrine.
Reference: Kaplan & Sadock's Synopsis of Psychiatry, 11th ed.
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