A patient taking phenelzine (an irreversible MAO inhibitor) for depression develops severe headache, hypertensive crisis, and palpitations after eating aged cheese. This is the 'cheese reaction.' Which enzyme substrate causes this by releasing norepinephrine from sympathetic terminals?
- A Tyramine ✓
- B Dopamine
- C Phenylethylamine
- D Tryptamine
Correct answer: A. Tyramine
Explanation
Aged cheeses contain high concentrations of tyramine, which is normally inactivated by intestinal and hepatic MAO-A. When MAO is irreversibly inhibited by phenelzine, dietary tyramine enters the circulation, is taken up by sympathetic nerve terminals, and displaces stored norepinephrine (acting as an indirect sympathomimetic), triggering hypertensive crisis. This is the classic 'cheese reaction'; phenylethylamine is implicated in chocolate-precipitated migraines in some individuals.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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