Question:medium
Fish odor syndrome is caused by deficiency of which enzyme?
Fish odor syndrome is caused by deficiency of which enzyme?
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Which liver monooxygenase converts smelly trimethylamine into odourless TMAO?
Updated On: Jun 23, 2026
- Fumarylacetoacetate hydrolase
- Methane monooxygenase
- Flavin monooxygenase 3 (FMO3)
- D-amino acid oxidase
Show Solution
The Correct Option is C
Solution and Explanation
Step 1: The disease is named for its smell, so the offending molecule is trimethylamine (TMA), which is produced in the gut from dietary choline and carnitine.
Step 2: A healthy liver detoxifies TMA by adding an oxygen to make odourless TMAO; the enzyme that performs this N-oxidation is flavin-containing monooxygenase 3 (FMO3).
Step 3: If FMO3 is genetically deficient (autosomal recessive), TMA escapes oxidation and spills into sweat, urine, and breath, giving the characteristic rotting-fish odor; the oxidation ratio $TMAO/(TMAO+TMA)\times100\%$ confirms the impaired conversion.
Step 4: None of the other listed enzymes participate in amine detoxification of this type, so only FMO3 fits. \[\boxed{\text{FMO3}}\]
Step 2: A healthy liver detoxifies TMA by adding an oxygen to make odourless TMAO; the enzyme that performs this N-oxidation is flavin-containing monooxygenase 3 (FMO3).
Step 3: If FMO3 is genetically deficient (autosomal recessive), TMA escapes oxidation and spills into sweat, urine, and breath, giving the characteristic rotting-fish odor; the oxidation ratio $TMAO/(TMAO+TMA)\times100\%$ confirms the impaired conversion.
Step 4: None of the other listed enzymes participate in amine detoxification of this type, so only FMO3 fits. \[\boxed{\text{FMO3}}\]
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