Ethanol is metabolised by alcohol dehydrogenase following zero-order kinetics even at relatively low blood concentrations. The biochemical reason is that:
- A Alcohol dehydrogenase has a very low Km for ethanol, so hepatic enzyme is saturated at concentrations well below those reached after drinking ✓
- B Alcohol dehydrogenase has an unusually high Km for ethanol, so the enzyme is saturated only at toxic levels
- C Ethanol induces synthesis of CYP2E1 immediately, making the reaction independent of substrate concentration
- D Zero-order behaviour arises because NAD+ regeneration, not enzyme concentration, is the true limiting factor at all doses
Correct answer: A. Alcohol dehydrogenase has a very low Km for ethanol, so hepatic enzyme is saturated at concentrations well below those reached after drinking
Explanation
The Km of hepatic alcohol dehydrogenase for ethanol is roughly 1 mM or less, while blood ethanol after even modest intake reaches 10 to 20 mM. The enzyme therefore operates at Vmax, and elimination proceeds at a constant rate regardless of concentration, which defines zero-order kinetics. CYP2E1 contributes more at higher concentrations and after chronic induction, but it does not explain the low-dose zero-order behaviour.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
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