A 45-year-old man with chronic alcoholism is found to eliminate blood ethanol at a constant rate of about 7 g per hour whether his blood level is 50 mg/dL or 300 mg/dL. This behaviour is best explained by:
- A Alcohol dehydrogenase operating at less than half-maximal velocity
- B Saturation of alcohol dehydrogenase so that velocity approximates Vmax and is independent of substrate concentration ✓
- C First-order kinetics because ethanol is cleared exponentially
- D Induction of microsomal ethanol oxidising system at all concentrations
Correct answer: B. Saturation of alcohol dehydrogenase so that velocity approximates Vmax and is independent of substrate concentration
Explanation
When substrate concentration greatly exceeds Km, the enzyme is saturated and the reaction follows zero-order kinetics: velocity equals Vmax and does not change with further rises in substrate concentration. Ethanol concentrations achieved after drinking are far above the Km of hepatic alcohol dehydrogenase, giving a fixed elimination rate. Option C describes first-order kinetics, seen only when substrate concentration is well below Km, which does not apply here.
Reference: Lippincott Illustrated Reviews: Biochemistry, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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