- In yeast, under anaerobic conditions, pyruvate is fermented to ethanol through two steps:
decarboxylation of pyruvate to acetaldehyde and NADH-mediated reduction of acetaldehyde to ethanol. The mammalian liver also expresses alcohol dehydrogenase (Liver ADH: L-ADH).
From the options given below, choose the one that best explains the physiological significance of L-ADH in the absence of fermentation in the liver.
(1) The direction of L-ADH reaction varies with the relative concentrations of acetaldehyde and ethanol.
In addition, the enzyme metabolizes the alcohols produced by intestinal microflora anaerobically.
(2) NAD+ produced by L-ADH drives glycolysis in the liver.
(3) Mammalian L-ADH converts pyruvate to lactate and the NAD thus generated drives
glycolysis.
(4) Mammalian L-ADH has non-metabolic moonlighting functions.
Alcohol dehydrogenase (ADH) enzymes play essential roles in the metabolism of alcohols across various organisms. In yeast, ADH facilitates fermentation by converting acetaldehyde to ethanol, regenerating NAD+ to sustain glycolysis under anaerobic conditions. Mammalian liver also expresses a form of ADH, known as liver alcohol dehydrogenase (L-ADH), but unlike yeast, the liver does not rely on fermentation for energy production.
This article discusses the physiological significance of L-ADH in mammals, particularly in the liver, highlighting how its function differs from yeast fermentation and why it remains crucial for alcohol metabolism.
L-ADH Reaction Direction Depends on Substrate Concentrations
L-ADH catalyzes a reversible redox reaction between ethanol and acetaldehyde:
Ethanol+NAD+↔Acetaldehyde+NADH+H+
The direction of this reaction depends largely on the relative concentrations of ethanol and acetaldehyde in the liver. When ethanol levels are high, such as after alcohol consumption, L-ADH oxidizes ethanol to acetaldehyde. Conversely, under certain conditions, the enzyme can reduce acetaldehyde back to ethanol.
This reversible nature allows L-ADH to maintain metabolic balance and process various alcohol substrates efficiently.
Metabolism of Alcohols Produced by Intestinal Microflora
Beyond metabolizing dietary ethanol, L-ADH also processes alcohols produced by intestinal microflora. Gut microbes generate small amounts of alcohols anaerobically during fermentation of dietary components. These alcohols enter the portal circulation and reach the liver, where L-ADH metabolizes them, preventing their accumulation and potential toxicity.
This function highlights L-ADH’s role in maintaining systemic metabolic homeostasis beyond ethanol clearance.
Why Other Options Are Less Relevant
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NAD+ produced by L-ADH drives glycolysis in the liver:
Unlike yeast, mammalian liver cells rely primarily on oxidative phosphorylation for energy, not fermentation. NAD+ regeneration via L-ADH is not a major driver of glycolysis in the liver. -
Mammalian L-ADH converts pyruvate to lactate:
This reaction is catalyzed by lactate dehydrogenase, not alcohol dehydrogenase. -
Mammalian L-ADH has non-metabolic moonlighting functions:
While some enzymes have secondary functions, the primary physiological role of L-ADH is metabolic, focusing on alcohol metabolism.
Summary
Option Explanation Correctness (1) Reaction direction varies with ethanol/acetaldehyde concentrations; metabolizes microflora alcohols Correct (2) NAD+ produced by L-ADH drives glycolysis in liver Incorrect (3) L-ADH converts pyruvate to lactate Incorrect (4) L-ADH has non-metabolic moonlighting functions Incorrect Conclusion
The best explanation for the physiological significance of liver alcohol dehydrogenase (L-ADH) in mammals, in the absence of fermentation, is that the direction of the L-ADH reaction varies with the relative concentrations of acetaldehyde and ethanol, and the enzyme also metabolizes alcohols produced by intestinal microflora anaerobically.
Correct answer: (1)
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12 Comments
Kirti Agarwal
September 18, 2025Microflora of intestinal
Neha Yadav
September 20, 2025The direction of L-ADH reaction varies with the relative concentrations of acetaldehyde and ethanol. In addition, the enzyme metabolizes the alcohols produced by intestinal microflora anaerobically.
Khushi Agarwal
September 21, 2025Option A is correct answer
Soniya Shekhawat
September 21, 2025In mammals, there is no ethanol fermentation pathway. Instead, Liver ADH (L-ADH) primarily functions in detoxification and metabolism of ethanol which is secreted by intestinal bacteria and micro flora by anaerobic fermentation.
Manisha choudhary
September 21, 2025Mammals m y reaction reversible hoti h jo substarte ki concentration p depend krti h
Intestinal micoflora bhi kuch amount m alcohol bnate h liver alcohol and acetaldehyde ki concentration ko maintain rkhta h jaise alcohol consumption k baad alcohol ki concentration hight hone p acetaldehyde m convert kr diya jata h L-ADH(liver-alcohol dehydrogenase enzyme) k dwara hota h
Minal Sethi
September 22, 2025Reaction direction varies with ethanol/acetaldehyde concentrations; metabolizes microflora alcohols
Option A
Pallavi Ghangas
September 22, 2025the direction of the L-ADH reaction varies with the relative concentrations of acetaldehyde and ethanol, and the enzyme also metabolizes alcohols produced by intestinal microflora
Aakansha sharma Sharma
September 22, 2025The direction of L-ADH reaction varies with the relative concentrations of acetaldehyde and ethanol. In addition, the enzyme metabolizes the alcohols produced by intestinal microflora anaerobically.
Divya rani
September 25, 2025Direction of liver ADH is varies with relative concentration of both acetaldehyde and alcohol . In intestine the gut microbes produce some alcohol components anaerobically .
Muskan Yadav
September 26, 2025The direction of the L-ADH reaction varies with the relative concentrations of acetaldehyde and ethanol, and the enzyme also metabolizes alcohols produced by intestinal microflora anaerobically.
Kajal
October 3, 2025Option A
Sakshi Kanwar
November 25, 2025Gut microbes generate small amounts of alcohols anaerobically during fermentation option 1