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[Biochemistry](https://durgical.com/en/biochemistry/) · Enzyme

# Alcohol dehydrogenase (ADH)

A liver enzyme that starts alcohol removal: it turns it into acetaldehyde using NAD⁺ and a zinc atom.

Updated 2026-11-01

Loading the 3D model…

## Key facts

- Ethanol + NAD⁺ → acetaldehyde + NADH + H⁺, in the cytosol; the active-site zinc holds the alcohol's oxygen.
- It is a dimer; the class I isoenzymes (ADH1A, 1B and 1C) do almost all the work in the liver; the stomach has ADH7 (first-pass metabolism, lower in women).
- It has a low Km for ethanol: it saturates early and elimination is zero order.
- It also oxidises methanol, ethylene glycol, isopropanol and retinol (retinoic acid synthesis).

**Also called**
Class I ADH

**EC number**
[1.1.1.1](https://www.enzyme-database.org/query.php?ec=1.1.1.1)

**Genes**
ADH1A, ADH1B, ADH1C

**Structure (PDB)**
[1HTB](https://www.rcsb.org/structure/1HTB)

## Structure

- Dimer; each subunit has an NAD⁺-binding domain (Rossmann fold) and a catalytic domain with two zinc ions (one catalytic, one structural).

## Regulation

- It has no important allosteric control; NAD⁺ availability limits it.
- Variants: ADH1B*2 (common in East Asia) and ADH1B*3 (in people of African descent) are faster and raise acetaldehyde.

## Inhibitors and toxins

- Fomepizole (4-methylpyrazole), competitive
- Pyrazole and 4-iodopyrazole (the one in the 3D structure)
- Ethanol against methanol or ethylene glycol

## Clinical notes

- Antidote in methanol and ethylene glycol poisoning: fomepizole or ethanol occupies the enzyme and the toxin is excreted unchanged.
- The 3D structure (1HTB) shows the β3 variant (ADH1B*3) with NAD⁺ and 4-iodopyrazole, a relative of fomepizole.
- Fast ADH1B variants make more acetaldehyde and protect against alcohol dependence.

## Drugs that act here

- [Fomepizole](https://durgical.com/en/pharmacology/fomepizole/)

## Pathways

- [Ethanol metabolism](https://durgical.com/en/biochemistry/pathways/ethanol-metabolism/)

## Related conditions

- [Methanol poisoning](https://durgical.com/en/conditions/methanol-poisoning/)
- [Ethylene glycol poisoning](https://durgical.com/en/conditions/ethylene-glycol-poisoning/)
- [Alcoholic liver disease (biochemistry)](https://durgical.com/en/conditions/alcoholic-liver-disease/)

## Related

- [Aldehyde dehydrogenase 2 (ALDH2)](https://durgical.com/en/biochemistry/enzymes/aldehyde-dehydrogenase-2/)
- [Cytochrome P450 2E1 (CYP2E1, MEOS)](https://durgical.com/en/biochemistry/enzymes/cytochrome-p450-2e1/)
- [Ethanol](https://durgical.com/en/biochemistry/molecules/ethanol/)
- [Acetaldehyde](https://durgical.com/en/biochemistry/molecules/acetaldehyde/)
- [Fomepizole (4-methylpyrazole)](https://durgical.com/en/biochemistry/molecules/fomepizole/)
- [NAD⁺ (nicotinamide adenine dinucleotide)](https://durgical.com/en/biochemistry/molecules/nad/)
- [Vitamin A (retinol)](https://durgical.com/en/biochemistry/molecules/vitamin-a/)
- [Zinc](https://durgical.com/en/biochemistry/topics/zinc/)

## Test yourself

Think of the answer, then tap the question to see it.

## Which metal and which coenzyme does alcohol dehydrogenase need?

Zinc in the active site and NAD⁺ as the hydrogen acceptor.

## Why is fomepizole useful in methanol poisoning?

It inhibits alcohol dehydrogenase and prevents methanol from becoming formaldehyde and formic acid.

## Why is ethanol removed at an almost constant rate per hour?

Because the enzyme has a low Km and is saturated by little alcohol: zero-order kinetics.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [ADH1B: All-trans-retinol dehydrogenase [NAD(+)] ADH1B (human), UniProt P00325](https://www.uniprot.org/uniprotkb/P00325/entry) (UniProt, 2026); [RCSB PDB - 1HTB: CRYSTALLIZATION OF HUMAN BETA3 ALCOHOL DEHYDROGENASE (10 MG/ML) IN 100 MM SODIUM PHOSPHATE (PH 7.5), 7.5 MM NAD+ AND 1 MM 4-IODOPYRAZOLE AT 25 C](https://www.rcsb.org/structure/1HTB) (RCSB Protein Data Bank, 2026); [Ethanol oxidation](https://reactome.org/PathwayBrowser/#/R-HSA-71384) (Reactome, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/enzymes/alcohol-dehydrogenase/