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
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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).
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
Pathways
Related conditions
Related
- Aldehyde dehydrogenase 2 (ALDH2)
- Cytochrome P450 2E1 (CYP2E1, MEOS)
- Ethanol
- Acetaldehyde
- Fomepizole (4-methylpyrazole)
- NAD⁺ (nicotinamide adenine dinucleotide)
- Vitamin A (retinol)
- 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.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: ADH1B: All-trans-retinol dehydrogenase [NAD(+)] ADH1B (human), UniProt P00325 (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 (RCSB Protein Data Bank, 2026); Ethanol oxidation (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.