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

# NAPQI (N-acetyl-p-benzoquinone imine)

The toxic metabolite of acetaminophen: glutathione neutralises it, but when glutathione runs out it destroys liver cells.

Updated 2026-11-01

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## Key facts

- Cytochromes P450 (mainly CYP2E1, also CYP3A4 and CYP1A2) make it from a small fraction of acetaminophen.
- It is an electrophile: it binds protein cysteines, especially mitochondrial ones.
- Glutathione (GSH) conjugates it and it is excreted as cysteine and mercapturic acid derivatives in urine.
- When hepatic GSH falls to about 30 % of normal, free NAPQI causes zone 3 necrosis.

**Also called**
Acetimidoquinone

**Formula**
C₈H₇NO₂

**PubChem**
[CID 39763](https://pubchem.ncbi.nlm.nih.gov/compound/39763)

## Structure

- A quinone imine ring with an acetyl group; reactive towards thiol (–SH) groups.

## Clinical notes

- N-acetylcysteine restores glutathione and protects best when given early after ingestion.
- Fasting, malnutrition and chronic alcohol lower GSH or raise CYP2E1 and make injury easier.
- Acetaminophen–protein adducts in blood are being studied as a marker of this toxicity.

## Related conditions

- [Acetaminophen toxicity](https://durgical.com/en/conditions/acetaminophen-toxicity/)

## Related

- [Acetaminophen metabolism and NAPQI](https://durgical.com/en/biochemistry/topics/acetaminophen-metabolism-and-napqi/)
- [Glutathione (GSH)](https://durgical.com/en/biochemistry/molecules/glutathione/)
- [N-acetylcysteine (NAC)](https://durgical.com/en/biochemistry/molecules/n-acetylcysteine/)
- [Cytochrome P450 2E1 (CYP2E1, MEOS)](https://durgical.com/en/biochemistry/enzymes/cytochrome-p450-2e1/)
- [Cytochrome P450 3A4 (CYP3A4)](https://durgical.com/en/biochemistry/enzymes/cytochrome-p450-3a4/)

## Test yourself

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

## Which molecule neutralises NAPQI under normal conditions?

Hepatic glutathione, which binds it and makes it harmless.

## Which zone of the liver lobule does NAPQI damage first, and why?

Zone 3 (centrilobular): it has more CYP2E1 and less oxygen.

## Which enzyme makes most NAPQI?

CYP2E1, with contributions from CYP3A4 and CYP1A2.

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Molecules are shown in their neutral form, as the databases deposit them; in the cell the acids and phosphates are ionised.

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

Sources: [Acetimidoquinone (CID 39763)](https://pubchem.ncbi.nlm.nih.gov/compound/39763) (PubChem, NCBI, 2026); [Paracetamol ADME](https://reactome.org/PathwayBrowser/#/R-HSA-9753281) (Reactome, 2026); [Acetaminophen overdose: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002598.htm) (MedlinePlus, US National Library of Medicine, 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/biochemistry/molecules/napqi/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/molecules/napqi/