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

# Reactive oxygen species and antioxidants

Reactive oxygen species are forms of oxygen with extra electrons that damage lipids, proteins and DNA; antioxidant enzymes keep them in check.

Updated 2026-11-01

## Key facts

- The main ones: superoxide (O₂•⁻), hydrogen peroxide (H₂O₂) and the hydroxyl radical (•OH), the most damaging.
- The respiratory chain is the main source: complexes I and III leak stray electrons to O₂.
- Free iron turns H₂O₂ into •OH (the Fenton reaction): that is why iron overload is oxidising.
- Defences: superoxide dismutase → catalase and glutathione peroxidase; dietary antioxidants: vitamins E and C, carotenoids; also selenium.

## In detail

- A stray electron reduces O₂ to superoxide.
- Superoxide dismutase turns it into H₂O₂.
- Catalase (peroxisomes) and glutathione peroxidase (cytosol, mitochondria) turn H₂O₂ into water.
- If H₂O₂ meets free iron or copper, it forms the hydroxyl radical, which attacks whatever is nearby.
- •OH starts chain peroxidation of membrane lipids; vitamin E breaks the chain and vitamin C regenerates vitamin E.
- NADPH (pentose phosphate pathway) keeps glutathione reduced, the cell's antioxidant reserve.

**Also called**
Free radicals, Oxidative stress

## Clinical notes

- Phagocytes make superoxide on purpose with NADPH oxidase (the respiratory burst); its absence causes chronic granulomatous disease.
- Oxidative damage takes part in ischemia–reperfusion injury, atherosclerosis (oxidised LDL) and ageing.
- Oxygen toxicity in preterm babies favours retinopathy and bronchopulmonary dysplasia.
- High-dose antioxidant supplements have not been shown to prevent cardiovascular disease or cancer in large trials.

## Pathways

- [Electron transport chain and oxidative phosphorylation](https://durgical.com/en/biochemistry/pathways/electron-transport-chain-and-oxidative-phosphorylation/)

## Related conditions

- [Carbon monoxide poisoning](https://durgical.com/en/conditions/carbon-monoxide-poisoning/)
- [Atherosclerosis and dyslipidaemia](https://durgical.com/en/conditions/atherosclerosis-and-dyslipidaemia/)

## Related

- [Superoxide dismutase (SOD)](https://durgical.com/en/biochemistry/enzymes/superoxide-dismutase/)
- [Catalase](https://durgical.com/en/biochemistry/enzymes/catalase/)
- [Glutathione peroxidase (GPX)](https://durgical.com/en/biochemistry/enzymes/glutathione-peroxidase/)
- [Glutathione (GSH)](https://durgical.com/en/biochemistry/molecules/glutathione/)
- [Complex I (NADH dehydrogenase)](https://durgical.com/en/biochemistry/enzymes/complex-i/)
- [Complex III (cytochrome bc₁)](https://durgical.com/en/biochemistry/enzymes/complex-iii/)
- [Ubiquinone (coenzyme Q₁₀)](https://durgical.com/en/biochemistry/molecules/ubiquinone/)

## Test yourself

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

## Which reactive oxygen species is the most damaging, and how does it form?

The hydroxyl radical; it forms when H₂O₂ reacts with free iron or copper (the Fenton reaction).

## In what order do the antioxidant enzymes act?

First superoxide dismutase (superoxide → H₂O₂), then catalase or glutathione peroxidase (H₂O₂ → water).

## Which vitamin breaks the peroxidation of membrane lipids?

Vitamin E (α-tocopherol); vitamin C regenerates it.

## Which phagocyte enzyme makes superoxide to kill microbes?

NADPH oxidase; its deficiency causes chronic granulomatous disease.

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

Sources: [Detoxification of Reactive Oxygen Species](https://reactome.org/PathwayBrowser/#/R-HSA-3299685) (Reactome, 2026); [Antioxidants: MedlinePlus](https://medlineplus.gov/antioxidants.html) (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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