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
Related conditions
Related
- Superoxide dismutase (SOD)
- Catalase
- Glutathione peroxidase (GPX)
- Glutathione (GSH)
- Complex I (NADH dehydrogenase)
- Complex III (cytochrome bc₁)
- Ubiquinone (coenzyme Q₁₀)
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.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Detoxification of Reactive Oxygen Species (Reactome, 2026); Antioxidants: MedlinePlus (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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Representative model, for educational purposes only.