Enhance your knowledge with our comprehensive guide and curated study materials.
A free radical is any atom or molecule with one or more unpaired electrons in its outer orbit, making it highly reactive.
Superoxide (O₂•⁻)
Hydroxyl radical (•OH)
Nitric oxide (NO•)
Peroxyl radicals (ROO•)
Mitochondrial ETC leakage
Cytochrome P450 reactions
Inflammation (neutrophils/macrophages)
Radiation (UV, X-ray)
Pollution, cigarette smoke
Drugs and toxins
Highly reactive oxygen-containing molecules, including radicals and non-radicals.
Superoxide (O₂•⁻)
Hydroxyl radical (•OH)
Peroxyl radicals (ROO•)
Hydrogen peroxide (H₂O₂)
Hypochlorous acid (HOCl)
Ozone (O₃)
Mitochondrial ETC leakage → 1–3% of O₂ becomes superoxide.
Phagocytosis respiratory burst (NADPH oxidase):
O₂ → O₂•⁻ → H₂O₂ → HOCl
(important for neutrophil killing)
Cytochrome P450 reactions
Prostaglandin/catecholamine metabolism
Xanthine oxidase during ischemia–reperfusion injury.
Ionizing radiation (splits water → •OH radicals)
UV light
Smoking, air pollution
Ischemia–reperfusion injury
Chronic inflammation
Heavy metals (Fe²⁺ via Fenton reaction)
ROS cause oxidative stress, damaging:
ROS attack polyunsaturated fatty acids in membranes.
Effects:
Loss of membrane integrity
Increased permeability
Cell swelling
Mitochondrial damage
Highly exam-relevant for:
Liver injury
Atherosclerosis
Radiation toxicity
ROS cause:
Cross-linking
Fragmentation
Enzyme inactivation
Seen in:
Aging
Cataract formation
Neurodegenerative diseases
•OH causes:
Base modification
Strand breaks
Mutations → cancer risk
Seen in:
UV radiation exposure
Smoking
Radiation therapy
Antioxidant mechanisms protect cells from ROS.
Converts:
O₂•⁻ → H₂O₂
Types:
Cytosolic (Cu/Zn-SOD)
Mitochondrial (Mn-SOD)
Present in peroxisomes.
Converts:
2H₂O₂ → 2H₂O + O₂
Contains selenium.
Reduces:
H₂O₂ → H₂O
Lipid peroxides → alcohol
Uses reduced glutathione (GSH).
Regenerates GSH using NADPH.
Most important lipid-phase antioxidant
Prevents lipid peroxidation in membranes
Regenerates vitamin E
Scavenges hydroxyl radicals
Important in smokers and chronic inflammation
Quench singlet oxygen
Protective in UV light exposure
Removes peroxides
Maintains redox balance
Reduced form necessary for GPx activity
Scavenges singlet oxygen & hydroxyl radicals
Powerful chain-breaking antioxidant in plasma
Elevated in newborns as protective mechanism
Bind Fe²⁺, prevent Fenton reaction.
NADPH: Needed for glutathione regeneration
Thioredoxin system
Ceruloplasmin: Neutralizes free copper
Metallothioneins: Bind heavy metals
Albumin: Scavenges free radicals in plasma
Atherosclerosis
Diabetes mellitus
Alcoholic liver disease
Parkinson’s & Alzheimer’s diseases
COPD
Ischemia–reperfusion injury
Cancer initiation (DNA damage)
Vitamin E & C
N-acetylcysteine (NAC)
Selenium sources
Polyphenols
Free radicals have unpaired electrons → high reactivity.
ROS include O₂•⁻, H₂O₂, •OH, ROO•, HOCl.
Major sources: ETC leakage, inflammation, radiation, P450, reperfusion.
Lipid peroxidation → membrane damage (most common).
Key enzymes: SOD, catalase, GPx.
GSH + NADPH are essential for antioxidant defense.
Vitamin E = membrane protector; Vitamin C = aqueous protector.
During acute and chronic inflammation, neutrophils and macrophages produce large amounts of ROS through:
O₂ → O₂•⁻ → H₂O₂ → HOCl
Superoxide produced by NADPH oxidase
H₂O₂ generated by superoxide dismutase
HOCl generated by myeloperoxidase (MPO)
Macrophages produce NO•
NO• reacts with O₂•⁻ → Peroxynitrite (ONOO⁻), a strong oxidant
Damage to invading microbes
Also injures host tissues → oxidative stress
Vascular endothelial damage
Increased permeability & edema
Promotion of chronic inflammation (arthritis, IBD)
Free radicals contribute to major lung diseases.
Sources of ROS:
Cigarette smoke (very rich in free radicals)
Activated neutrophils in airways
Damaged mitochondria
Effects:
Destruction of alveolar walls
Elastin fragmentation → emphysema
Persistent inflammation
Eosinophils produce ROS
Oxidative stress → airway hyperresponsiveness
Decreases activity of antioxidants (SOD, catalase)
Persistent inflammation → ROS-mediated fibrosis
Seen in pneumoconiosis, radiation lung injury
Neutrophil burst → HOCl, O₂•⁻
Endothelial damage → capillary leakage
Surfactant destruction → respiratory failure
Oxidative stress is one of the central mechanisms of chronic and acute respiratory diseases.
A classic exam favourite.
Occurs in premature infants exposed to high oxygen therapy.
High O₂ → retinal vessels constrict (vaso-occlusion).
Withdrawal of oxygen → retina becomes relatively hypoxic.
Hypoxia triggers VEGF release → excessive neovascularization.
Fragile new vessels bleed →
Fibrous scarring
Retinal detachment
Blindness
High oxygen environment increases ROS (especially in immature retina lacking antioxidant enzymes), contributing to capillary damage.
Controlled oxygen therapy
Pulse oximetry targeting 90–95% saturation
Screening of preterm infants
Tissue damage that occurs after blood supply returns to previously ischemic tissue.
Reintroduction of oxygen leads to sudden burst of ROS:
Mitochondrial respiration restart
Xanthine oxidase converts accumulated hypoxanthine → uric acid, producing O₂•⁻
Activated neutrophils produce ROS
Cytochrome P450 leakage
Lipid peroxidation of membranes
Mitochondrial permeability transition → cell death
Calcium overload
Endothelial swelling & thrombosis
Worsened infarct size
Myocardial infarction after thrombolysis
Stroke after revascularization
Organ transplantation
Tourniquet release in trauma
Mesenteric ischemia
SOD, catalase, GPx
Vitamin C/E
Allopurinol (prevents xanthine oxidase-mediated ROS)
N-acetylcysteine
Inflammation generates ROS via NADPH oxidase & MPO.
Respiratory diseases involve excessive oxidative stress from smoking & inflammation.
Retrolental fibroplasia: hyperoxia → vasoconstriction → hypoxia → VEGF → blindness.
Reperfusion injury: sudden ROS overload after restoring blood supply.
Free radicals and oxidative stress play a central role in the initiation and progression of atherosclerosis.
ROS convert LDL → oxidized LDL (oxLDL).
oxLDL is taken up by macrophages → foam cells.
Foam cells form fatty streaks, the earliest atherosclerotic lesions.
ROS injure endothelial cells → reduced nitric oxide → vasoconstriction.
Damaged endothelium becomes adhesive, promoting leukocyte entry.
ROS stimulate smooth muscle migration into intima → plaque growth.
ROS activate NF-κB and inflammatory cytokines → chronic vascular inflammation.
Hyperlipidemia + ROS = accelerated plaque formation
Smokers have extremely high oxidative LDL burden
Antioxidants (Vitamin E, polyphenols) may reduce progression
Diabetes increases ROS → enhances atherosclerotic risk
Many skin pathologies are driven or worsened by oxidative stress.
UV light → generates ROS in skin
ROS damage collagen, elastin → wrinkles, laxity
Increases matrix metalloproteinases → collagen breakdown
ROS stimulate melanocytes
Increase melanin production → dark patches
Antioxidants used in cosmetic dermatology (vitamin C, niacinamide)
Neutrophils in comedones generate ROS
Increase inflammation & tissue damage
Some anti-acne formulations include antioxidants
Hyperproliferation of keratinocytes produces ROS
Oxidative stress amplifies inflammation
Excess hydrogen peroxide in skin damages melanocytes
Antioxidants (calcium folinate, vitamin C/E, topical catalase mimetics) support treatment
Chronic inflammation → ROS generation
Barrier disruption increases oxidative injury
Aging is strongly connected with cumulative oxidative damage — the basis of the Free Radical Theory of Aging.
β-amyloid generates ROS
Lipid peroxidation + protein carbonylation
Mitochondrial dysfunction
Dopamine oxidation → ROS
Mitochondrial Complex I defect → more ROS
Damages substantia nigra neurons
Lens proteins oxidize → clouding
UV exposure accelerates oxidation
Reduced glutathione (GSH) in older people worsens progression
UV → ROS → collagen degradation
Elastin damage → wrinkles
Reduced antioxidant enzymes with age
Mitochondrial ROS damage muscle fibers
Decreased ATP → reduced muscle strength
Impaired repair mechanisms
Endothelial nitric oxide decreases due to ROS
Increased arterial stiffness
Higher risk of hypertension, thrombosis, and MI
ROS damage lymphocytes and stem cells
Reduced adaptive immunity
Increased infections and cancer susceptibility
Hyperglycemia → advanced glycation → ROS burst
Mitochondrial oxidative stress → insulin resistance
Vascular complications (retinopathy, nephropathy)
Atherosclerosis: Oxidized LDL → foam cells → plaques.
Skin diseases: ROS central in UV-damage, melasma, vitiligo, psoriasis, acne.
Age-related diseases: Alzheimer’s, Parkinson’s, cataract, skin aging, sarcopenia linked to cumulative ROS injury.
Antioxidant enzymes (SOD, catalase, GPx) weaken with age → more oxidative damage.
Oxidative degradation of polyunsaturated fatty acids (PUFAs) in cell membranes by free radicals.
Major mechanism of cell membrane damage
Leads to loss of membrane fluidity, integrity & function
Produces toxic aldehydes (e.g., malondialdehyde – MDA), a marker of oxidative stress
Biological membranes rich in PUFAs (liver, brain, skin)
Lipoproteins (oxidized LDL → atherosclerosis)
Lipid peroxidation follows three classic phases:
ROS + PUFA → Lipid radicals (L•)
Key initiators:
Hydroxyl radical (•OH) — most potent
Peroxynitrite (ONOO⁻)
UV radiation
Ionizing radiation
Heavy metals (Fe²⁺ via Fenton reaction)
Reaction:
PUFA–H + •OH → PUFA• + H₂O
(The •OH removes H → creates lipid radical)
Lipid radical reacts with oxygen → lipid peroxyl radical (LOO•)
LOO• reacts with nearby PUFA → new lipid radical (L•)
This forms a chain reaction.
Cycle:
L• + O₂ → LOO•
LOO• + PUFA–H → LOOH + L•
Products:
Lipid hydroperoxide (LOOH)
Malondialdehyde (MDA)
4-HNE (4-hydroxynonenal)
These molecules damage:
Membrane proteins
DNA
Mitochondria
Lipoproteins
Occurs when two radicals combine → non-radical stable product.
LOO• + LOO• → non-radical
L• + LOO• → stable complex
OR
Stopped by chain-breaking antioxidants (vitamin E, vitamin C).
These prevent formation of free radicals before they start damaging lipids.
Prevent Fenton reaction (Fe²⁺ → •OH).
Transferrin
Lactoferrin
Ferritin
Ceruloplasmin
EDTA (lab)
Superoxide dismutase (SOD) → removes O₂•⁻
Catalase → removes H₂O₂
Glutathione peroxidase → removes peroxides
Scavenges singlet oxygen.
Protective chain-stopping antioxidant in plasma.
Binds free copper/iron → prevents radical formation.
Purpose: Stop ROS formation upstream, preventing initiation stage.
Interrupt the propagation phase by reacting with lipid peroxyl radicals (LOO•) and converting them into non-radical products.
Lipid-soluble
Present in membranes
Neutralizes LOO• → stops chain reaction
First line of defense against lipid peroxidation
Water-soluble
Regenerates vitamin E
Scavenges •OH
Removes LOOH via glutathione peroxidase
Restored by glutathione reductase + NADPH
Membrane antioxidant
Neutralizes lipid radicals
Quench singlet oxygen
Protect lipids in membranes and skin
Donate hydrogen to radicals
Stop propagation
LDL oxidation via lipid peroxidation → foam cells → plaque formation.
UV-induced lipid peroxidation damages collagen/elastin.
Brain is PUFA-rich → vulnerable to ROS.
Alcohol increases CYP2E1 → ROS → lipid peroxidation.
Lipid peroxidation byproducts like MDA are mutagenic.
Initiation: ROS attacks PUFA → lipid radical.
Propagation: Lipid radical + O₂ → peroxyl radical → chain reaction.
Termination: Radicals combine OR antioxidants stop chain.
Preventive antioxidants: Stop free radicals from forming.
Chain-breaking antioxidants: Stop ongoing lipid peroxidation.
Occurs mainly in polyunsaturated fatty acids (PUFAs) of membranes.
One of the most important mechanisms of cell membrane injury.
Produces toxic byproducts like malondialdehyde (MDA) and 4-HNE, used as markers of oxidative stress.
Particularly important in liver, brain, skin, and LDL particles.
Triggered by hydroxyl radical (•OH) → most reactive species.
Other initiators: UV, ionizing radiation, Fe²⁺ (Fenton reaction), peroxynitrite.
ROS abstracts hydrogen from PUFA → lipid radical (L•).
L• + O₂ → lipid peroxyl radical (LOO•).
LOO• reacts with nearby PUFA → LOOH + new L• → chain reaction.
Responsible for rapid spread of membrane damage.
Generates lipid hydroperoxides (LOOH) and MDA.
Occurs when two radicals combine → stable product.
Chain-breaking antioxidants (e.g., vitamin E) stop radical propagation.
Marks the end of lipid peroxidation.
SOD → removes superoxide (O₂•⁻).
Catalase → removes H₂O₂.
Glutathione Peroxidase (selenium-dependent) → removes peroxides (H₂O₂, LOOH).
Metal-chelating proteins: transferrin, lactoferrin, ferritin, ceruloplasmin → prevent Fenton reaction.
Albumin → binds free metals, prevents radical formation.
Uric acid and bilirubin → neutralize singlet oxygen.
Vitamin E (α-tocopherol) → most important lipid-phase antioxidant; protects membranes.
Vitamin C (ascorbate) → regenerates vitamin E; scavenges aqueous radicals.
Glutathione (GSH) → detoxifies LOOH through GPx.
Coenzyme Q (ubiquinol) → membrane antioxidant.
Carotenoids (β-carotene, lycopene) → quench singlet oxygen.
Plant polyphenols → donate electrons to stop radical chains.
Oxidized LDL from lipid peroxidation → atherosclerosis.
UV-induced lipid peroxidation → photoaging, melasma, skin inflammation.
Alcohol metabolism (CYP2E1) increases ROS → fatty liver and hepatotoxicity.
High ROS in brain → linked to Alzheimer’s and Parkinson’s.
Newborn antioxidant systems are immature → prone to retinopathy of prematurity.
Reperfusion after ischemia produces ROS → further tissue injury.
Lipid peroxidation = PUFA destruction by ROS.
Steps: Initiation → Propagation → Termination.
Preventive antioxidants stop radicals from forming.
Chain-breaking antioxidants stop chain reactions already underway.
Vitamin E = main membrane protector; GPx = removes peroxides.
A. Excess electrons
B. Paired electrons
C. Unpaired electrons
D. No electrons
Answer: C
A. Superoxide
B. Hydrogen peroxide
C. Singlet oxygen
D. Hydroxyl radical (•OH)
Answer: D
A. Xanthine oxidase
B. NADPH oxidase
C. Catalase
D. Nitric oxide synthase
Answer: B
A. GPx
B. SOD
C. Catalase
D. MPO
Answer: C
A. O₂•⁻
B. •OH
C. NO•
D. H₂O₂
Answer: D
A. Sodium
B. Potassium
C. Iron (Fe²⁺)
D. Zinc
Answer: C
A. Golgi apparatus
B. ER
C. Electron Transport Chain (ETC)
D. Peroxisome
Answer: C
A. Acetaldehyde
B. Formaldehyde
C. Malondialdehyde (MDA)
D. Uric acid
Answer: C
A. Initiation
B. Propagation
C. Termination
D. Stabilization
Answer: B
A. Vitamin C
B. Glutathione
C. Uric acid
D. Vitamin E
Answer: D
A. Coenzyme Q
B. Vitamin C
C. Bilirubin
D. Selenium
Answer: B
A. Catalase
B. SOD
C. Glutathione peroxidase (GPx)
D. Xanthine oxidase
Answer: C
A. Breaking lipid radicals
B. Blocking LOO•
C. Preventing formation of new ROS
D. Stabilizing MDA
Answer: C
A. Vitamin E
B. Vitamin C
C. Transferrin
D. Coenzyme Q
Answer: C
A. DNA only
B. Proteins only
C. Biological membranes
D. Cholesterol
Answer: C
A. Glaucoma
B. Cataract
C. Retrolental fibroplasia (ROP)
D. Strabismus
Answer: C
A. O₂•⁻
B. •OH
C. NO•
D. H₂O₂
Answer: C
A. LDL hydrolysis
B. LDL oxidation
C. LDL polymerization
D. LDL glycation
Answer: B
A. Catalase
B. GPx
C. SOD
D. MPO
Answer: C
A. Creating more radicals
B. Combining two radicals into a stable molecule
C. Breaking lipids into aldehydes
D. Quenching hydroxyl radicals
Answer: B
A. O₂•⁻
B. O₃
C. H₂O₂
D. •OH
Answer: D
A. Hypothyroidism
B. COPD
C. Gallstone disease
D. Asthma
Answer: B
A. ATP accumulation
B. Increased antioxidants
C. Sudden ROS burst after restoring oxygen
D. Excess nitric oxide removal
Answer: C
A. Vitamin A
B. Ceruloplasmin
C. Myeloperoxidase
D. Albumin
Answer: B
A. Catalase
B. SOD
C. Vitamin E
D. Glutathione reductase
Answer: C
A. Vitamin E
B. Coenzyme Q
C. Vitamin C
D. Bilirubin
Answer: C
A. Melasma
B. Acne
C. Vitiligo
D. Psoriasis
Answer: C
A. Lactate dehydrogenase
B. Myeloperoxidase
C. G6PD deficiency
D. Catalase deficiency
Answer: C
A. Cholesterol
B. Mitochondrial DNA
C. Lipoproteins
D. Lysosomes
Answer: B
A. Catalase
B. GPx
C. Mn-SOD
D. NADPH oxidase
Answer: C
Cigarette smoke delivers thousands of free radicals per puff.
ROS damage alveolar walls → elastin breakdown → emphysema.
Answer: Excess oxidative stress leading to lipid/protein oxidation in lungs.
Excess O₂ → ROS overload → capillary damage → hypoxia on withdrawal → VEGF-mediated neovascularization.
Answer: ROS-mediated vascular injury → retrolental fibroplasia (ROP).
RBCs lack NADPH → cannot regenerate glutathione.
Peroxides accumulate → membrane lipid peroxidation → hemolysis.
Answer: Inability to detoxify ROS due to reduced glutathione.
Reperfusion → sudden supply of oxygen → ROS burst.
Xanthine oxidase, neutrophils contribute to ROS.
Answer: Reperfusion injury due to sudden ROS generation.
Bilirubin acts as a natural antioxidant.
Answer: Elevated bilirubin functioning as an antioxidant.
MDA = marker of lipid peroxidation.
Answer: Increased lipid peroxidation due to chronic oxidative stress.
UV generates ROS in skin → stimulates melanocytes → melasma.
Answer: UV-induced ROS activation of melanogenesis.
Mitochondrial ROS damage neuronal lipids/proteins in Alzheimer’s disease.
Answer: Oxidative stress-mediated neurodegeneration.
ROS convert LDL → oxidized LDL → foam cells → plaques.
Answer: Free radical–mediated oxidation of LDL in endothelium.
CYP2E1 generates ROS → lipid peroxidation → liver injury.
Answer: Alcohol-induced ROS formation causing hepatic lipid peroxidation.
Catalase deficiency → H₂O₂ builds up → melanocyte damage.
Answer: Reduced catalase activity causing H₂O₂ accumulation.
Sudden oxygen reentry → ROS burst → inflammation, endothelial injury.
Answer: Reperfusion injury from ROS excess.
NADPH oxidase → O₂•⁻ → H₂O₂ → HOCl (myeloperoxidase).
Answer: Respiratory burst generating ROS in inflammation.
NAPQI depletes glutathione → ROS accumulate → hepatocyte necrosis.
Answer: GSH depletion → inability to neutralize ROS.
UV-induced ROS → collagen & elastin destruction.
Answer: ROS-mediated photoaging of skin.
They destroy alveolar walls, activate neutrophils, damage surfactant.
Answer: Oxidative injury leading to emphysema development.
ETC leakage generates more ROS → increased SOD, catalase, GPx.
Answer: Upregulation of enzymatic antioxidants due to increased ROS.
Oxygen toxicity → •OH formation → alveolar damage.
Answer: Hydroxyl radical–mediated pulmonary injury.
oxLDL due to lipid peroxidation.
Answer: Oxidized LDL formed by ROS.
Dopamine metabolism produces ROS → mitochondrial dysfunction.
Answer: ROS generation from dopamine oxidation.
A molecule with one or more unpaired electrons, making it highly reactive.
Superoxide (O₂•⁻), hydroxyl radical (•OH), nitric oxide (NO•).
A reactive oxygen-containing molecule, radical or non-radical (e.g., H₂O₂, HOCl).
Hydroxyl radical (•OH).
Mitochondrial electron transport chain leakage.
Rapid ROS generation by neutrophils via NADPH oxidase.
Superoxide dismutase (SOD).
Catalase or glutathione peroxidase (GPx).
Glutathione peroxidase.
Neutralizes peroxides and maintains cellular redox balance.
ROS-mediated oxidation of PUFAs in membranes.
Initiation, propagation, termination.
Attack by hydroxyl radical on membrane lipids.
Lipid peroxyl radicals (LOO•) and lipid hydroperoxides (LOOH).
Two radicals combine to form a stable non-radical.
A toxic product of lipid peroxidation; marker of oxidative stress.
SOD, catalase, GPx, transferrin, ferritin, ceruloplasmin.
Prevent formation of ROS (act before initiation phase).
Antioxidants that stop propagation by neutralizing LOO•.
Vitamin E (α-tocopherol).
Regenerates vitamin E and scavenges aqueous radicals.
Ceruloplasmin.
Thermogenin (UCP-1).
Atherosclerosis.
Introduces large numbers of free radicals into lungs → COPD.
UV-generated ROS damage collagen and elastin.
High PUFA content + high oxygen use + low antioxidants.
Sudden ROS burst when oxygen returns to ischemic tissue.
Neutrophils generate ROS (respiratory burst) for microbial killing.
Hyperoxia → ROS → retinal vessel damage → neovascularization.
Hydroxyl radical (•OH).
Converts LOOH → alcohol, preventing propagation.
Vitamin E, Vitamin C, glutathione, carotenoids, uric acid, bilirubin.
Vitamin E.
Peroxisomes generate large amounts of H₂O₂.
Bilirubin.
Low NADPH → low GSH → peroxide accumulation in RBCs.
Peroxynitrite → formed from NO• + O₂•⁻ → potent oxidant.
Vitamin E and Vitamin C.
Mn-SOD (manganese SOD).
Get the full PDF version of this chapter.
Continue with Google