What is free radical injury?

Free radical injury is cell damage caused by reactive oxygen species and other free radicals when they are produced faster than the cell's antioxidant defenses can remove them. A free radical has an unpaired electron, which makes it highly reactive; the key species are superoxide, hydrogen peroxide (not itself a radical) and the hydroxyl radical. The damage takes the form of lipid peroxidation of membranes, oxidation of proteins and breaks in DNA.

What are examples of free radical injury?

Classic examples are reperfusion injury after an MI is treated with PCI or thrombolysis, acetaminophen overdose, carbon tetrachloride poisoning, radiation injury, and oxygen toxicity in preterm infants (bronchopulmonary dysplasia and retinopathy of prematurity). Metal overload causes it too, through iron in hemochromatosis and copper in Wilson disease, as does doxorubicin cardiotoxicity. Lipofuscin, the wear-and-tear pigment of aging, is residue left behind by lipid peroxidation.

How do free radicals damage cells?

Free radicals attack polyunsaturated fatty acids in membranes, starting a lipid peroxidation chain reaction that makes plasma, mitochondrial and lysosomal membranes leaky. They also oxidize and cross-link proteins, which inactivates enzymes, and they damage DNA bases and cause strand breaks. Severe membrane damage leads to necrosis, while DNA and mitochondrial damage often trigger apoptosis.

What enzymes protect cells against free radicals?

Superoxide dismutase converts superoxide into hydrogen peroxide, and catalase and glutathione peroxidase then convert hydrogen peroxide into water. Glutathione peroxidase depends on selenium and on reduced glutathione, which glutathione reductase regenerates using NADPH from the G6PD step of the pentose phosphate pathway. Vitamins E, C and A and metal-binding proteins such as transferrin, ferritin and ceruloplasmin add non-enzymatic protection.

Why does reperfusion cause injury?

When blood flow returns to ischemic tissue, xanthine oxidase in endothelial cells, NADPH oxidase in incoming neutrophils and damaged mitochondria release a burst of reactive oxygen species. Calcium overload, opening of the mitochondrial permeability transition pore and rapid correction of acidosis add to cell death. In the heart this produces reperfusion arrhythmias, myocardial stunning, no-reflow and lethal reperfusion injury, although prompt reperfusion remains the treatment of choice.

What is the Fenton reaction?

The Fenton reaction is the reaction of ferrous iron with hydrogen peroxide that produces ferric iron, hydroxide and the highly reactive hydroxyl radical; copper can drive similar chemistry. It is why the body keeps iron and copper bound to proteins such as transferrin, ferritin and ceruloplasmin. It helps explain oxidative tissue damage in hemochromatosis, transfusional iron overload and Wilson disease.

Free Radical Injury: Sources, Damage, Defenses & Examples

Free radical injury is cell damage caused by reactive oxygen species (ROS), highly reactive oxygen derivatives (many with an unpaired electron) that pile up when they are made faster than the cell can remove them. They come from normal mitochondrial respiration, ionizing radiation, inflammation (the neutrophil respiratory burst), drugs and toxins metabolized by cytochrome P450 (acetaminophen, carbon tetrachloride), free iron and copper (the Fenton reaction), high-dose oxygen and reperfusion of ischemic tissue. They injure cells through lipid peroxidation of membranes, oxidation of proteins and DNA strand breaks, and are removed by superoxide dismutase, catalase and glutathione peroxidase, by antioxidant vitamins E, C and A, and by metal-binding proteins such as transferrin and ceruloplasmin that keep iron and copper tightly bound.

Quick check · free practice questionStep 2 CK

A 63-year-old obese woman presents 2 hours after a left femoral artery embolectomy with severe pain, numbness, and tingling in the left leg. The procedure was uncomplicated, and peripheral pulses were weakly palpable immediately after surgery. Her medical history is significant for type 2 diabetes mellitus, peripheral artery disease, hypertension, and hypercholesterolemia. Home medications include insulin, enalapril, carvedilol, aspirin, and rosuvastatin. She appears uncomfortable. Temperature is 37.1°C (99.3°F), pulse is 98/min, and blood pressure is 132/90 mm Hg. Examination reveals a left groin surgical incision. The left lower extremity is swollen, stiff, and markedly tender to palpation. Dorsiflexion of the left foot elicits severe calf pain. Femoral pulses are present bilaterally, and pedal pulses are weaker on the left compared with the right. Urinalysis is normal. Labs: - Hemoglobin: 12.1 g/dL - Leukocyte count: 11,300/mm³ - Platelet count: 189,000/mm³ - Glucose: 222 mg/dL - Creatinine: 1.1 mg/dL Which of the following is the most likely cause of these findings?

The reactive species at a glance

Species	Radical?	Main source	Key point
Superoxide, $O_2^{\bullet -}$	Yes	Electron leak from the mitochondrial respiratory chain; NADPH oxidase in phagocytes; xanthine oxidase	Superoxide dismutase (SOD) turns it into $H_2O_2$
Hydrogen peroxide, $H_2O_2$	No, but reactive	SOD; oxidation reactions in peroxisomes	Removed by catalase and glutathione peroxidase; feeds the Fenton reaction and myeloperoxidase
Hydroxyl radical, ${}^{\bullet}OH$	Yes	Fenton reaction; radiation acting on water	The most reactive; attacks lipids, proteins and DNA
Hypochlorous acid, HOCl	No	Myeloperoxidase in neutrophils ( $H_2O_2 + Cl^-$ )	"Bleach" for killing microbes
Nitric oxide and peroxynitrite	Nitric oxide is	Inducible NO synthase in macrophages; nitric oxide plus superoxide gives peroxynitrite	Reactive nitrogen species

Core idea: injury happens when production exceeds removal, either because generation spikes (radiation, reperfusion, toxins) or because defenses fail (glutathione used up, selenium or vitamin E deficiency).

Where free radicals come from

Source	How it makes radicals	Classic exam setting
Normal metabolism	Electrons leak from the respiratory chain and partly reduce $O_2$ to superoxide	Low-level lifelong exposure; lipofuscin builds up with age
Ionizing radiation	Most DNA damage is indirect, from free radicals formed when radiation hits intracellular water	Radiation therapy and exposure; lymphoid cells, germ cells, marrow and gut epithelium are most sensitive
Inflammation	Respiratory burst: NADPH oxidase makes superoxide, SOD makes $H_2O_2$ , myeloperoxidase makes HOCl	Microbial killing plus bystander tissue damage; absent in chronic granulomatous disease
Drugs and toxins	Cytochrome P450 (mainly CYP2E1) makes reactive metabolites: NAPQI from acetaminophen, the trichloromethyl radical ( ${}^{\bullet}CCl_3$ ) from carbon tetrachloride	Acetaminophen overdose; carbon tetrachloride causing fatty liver, then necrosis
Transition metals	Free iron and copper drive Fenton chemistry: $Fe^{2+} + H_2O_2 \rightarrow Fe^{3+} + OH^- + {}^{\bullet}OH$	Hemochromatosis, transfusional iron overload, Wilson disease, anthracycline cardiotoxicity
High-dose oxygen	Breathing high oxygen concentrations increases ROS production	Bronchopulmonary dysplasia and retinopathy of prematurity in preterm infants
Reperfusion	Burst of ROS when oxygen returns (xanthine oxidase in endothelium, NADPH oxidase in neutrophils, damaged mitochondria), plus calcium overload	PCI or thrombolysis after an MI

How free radicals damage cells

Target	What happens	What you see
Membrane lipids	Lipid peroxidation: radicals attack polyunsaturated fatty acids, and the lipid peroxyl radicals formed propagate a chain reaction	Leaky plasma, mitochondrial and lysosomal membranes; cell contents leak into blood (troponin, AST, ALT)
Proteins	Side-chain oxidation, cross-linking and fragmentation	Lost enzyme activity, misfolding, degradation
DNA	Oxidized bases and single- and double-strand breaks	Cell-cycle arrest or apoptosis; surviving cells can carry mutations that lead to cancer

Mild oxidative stress is repaired. Severe membrane damage pushes the cell into necrosis, while DNA and mitochondrial damage often trigger apoptosis; the morphologic dividing line is laid out in reversible vs irreversible cell injury. Peroxidized lipids and proteins that lysosomes cannot fully digest persist as lipofuscin, the yellow-brown "wear-and-tear" pigment of aging, most abundant in long-lived, non-dividing cells such as neurons and cardiac myocytes.

Antioxidant defenses

Defense	What it does	Board hook
Superoxide dismutase	$2O_2^{\bullet -} + 2H^+ \rightarrow H_2O_2 + O_2$	Copper-zinc SOD1 mutations cause some familial ALS
Catalase	$2H_2O_2 \rightarrow 2H_2O + O_2$	Concentrated in peroxisomes; catalase-positive organisms are the problem in CGD
Glutathione peroxidase	$2GSH + H_2O_2 \rightarrow GSSG + 2H_2O$ ; also reduces lipid peroxides	Selenium-dependent; glutathione reductase regenerates GSH using NADPH from G6PD
Vitamin E (α-tocopherol)	Fat-soluble, chain-breaking antioxidant that stops lipid peroxidation in membranes	Deficiency (fat malabsorption, cystic fibrosis): spinocerebellar ataxia, neuropathy, myopathy, retinopathy, hemolysis
Vitamin C	Water-soluble antioxidant that regenerates vitamin E	Also needed for collagen hydroxylation (scurvy)
Vitamin A (β-carotene)	Dietary antioxidant	Listed with C and E as the antioxidant vitamins
Metal-binding proteins	Transferrin and ferritin hold iron; ceruloplasmin's ferroxidase activity keeps $Fe^{2+}$ out of Fenton chemistry	Overwhelmed in hemochromatosis and Wilson disease

Read the enzymes as a relay: SOD converts superoxide into $H_2O_2$ , and catalase or glutathione peroxidase finishes the job. If $H_2O_2$ lingers near free iron, it becomes the hydroxyl radical.

Classic free radical injuries

Example	The free radical story	Exam facts
Reperfusion injury after PCI or thrombolysis for MI	ROS burst from xanthine oxidase, neutrophil NADPH oxidase and damaged mitochondria; calcium overload; mitochondrial permeability transition pore opens	Reperfusion arrhythmias, myocardial stunning, no-reflow and lethal reperfusion injury; hypercontracted myocytes show contraction band necrosis
Acetaminophen overdose	CYP2E1 makes NAPQI; once glutathione is used up, NAPQI binds cell proteins	Centrilobular (zone 3) necrosis; risk rises with chronic alcohol use and fasting; N-acetylcysteine restores glutathione
Carbon tetrachloride	CYP2E1 makes the trichloromethyl radical, which drives lipid peroxidation	Fatty change at lower exposures, necrosis at higher ones; liver and kidney
Ionizing radiation	Radiation acting on water makes free radicals that break DNA	Rapidly dividing cells die first; late cancer risk
Oxygen toxicity in preterm infants	High inspired oxygen	Bronchopulmonary dysplasia; retinopathy of prematurity (abnormal retinal vessel growth)
Hemochromatosis	Free iron drives Fenton chemistry	Oxidative damage in the liver, heart and pancreas
Wilson disease	Redox-active copper accumulates	Liver, brain and cornea
Doxorubicin cardiotoxicity	Iron-dependent free radical generation in the heart	Dilated cardiomyopathy; dexrazoxane, an intracellular iron chelator, is cardioprotective

Acetaminophen's toxic metabolite NAPQI is an electrophile rather than a free radical, but the injury hinges on losing glutathione, the cell's main antioxidant, which is why it sits on every free radical list.

Practice question

Vignette. A 62-year-old man arrives 2 hours after the onset of crushing substernal chest pain with ST elevation in leads V1–V4. Primary PCI reopens a totally occluded proximal left anterior descending artery. Minutes after flow is restored, he has a brief, self-limited run of ventricular tachycardia. Which mechanism most directly causes the additional myocyte injury that occurs when blood flow returns?

A. Continued ATP depletion from loss of oxidative phosphorylation
B. Lactic acidosis from anaerobic glycolysis
C. A burst of reactive oxygen species with intracellular calcium overload
D. Fas ligand–mediated apoptosis by cytotoxic T cells
E. Immune complex deposition in the coronary microvasculature

Answer: C. When oxygen returns, xanthine oxidase in endothelial cells, NADPH oxidase in incoming neutrophils and damaged mitochondria release a burst of ROS, while calcium floods the cytosol and mitochondria, the mitochondrial permeability transition pore opens and myocytes hypercontract. His arrhythmia is one of the four forms of reperfusion injury, alongside myocardial stunning, microvascular obstruction (no-reflow) and lethal reperfusion injury, which Hausenloy and Yellon estimate may account for up to 50% of final infarct size. Reperfusion is still the treatment of choice; the aim is to limit reperfusion injury, not to avoid reperfusion.

Why not the others?

A. ATP depletion is the injury of ischemia itself; restoring oxygen lets mitochondria make ATP again.
B. Lactic acid accumulates during ischemia. At reperfusion it is washed out, and it is the rapid return to normal pH that contributes to cell death.
D. Fas ligand–driven (extrinsic) apoptosis is one way cytotoxic T cells kill target cells and how activated lymphocytes are deleted; it is not the acute mechanism of reperfusion injury.
E. Immune complex deposition is type III hypersensitivity (serum sickness, vasculitis) and plays no part here.

Exam traps

$H_2O_2$ is not a free radical (no unpaired electron), yet it is the substrate for both the Fenton reaction and myeloperoxidase.
SOD does not neutralize ROS on its own: it converts superoxide into $H_2O_2$ , which catalase or glutathione peroxidase must remove.
Chronic granulomatous disease: no NADPH oxidase, no respiratory burst, so catalase-positive organisms (Staphylococcus aureus, Aspergillus) cause recurrent infections; see the CGD guide.
G6PD deficiency: without NADPH, red cells cannot keep glutathione reduced, so oxidant drugs, fava beans or infection cause hemolysis with Heinz bodies and bite cells; see the G6PD deficiency guide.
Acetaminophen risk goes up with chronic alcohol use (more CYP2E1 activity) and with fasting or malnutrition (less glutathione).
Selenium deficiency weakens glutathione peroxidase; Keshan disease is a selenium-deficiency cardiomyopathy.
Vitamin E deficiency causes ataxia and neuropathy that can mimic B12 deficiency, but without megaloblastic anemia.

Key takeaways

Free radical injury = ROS production outpacing removal.
Sources: radiation (hydroxyl radicals from water), inflammation (NADPH oxidase, myeloperoxidase), CYP450 metabolism (acetaminophen, carbon tetrachloride), Fenton metals (iron, copper), hyperoxia and reperfusion.
Damage: lipid peroxidation, protein oxidation and DNA breaks, ending in necrosis or apoptosis.
Defenses: SOD, then catalase or glutathione peroxidase (selenium, NADPH-dependent glutathione recycling), vitamins E, C and A, and metal-binding proteins.
Classic examples: reperfusion injury, acetaminophen (treat with N-acetylcysteine), carbon tetrachloride (fatty liver), BPD and retinopathy of prematurity, hemochromatosis, Wilson disease, doxorubicin (dexrazoxane) and lipofuscin.