Free Radicals: What They Are & How They Affect Your Cells

Free radicals are atoms or molecules with one or more unpaired electrons in their outer shell. This unpaired electron makes them highly reactive, as they seek to stabilize by stealing electrons from or donating electrons to other molecules. Common examples include superoxide radical (O₂•⁻), hydroxyl radical (•OH), and nitric oxide (NO•).

Free radicals are produced through multiple pathways: (1) Mitochondrial oxidative phosphorylation — the electron transport chain leaks 1-3% of electrons that form superoxide; (2) NADPH oxidases in immune cells that deliberately produce ROS to kill pathogens; (3) Cytochrome P450 enzymes during detoxification; (4) Peroxisomes during fatty acid oxidation; (5) Inflammation and immune responses; (6) Exposure to UV radiation, pollution, and toxins.

No. Free radicals serve essential physiological functions at appropriate concentrations: cell signaling and gene expression regulation, immune defense against pathogens, vasodilation through nitric oxide, thyroid hormone synthesis, and apoptosis (programmed cell death) to remove damaged cells. The key is balance — both deficiency and excess cause problems.

ROS (reactive oxygen species) is a broader term that includes both free radicals (superoxide, hydroxyl radical) and non-radical oxidants (hydrogen peroxide, singlet oxygen, peroxynitrite). All oxygen-centered free radicals are ROS, but not all ROS are free radicals. Hydrogen peroxide (H₂O₂) is the most important non-radical ROS — it's not a free radical but can generate the highly reactive hydroxyl radical via the Fenton reaction.

Free radicals, especially the hydroxyl radical, attack DNA bases and the sugar-phosphate backbone. The most common lesion is 8-oxo-deoxyguanosine (8-OHdG), which can mispair with adenine during replication, causing G→T mutations. Each cell sustains 10,000-100,000 oxidative DNA lesions daily, but base excision repair pathways fix most damage. Unrepaired damage accumulates with age and contributes to cancer and aging.

Through lipid peroxidation — a chain reaction where free radicals attack polyunsaturated fatty acids in cell membranes. The process: (1) Initiation — a free radical abstracts a hydrogen from a fatty acid; (2) Propagation — the resulting lipid radical reacts with oxygen to form a lipid peroxyl radical, which attacks neighboring fatty acids; (3) Termination — when two radicals combine or antioxidants intervene. One initiated radical can oxidize hundreds of fatty acids, producing reactive aldehydes like malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE).

The hydroxyl radical (•OH) is the most reactive and damaging free radical in biological systems. It reacts with virtually all cellular components at diffusion-limited rates (~10¹⁰ M⁻¹s⁻¹), meaning it attacks the first molecule it encounters. There are no enzymatic defenses against it — the only protection is preventing its formation. It's produced primarily via the Fenton reaction (iron + hydrogen peroxide) and radiation exposure.

The Fenton reaction is a chemical reaction where ferrous iron (Fe²⁺) reacts with hydrogen peroxide (H₂O₂) to produce the hydroxyl radical (•OH), ferric iron (Fe³⁺), and hydroxide ion: Fe²⁺ + H₂O₂ → Fe³⁺ + •OH + OH⁻. This is the primary mechanism by which iron overload causes tissue damage in conditions like hemochromatosis. It explains why free iron is tightly bound to proteins (transferrin, ferritin) in the body.

Antioxidants donate electrons to free radicals without becoming dangerously reactive themselves. They work through: (1) Direct scavenging — vitamins C and E donate electrons to neutralize radicals; (2) Enzymatic dismutation — SOD converts superoxide to hydrogen peroxide; (3) Decomposition — catalase and glutathione peroxidase break down H₂O₂ to water; (4) Metal chelation — binding iron and copper to prevent Fenton reactions; (5) Regeneration — vitamin C regenerates oxidized vitamin E.

Superoxide dismutase (SOD) is an enzyme that catalyzes the dismutation of superoxide radical into oxygen and hydrogen peroxide: 2O₂•⁻ + 2H⁺ → O₂ + H₂O₂. There are three isoforms: SOD1 (cytoplasmic, Cu/Zn-dependent), SOD2 (mitochondrial, Mn-dependent), and SOD3 (extracellular). SOD2 knockout mice die within 3 weeks of birth, demonstrating superoxide's lethality when unchecked.

Free radicals contribute to cancer development through multiple mechanisms: DNA mutations that activate oncogenes or inactivate tumor suppressors, chronic inflammation that promotes tumor growth, activation of signaling pathways (NF-κB, MAPK) that drive proliferation, and damage to proteins involved in DNA repair. However, cancer is multifactorial — free radicals are one piece of a complex puzzle involving genetics, environment, and lifestyle.

Cigarette smoke contains over 10¹⁵ free radicals per puff — among the highest environmental exposures. It delivers: (1) Gas-phase radicals that directly damage lung tissue; (2) Tar-phase radicals that persist in tissues; (3) Heavy metals (cadmium, lead) that catalyze ROS production; (4) Depletion of lung antioxidants, particularly glutathione. Smokers have 20-30% lower vitamin C levels and significantly elevated oxidative DNA damage markers.

Peroxynitrite (ONOO⁻) is a reactive nitrogen species formed when superoxide radical reacts with nitric oxide at near-diffusion-limited rates (~10¹⁰ M⁻¹s⁻¹) — faster than SOD can dismutate superoxide. It's extremely damaging: nitrates tyrosine residues in proteins (disrupting function), oxidizes lipids and DNA, and activates PARP enzymes (depleting NAD⁺). Peroxynitrite formation is a key mechanism in inflammation, neurodegeneration, and cardiovascular disease.

Exercise acutely increases ROS production 2-10 fold through increased mitochondrial oxygen consumption, but this triggers beneficial adaptations (mitohormesis): upregulation of antioxidant enzymes (SOD, catalase, GPx), increased mitochondrial biogenesis via PGC-1α, improved insulin sensitivity, and enhanced DNA repair capacity. Regular exercisers have lower baseline oxidative stress despite acute increases — the key is consistent moderate-to-vigorous activity with adequate recovery.

Proposed by Denham Harman in 1956, the free radical theory of aging posits that accumulated oxidative damage to cellular components causes aging. While influential, it has evolved: the mitochondrial theory of aging emphasizes mitochondrial DNA mutations and dysfunction; the redox stress hypothesis focuses on disrupted redox signaling rather than simple damage accumulation. Antioxidant supplementation trials largely failed to extend lifespan, suggesting the reality is more nuanced — ROS are both damaging agents and essential signaling molecules.