Free Radicals, Antioxidants & Xenobiotic Metabolism

Free radicals are molecules with an unpaired electron, hungry to steal electrons from DNA, proteins, and membranes. The most important are reactive oxygen species (ROS) leaking from mitochondria. Antioxidants are the defence: molecules and enzymes that neutralise radicals safely. Xenobiotic metabolism is the liver's chemical plant for dismantling foreign compounds — drugs, pollutants, food toxins — mostly by making them water-soluble enough for the kidneys to excrete.

This matters because these systems decide drug safety. Paracetamol overdose kills through a toxic metabolite, grapefruit juice dangerously raises drug levels by blocking a key enzyme, and antioxidant defences determine how much smoke or radiation damage a cell survives.

Key Concepts

• ROS trio: superoxide (O₂•⁻), hydrogen peroxide (H₂O₂), and the hydroxyl radical (•OH) — the last being the most destructive molecule in biology, reacting within nanoseconds.
• Radical sources: mitochondrial electron leak, immune-cell respiratory burst, cigarette smoke, UV radiation, and air pollution.
• Antioxidant enzymes: superoxide dismutase (SOD) converts superoxide to H₂O₂; catalase and glutathione peroxidase convert H₂O₂ to water. Non-enzymatic defenders include glutathione and vitamins C and E.
• Oxidative stress: the imbalance when radical production overwhelms antioxidant defence — implicated in ageing, atherosclerosis, and neurodegeneration.
• Phase I metabolism: mostly oxidation by cytochrome P450 enzymes (CYPs) — adds or exposes reactive groups, often a single oxygen atom.
• Phase II metabolism: conjugation — attaching glucuronic acid, sulfate, or glutathione, making the product large, polar, and excretable.

Formulas

Phase I hydroxylation mass change: adding one oxygen atom increases a molecule's mass by 16 Da (the atomic mass of oxygen). So: metabolite mass = parent drug mass + 16 Da. Assumptions: a single hydroxylation event with no other change; limits: further Phase II conjugation adds much larger masses (glucuronic acid adds 176 Da), and some reactions remove groups instead.

Worked Example

Predict a hydroxylated metabolite's mass. A drug has a molecular mass of 300 Da and undergoes one Phase I hydroxylation. Step 1: add the oxygen — 300 + 16 = 316 Da. Step 2: the percentage mass gain — 16 ÷ 300 = 0.0533, or about 5.3%. Mass spectrometers in drug labs hunt for exactly this +16 Da signature to identify metabolites in blood samples — a tiny mass shift that proves the liver has processed the drug.

Common Misconceptions

• Antioxidant supplements prevent disease. Large trials of high-dose vitamin E and beta-carotene showed no benefit and sometimes harm; the body's own enzyme defences do the heavy lifting.
• Free radicals are purely evil. Immune cells deliberately make ROS to kill engulfed bacteria, and low-level ROS act as cellular signals — the dose makes the poison.
• Detox diets cleanse the liver. The liver detoxifies itself through CYP enzymes and conjugation; no juice adds a pathway the liver lacks.

Real-World Applications

• Paracetamol overdose depletes glutathione, letting a toxic metabolite destroy the liver — N-acetylcysteine treatment restores glutathione.
• Grapefruit juice blocks intestinal CYP3A4, raising blood levels of many drugs into the toxic range.
• Drug interactions are predicted from which CYP enzyme metabolises each drug — shared enzymes mean competition.
• Smokers' high ROS load explains part of their accelerated ageing and cancer risk.

Free radicals are unpaired-electron molecules whose damage is held in check by SOD, catalase, and glutathione, while xenobiotic metabolism runs drugs through Phase I oxidation (+16 Da per hydroxylation) and Phase II conjugation for excretion. Oxidative stress marks the imbalance, first-pass metabolism taxes every oral drug, and CYP enzymes explain grapefruit interactions and paracetamol poisoning alike.