Free Radical Generation and Antioxidant Defense

Free Radicals: Basics - Unstable Atoms Alert

• Atoms or molecules with one or more unpaired electrons in their outer orbital; highly unstable and reactive.
• Seek stability by oxidizing (stealing electrons from) or reducing (donating electrons to) other molecules.
• Key Types:
  • Reactive Oxygen Species (ROS):
    • Superoxide ($O_2^{\cdot-}$)
    • Hydroxyl radical ($\cdot OH$) - extremely reactive.
    • Hydrogen peroxide ($H_2O_2$) (not a radical, but a key ROS)
  • Reactive Nitrogen Species (RNS):
    • Nitric oxide ($NO\cdot$)
    • Peroxynitrite ($ONOO^-$)

⭐ The hydroxyl radical ($\cdot OH$) is the most reactive free radical, causing significant damage to DNA, lipids, and proteins. It has a very short half-life (nanoseconds).

FR Generation - Where Mischief Brews

• Free Radicals (FRs): Unstable molecules, unpaired electrons, cause cellular damage.
• Endogenous Sources (Cellular Metabolism):
  • Mitochondrial ETC: Major site; 'leaks' superoxide ($O_2^{\cdot-}$).
  • NADPH Oxidase: Phagocytes for microbial killing (respiratory burst).
  • Xanthine Oxidase: Purine degradation to uric acid.
  • Fenton Reaction: $Fe^{2+} + H_2O_2 \rightarrow Fe^{3+} + \cdot OH + OH^-$. Iron/copper catalyzes hydroxyl radical ($\cdot OH$) formation.
  • Peroxisomal metabolism: e.g., fatty acid oxidation.
• Exogenous Sources (Environmental):
  • Radiation: UV light, X-rays, gamma rays.
  • Pollutants: Air pollution, industrial chemicals.
  • Drugs: e.g., paracetamol overdose (OD).
  • Cigarette smoke: Rich in FRs.
  • Inflammation: Chronic inflammation ↑ FRs.

⭐ The hydroxyl radical, often formed via the Fenton reaction, is considered the most reactive and damaging oxygen-derived free radical.

FR Damage - Radicals on Rampage

Free radicals wreak havoc on cellular components:

• Lipid Peroxidation: Polyunsaturated fatty acids (PUFAs) in membranes targeted.
  • Chain reaction: initiation, propagation, termination.
  • Markers: Malondialdehyde (MDA), 4-hydroxynonenal (4-HNE).
  • Damage: ↑ Membrane permeability, ↓ fluidity, cell lysis.
• Protein Oxidation: Amino acid residues (Cys, Met) & protein backbone attacked.
  • Markers: Protein carbonyls, 3-nitrotyrosine.
  • Damage: Enzyme inactivation, protein aggregation & misfolding.
• DNA Damage: Bases (esp. Guanine) & deoxyribose sugar affected.
  • Marker: $8-OHdG$ (8-hydroxy-2'-deoxyguanosine).
  • Damage: Mutations, DNA strand breaks, carcinogenesis.

⭐ 8-OHdG is a critical biomarker for oxidative DNA damage, frequently elevated in various cancers and degenerative diseases, making it a common exam topic related to free radical injury assessment and antioxidant efficacy studies.

Antioxidant Defenses - Body's Superheroes

• Enzymatic Systems: Neutralize ROS. 📌 Mnemonic: Can Super Glue Protect? (Catalase, SOD, GPx)
  • Superoxide Dismutase (SOD): Converts $O_2^{\cdot-}$ to $H_2O_2$. (Cu-Zn, Mn, EC types).
  • Catalase (Peroxisomes): Degrades $H_2O_2$: $2H_2O_2 \rightarrow 2H_2O + O_2$.
  • Glutathione Peroxidase (GPx-Se): Selenium-dependent. Reduces $H_2O_2$/lipid peroxides: $H_2O_2 + 2GSH \rightarrow GSSG + 2H_2O$.
  • Glutathione Reductase: Regenerates GSH from GSSG (uses NADPH).
• Non-Enzymatic Scavengers:
  • Glutathione (GSH): (\gamma)-Glu-Cys-Gly. Major intracellular antioxidant.
  • Vitamin E ((\alpha)-tocopherol): Lipid-soluble; protects membranes. Chain-breaking.
  • Vitamin C (Ascorbic acid): Water-soluble; regenerates Vit E; scavenges $O_2^{\cdot-}, OH^{\cdot}$.
  • Uric Acid: Plasma antioxidant (purine metabolism).
  • Bilirubin: Antioxidant (heme breakdown).
  • Others: Carotenoids, flavonoids, melatonin.

⭐ Glutathione (GSH) depletion ↑ oxidative stress susceptibility (e.g., Parkinson's, liver damage).

Oxidative Stress & Disease - Health Under Siege

• Definition: Imbalance: ↑Reactive Oxygen/Nitrogen Species (ROS/RNS) vs. ↓Antioxidant capacity, causing cellular damage.
• Key Damage Markers:
  • Lipids: Malondialdehyde (MDA).
  • Proteins: Carbonyls.
  • DNA: 8-hydroxy-2'-deoxyguanosine (8-OHdG).
• Associated Diseases:
  • Atherosclerosis (LDL oxidation).
  • Neurodegeneration (Alzheimer's, Parkinson's).
  • Cancer (mutagenesis).
  • Ischemia-reperfusion injury.
  • Diabetes, chronic inflammation, aging.

⭐ 8-OHdG is a key biomarker for oxidative DNA damage, often elevated in various cancers and chronic diseases associated with oxidative stress.

High‑Yield Points - ⚡ Biggest Takeaways

• Key Reactive Oxygen Species (ROS): superoxide (O₂⁻•), hydroxyl radical (•OH), hydrogen peroxide (H₂O₂).
• Fenton reaction (Fe²⁺ + H₂O₂) generates highly reactive hydroxyl radicals (•OH).
• Superoxide Dismutase (SOD) (Mn-mitochondrial, Cu/Zn-cytosolic) converts O₂⁻• to H₂O₂.
• H₂O₂ is neutralized by Catalase and Selenium-dependent Glutathione Peroxidase.
• Glutathione (GSH), a key antioxidant, is regenerated by Glutathione Reductase (uses NADPH).
• Important antioxidant vitamins: Vitamin E (lipid-soluble), Vitamin C, and β-carotene.
• Lipid peroxidation damages membranes; marker: Malondialdehyde (MDA).