Oxygen Toxicity and Free Radicals

Free Radicals - Unstable Troublemakers

• Highly reactive species with unpaired electrons, causing cellular damage (oxidative stress).
• Types:
  • Reactive Oxygen Species (ROS):
    • Superoxide: $O_2•⁻$
    • Hydroxyl radical: $•OH$ (most reactive)
    • Hydrogen peroxide: $H_2O_2$ (key ROS precursor)
  • Reactive Nitrogen Species (RNS):
    • Nitric oxide: $NO•$
    • Peroxynitrite: $ONOO⁻$
• Sources:
  • Endogenous:
    • Mitochondrial ETC (electron leaks at Complex I & III).
    • Phagocytosis (NADPH oxidase).
    • Peroxisomal metabolism.
    • Cytochrome P450 enzymes.
    • Inflammation.
  • Exogenous:
    • Radiation (UV, X-rays).
    • Pollution, smoking.
    • Drugs (e.g., doxorubicin).
    • Heavy metals (e.g., Fe, Cu via Fenton reaction: $Fe^{2+} + H_2O_2 \rightarrow Fe^{3+} + •OH + OH⁻$).

⭐ NADPH oxidase in neutrophils generates superoxide ($O_2•⁻$) during respiratory burst to kill bacteria.

Oxidative Damage - Cellular Mayhem

Excess Reactive Oxygen/Nitrogen Species (ROS/RNS) cause widespread cellular injury.

• Lipids (Cell Membranes):
  • Peroxidation of Polyunsaturated Fatty Acids (PUFAs).
  • Products: Malondialdehyde (MDA), 4-hydroxynonenal (4-HNE) - key biomarkers of lipid damage.
  • Results in ↑ membrane permeability, altered fluidity, potential cell lysis.
• Proteins:
  • Oxidation of amino acid side chains (e.g., sulfhydryl groups, methionine).
  • Formation of protein carbonyls (markers).
  • Leads to fragmentation, cross-linking, aggregation, enzyme inactivation.
• DNA (Nuclear & Mitochondrial):
  • Base modifications (e.g., 8-hydroxydeoxyguanosine/8-OHdG - a mutagenic lesion).
  • Single/double-strand breaks, DNA-protein crosslinks.
  • Contributes to mutagenesis, carcinogenesis, apoptosis.
• Carbohydrates:
  • Oxidation and fragmentation, advanced glycation end-products (AGEs) precursors.

Key Reactions Producing •OH (Hydroxyl Radical):

• Fenton Reaction: $Fe^{2+} + H_2O_2 \rightarrow Fe^{3+} + \cdot OH + OH^{-}$ (metal-catalyzed)
• Haber-Weiss Reaction: $O_2^{\cdot-} + H_2O_2 \rightarrow \cdot OH + OH^{-} + O_2$

⭐ 8-hydroxydeoxyguanosine (8-OHdG) is a critical biomarker for oxidative DNA damage; its presence indicates mutagenic lesions and is linked to ↑ cancer risk.

![Image of cellular components damaged by free radicals]

Antioxidant Defenses - Body's Guardians

• Enzymatic Defenses:

  • Superoxide Dismutase (SOD): Converts superoxide ($O_2•⁻$) to $H_2O_2$. $2O_2•⁻ + 2H⁺ \rightarrow H_2O_2 + O_2$ (Cytosolic Cu/Zn-SOD, Mitochondrial Mn-SOD).
  • Catalase: Decomposes $H_2O_2$ to $H_2O + O_2$. $2H_2O_2 \rightarrow 2H_2O + O_2$ (Located in Peroxisomes).
  • Glutathione Peroxidase (GPx): Reduces $H_2O_2$ & lipid peroxides using GSH. Requires Selenium (Se). $2GSH + H_2O_2 \rightarrow GSSG + 2H_2O$
  • Glutathione Reductase (GR): Regenerates GSH from GSSG. Requires NADPH (Niacin) & FAD (Riboflavin). $GSSG + NADPH + H⁺ \rightarrow 2GSH + NADP⁺$

• Non-Enzymatic Antioxidants:

  • Vitamins:
    • Vit E (α-tocopherol): Lipid-soluble; protects membranes from lipid peroxidation.
    • Vit C (Ascorbic acid): Water-soluble; regenerates Vit E; direct ROS scavenger.
    • Vit A (Carotenoids, e.g., β-carotene): Lipid-soluble; singlet oxygen quenchers.
  • Endogenous Molecules:
    • Glutathione (GSH): Tripeptide (γ-glutamylcysteineglycine); major intracellular redox buffer.
    • Uric Acid: End product of purine metabolism; potent scavenger.
    • Melatonin: Neurohormone; effective radical scavenger.
  • Dietary Phytochemicals:
    • Flavonoids: Polyphenolic compounds from plants; antioxidant activity.

⭐ Glutathione (GSH) is the most abundant intracellular antioxidant; its depletion is a key marker of oxidative stress.

Oxygen Toxicity - When Good Gas Turns Bad

Hyperoxia (↑ $FiO_2$) → ↑ Reactive Oxygen Species (ROS) like $O_2^{\cdot-}$, $H_2O_2$, $\cdot OH$. Overwhelms antioxidant defenses (e.g., SOD, catalase, glutathione peroxidase).

• Lungs (Lorrain Smith Effect):
  • Acute tracheobronchitis, cough, substernal pain.
  • Diffuse Alveolar Damage (DAD), ARDS-like changes; ↓ Surfactant, atelectasis.
• CNS (Paul Bert Effect):
  • Seizures (tonic-clonic), visual changes (tunnel vision), tinnitus, muscle twitching.
  • Nausea, anxiety.
• Eyes:
  • Retinopathy of Prematurity (ROP) in neonates: retinal vasoconstriction → neovascularization.

⭐ Paul Bert effect (CNS toxicity) typically occurs with $O_2$ pressures >2-3 atmospheres absolute (ATA); Lorrain Smith effect (pulmonary toxicity) with prolonged exposure to $O_2$ partial pressures >0.5 ATA.

• Related Conditions: Ischemia-Reperfusion Injury, ARDS (oxygen can exacerbate).

High‑Yield Points - ⚡ Biggest Takeaways

• Reactive Oxygen Species (ROS) include superoxide (O₂⁻•), hydrogen peroxide (H₂O₂), and hydroxyl radical (•OH).
• Key enzymatic antioxidants: Superoxide Dismutase (SOD), Catalase, Glutathione Peroxidase.
• Non-enzymatic antioxidants: Vitamin E, Vitamin C, glutathione.
• ROS cause lipid peroxidation, protein damage, and DNA damage.
• Fenton reaction (Fe²⁺ + H₂O₂) generates the highly reactive hydroxyl radical (•OH), the most damaging ROS.
• NADPH oxidase in phagocytes produces superoxide for bacterial killing (respiratory burst).
• Glutathione peroxidase requires selenium; SOD requires Cu, Zn (cytosolic) or Mn (mitochondrial).