Study Notes on Free Radicals, Antioxidants, and Oxidative Stress

Overview of Free Radicals, Antioxidants, and Oxidative Stress

This study guide compiles exhaustive notes on the topics concerning free radicals, their characteristics, roles in the body, and the antioxidant defense system. The content is structured according to various definitions, types, physiological roles, and implications for health.

Objectives

  • To understand the types and characteristics of free radicals.
  • To explore the role of free radicals in the human body.
  • To examine the antioxidant defense system present in the body.

Definitions of Key Terms

  • Oxidation:

    • Gain in oxygen.
    • Loss of hydrogen.
    • Loss of electrons.

  • Reduction:

    • Loss of oxygen.
    • Gain of hydrogen.
    • Gain of electrons.

  • Oxidant:

    • A substance that oxidizes another chemical by taking electrons, hydrogen, or by adding oxygen.

  • Reductant:

    • A substance that reduces another chemical by supplying electrons, hydrogen, or by removing oxygen.

Understanding Free Radicals

  • Free Radical:
    • A molecule or molecular fragment that contains one or more unpaired electrons in its outer orbit, denoted by a superscript dot (e.g., R•).

Reactive Oxygen Species (ROS)

  • Definition:

    • Highly reactive products of molecular oxygen's partial reduction.

  • Types of Reactive Oxygen Species:

    • Radicals:

    • $O_2^−$ (Superoxide)

    • $H2O2$ (Hydrogen Peroxide)

    • $•OH$ (Hydroxyl Radical)

    • $RO_2•$ (Peroxyl)

    • $O_3$ (Ozone)

    • $RO•$ (Alkoxyl)

    • $HO_2•$ (Hydroperoxyl)

    • Non-Radicals:

    • These are strong oxidizing agents that facilitate the formation of strong oxidants, such as transition metals (e.g., $Fe^{3+}$).

Reactive Nitrogen Species (RNS)

  • Types of RNS:

    • Radicals:

    • $NO•$ (Nitric Oxide)

    • $ONOO−$ (Peroxynitrite)

    • $NO_2•$ (Nitrogen Dioxide)

    • Non-Radicals:

    • $ROONO$ (Alkyl Peroxynitrites)

Sources of Reactive Oxygen Species (ROS)

  • Exogenous Sources:

    • Fried foods.
    • Tobacco smoke.
    • Alcohol.
    • Pesticides.
    • Air pollutants.

  • Endogenous Sources:

    • Mitochondrial Electron Transport Chain (ETC): Leakage of superoxide anions occurs.
    • Enzymatic actions such as those from aldehyde oxidase, nitric oxide synthase, cyclooxygenase, and lipoxygenase.
    • Action of Xanthine oxidase produces superoxide and hydrogen peroxide.
    • Macrophages produce nitric oxide (NO•) that combines with free radicals.

Physiological Functions of Free Radicals

  • Functions Include:
    • Smooth muscle relaxation.
    • Neurotransmission.
    • Immune regulation, such as phagocytosis, which helps destroy invading microorganisms.

Cellular Damage by Free Radicals

  • Impacts on Biomolecules:
    • Free radicals damage cellular integrity due to their high reactivity.
    • They react with proteins, membrane lipids, carbohydrates, and nucleic acids, affecting their normal functions.
Specific Types of Damage:
  1. Lipid Peroxidation:
    • Lipids, particularly polyunsaturated fatty acids, undergo peroxidation, damaging membrane structures.
    • Initiation of lipid peroxy radicals leading to the formation of malondialdehyde as a marker for lipid peroxidation.
  2. Membrane Structural Changes:
    • Altered fluidity and ion permeability.
    • Formation of adducts/crosslinks with non-lipids (e.g., DNA) associated with DNA damage and mutagenesis.
  3. Effects on Carbohydrates:
    • Hydroxyl radicals react with carbohydrates, affecting synovial fluid levels in joints.
  4. Effects on Proteins:
    • Amino acids such as proline and methionine are susceptible to oxidative damage, leading to fragmentation and susceptibility to proteolytic digestion.
  5. Oxidative Damage to DNA:
    • Causes strand breaks and base alterations, which can lead to mutations and cancer (notably through the formation of 8−OHdG).
  6. Cellular Consequences:
    • Increase in calcium ion flux, cell swelling, and sodium ion permeability leading to further cellular damage.

Antioxidant Defense System in Cells

General Overview
  • Function:
    • Cells utilize an antioxidant defense mechanism to combat ROS toxicity through scavenging actions.
Types of Antioxidants
  • Preventive Antioxidants:
    • Inhibit initial production of free radicals (e.g., catalase, glutathione peroxidase).
  • Chain-Breaking Antioxidants:
    • Interrupt chain reactions (e.g., superoxide dismutase, Vitamin E, selenium, glutathione peroxidase).

Major Antioxidant Enzymes

  1. Superoxide Dismutase (SOD):
    • Catalyzes the dismutation of superoxide anion in cytoplasm and mitochondria.
  2. Catalase:
    • Mainly present in peroxisomes, decomposes excess hydrogen peroxide into water and oxygen.
  3. Glutathione Peroxidase (GPx):
    • Reduces hydrogen peroxide and lipid hydroperoxides using reduced glutathione.
  4. Glutathione Reductase (GR):
    • Converts oxidized glutathione (GSSG) back into its reduced form (GSH) in presence of NADPH.

Non-Enzymatic Antioxidants

  • Water-Soluble:
    • Glutathione (GSH), uric acid, ascorbate (Vitamin C).
  • Lipid-Soluble:
    • Alpha-tocopherol (Vitamin E), beta-carotene, coenzyme Q.

Implications of Oxidative Stress

  • Definition:
    • An imbalance between prooxidants (free radicals) and antioxidants leading to cellular injury.
Associated Diseases and Disorders
  1. Chronic and Acute Inflammation:
    • Various inflammatory conditions produce free radicals.
  2. Respiratory Diseases:
    • Prolonged high oxygen exposure leads to issues like bronchopulmonary dysplasia and ARDS.
  3. Cataracts:
    • Oxidative stress contributes to cataract formation and retinopathy in prematurity.
  4. Atherosclerosis and Myocardial Infarction:
    • Oxidized LDL contributes to plaque formation via macrophage recruitment.
  5. Skin Diseases:
    • Certain plant products used in treatment lead to radical generation but yield clinical benefits.
  6. Chronic Granulomatous Disease:
    • Leads to increased susceptibility to infections due to defective phagocytic function.
  7. Roles in Aging:
    • Free radicals contribute to neurodegenerative disorders and overall aging processes.
  8. Carcinogenesis:
    • Free radicals induce mutations in DNA, leading to cancer.

Summary of Key Points

  • Free radicals are defined as molecular species with unpaired electrons, displaying high reactivity with short life spans.
  • Types include superoxide, hydroxyl radical, hydrogen peroxide, nitric oxide, lipid peroxide radical, etc.
  • They are involved in both physiological functions (e.g., neurotransmission, immune regulation) and cellular damage, particularly oxidizing biomolecules like lipids, proteins, DNA, and carbohydrates.

Final Thought

  • Understanding the roles of free radicals and antioxidants is crucial for recognizing their relationships to various diseases and for exploring potential therapeutic interventions.