Reactive oxygen species, formation and significance, antioxidants.

Introduction

Reactive oxygen species (ROS) are highly reactive molecules derived from molecular oxygen. They are continuously produced in cells as byproducts of aerobic metabolism, especially in the mitochondria. While low levels of ROS function in cell signaling, excessive accumulation leads to oxidative stress, damaging lipids, proteins, and DNA. To counteract this, organisms have evolved a complex system of antioxidants and enzymatic defenses to maintain redox homeostasis.

Sources of reactive oxygen species

1. Mitochondrial electron transport chain (ETC)

  • Main source of ROS under physiological conditions
  • Complexes I and III leak electrons to oxygen, forming superoxide (O₂⁻·)

2. Enzymatic reactions

  • Xanthine oxidase
  • Cytochrome P450 enzymes
  • NADPH oxidase (NOX) – important in phagocytes during respiratory burst

3. Environmental factors

  • UV radiation
  • Ionizing radiation
  • Pollutants, drugs, toxins

Types of reactive oxygen species

ROS Description
Superoxide (O₂⁻·) Formed by one-electron reduction of O₂
Hydrogen peroxide (H₂O₂) Not a radical, but membrane-permeable
Hydroxyl radical (·OH) Highly reactive, damages all biomolecules
Singlet oxygen (¹O₂) Excited-state form of oxygen
Peroxyl radicals (ROO·) Lipid peroxidation intermediates

Source: Halliwell B, Gutteridge JMC. Free Radicals in Biology and Medicine. 5th ed. 2015.

Oxidative damage

Reactive oxygen species can damage all classes of biomolecules, especially under conditions of oxidative stress. Key types of oxidative damage include:

1. Lipid peroxidation

ROS attack polyunsaturated fatty acids in membrane lipids, initiating a chain reaction that compromises membrane integrity. The process consists of three stages:

  • Initiation: RH (lipid) + ⋅OH → R⋅ + H2​O
  • Propagation: R⋅ + O2 ​→ ROO⋅ and ROO⋅ + RH → ROOH + R⋅
  • Termination: R⋅ + R⋅ → R−R / ROO⋅ + ROO⋅ → non−radical products

Byproducts like malondialdehyde (MDA) and 4-hydroxynonenal (HNE) are commonly measured markers of lipid peroxidation.

2. Protein oxidation

ROS can oxidize amino acid side chains (notably cysteine and methionine), induce protein fragmentation, or form carbonyl groups. This results in:

  • Loss of enzymatic activity
  • Protein misfolding
  • Increased susceptibility to proteolysis
  • Aggregation (as seen in neurodegenerative diseases)

3. DNA damage

ROS, especially hydroxyl radicals, can cause:

  • Base modifications (e.g., 8-oxo-guanine, a mutagenic lesion)
  • Single- and double-strand breaks
  • Cross-linking between DNA and proteins

These changes can result in mutations, carcinogenesis, or cell death if not repaired.

Antioxidant defense systems

Divided into enzymatic and non-enzymatic systems:

Enzymatic antioxidants

1. Superoxide dismutase (SOD)

2O2−⋅​ + 2H+ → H2​O2 ​+ O2​

  • Cytosolic: Cu/Zn-SOD (SOD1)
  • Mitochondrial: Mn-SOD (SOD2)
  • Extracellular: EC-SOD (SOD3)

2. Catalase

2H2​O2 ​→ 2H2​O + O2​

Located mainly in peroxisomes

3. Glutathione peroxidase (GPx)

H2​O2​+ 2GSH → 2H2​O + GSSG

  • Selenium-dependent
  • Reduces lipid hydroperoxides and H₂O₂
  • Requires glutathione reductase and NADPH to regenerate GSH

Source: Devlin TM. Textbook of Biochemistry with Clinical Correlations. 7th ed. 2010.

Non-enzymatic antioxidants

Antioxidant Source Function
Vitamin E (α-tocopherol) Vegetable oils, nuts Lipid-soluble; protects membranes
Vitamin C (ascorbate) Citrus fruits, vegetables Water-soluble; regenerates vitamin E
Glutathione (GSH) Endogenous tripeptide Detoxifies peroxides via GPx
Uric acid Endogenous (purine catabolism) Scavenger of singlet oxygen
Flavonoids, carotenoids Fruits, vegetables Quench free radicals

Redox signaling

Low/moderate ROS levels act as second messengers in pathways controlling:

  • Cell proliferation
  • Differentiation
  • Inflammation
  • Apoptosis

Regulation is mediated through:

  • NF-κB
  • Nrf2 (nuclear factor erythroid 2–related factor 2) → Activates transcription of antioxidant response genes

Oxidative stress

Occurs when ROS production exceeds antioxidant capacity. Linked to numerous diseases:

Condition Mechanism of oxidative injury
Atherosclerosis LDL oxidation → foam cells, inflammation
Cancer DNA damage → mutations, proliferation
Neurodegeneration Protein aggregation (e.g., in Alzheimer’s, Parkinson’s)
Diabetes mellitus ROS from hyperglycemia damages endothelium
Ischemia-reperfusion Sudden ROS burst damages tissues after reoxygenation
Inflammatory diseases Neutrophils/macrophages generate ROS

Source: Valko M et al. Free radicals and antioxidants in normal physiological functions and human disease. Int J Biochem Cell Biol. 2007;39(1):44–84.

Antioxidant therapies

Lifestyle & dietary

  • Diet rich in fruits and vegetables
  • Avoidance of smoking and pollutants
  • Physical activity (improves endogenous antioxidant response)

Pharmacological

  • N-acetylcysteine (NAC) → GSH precursor
  • Vitamin C and E supplements (controversial efficacy)
  • Coenzyme Q10, melatonin, flavonoids

→ Note: Excessive antioxidant supplementation may blunt beneficial effects of physiological ROS

Summary

Reactive oxygen species are normal byproducts of aerobic metabolism and play important roles in cell signaling. However, when in excess, they cause oxidative damage to cellular macromolecules. Cells are protected by a wide array of enzymatic and non-enzymatic antioxidants. Imbalance in this system can lead to oxidative stress, contributing to a wide spectrum of diseases. Understanding the dual role of ROS is crucial for targeting oxidative stress in clinical practice.