FORMATION

Reactive oxygen species (ROS) are formed during incomplete reduction of O₂:

• Superoxide radical, •O₂^- - from the addition of one electron, mostly from electron transport reactions
• Hydrogen peroxide, H₂O₂ - from the addition of two electrons
• Hydroxyl radical, •OH - from the addition of three electrons
• Singlet oxygen, ¹O₂ - mostly energy from excited chlorophyll
• Ozone, O₃ - from air pollution

There are now known to be numerous pathways generating endogenous reactive oxygen species. These include:

• Respiration: leakage from the electron transport chain, e.g. from ubiquinone (producing •O₂^-)
• Stimulation of phagocytes:
  e.g. the enzyme complex NADPH oxidase of macrophages and neutrophils (producing •O₂^-), or the enzyme myeloperoxidase in neutrophils (producing HOCl)
• Fenton reactions:
  in the presence of transition metals (producing •OH)

Free radicals

Processes in the organism itself are responsible for formation of free radicals. These include:

• The electron transport chain
• Oxidizing enzyme systems

Environmental factors which create free radicals:

• Alcohol, recreational drugs
• Pharmaceutical drugs
• Smoking
• Pollution
• Exercise
• Ionizing radiation
• UV radiation