Reactive oxygen species

Reactive oxygen species (ROS) are chemically reactive chemical species containing oxygen. Examples include peroxides, superoxide, hydroxyl radical, and singlet oxygen.

In a biological context, ROS are formed as a natural byproduct of the normal metabolism of oxygen and have important roles in cell signaling and homeostasis. However, during times of environmental stress (e.g., UV or heat exposure), ROS levels can increase dramatically. This may result in significant damage to cell structures. Cumulatively, this is known as oxidative stress. ROS are also generated by exogenous sources such as ionizing radiation.

The reduction of molecular oxygen (O₂) produces superoxide (^•O−
2) and is the precursor of most other reactive oxygen species:

Dismutation of superoxide produces hydrogen peroxide (H₂O₂):

Hydrogen peroxide in turn may be partially reduced to hydroxyl radical (^•OH) or fully reduced to water:

Exogenous ROS can be produced from pollutants, tobacco, smoke, drugs, xenobiotics, or radiation.

Ionizing radiation can generate damaging intermediates through the interaction with water, a process termed radiolysis. Since water comprises 55–60% of the human body, the probability of radiolysis is quite high under the presence of ionizing radiation. In the process, water loses an electron and becomes highly reactive. Then through a three-step chain reaction, water is sequentially converted to hydroxyl radical (^•OH), hydrogen peroxide (H₂O₂), superoxide radical (^•O−
2) and ultimately oxygen (O₂).

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