Reactive oxygen species

In chemistry and biology, reactive oxygen species (ROS) are highly reactive chemicals formed from diatomic oxygen (O2), water, and hydrogen peroxide. Some prominent ROS are the hydroperoxide radical or hydroperoxyl (HO2), superoxide (O2−), the hydroxyl radical (OH.), and singlet oxygen (1O2). ROS are pervasive because they are readily produced from O2, which is abundant. Biologically, ROS function as cell signals, are intermediates in the redox behavior of O2, and impact the process of aging.

Definition Reactive oxygen species (ROS) are not uniformly defined, but generally include superoxide, singlet oxygen, and hydroxyl radical. Hydrogen peroxide is not nearly as reactive as these species, but is readily activated and is thus included. Peroxynitrite and nitric oxide are also reactive oxygen-containing species.

Hydroxyl radical (HO·) is generated by Fenton reaction of hydrogen peroxide with ferrous compounds and related reducing agents: Fe(II) + H2O2 → Fe(III)OH + HO· In its fleeting existence, the hydroxyl radical reacts rapidly irreversibly with all organic compounds.

Superoxide (O−2) is produced by reduction of O2. Several grams are produced per day in the human body within the mitochondria. O2 + e− → O−2 Competing with its formation, superoxide is destroyed by the action of superoxide dismutases, enzymes that catalyze its disproportionation:

2 O−2 + 2H+ → O2 + H2O2 Hydrogen peroxide (H2O2) is also produced as a side product of respiration. Peroxynitrite (ONO−2) results from the reaction of superoxide and nitric oxide. Singlet oxygen (1O2) is sometimes included as an ROS. Photosensitizers such as chlorophyll may convert triplet (3O2) to singlet oxygen. Singlet oxygen is highly reactive with unsaturated organic compounds. Carotenoids, tocopherols, and plastoquinones contained in chloroplasts quench singlet oxygen and protect against its toxic effects. Oxidized products of β-carotene arising from the presence of singlet oxygen act as second messengers that can either protect against singlet oxygen induced toxicity or initiate programmed cell death. Levels of jasmonate play a key role in the decision between cell acclimation or cell death in response to elevated levels of this reactive oxygen species.

Function In a biological context, ROS are byproducts of the normal metabolism of oxygen. ROS have roles in cell signaling and homeostasis. ROS are intrinsic to cellular functioning, and are present at low and stationary levels in normal cells. In plants, ROS are involved in metabolic processes related to photoprotection and tolerance to various types of stress. However, ROS can cause irreversible damage to DNA as they oxidize and modify some cellular components and prevent them from performing their original functions. This suggests that ROS has a dual role; whether they will act as harmful, protective or signaling factors depends on the balance between ROS production and disposal at the right time and place. In other words, oxygen toxicity can arise both from uncontrolled production and from the inefficient elimination of ROS by the antioxidant system. ROS are intermediates in the redox behavior of O2, which is central to fuel cells. 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. The production of ROS is strongly influenced by stress factor responses in plants, these factors that increase ROS production include drought, salinity, chilling, defense of pathogens, nutrient deficiency, metal toxicity and UV-B radiation. ROS are also generated by exogenous sources such as ionizing radiation generating irreversible effects in the development of tissues in both animals and plants. ROS have been demonstrated to modify the visual appearance of fish. This potentially affects their behavior and ecology, such as their temperature control, their visual communication, their reproduction and survival. ROS are central to the photodegradation of organic pollutants in the atmosphere.

Sources of ROS production

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