Box Extension 8.1

Reactive Oxygen Species (ROS)

The ordinary metabolism of aerobic cells produces molecules—chemical species—that because of the status of their oxygen atoms have a high potential to react with (e.g., oxidize) cell lipids, proteins, and nucleic acids. These are called reactive oxygen species (ROS). ROS can have benign functions; they play signaling roles, for example. ROS are more famous, however, for their ability to damage macromolecules on which health and life depend. ROS, in fact, are believed by some biologists to be the primary causative agents of aging. According to this hypothesis, we age because of incessant damage done to vital macromolecules by metabolically produced ROS; this damage is repaired, but not entirely, so defects accumulate over time. ROS are also implicated in muscle fatigue and many other phenomena.

The superoxide anion—an O₂ molecule with an extra electron—is often the initial ROS produced by metabolism (see figure). Mitochondria routinely generate superoxide as a by-product of electron transport, and in turn, superoxide and its ROS products can injure mitochondria by reacting in destructive ways with mitochondrial macromolecules. Superoxide is produced also by several extramitochondrial reactions. Superoxide is converted to two other ROS: hydrogen peroxide and hydroxyl radicals (see figure). All ROS are short-lived—sometimes exceedingly so—precisely because they are extraordinarily reactive. Hydroxyl radicals, for example, exist for less than 1-billionth of a second after they are formed.

An outline of the chemical relationships of O₂ and four reactive oxygen species (ROS) The ROS are shown in boxes. A rough estimate of the average lifespan of each ROS is shown below the name of the ROS; for example, superoxide radicals typically react—and therefore cease to be superoxide radicals—within 1-millionth of a second (10−6 s) after they are formed. Superoxide dismutase (SOD) and catalase are enzymes. A superscript dot next to a compound symbolizes an unpaired electron. The reactions are shown in outline only, not as fully balanced reactions. (After Allen et al. 2008.)

Cells possess antioxidant mechanisms—enzymatic and non-enzymatic mechanisms of detoxifying ROS. Enzyme antioxidants catalyze the transformation of ROS to less-reactive chemical forms. Superoxide dismutase, for example, converts superoxide to hydrogen peroxide, which is then converted by catalase to water and O₂ (see figure). Non-enzymatic antioxidants often function in a sacrificial way: By reacting with ROS, they prevent the ROS from reacting with other, more critical molecules.

Oxidative stress (oxidant stress) refers to damage inflicted by ROS on cellular integrity. It occurs when the rate of ROS production exceeds the rate at which antioxidant mechanisms can dispose of ROS. Box Extension 8.1 discusses antioxidants and the roles of ROS in more detail.

Some ROS are free radicals, meaning they have one or more unpaired electrons. These include the superoxide anion and hydroxyl radical. Other ROS, such as hydrogen peroxide (H₂O₂), although fairly reactive, are not free radicals.

An important distinction between enzymatic and non-enzymatic antioxidants is that enzyme antioxidants (in common with all enzymes) remain unaltered as they go through repeated catalytic cycles in which they clear cells of ROS, whereas non-enzymatic antioxidants are often consumed by their reactions with ROS. Unless a non-enzymatic antioxidant is regenerated after reacting with a ROS, it is modified by the reaction in such a way that it cannot react similarly with another ROS. A large variety of dietary molecules have the potential to act as non-enzymatic antioxidants after having been absorbed and delivered to cells. These include carotenoids, vitamin C, vitamin E, and some polyphenols and flavonoids.

A common way of testing for ROS effects is to determine if addition of antioxidants alters a system of interest. For example, to test for ROS effects on muscle fatigue, antioxidants of several types—including N-acetyl-cysteine, dimethylsulfoxide, and superoxide dismutase—have been injected into isolated muscles. These supplementary antioxidants often reduce fatigue, indicating that ROS promote fatigue. Despite such laboratory results, there is far less evidence, of a definitive sort, that supplementary antioxidants alter function in whole animals.

Mitochondria are the focus of a great deal of ROS research. One of the remaining mysteries of mitochondrial function is why mitochondria do not continuously maximize their efficiency of ATP synthesis. A current working hypothesis is that mitochondria may need to trade off ROS damage and efficiency of ATP production (measured by the P/O ratio). Under some circumstances, allowing proton leakage across the inner mitochondrial membrane is thought to reduce ROS production. Increased proton leakage, however, diverts energy from ATP synthesis and therefore reduces the efficiency of ATP synthesis.

Most research on ROS is focused on chronic and degenerative disease states. ROS are implicated in cancer, vascular diseases, cardiac diseases, diabetes, neurodegenerative diseases, and arthritis—as well as aging.

References

Allen, D. G., G. D. Lamb, and H. Westerblad. 2008. Skeletal muscle fatigue: cellular mechanisms. Physiol. Rev. 88: 287–332.

Becker, L. B. 2004. New concepts in reactive oxygen species and cardiovascular reperfusion physiology. Cardiovasc. Res. 61: 461–470.

Brand, M. D. 2005. The efficiency and plasticity of mitochondrial energy transduction. Biochem. Soc. Trans. 33: 897–904.

Cai, H., and D. G. Harrison. 2000. Endothelial dysfunction in cardiovascular diseases. The role of oxidant stress. Circ. Res. 87: 840–844.

Halliwell, B., and J. M. C. Gutteridge. 2007. Free Radicals in Biology and Medicine, 4th ed. Oxford University Press, Oxford, UK.

Lesser, M. P. 2006. Oxidative stress in marine environments: biochemistry and physiological ecology. Annu. Rev. Physiol. 68: 253–278.