Oxidative stress

Definition

Oxidative stress refers to a state in which the generation of reactive oxygen species and other oxidizing substances within cells exceeds the capacity of antioxidant and repair systems, thereby causing oxidative damage and disrupting cellular homeostasis.

Detailed Description

Under normal conditions, cells continuously generate substances such as reactive oxygen species (ROS) and reactive nitrogen species (RNS). Common ROS in cells include superoxide anion (•O₂⁻), hydroxyl radical (•OH), peroxyl radical (ROO•), and hydrogen peroxide (H₂O₂). RNS include nitric oxide (NO•) and peroxynitrite (ONOO⁻).

To understand oxidative stress, it is first necessary to recognize that free radicals and reactive oxygen species are not exactly the same. A free radical is a molecule or atom that contains an unpaired electron, making it highly reactive and prone to abstracting electrons from surrounding molecules in order to achieve stability. Reactive oxygen species, by contrast, are oxygen-derived substances with high chemical reactivity. Some are free radicals, whereas others are not. Hydrogen peroxide (H₂O₂), for example, is highly reactive but is not itself a free radical.

These substances are not entirely harmful. At low concentrations, some of them form part of normal cellular signaling and participate in cell proliferation, differentiation, apoptosis, immune responses, and metabolic regulation. For that reason, lower levels are not always better. The real problem begins when the rate of generation of free radicals and other oxidizing substances exceeds the processing capacity of the cell's antioxidant and repair systems. At that point, oxidative stress arises. Controlled oxidative signaling is then transformed into a chemically destructive force that damages cellular structure and function.

Reactive oxygen species（圖片來源：Masenga SK et al. (2023)，採用 CC BY 4.0 授權）

Where do these oxidizing substances come from?

The answer is that nearly all aerobic life processes generate them, and mitochondria are among the most important sources. We breathe in order to supply oxygen so that mitochondria can carry out oxidative phosphorylation and produce ATP. During this process, electrons move along the electron transport chain. In theory, they should ultimately be fully accepted by oxygen to form water. In reality, however, a small proportion of electrons leak prematurely and react with oxygen to generate superoxide anion. This occurs especially readily at complex I (NADH ubiquinone oxidoreductase) and complex III (coenzyme Q-cytochrome c reductase) of the electron transport chain, which is why mitochondria are a major site of ROS production. Superoxide anion can then be converted into hydrogen peroxide by superoxide dismutase (SOD). If hydrogen peroxide subsequently encounters iron ions, copper ions, or other transition metals, it can give rise to the extremely reactive hydroxyl radical, potentially amplifying local oxidative events into broader cellular damage.

Electron transport chain in mitochondria（圖片來源：Seba4108，採用 CC BY-SA 4.0 授權）

In addition to mitochondria, the endoplasmic reticulum and peroxisomes are also important sources of ROS. During protein folding in the endoplasmic reticulum, disulfide bonds must be formed, and this is itself an oxidative process. Enzymes involved in protein maturation, such as ERO1 and PDI, therefore generate hydrogen peroxide while helping proteins reach their correct conformation. Meanwhile, the cytochrome P450 system, during drug metabolism and lipid metabolism, can also produce ROS when electron transfer is incomplete. Peroxisomes are highly active in fatty acid oxidation and a variety of oxidase reactions, and as a result they generate large amounts of hydrogen peroxide. In other words, cellular metabolism is constantly producing reactive oxygen species.

Peroxisomes are organelles within the cell（圖片來源：Anthony Atkielski，採用 CC BY-SA 3.0 授權）

Why, then, do ROS not usually cause obvious damage under normal conditions?

Cells possess two major systems for removing ROS: the enzymatic antioxidant system and the non-enzymatic antioxidant system. The former includes superoxide dismutase (SOD), catalase, and glutathione peroxidase (GPx). These enzymes rapidly convert reactive oxygen species into less dangerous molecules. For example, superoxide dismutase converts superoxide anion into hydrogen peroxide and oxygen (2 •O₂⁻ + 2 H⁺ → H₂O₂ + O₂), and catalase then converts hydrogen peroxide into water and oxygen (2 H₂O₂ → 2 H₂O + O₂).

The non-enzymatic system includes small molecules such as glutathione, vitamin C, vitamin E, and uric acid. These molecules help scavenge free radicals, maintain a reducing intracellular environment, and protect membrane lipids and proteins from oxidation by ROS. As long as ROS production remains within a controllable range, cells do not enter a state of oxidative stress. But when ROS generation rises sharply, the antioxidant system becomes depleted, or repair mechanisms can no longer keep up, the cell shifts from a reversible regulatory state toward irreversible oxidative damage.

What happens under oxidative stress?

Once oxidative stress develops, the first major targets are usually the three major classes of biomolecules in the cell: lipids, proteins, and nucleic acids. Lipids are especially vulnerable because the polyunsaturated fatty acids in cellular membranes are readily subjected to hydrogen abstraction by free radicals, initiating lipid peroxidation. This ultimately produces highly reactive secondary products such as 4-hydroxynonenal (4-HNE) and malondialdehyde (MDA), which can further damage proteins, DNA, and membrane architecture, thereby amplifying cellular injury.

Proteins also undergo profound changes under oxidative stress. Once oxidized, proteins may experience side-chain modification, thiol oxidation, carbonylation, misfolding, and aggregation, ultimately leading to loss of function. Because proteins govern a wide range of signaling and metabolic processes, their dysfunction can disrupt cellular physiology at many levels. In addition, abnormally aggregated proteins may themselves be toxic to cells and can trigger cell death.

DNA and RNA are likewise direct targets of ROS during oxidative stress. Reactive oxygen species can attack both the nucleotide bases and the sugar-phosphate backbone, causing base modifications, single-strand or double-strand breaks, mismatches, and mutations. One of the most classic forms of oxidative DNA damage is 8-Oxo-dG. This damaged guanine derivative readily mispairs with adenine, leading to G:C → T:A transversion mutations during DNA replication. If such damage occurs in tumor suppressor genes, DNA repair genes, or genes that regulate the cell cycle, it can increase the risk of carcinogenesis.

Author: Shui-Ye You

References:

1. Masenga SK et al. (2023). Mechanisms of Oxidative Stress in Metabolic Syndrome. International Journal of Molecular Sciences.

2. Qin Y et al. (2024). Oxidative Stress: Molecular Mechanisms, Diseases, and Therapeutic Targets. MedComm.

3. Tumilaar SG et al. (2024). A Comprehensive Review of Free Radicals, Oxidative Stress, and Antioxidants: Overview, Clinical Applications, Global Perspectives, Future Directions, and Mechanisms of Antioxidant Activity of Flavonoid Compounds. Journal of Chemistry.