The mitochondrion is the major cell organelle responsible for ROS production. It generates ATP through a series of oxidative phosphorylation processes. During this process, one- or two-electron reductions instead of four electron reductions of O2 can occur, leading to the formation of superoxide and H2O2, and these can be converted to other ROS. Other sources of ROS may be reactions involving peroxisomal oxidases, cytochrome P-450 enzymes, NAD (P)H oxidases, or xanthine oxidase (黄嘌呤；黄质.
The central nervous system (CNS) is extremely sensitive to free radical damage because of a relatively small defensive antioxidant capacity. The ROS produced in the tissues can inflict direct damage to macromolecules, such as lipids, nucleic acids, and proteins. Oxygen-free radicals, particularly superoxide anion radical, hydroxyl radical (OH•−), and alkylperoxyl radical (•OOCR), are potent initiators of lipid peroxidation. Once lipid peroxidation is initiated, a propagation of chain reactions takes place until termination products are produced. The end products of lipid peroxidation, are such as malondialdehyde (MDA), 4-hydroxy-2-nonenol (4-HNE), and F2-isoprostanes, are accumulated in biological systems.
DNA bases are very susceptible to ROS oxidation, and the predominant detectable oxidation product of DNA bases in vivo is 8-hydroxy-2-deoxyguanosine. Oxidation of DNA bases can cause mutations and deletions in both nuclear and mitochondrial DNA. Mitochondrial DNA is prone to oxidative damage due to its proximity to a primary source of ROS and its deficient repair capacity compared with nuclear DNA. These oxidative modifications lead to functional changes in various types of proteins (enzymatic and structural), which can have a substantial physiological impact. Similarly, redox modulation of transcription factors produces an increase or decrease in their specific DNA binding activities, thus modifying the gene expression.
Among different markers of oxidative stress, malondialdehyde (MDA) and the natural antioxidants, metalloenzymes Cu, Zn-superoxide dismutase (Cu, Zn-SOD), and selenium-dependent glutathione peroxidase (GSHPx), is currently considered to be the most important markers. Malondialdehyde (MDA) is a three-carbon compound formed from peroxidized polyunsaturated fatty acids, mainly arachidonic acid. It is one of the end products of membrane lipid peroxidation. Since MDA levels are increased in various diseases with an excess of oxygen free radicals, many relationships with free radical damage were observed.
Cu, Zn-SOD is an intracellular enzyme present in all oxygen-metabolizing cells, which dismutases the extremely toxic superoxide radical into potentially less toxic hydrogen peroxide. Cu, Zn-SOD is widespread in nature, but being a metalloenzyme, its activity depends upon the free copper and zinc reserves in the tissues. GSHPx, an intracellular enzyme, belongs to several proteins in mammalian cells that can metabolize hydrogen peroxide and lipid hydroperoxides.
The Body’s Natural Antioxidant Defenses
To detoxify ROS, the body uses a system of antioxidants, such as antioxidative enzymes, e.g. superoxide dismutase, catalase, glutathione peroxidase. This system consists of degradative yet and other enzymes such as proteases, peptidases, phospholipases, acyltransferases, endonucleases, exonucleases, polymerases, ligases, etc., to leave and replace irreversibly damaged macromolecules. Importantly, the systems are integrated, they work in to continue the close interaction.
Superoxide Dismutase (SOD) catalyzes the reduction of superoxide into hydrogen peroxide and water. In mammals, there are three isoforms which function in distinct cellular compartments. SOD1 is found in the cytosol and mitochondrial intermembrane. SOD2 is located in the mitochondrial matrix; and SOD3 functions in the extracellular space.
Glutathione Peroxidase (Gpx) transforms peroxides, especially lipid hydroperoxides, into water and alcohol. Specialized GPx forms function in distinct cellular compartments in specific tissue types. Analysis of the selenoproteome identified five glutathione peroxidases (GPxs) in mammals: cytosolic GPx (cGPx, GPx1), phospholipid hydroperoxide GPx (PHGPX, GPx4), plasma GPx (pGPX, GPx3), gastrointestinal GPx (GI-GPx, GPx2) and, in humans, GPx6, which is restricted to the olfactory system. GPxs reduce hydroperoxides to the corresponding alcohols by means of glutathione (GSH). They have long been considered to only act as antioxidant enzymes. Increasing evidence, however, suggests that nature has not created redundant GPxs just to detoxify hydroperoxides. In conclusion, cGPx, PGPX, and GI-GPx have distinct roles, particularly in cellular defense mechanisms.
Catalase (CAT) uses an iron to reduce peroxides. Hundreds of different forms are widely distributed in animal, plant and fungi tissues. Some contain manganese, and some are bifunctional catalase-peroxidases.
In addition to these principal antioxidant enzymes, the secondary antioxidant enzymes, thioredoxin, glutaredoxin, and peroxiredoxin systems also aid in the control, and selective removal, of ROS. The body is able to increase or decrease their activity in target locations, as needed, to maintain ideal redox homeostasis. Antioxidant enzymes cannot be taken orally; it would not be advisable to do so, even if possible.
Oxidative Stress and Altered Immune Function
The relationship between oxidative stress and immune function of the body is well established. The immune defense mechanism uses the lethal effects of oxidants in a beneficial manner with ROS and RNS playing a pivotal role in the killing of pathogens. The skilled phagocytic cells (macrophages, eosinophils, heterophils), as well as B and T lymphocytes, contain an enzyme, the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (还原型辅酶Ⅱ). It is made up of six subunits.
These subunits are a Rho guanosine triphosphatase (GTPase), usually Rac1 or Rac2 (Rac stands for Rho-related C3 botulinum toxin substrate), Five “phox” units. (Phox stands for phagocytic oxidase.) gp91-PHOX (contains heme)(Nox2)/p22phox / p40phox/ p47phox / p67phox, which is responsible for the production of ROS following an immune challenge. At the onset of an immune response, phagocytes increase their oxygen uptake as much as 10–20 folds (respiratory burst). The generated superoxide by this enzyme serves as the starting material for the production of a suite of reactive species. Direct evidence also certifies production of other powerful pro-oxidants, such as hydrogen peroxide (H2O2), hypochlorous acid (HOCl), peroxynitrite (ONOO–), and, possibly, hydroxyl (OH•) and ozone (O3) by these cells. Although the use of these highly reactive endogenous metabolites in the cytotoxic response of phagocytes also injures the host tissues, the nonspecificity of these oxidants is an advantage since they take care of all the antigenic components of the pathogenic cell.
Several studies have demonstrated the interdependency of oxidative stress, immune system, and inflammation. Increased expression of NO has been documented in dengue and in monocyte cultures infected with different types of viral infections. Increased production of NO has also been accompanied with enhancement in oxidative markers like lipid peroxidation and an altered enzymatic and nonenzymatic antioxidative response in dengue-infected monocyte cultures. More specifically, the oxygen stress related to immune system dysfunction seems to have a key role in senescence, in agreement with the oxidation/inflammation theory of aging. Moreover, it has been revealed that reduced NADPH oxidase is present in the pollen grains and can lead to the induction of airway associated oxidative stress. Such oxidative insult is responsible for developing allergic inflammation in sensitized animals.
The immune status directly interplays with disease production process. The role of physical and psychological stressors contributes to incidences and severity of various viral and bacterial infections. Both innate, as well as acquired immune responses, are affected by the altered IFN-γ secretion, expression of CD14, production of the acute-phase proteins, and induction of TNF-α. Fatal viral diseases produce severe oxidative stress (OS) leading to rigorous cellular damage. However, initiation, progress, and reduction of damages are governed by the redox balance of oxidation and antioxidation. The major pathway of pathogenesis for cell damage is via lipid peroxidation particularly in microsomes, mitochondria, and endoplasmic reticulum due to OS and free radicals. All the factors responsible for the oxidative stress directly or indirectly participate in the immune system defense mechanism. Any alteration leading to immunosuppression can trigger the disease production.
Oxidative Stress and Aging
Aging is an inherent mechanism existing in all living cells. There is a decline in organ functions progressively along with the age-related disease development. Two most important theories related to aging are free radical and mitochondrial theories, and these have passed through the test of time. There is claim by such theories that a vicious cycle is generated within mitochondria wherein reactive oxygen species (ROS) is produced in increased amount thereby augmenting the damage potential. Oxidative stress is present at genetic, molecular, cellular, tissue, and system levels of all living beings and is usually manifested as a progressive accumulation of diverse deleterious changes in cells and tissues with advancing age that increase the risk of disease and death. Recent studies have shown that with age, ROS levels show accumulation in major organ systems such as liver, heart, brain, and skeletal muscle either due to their increased production or reduced detoxification. Thus, aging may be referred to as a progressive decline in biological function of the tissues with respect to time as well as a decrease in the adaptability to different kinds of stress or briefly an overall increase in susceptibility to diseases. Oxidative stress theory is presently the most accepted explanation for the aging which holds that increases in ROS lead to functional alterations, pathological conditions and other clinically observable signs of aging, and finally death. No matter whether mitochondrial DNA damage is involved or electron transport chain damage is responsible for aging, modulation of cellular signal response to stress or activation of redox-sensitive transcriptional factors by age-related oxidative stress causes the upregulation of pro-inflammatory gene expression, finally leading to an increase in the ROS levels.