Involvement of Antioxidant in the Prevention of Cellular Damage

Abstract

Oxidative stress occurs when the body’s enzymatic or non-enzymatic antioxidants are outweighed by endogenous or exogenous free radicals. Oxidative radicals, reactive oxygen species, and other biomolecule-damaging free radicals can be generated during normal cellular metabolism and react with proteins, lipids, and DNA. In the domains of biology and medicine, free radicals have become increasingly important. They can accumulate in a variety of ways, both endogenously and exogenously. Mitochondria are the primary source of cell-level endogenous reactive oxygen species. In several chronic and degenerative disorders, this results in tissue destruction. In addition to being produced endogenously, antioxidants can also be delivered exogenously to the biological system, most frequently through nutrition. Antioxidants are generally used to counteract the effects of free radicals produced by metabolic processes. In this chapter, the crucial function of reactive oxygen species in human health, as well as exploring the functioning of antioxidative defense systems in reducing toxicity caused by excess reactive oxygen species were discussed.

1. Introduction

Defending cells against the harm produced by free radicals is the goal of antioxidants. Taking antioxidants may help to counteract some of the damage that free radicals can inflict. Carotenoids and other nutrients like beta carotene and lycopene have been shown to be effective antioxidants [1]. Free radicals can be generated during oxidation reactions, which can set off a cascade of events that damage cells. By eliminating free radical intermediates, antioxidants put an end to these chain events and block further oxidation reactions. They participate in processes that maintain cell health and repair DNA. Because of this, oxidizing substances like thiols or ascorbic acid frequently serve as antioxidants. Plants and animals utilize a wide variety of antioxidants, such as glutathione and vitamins C and E, as well as enzymes such as catalase, superoxide dismutase, glutathione peroxidase to mitigate the deleterious effects of oxidation reactions [2]. If a cell’s antioxidant levels are low or its antioxidant enzymes are blocked, oxidative stress can cause the cell to become damaged [3, 4].

A wide variety of dietary supplements contain antioxidants which help people stay healthy while also reducing their risk of developing ailments like cancer and heart disease. In later, more extensive clinical tests, researchers were unable to demonstrate any benefit to taking antioxidant supplements; instead, they discovered that taking an excessive amount may be harmful [5]. Natural antioxidants have a variety of applications in industry, in addition to their usage in medicine. Some of these applications include acting as preservatives in food and cosmetics [6, 7].

2. Oxidants and free radicals

Any molecular species that has an unpaired electron in an atomic orbital and is capable of independent existence is referred to as a free radical. When an electron is missing from a pair, it causes the resulting species to be extremely reactive. Free radicals are capable of a diverse set of reactions, the most common of which are electron transfer and addition processes that lead to the creation of covalent bonds [8, 9]. Reducing free radicals are those that give up an electron to an acceptor, while oxidizing free radicals are those that take in electrons (accepting an electron from a donor). There is a thermodynamic hierarchy, often known as a pecking order, for the many types of electron transfer reactions. This is because radicals can have a wide variety of reactivities [1, 9].

2.1 Generation of free radicals and oxidants

Non-radicals can be converted into radicals through a variety of methods, including the addition of a single electron to the molecule. A covalent bond (C–H, C–O or C–C) can be broken via homolytic fission, in which one electron from the bonding pair remains on each atom. While disulfide links can readily be broken, the O–O bond in H₂O₂ can be broken by exposing it to UV light, resulting in the formation of ^•OH, these covalent bonds are extremely difficult to dissociate. Exogenous and endogenous sources of free radicals exist [10, 11]. Different cell organelles, such as mitochondria, peroxisomes, and endoplasmic reticulum, as well as various enzyme activity, fatty acid metabolism, and phagocytic cells, are examples of endogenous sources. High temperatures and environmental pollutants, such as those produced by cooking (smoked meat or used cooking oil), H₂O₂, N₂O₂, deoxyosones, and ketamine, are examples of exogenous sources of radiation. Other exogenous sources include X-ray and beta-ray light, ultraviolet A light in the presence of a sensitizer and chemical reagents such as these (aromatic hydrocarbons, pesticides, polychlorinated biphenyls, dioxins, and many others), microbial infections, drugs, and their metabolites [9, 11]. To combat bacteria and other invaders, activated immune cells (eosinophils, neutrophils, etc.) produce endogenous free radicals, as does the mitochondrial respiratory chain, enzymatic activity (xanthin oxidase, NADPH oxidase, lipo-oxygenase, NO synthase, etc.), and various pathological conditions and diseases [12, 13]. Air and water pollution, cigarette smoke, heavy metals or transition and other medications and chemicals, radiation and extreme temperatures produce exogenous free radicals.

3. Oxidative damage to cellular molecules

3.1 Oxidative damage to protein

Free radicals, amino acid modification, cross-linkage formation due to lipid peroxidation, and protein fragmentation are all methods by which proteins can be damaged. Methionine, cysteine, arginine, and histidine are the most susceptible to oxidation in proteins. Proteins that have already been damaged by free radicals are more vulnerable to enzyme proteolysis. The oxidation of protein products can influence enzymes, receptors, and the transport of molecules across membranes [13].

Since oxidatively damaged protein products contain highly reactive groups, membrane damage and other cellular activities may be impaired because of their existence. Peroxyl radicals, a type of free radical, are hypothesized to be responsible for protein oxidation. Carbonyls and other amino acid modifications can be created because of ROS damaging proteins and causing carbonyl and other amino acid alterations, such as the production of methionine sulfoxide and protein peroxide. From signaling pathways to enzyme activity to heat stability to proteolysis susceptibility, many elements of protein oxidation are affected [4, 9].

3.2 Lipid peroxidation

In a variety of physiological and pathological processes, Including aging, arterial hardening, inflammation, and cancer development and progression, oxidative stress play an important role to increase biochemical lesions by reacting with other biomolecules [14]. Cell membrane-bound polysaturated fatty acids are subjected to lipid peroxidation, which progresses via radical chain reaction. ROS is hypothesized to be triggered by hydroxyl radicals, which remove hydrogen atoms, resulting in the formation of lipid radicals and diene conjugates. In addition, it generates a peroxyl radical when oxygen is added; this extremely reactive radical then attacks a different fatty acid, resulting in lipid hydroperoxide (LOOH) and a brand-new radical. As a result, lipid peroxidation grows. Several chemicals are generated because of lipid peroxidation, including alkanes, malonaldehyde, and isoprotanes. Researchers have shown these chemicals to be biomarkers for lipid peroxidation in a variety of conditions including diabetes, ischemia reperfusion injury and neurodegenerative disorders [4, 15].

3.3 Oxidative damage to DNA

Oxidative DNA damage is an inevitable consequence of cellular metabolism. While guanine typically pairs with cytosine, the most common form of oxidative base damage, 8-oxo-7,8-dihydroguanine (8-oxoG), can lead to adenine mispairing through a conformational change. DNA and RNA can be damaged by oxidative stress, as numerous studies have demonstrated beyond reasonable doubt. Many diseases, including aging and cancer, have been linked to mutations in DNA. When free radicals or ultraviolet radiation cause oxidative damage to DNA, the levels of oxidative nucleotides including glycol, dTG, and 8-hydroxy-2-deoxyguanosine rise [16]. One of the many illnesses linked to oxidative damage is cancer, and mitochondrial DNA has been found to be particularly vulnerable. The use of 8-hydroxy-2-deoxyguanosine as a biomarker for oxidative stress is well adopted. Oxidatively stressed cells have high levels of this marker [17].

4. Biological activities of free radicals and oxidants

It is necessary for the maturation of cellular structures that both reactive oxygen species (ROS) and reactive nitrogen species (RNS) are present in low to moderate concentrations since they can operate as weapons for the host defense system. It is true that phagocytes (which include neutrophils, macrophages, and monocytes) create free radicals as a part of the body’s immune system’s fight against sickness. Reactive oxygen species (ROS) production by the immune system is clearly demonstrated in patients with granulomatous illness (ROS) [18]. The membrane-bound NADPH oxidase machinery is faulty in these individuals; therefore, they are unable to create the superoxide anion radical (O₂^•). The effect is that people get sick and become infected with numerous diseases that endure for a long time.

Several cellular signaling systems benefit from the physiological functions ROS and RNS play in their operation, as well (ROS and RNS). Nonphagocytic NADPH oxidase isoforms are crucial for the control of intracellular signaling cascades in fibroblasts, endothelial cells, vascular smooth muscle cells, cardiac myocytes, and thyroid tissue. Blood flow, clotting, and cognitive function are all affected by nitric oxide (NO), an intercellular messenger [18, 19].

In addition to its role in nonspecific host defense, NO is essential for the eradication of intracellular infections and malignancies. A mitogenic reaction is one of the many good effects of free radicals. At low to moderate levels of intensity, ROS and RNS are required for human health.

5. Mechanism of cell signaling mediated by RNS/ROS

Oxidation can take place in any of these macromolecules: DNA, proteins, and lipids when ROS are present. Oxidative stress is primarily caused by reactive oxygen species (ROS) in cells. Signaling molecules such as ROS are critical to the proper operation of the body’s physiological systems. In a physical sense, this is what is going on. Autophagy, apoptosis, necrosis, and other mechanisms that lead to cell death are activated when ROS levels are too high.

5.1 ROS induce autophagy

Lysosomes of the cell remove damaged organelles, protein aggregates, and foreign invaders via autophagy, a cellular breakdown process. Several human diseases, including as cancer, neurological disorders, infectious diseases, metabolic disorders, and the aging process, may be caused in part by problems with autophagy. Autophagy can be triggered in response to a variety of stresses, including starvation, ER stress, organelle breakdown, and pathogen infection. Activation of autophagy has been linked to reactive oxygen species (ROS). H₂O₂ will eventually cause oxidative stress because of its buildup in the cell. The autophagic process relies heavily on the autophagy gene ATG4. There is evidence to suggest that H₂O₂ oxidizes this gene specifically in the absence of food. If H₂O₂ builds up, the ATG4’s activity can be oxidized. The lipidation of LC3/ATG8 is essential for the initiation of autophagy, which is facilitated by oxidized ATG4. For the buildup of LC3-PE on autophagosome membranes and the subsequent stimulation of autophagosome formation, ROS is necessary [20, 21].

Reactive oxygen species (ROS) have the potential to regulate autophagy via activating the mitogen-activated protein kinase [MAPK] family. JNK1c-Jun-N-terminal kinase (JNK), p38, and extracellular signal-regulated kinase are all members of this family (ERK). In the three-tier kinase cascade that activates the members of the MAPK family, MAPK kinase (MAPKK), MAPK kinase (MAPKK), and MAPK all participate. When JNK is activated for an extended period, the cell’s production of reactive oxygen species (ROS) increases significantly, increasing the risk of DNA damage. The identification of cellular redox stress is the final step in the activation of the p53 pathway. Many autophagy-inducing genes can be activated by p53 as a transcription factor. As a result, JNK and Sestrin2 may be activated, resulting in the phosphorylation and activation of TSC2 and the resulting autophagy [22].

Additional signaling pathways that participate in ROS-mediated autophagy and contribute to the process include Akt/mTOR (mechanistic target of rapamycin), as well as AMPK The well-known kinase Akt/mTOR, which in turn oxidizes the phosphatase and tensin homolog, is controlled by reactive oxygen species (ROS) (PTEN). Inhibition of mTOR and activation of AMPK are required for autophagy activation, and these two processes are controlled by the VPS34 complex [23].

5.2 ROS trigger apoptosis

Death receptors and mitochondrial pathways initiate cell apoptosis in response to both external and internal stimuli. Because an increase in oxidative stress disturbs the homeostatic equilibrium within cells and causes long-term oxidative changes to fat, protein, or DNA, ROS levels rise. TRAIL and nuclear factor kappa B (NF-kB) are activated by reactive oxygen species (ROS) and result in the death of cancer cells. Apoptosis can be induced by ROS-driven activation of JNK, a MAPK family member like JNK. Mitochondrial malfunction and apoptosis are becoming more obvious roles for JNK [24]. Several studies have shown that Shikonin, a naturally occurring naphthoquinone derivative, can kill cancer cells. Shikonin boosted ROS production and apoptosis, as well as the production of JNK and p38 in K562 cells, which were then treated. Programed cell death in cancer cells increases because of ROS/JNK activation [25]. The redox sensitive MAPK kinase and Apoptosis Signal Regulation Kinase 1 (ASK1) are positioned upstream of ROS/JNK. The antioxidant protein ASK1 is prevented from conducting its work by Grx and Trx1, which are antioxidant proteins. Components associated with the tumor necrosis factor receptor are recruited to the complex when ROS cause Trx1 to dissociate from the ASK1-Trx1. Activated ASK1 signals can activate AP-1-dependent proapoptotic genes and mitochondrial signaling. By altering the mitochondrial ASK1/ASK2/Trx2 complex, ROS can also lead to the release of cytochrome C. Increase ROS levels in the ER and stimulate mitochondria to do this. Antioxidant flavone can protect against myocardial ischemia/reperfusion injury, which can lead to apoptosis [26, 27].

5.3 Necrosis induced by ROS

In contrast to apoptosis, necrosis is a unique form of cell death. The receptor-interacting serine/threonine 3-like (RIP3) protein kinase has the potential to destroy cells because it is highly expressed in so many different cell lines. The RIP1 and RIP3 serine/threonine kinases both regulate necrosis in a similar manner. To activate the transmission of the pro-necrotic signal, RIP1 and RIP3 must be phosphorylated to form necrosome, an amyloid-like complex. Depletion of RIP3 in necrosis-inducing cells reduces ROS concentration, but RIP3 overexpression raises ROS levels. The involvement of RIP3 in necrosis induction is performed via increasing ROS production associated to energy metabolism [28, 29]. When RIP1 and RIP3 are phosphorylated, the pronecrotic kinase activity is triggered, and ROS are generated. According to this study, the phosphorylation of pronecrotic complexes stabilizes their interactions. A link between STAT3 and the mitochondrial electron transport chain complex I component GRIM-19 governs enhanced ROS production from RIP1 phosphorylation-dependent activation of the mitochondrial electron transport chain. In the mitochondria, STAT3 and GRIM-19 accumulate and increase ROS production and necroptosis, the process by which cells die, because of this interplay [30]. They all work together to increase energy consumption and mitochondrial ROS generation by enhancing the interaction between RIP3 and the enzymes glutamate dehydrogenase 1 [GDH], glutamate ligase (GLUL), and PYGL. During necrosis induction, there may be an interaction between the TNF receptor and the necrosis-inducing ROS generated by the NADPH oxidase NOX enzyme complex. As an important RIP3 downstream component of TNF-induced necrosis in ROS-induced necrosis, MLKL has been found to be an important player in the process. In the last phases of necrosis caused by TNF, MLKL also plays a role in ROS production and JNK activation [31].

6. Oxidative stress and human diseases

An organ or tissue is said to be under oxidative stress when the endogenous antioxidant defense system is overwhelmed by the production of highly reactive molecules like ROS, RNS and RSS, resulting in cellular damage and malfunction and a wide spectrum of illnesses. As a result of normal metabolic activities, the reactive species are created in low concentrations within the cells themselves. Radiation (X-rays and UV), pollution, cigarette smoke, bacteria, viruses, and drugs can also cause them, as can acute or chronic cellular stress (acute or chronic) [24]. They include free radicals and nonradical oxidants. Free radicals are unstable because of the presence of unpaired electrons in their outer electron orbit. Free radicals tend to neutralize themselves by reacting with other molecules and triggering their oxidation because they are so unstable and reactive. Thus, they have the potential to disrupt a wide spectrum of biological components, such as DNA, lipids, and proteins. Proteins are a common target for free radicals because of their critical role in cellular activity. Although free radicals have been shown to cause some protein modifications, such as protein unfolding or structural alteration, the majority are absolutely harmless. It is possible for protein inactivation and long-term cellular damage to be caused by irreversible protein alterations, even if reversible oxidative changes govern protein activity [1].

6.1 Oxidative stress in atherosclerosis

Atherosclerosis is a chronic inflammatory disease of the vascular system that is marked by chronic inflammation. There is a strong correlation between cardiovascular disease and atherosclerosis in most developed countries (CVD). The endothelium is injured because of inflammation and oxidative stress, resulting in arterial lesions and plaque deposition [32]. It is easier for plaque, which is mostly composed of blood cells and foam cells as well as lipids and proteins to impede the vascular system and prevent blood flow. Infarctions and strokes resulting from coronary artery disease characterize cardiovascular disease (CVD). Diabetes, high blood pressure, smoking, cholesterol difficulties, obesity, and other metabolic illnesses are linked to endothelial degradation. In the early phases of atherosclerosis, oxidative stress has a negative impact on endothelial function. Endothelial function, inflammation, bleeding, and oxidative damages are all influenced by the endocrine system (RAS) [33, 34]. As a result of activating NADPH oxidase in the cardiovascular system, reactive oxygen species (ROS) are produced that damage the endothelium, resulting in endothelial dysfunction (ROS).

6.2 Oxidative stress in hypertension

The most common cause of cardiovascular disease and death around the world is high blood pressure. About 90% of instances of hypertension are categorized as essential hypertension, when the exact reason is unknown. Hypertensive stimuli, such as salt and the hyperactive RAS and OS systems as well as endogenous hormones such as Ang II and aldosterone, produce protein modification that results in a rise in blood pressure. Neoantigens are proteins that have been altered so that they are no longer identified by activated T cells as being their own. Macrophages in the blood and kidneys are stimulated to release proinflammatory cytokines by T cell-derived signals. Activated T cells in the vasculature enhance renal salt and water retention, as well as renal vasoconstriction and remodeling. Chronic inflammation can lead to high blood pressure, which is a risk factor for OS. In the presence of Ang II-induced hypertension, T cells show substantial amounts of p47phox, p22phox, and NOX₂ oxidase components. To put it another way, the transfer of faulty T cells results in arterial hypertension and a decreased generation of oxygen. Angiotensin II (Ang II) is one of the most major ROS generators, while NADPH oxidase is one of the most prominent ROS producers [35, 36]. The production of Ang II reaches its peak under hypertensive conditions. In addition, increased angiotensin II levels can promote necrosis and apoptosis in renal tissue during the period of reperfusion. Ang II inhibits the SR-BI HDL receptor in proximal tubular cells. Statins were intended to inhibit HMG-CoA reductase to lower cholesterol production. However, these medications have anti-inflammatory properties as well as the potential to reduce systolic blood pressure in people with high cholesterol as part of their pleiotropic effects. Patients with elevated blood pressure feel the effects more intensely [37].

6.3 Oxidative stress in diabetes mellitus

The body’s ability to neutralize free radicals and produce antioxidants is out of whack, resulting in diabetes mellitus (DM). Diabetes can be triggered by changes in blood glucose levels. OS has a significant impact on the emergence of DM problems. A high blood sugar level can have a significant impact on a person’s overall health [38]. As a result, chronic hyperglycemia has a lower OS than any other kind of glucose oscillation. Long-term and severe chronic hyperglycemia, as well as frequent blood glucose fluctuations, are hallmarks of many glycemic disorders. Hyperglycemia triggers ROS production in the body. Even if the cells of persons with type 2 diabetes are still functioning and intact, ROS produces OS because of the existence of ROS. Insulin production is reduced as a result. Diabetes mellitus has been linked to an increase in the radical O₂ - in both animal and vitro investigations. Many mechanisms, such as enzymatic, nonenzymatic, and mitochondrial processes, exist in DM for the generation of oxidative stress. There are numerous reasons for the rise in OS in DM. The most major oxidizing activity, glucose autooxidation, generates free radicals [38, 39]. Reduced antioxidant defenses (lower levels of cellular antioxidants and decreased enzyme activity against free radicals) and unbalanced reduction/oxidation are also contributing factors. High blood glucose levels activate numerous pathways when O₂ - is generated, for the same reasons the hexosamine route is more active and the protein kinase C isoform is activated in DM. When studying DM in mitochondria, researchers look at how much energy is produced, ROS are produced, signals are transmitted, and cells die. The processes of mitochondrial fusion and fission are essential for the preservation of homeostasis. The expansion of the mitochondrial network via mitochondrial fusion appears to be beneficial. Excessive mitochondrial fission, which results in a buildup of mitochondrial fragments and a shortened electron transport chain, can aggravate cellular mitochondrial ROS generation [40].

6.4 Oxidative stress in neurodegenerative diseases

Alzheimer’s, Parkinson’s, and depression are all linked to OS. The emergence of neurological diseases like Alzheimer’s and Parkinson’s, both of which are intimately linked to aging, is a key risk factor for OS. Oxidative stress and mitochondrial dysfunction are two of OS’s long-term side effects. The hippocampus of Alzheimer’s disease animal models shows decreased activation of mitochondrial complex IV. As well as causing mitochondrial oxidative damage, increased OS also generates harmful byproducts for the brain [41, 42]. Alzheimer’s disease neurodegeneration is linked to the production of a potentially hazardous peptide known as -amyloid by ROS. Neocortical neurons produce more H₂O₂ when -amyloid is present. Activation of NADPH by microglia cells in Parkinson’s disease mice is also linked to the progression of dopaminergic neurodegeneration. Multiple sclerosis (MS) and depressive and autoimmune illnesses are all connected to OS. Multiple sclerosis patients have lower GPx enzyme activity and higher levels of oxidative damage to DNA (8-OHdG). Patients with unipolar depression have been shown to have low levels of SOD, ascorbic acid, and MDA [43].

6.5 Oxidative stress in cancer

Cancerous cells can proliferate more quickly when ROS is present. OH, the major ROS that damages mitochondrial and nuclear DNA, hydrolyzes bases to form 8-OHdG and 8-oxodG, two examples of hydrolyzed base products. Various enzymatic pathways can be used by cells to repair damaged DNA. However, mutations caused by base change or deletion can cause cancer if DNA damage is too severe to repair. Insufficient DNA repair is more likely when there are twice as many DNA oxidative damages [44]. As we age, the bodies’ ability to repair oxidative damage and other forms of DNA damage decreases. Cytotoxicity and chromosomal disorders can result from DNA oxidation. Genetic mutations may be generated through free radical interactions with other biological components, in addition to DNA damage. The carcinogen LPO is a known carcinogen. In the presence of guanine bases, MDA may create adducts, which are toxic. It’s yet unclear how OS-induced carcinogenesis affects the human body. The ability of OS to alter gene and protein expression that signals cell growth and proliferation has been demonstrated through new techniques [1, 24].

7. Classification of antioxidants

In order to protect cells from oxidative stress, antioxidant enzymes network together to create a protective barrier. Oxidative phosphorylation and other cellular processes generate H₂O₂, which is then reduced to water. The initial stage in this detoxification pathway is initiated by superoxide dismutase, and hydrogen peroxide is eliminated by catalases and other peroxidases [2, 45].

7.1 Enzymatic antioxidants

7.1.1 Superoxide dismutase

A set of enzymes known as superoxide dismutases (SODs) breaks down the superoxide anion to produce oxygen and hydrogen peroxide. SOD enzymes are found in almost all aerobic cells and fluids. It is possible to categorize superoxide dismutases into three main types based on their ability to bind iron or manganese as cofactors: the Cu/Zn, Fe, and Mn, and finally the Ni subtypes [46]. It has been discovered that SOD isozymes are present in several cell compartments in higher plants. Mn-SOD is found in both mitochondria and peroxisomes. CuZn-SOD was found in all four chloroplasts, peroxisomes, and apoplasts using fluorescent microscopy. Fe-SOD was also found in these organelles, albeit at lower concentrations [47].

Superoxide dismutase enzymes are found in all mammals and most chordates, including humans. SOD1 and SOD2 can be found in the cytoplasm, mitochondria, and extracellular space. In comparison, the other three are tetramers (four units) in structure (four subunits). SOD1 and SOD3 include copper and zinc, while SOD2 contains manganese in the reactive core [48, 49].

7.1.2 Catalase

Catalase is an antioxidant enzyme that also plays the role of a catalyst in the process of converting hydrogen peroxide into oxygen and water. It does this by nullifying the effect that the hydrogen peroxide that is present inside the cell would otherwise have. It is not possible to determine the exact quantity of catalase that is present in the cytoplasm due to the fact that the majority of it is destroyed whenever the tissue is handled. The interaction of reactive oxygen species and antioxidants can lead to an imbalance, which in turn can lead to oxidative stress. Oxidative stress is both a disease-causing and disease-aggravating factor, and it plays a role in the development of many different diseases [1, 50].

7.1.3 Glutathione systems

The glutathione system includes glutathione, glutathione reductase, glutathione peroxidases, and glutathione S-transferases. This system can be found in all living things, including bacteria, plants, and mammals. Hydroperoxides and hydrogen peroxide can be broken down by glutathione peroxidase, an enzyme found in the body. The enzyme Glutathione peroxidase possesses all four of the necessary selenium cofactors for this procedure. There are at least four unique isozymes of glutathione peroxidase reported in different species of animals. As far as scavenging hydrogen peroxide is concerned, glutathione peroxidase 1 is more common and more effective than glutathione peroxidase 4 when it comes to lipid hydroperoxides. S-transferase activity increases when lipid peroxides are present. These enzymes are particularly abundant in the liver, where they contribute to the metabolic process of detoxification [1].

7.2 Non-enzymatic antioxidants

7.2.1 Ascorbic acid

Both plants and animals contain ascorbic acid, a monosaccharide antioxidant. For this reason, the nutrient is categorized as a vitamin and must be ingested through food. Most other animals can produce this chemical on their own and do not need it in their diets because of that. Cells can continue to function effectively because glutathione is kept in a reduced state by protein disulfide isomerase and glutaredoxins. Ascorbic acid, for example, can neutralize ROS such as hydrogen peroxide (H₂O₂). In addition to being a direct antioxidant, ascorbic acid also provides a substrate for an enzyme called ascorbate peroxidase. Because of this function, plants are better equipped to deal with a wide range of stresses [51, 52].

7.2.2 Glutathione

In all aerobic living forms, the cysteine-containing peptide known as glutathione can be detected. It can be made in the cells of the body from the amino acids that make up its components, therefore getting it in one’s food is not necessary for getting it. Because the thiol group in the cysteine that makes up glutathione is a reducing agent, glutathione could both oxidize and reduce itself in a reversible fashion, giving it antioxidant properties [53, 54]. Within cells, the enzyme glutathione reductase is responsible for maintaining the reduced form of glutathione. Glutathione, in this state, can reduce the levels of other metabolites and enzyme systems, as well as react directly with oxygen. A key antioxidant in cells, glutathione has an extremely high concentration and plays a critical role in maintaining the redox balance within cells. Mycothiol and trypanothione, two additional thiols can be substituted for glutathione in some organisms, such as actinomycetes and Kinetoplastids [55].

7.2.2.1 Thiols

The group of organic compounds with a sulfhydryl group includes thiols (-SH). They are made up of a carbon atom joined to a hydrogen atom and a sulfur atom. In the organism, extra electrons pass to thiols during the oxidation caused by ROS, resulting in the formation of disulphide bonds. These reversible bonds allow electrons to transfer back to thiols due to the oxidative balance. In enzymatic reactions, signal transduction, detoxification, transcription, regulation of enzymatic activation, cellular signaling mechanisms, and apoptosis reaction, thiol-disulphide homeostasis’ antioxidant capacity plays a crucial role [55].

7.2.3 Tocopherols and tocotrienols (vitamin E)

Tocopherols and tocotrienols make form a set of eight different fat-soluble vitamins with antioxidant properties. These vitamins are closely connected to one another. The umbrella term “vitamin E” is used to refer to all these different vitamins. The most research has been done on beta-tocopherol because of its high bioavailability compared to the other tocopherols [56]. This demonstrates that the body can absorb beta-tocopherol and metabolize it more efficiently than it does the other forms. It has been hypothesized that the form of tocopherol known as gamma-tocopherol is the most effective lipid-soluble antioxidant, and that it shields membranes from oxidation by engaging in a chain reaction with lipid radicals that are generated during the process of lipid peroxidation [57]. In other words, gamma-tocopherol protects membranes from oxidation by reacting with lipid radicals. There are no free radical intermediates left after this process and the reaction comes to a complete and total stop. Oxidized-tocopheroxyl radicals are generated because of this procedure. Activated radicals can be regenerated back into the reduced form by other antioxidants as ascorbate, retinol, or ubiquinol [58, 59].

7.3 Exogenous antioxidants

Several pharmacological properties are attributed to flavonoids, a class of polyphenolic chemicals with a benzo—pyrone structure that is abundantly found in plants. Researchers have been looking into the antioxidant properties of these compounds because of the free radical scavenging and metal ion chelating abilities of their functional hydroxyl groups. Functional groups play a critical role in determining activities such as ROS/RNS-scavenging and metal chelation, which are dependent on functional groups’ configuration and substitution. Flavonoid suppresses ROS creation, inhibits enzymes, or chelates trace elements that generate free radicals; Flavonoid scavenges ROS; and Flavonoid increases antioxidant protection [60].

Due to Genistein’s wide range of pharmacological properties, it is perhaps the most interesting and thoroughly researched flavonoid component in soy. An abundance of research has shown that genistein can scavenge ROS and RNS with great efficiency. Through gene and protein regulation, this flavonoid molecule can increase a cell’s antioxidant defenses, hence preventing apoptosis. Many plant-based foods (fruits, oils, seeds, etc.) include flavonoids, a class of naturally occurring substances that have been proved to be beneficial to human health. However, there are certain possible hazards to human health and well-being when these foods are included in the diet (as food enrichment or as nutraceuticals) [61]. Flavonoids, a group of polyphenolic compounds with a benzo—pyrone structural arrangement that is abundant in plants, are known to have several pharmacological characteristics. Because of their functional hydroxyl groups’ ability to scavenge free radicals and chelate metal ions, these compounds have been investigated for their antioxidant capabilities [62].

It is important to note that the methods of antioxidant activity such as ROS/RNS removal and metal chelation are all dependent on a variety of factors. The conformational disposition of functional groups in these compounds determines their antioxidant activity. Increased antioxidant defenses and suppression of ROS generation are both caused by flavonoid-induced ROS scavenging, as are enzyme inhibition and trace element chelation [61].

In terms of pharmacological effects, genistein is perhaps the most fascinating and well-studied flavonoid molecule. It is an isoflavone found in soy. Antioxidant genistein has been used widely in a wide range of investigations, indicating its ability to scavenge ROS and RNS. Antioxidant defenses of a cell can be improved by this flavonoid molecule, which modulates numerous genes and proteins. Flavonoids are a class of naturally occurring compounds found in a wide variety of plant-based foods (fruits, oils, seeds, etc.) that have been shown to be beneficial to human health, both as antioxidant molecules and for their other, less obvious but no less intriguing, pharmacological qualities. It’s still important to take precautions when using these supplements, and there may be some negative effects on human health and well-being if they are used in this way (as dietary supplements or nutraceuticals). Lipid soluble vitamins acts as an antioxidant, blocking free radicals from causing damage to cell membranes through a process known as lipid peroxidation (Figure 1) [8, 61].

Figure 1.

Classes of antioxidants. Source: [24, 26].

8. Diets rich in antioxidants

8.1 Fruits

A lot of different fruits have a lot of health benefits, like being high in antioxidants, filled with vitamins, and having a lot of different vitamin content. Cranberries, red grapes, peaches, raspberries, strawberries, red currants, figs, cherries, pears, guava, oranges, apricots, mango, red grapes, cantaloupe, watermelon, papaya, and tomatoes are some of the fruits that fall into this category [63].

8.2 Dried fruits

Dried fruits have a greater antioxidant ratio than fresh fruits since the water have been removed during drying. They are convenient to carry around in a handbag, briefcase, or car because they can be eaten on the go and are high in nutritional value.

8.3 Vegetables

In addition to broccoli, spinach, carrots, and potatoes, other vegetables and fruits that are high in antioxidants include artichokes, cabbage, asparagus, avocados, beetroot, radish, lettuce, sweet potatoes, squash, pumpkin, collard greens, and kale. Broccoli, spinach, carrots, and potatoes are all high in antioxidants.

8.4 Herbs and other seasonings

Antioxidants can be found in a variety of spices, including cinnamon, cardamom, paprika, oregano, and turmeric. Curry powder and mustard seed powder are also rich in antioxidants. In addition to spices like sage and tarragon and herbs like peppermint and oregano and basil, herbs also include dill weed and marjoram. Dill weed is one of several herbs that can be found. They are a fantastic source of antioxidants, as well as taste and complexity to your meals.

Grains and nuts can be found in a wide variety of cuisines. Everything from cereal to nuts to a peanut butter and jelly sandwich contains nutritional powerhouses including peanut butter and jelly, granola bars, corn flakes, and granola.

8.5 Beverages

In contrast to popular belief, the great majority of our body’s antioxidants can be found in beverages rather than in food. Apple juice, cider, tomato juice, pomegranate juice, and pink grapefruit juice are among the most common sources. In addition to green tea, black tea and ordinary tea contain a significant number of antioxidants. Coffee aficionados, welcome! Coffee is heavy in calories; however, it should be used in moderation because it can boost blood pressure and heart rate, which is why it is important to drink it in moderation. The antioxidants in coffee or tea are inhibited from being released when milk is added. While red wine and beer [which are both brewed from grains] provide a large amount of alcohol in moderation, the health benefits of moderate alcohol use have been extensively studied. Colorful fruits and vegetables are vital to have in your diet. Consider all selections, not just the most popular ones. More antioxidants can be found in foods that are deeper and brighter in color, such as oranges and yellows. With so many options, you’ll never get bored or run out of tasty and nutritious dishes to pick from. Variety, it is claimed, is the flavor of life [64].

9. Mechanism of action of antioxidants

It has been found that antioxidants can have two primary effects. The primary antioxidant breaks the chain by supplying an electron to the system’s free radical. To eliminate ROS/reactive nitrogen species initiators, the second procedure involves quenching the chain-initiating catalyst (secondary antioxidants). Co-antioxidants, electron donation, and gene expression control are some of the ways antioxidants can alter biological systems.

9.1 Preventive antioxidants

The production of ROS, such as H₂O₂ and O₂^•, cannot be prevented during metabolism. To decrease the harm caused by oxidative stress, numerous techniques have been developed. One of the finest defenses against the creation of free radicals is the cell’s own synthesis of antioxidant enzymes. SOD, CAT, GPx, GR, GST, thioredoxin reductase, and hemeoxygenase are some of the most significant antioxidant enzymes in the body. Using the Fenton reaction, the CAT, GPx, and CAT reactions, SOD decomposes O₂^• into water [65].

GST and GPX work together to help the body get rid of peroxides. The GSH/GSSG ratio is a well-established biomarker of oxidative stress since GRd controls it. GRd plays a vital role in boosting GSH concentration to maintain a steady oxido-redox state. As a result, researchers have discovered a link between oxidative stress and autism. Free radical generation and GSH/GSSG ratio in autistic cells were shown to be lower in comparison to those in control cells in an experiment.

Thus, GPx is widely distributed in cells, as opposed to CAT, which is typically restricted to the peroxisomes. Seven times more GPx activity in the brain than CAT activity, which is more susceptible to free radical damage, is found in the brain. Catalase (CAT) can breakdown H₂O₂ in the liver, kidneys, and erythrocytes at high doses [66].

9.2 Free radical scavengers

9.2.1 Scavenging superoxide and other ROS

The most prevalent kind of cellular free radical, which is referred to as superoxide (O₂^•), is accountable for a wide array of damaging modifications. Alterations in peroxidative processes and low antioxidant concentrations are commonly related with these changes. As a means of generating more powerful OH^• and ONOO even when it itself is not reactive with biomolecules, O₂• is nevertheless beneficial. The phagocyte enzyme known as NADPH oxidase is responsible for the production of massive volumes of oxygen dioxide (O₂) during the process of phagocytes eliminating microorganisms. In addition to this, it is a direct result of the respiration that takes place in the mitochondria of the cell [2, 14].

9.2.2 Scavenging hydroxyl radical and other ROS

When compared to other radical species, the hydroxyl radical, which is represented by the symbol OH^• and has the chemical formula OH, is a very active radical that can cause significant damage to biological components such as DNA, lipids, and proteins. Usually, it is believed that the synthesis of OH^• originates from the Fenton reaction system. This reaction system involves the interaction of FeSO₄ and H₂O₂ and is then carried out in an aqueous medium. Because of this, the activity of antioxidants as OH^• scavengers can be achieved through direct scavenging, the restriction of OH^• generation through the chelation of free metal ions, or the transformation of H₂O₂ into other molecules that are not harmful to the body [9].

9.2.3 Metal ion (Fe²⁺, Fe³⁺, cu²⁺, and Cu⁺) chelating

Even though trace minerals are essential for human nutrition, they are also capable of performing the function of antioxidants (through enhancing formation of free radicals). During the process of dismutation of SOD, H₂O₂ is produced as a byproduct. This H₂O₂ combines with the ions Fe²⁺ and Cu⁺ to produce the highly reactive OH^•. Although this is not the case with OH^•, the reaction of iron and copper with H₂O₂ results in the generation of more singlet oxygen than OH^•. Oxidation has occurred for both the Fe²⁺ and the Cu⁺¹ ions. When vitamin C is available, it is feasible to recycle the cellular reductants, such as NADH as well as Fe³⁺ and Cu²⁺, to produce OH^• radicals [1]. This can be accomplished when vitamin C is present. OH, is one of the most reactive elements in the body. It may react directly with proteins and fats to generate carbonyls [aldehydes and ketones], as well as cause lipid peroxidation and cross-linking. Chelating metal ions can lower their activity, which in turn lowers the formation of reactive oxygen species (ROS). Cu²⁺ and ascorbic acid are responsible for the generation of Cu⁺. This Cu⁺ is then chelated by the Se antioxidant, which prevents DNA damage caused by the OH^• radical that is generated when Cu⁺ is combined with H₂O₂ [67].

9.3 Free radical generating enzyme inhibitors

The production of free radicals by specific enzymes has been shown to occur in a wide range of physiological and pathological situations. The plasma membrane is home to a group of enzymes known as NADPH oxidases. A cytosolic donor NADPH is transferred to an extracellular oxygen molecule by these electron transporters. Hypoxanthine and hypoxanthin are converted into uric acid in the organism when catalysts for the oxidation of these compounds are present [2]. Hydrogen peroxide and oxygen radicals are formed because of this process. Aside from mitochondrial respiration, additional enzymes produce oxygen dioxide as a waste product, including NADH oxidase, monooxygenases, and cyclooxygenases. The enzyme NADPH oxidase produces a large amount of oxygen that is poisonous to all living things to fight infections in a way that is dependent on oxygen. The dying mechanism then makes use of this oxygen. Regulating the generation of reactive oxygen derivatives is critical during a respiratory burst to prevent tissue damage. This is done so that an organism can defend itself against invading pathogens [68]. On the other side, excessive ROS can cause oxidative stress, which can lead to processes like the oxidation of low-density lipoprotein (LDL). An increase in the amount of oxidized LDL that is circulating in the blood of patients who have metabolic syndrome has been linked to increased phagocytic NADPH oxidase activity. To lessen the negative consequences of oxidative stress, several studies have demonstrated that hemodialysis patients can gain advantages from inhibiting NADPH oxidase and taking antioxidants. In recent years, a great number of naturally occurring antioxidants have demonstrated the potential to inhibit enzymes that enhance O₂ generation, which has led to the development of new therapy agents for illnesses related to oxidative stress [61].

9.4 Prevention of lipid peroxidation

Some of the most frequent C-C double bonds can be found in unsaturated fatty acids, glycolipids, cholesterol, the cholesterol ester, and phospholipids, although there are many others. Lipid peroxidation is the process by which these compounds are oxidized. As a result of this chain reaction, ROS begin to damage unsaturated fats. There are several double bonds and methylene-CH₂-group groups in unsaturated fatty acids, which are very reactive hydrogens. Antioxidants can quench peroxide radicals directly, stopping the chain reaction and preventing further damage [1]. Chronic diseases such as cancer and atherosclerosis, which can cause early death, have been related to lipid peroxidation. It is possible for antioxidant compounds to neutralize or inhibit the generation of ROS and peroxide radicals, respectively. Lipid peroxidation is an important method for discovering naturally occurring antioxidants and figuring out the mechanism by which they work. Lipoproteins and red blood cells as well as low density lipoprotein (LDL) have been studied for their ability to protect against lipid peroxidation due to free radicals. An important aspect influencing the antioxidant activity of these polyphenols is their structure and starting circumstances. The microenvironment in which this reaction occurs also has a substantial impact [61].

9.5 Prevention of DNA damage

Direct interactions between nitric oxide radicals (OH^•) and O₂^• radicals (OO^•) in living cells can sever a single strand of DNA. This causes damage to the DNA that is not repairable in any way. DNA damage, which results in cell death and mutation, has been linked to degenerative diseases such as Alzheimer’s. As a result of this, DNA or plasmid damage has evolved into a model for the investigation and characterization of antioxidants. On the other hand, a study that used metal-free plasmid DNA found that the reaction of Cu²⁺ with ascorbic acid and hydrogen peroxide at pH 7 caused DNA damage. This was observed in the study. During the process, which takes place in the presence of Cu²⁺, ascorbic acid is utilized to bring about the reduction of Cu²⁺ to Cu⁺. Cu⁺ and H₂O₂ react to produce an OH^• radical, which then cleaves one strand of DNA. This results in the typically supercoiled plasmid DNA unraveling and becoming more helical [69].

10. Conclusion

Oxidative stress occurs in cells as a result of normal physiological processes and interactions with the environment, and cells are protected from oxidative damage by a sophisticated network of antioxidant defense systems. In general, our body’s innate antioxidant defense system or antioxidants added to our diet counteract the creation of reactive species. Oxidative stress arises when this balance is disrupted. Oxidative stress plays a role in the development of a wide range of diseases, including those of the gastrointestinal system. The development of antioxidant therapies is a viable route for the treatment of gastrointestinal illnesses, as data suggests that using antioxidants can improve the progression of many diseases. As a result, understanding the unique oxidative route implicated in each disease may help to identify disease signs as well as design preventive and curative therapy techniques. Inhibiting radical generation, scavenging radicals, or stimulating their breakdown are some of the ways antioxidants protect tissue against free radical harm. In the last few years, synthetic antioxidants have been connected to human health issues. As a result, the search for natural antioxidative compounds that are safe and effective has intensified in recent years. Dietary and plant-derived antioxidants may offer a feasible alternative to the body’s own antioxidant defenses. A wide variety of plant and food-derived antioxidants can be found.

Conflict of interest

The authors do not have any conflict of interest to declare.