Antioxidant Roles/Functions of Ascorbic Acid (Vitamin C)

Abstract

Antioxidant plays important roles in cellular function and has been implicated in processes associated with aging, vascular and inflammatory damage, and cancer. Ascorbic acid is a water-soluble essential vitamin with antioxidant properties found in both animals and plants but cannot be synthesized by humans and must be obtained from the diet. Ascorbic acid possessed antioxidant property and readily scavenge reactive oxygen and nitrogen species which are associated with lipid peroxidation, damage of DNA, and proteins. Ascorbic acid as an antioxidant contributes to the maintenance of the vascular system, the reduction of atherogenesis through regulation of collagen synthesis, and the production of prostacyclin and nitric oxide. It reacts with compounds like histamine and peroxides to reduce inflammatory responses. Its antioxidant property is also associated with the reduction of cancer incidences. Ascorbic acid plays a role as a redox cofactor and catalyst in a biological system such as in the conversion of the neurotransmitter dopamine to norepinephrine, in peptide amidation, and in tyrosine metabolism. In the food industry, ascorbic acid is often added to food treated with nitrite in order to reduce the generation of nitrosamines (a carcinogen), found in sausages and cold cuts.

1. Introduction

Many biological processes in humans and animals such as digestion of food, breathing, drug, and alcohol metabolism, and even the conversion of fats to energy produce free radicals or reactive oxygen species (ROS) which are harmful to the body [1]. If the system cannot cope well, free radicals can lead to a negative chain reaction in the body known as oxidative stress. Oxidative stress is the reflection of an imbalance between the body system’s antioxidants and reactive oxygen species due to a decrease in antioxidants or ROS buildup. Increased ROS production in the body may alter DNA structure, protein, and lipid alteration, the awakening of several stress-induced transcription factors as well as the creation of pro-inflammatory and anti-inflammatory cytokines. [2, 3, 4]. Oxidative stress is shown to be associated with the development of certain disease conditions such as neurodegenerative disease, cardiovascular disease, diabetes, atherosclerosis, hypertension, stroke, heart failure, Parkinson’s disease, and cancer [5, 6, 7]. One solution to this problem is to supplement the diet with antioxidant compounds [8].

Antioxidants are molecules that are capable of reducing or inhibiting the consumption or decomposition of other molecules. Free radicals are produced from oxidative processes, which can trigger several chemical reactions resulting in damage of cells. Antioxidants limit these chemical reactions by clearing or eliminating free radical intermediates and hindering other oxidative reactions by being oxidized themselves [4]. As such, antioxidants are frequently known as deoxidizing agents such as thiols or polyphenols [9]. The body’s antioxidant defense system operates/works by inhibiting the initial production of ROS, scavenge of oxidizing agent, changing oxidizing agent to less-harmful compounds, obstructing the secondary production of harmful metabolites or mediators of inflammation, stopping the circulation of secondary oxidants, restoring the molecular damage generated by ROS, or boosting the endogenous antioxidant defense system of the body.

Ascorbic acid or vitamin C is a water-soluble essential vitamin with antioxidant properties found in both animals and plants but cannot be synthesized by humans and must be obtained from the diet [10]. Ascorbic acid as an antioxidant plays essential function in the prevention of oxidative stress in various tissues in the body [11]. Ascorbic acid can protect several molecules in the body such as carbohydrates, lipids, and nucleic acids (DNA and RNA) from the damaging effects of reactive oxygen species and free radicals that are generated during cellular metabolism as a result of exposure to toxins and pollutants (drugs, cigarettes) or by immune cells [12].

2. Mechanism of action

The mechanism of action of ascorbic acid as an antioxidant is focused on the donation of a hydrogen atom to lipid radicals, removal of molecular oxygen, and quenching of singlet oxygen [13]. Regeneration of α-tocopherol from the tecopheroxyl radical species and aqueous radical scavenging is also the antioxidant mechanism of ascorbic acid [14]. According to [15], ascorbic acid has the ability to bind and reduce transition metal ions especially Fe³⁺ and Cu²⁺, and this characteristic contributes to its capacity to promote iron uptake from the diet. The proposed mechanism of action of vitamin C in mammalian cells is shown in Figure 1.

Figure 1.

Proposed mechanism of action of ascorbic acid in mammalian cells.

3. Ascorbic acid as an electron donor

Ascorbic acid is a reducing agent and thus an electron donor. The electron donation ability of ascorbic acid is responsible for its biochemical and physiological roles [14]. The double-bond carbon between the second and third of the 6-carbon molecules of ascorbic acid give two electrons. Ascorbic acid is often referred to as an antioxidant owing to the fact that it prevents other molecules from being consumed by donating electrons [15]. Ascorbic acid as an antioxidant and its interaction with reactive nitrogen species and singlet oxygen has been extensively documented [16]. Ascorbic acid has been utilized as an antioxidant for the stabilization of oxidation-prone medications. These medications include vitamin A, morphine, cholecalciferol, rifampin, promethazine, and sulphacetamide [17].

Electrons from ascorbic acid can react with metals such as copper and iron, leading to the generation of superoxide and hydrogen peroxide, which may later result in the formation of ROS. Consequently, in certain situations, ascorbic acid will give rise to an oxidizing agent by way of its operation as a reducing agent. This reaction occurs in the biological system when different pharmacological doses of ascorbic acid are absorbed in the plasma and extracellular fluid compartment as well as low doses of ascorbic acid in cell culture media containing metals [18].

When vitamin C donates electrons, the electrons are lost sequentially. The product formed after the loss of one electron is a free radical, semidehydroascorbic acid, or ascorbyl radical. When compared with other free radicals, the ascorbyl radical is different because its half-life can be measured in many seconds or even minutes depending on the absence or the presence of oxygen or electron acceptors, especially iron [19]. Ascorbyl radical is relatively stable and fairly reactive. This attribute explains why ascorbic acid may be a preferred antioxidant. In other words, a reactive and a harmful free radical can react with ascorbic acid [20]. The reduction of a reactive free radical with the generation of a less-reactive product is sometimes called free radical scavenging or quenching. Therefore, ascorbic acid is a good free radical scavenger due to its chemical composition [21]. For instance, under some conditions, ascorbyl radical could be estimated in blood and extracellular fluid samples [22].

Upon the loss of second electron, a more stable species, dehydroascorbic acid is formed when compared with ascorbyl free radical. The stability of dehydroascorbic acid depends on temperature and pH of the medium, but is often only in minutes. Dehydroascorbic acid has affinity for facilitated glucose transporters and is transported by a number of them [23]. Variety of oxidants in biological system mediate the formation of both ascorbyl radical and dehydroascorbic acid which includes molecular oxygen, superoxide, hydroxyl radical, hypochlorous acid, reactive nitrogen species, and trace metals like iron and copper. Both dehydroascorbic acid and ascorbyl radical could be reversibly broken down to ascorbic acid via different metabolic pathways and also through oxidizing compounds inside the body system including glutathione [24]. All the ascorbic acids consumed by human beings cannot be regained because there exists just fragmental depletion back to ascorbic acid.

In case of nonreduction of dehydroascorbic acid to ascorbic acid, it will be broken down to 2, 3-diketogulonic acid irreversibly due to the rupture of the lactone ring structure that is a component of ascorbic acid, ascorbyl radical, and dehydroascorbic acid. The 2, 3-diketogulonic acid is metabolized further into other compounds such as xylose, xylonite, lyxonate, and oxalate [25]. The development of oxalate is clinically significant because some persons with hyperoxaluria (excessive oxalate excretion) develop oxalate kidney stones (Figure 2).

Figure 2.

Redox chemistry of vitamin C.

4. Ascorbic acid as a co-antioxidant

Ascorbic acid has been used in combination with other vitamins and compounds to achieve a greater synergistic effect [26]. The synergistic effect of both the vitamins C and E has been reported earlier to prevent the phenomenon of autoxidation [27, 28, 29]. According to Shad et al. [30], ascorbic acid alone has little effect in preventing oxidation of lard oil. It was reported that a combination of ascorbic acid and α-tocopherol provides powerful synergistic antioxidative effect. The above researchers conclude that ascorbic acid role was to protect α-tocopherol from consumption. This characteristic of ascorbic acid is termed as co-antioxidant effect. Barcalay et al. [31] figure that ascorbic acid in sodium dodecyl sulfate (SDS) micelle system provides essential synergistic role in the autoxidation of linoleic acid acting from the aqueous phase to recreate tocopherol in the micellar phase. In a study by Gitto et al. [32], on the antioxidant action of melatonin with tocopherol, ascorbic acid, glutathione, and deferoxamine on homogenates of rat liver revealed combined effects with ascorbic acid and tocopherol. A similar study by Lui et al. [28] illustrated that the collaborative antioxidant effects of lycopene, vitamins E and C, and β-carotene were significantly higher than their antioxidant effects individually. An investigation on the fortification of water-in-extra virgin olive oil emulsion with vitamins by Cuomo et al. [26] showed that the presence of ascorbic acid controls the slowdown of oil oxidation. They also reported that the presence of vitamin C limited the peroxide value variations compared to the olive oil and to emulsions without vitamin fortification. The presence of ascorbic acid obstructs the pro-oxidant effect of α-tocopherol (vitamin E) radical since it could be changed back to α-tocopherol [33] as shown in the reaction below:

$$\text{TocO}•+\text{AscH}-\to \text{TocOH}+Asc•-.$$

The mechanism of vitamin E renewal in association with co-antioxidants makes use of greater concentrations of less potent α-tocopherol than lower concentrations. This demonstrates that food containing little amounts of vitamin E with co-antioxidants gives a higher health advantage compared to vitamin E supplements.

Tian et al. [34] observed that co-administration of araloside A and L-ascorbic acid produce coordinated antioxidant activity, and a relationship involving antioxidant cellular indexes with ROS clearing ability was identified. The pretreatment of araloside A and L-ascorbic acid singly or in combination improves both cell sustainability and function of antioxidant enzyme and obstructed the discharge of lactate dehydrogenase (LDH), the agglomeration of Malondialdehyde (MDA), lipid peroxidation (LPO) products, and H₂O₂; and the manufacturing of intracellular responsive oxygen species (ROS), protein carbonyls and 8-hydroxy-2-deoxy guanosine (8-OHdG).

5. Ascorbic acid as a co-factor of enzymes

Ascorbic acid is an important cofactor in numerous enzymatic reactions such as in the biosynthesis of collagen, neuropeptides, and in the regulation of gene expression. The function of ascorbic acid as a cofactor is also associated with its redox potential. Ascorbic acid aids mixed-function oxidases in the synthesis of several critical biomolecules by maintaining enzyme-bound metals in their reduced forms [12]. These enzymes are either monooxygenases or dioxygenases.

One of these enzymes is dopamine β-hydroxylase (DBH). Ascorbic acid serves as a cofactor for dopamine β-hydroxylase in the conversion of dopamine to norepinephrine through hydroxylation of dopamine in different neuroendocrine tissues synthesizes [35]. Synthesis of catecholamine is ascorbic acid-dependent as ascorbic acid levels are known to be in the millimolar range in the adrenal gland. Ascorbic acid in chromaffin granule is secreted together with catecholamines from chromaffin-cultured cells and in vivo by human adrenal glands, the latter in response to adreno-corticotrophin stimulation [36]. Ascorbic acid can act as a neuromodulator. Its release into the extracellular fluid of the brain, it regulates dopaminergic and glutamatergic transmission. Ascorbic acid is released from glutamatergic neurons as a result of the high-affinity glutamate transporter-exchanging ascorbic acid for glutamate during the glutamate reuptake process [37]. This hetero-exchange process ensures a comparatively high quantity of extracellular ascorbic acid in the forebrain, which may also occur in glial cells [38]. Therefore, it protects nerve cells against glutamate excitotoxicity. As the interaction between ascorbic acid and glutamate is crucial in neuron metabolism, ascorbic acid acts as a metabolic switch that modulates neural metabolism between resting and activation periods [39]. In addition, glutamate from astrocytes in the CNS regulates ascorbic acid release [40]. Several studies have reported the neuroprotective functions of ascorbic acid. Akbari et al. [41] observed that after exposure of rats to radiofrequency waves produced by the BTS mobile antenna, the rats’ brains were preserved from oxidative stress with the help of ascorbic acid. Naseer et al. [42] showed how ascorbic acid can hinder part of the negative impacts of nervous deterioration caused by PTZ in the brain of adult rats. Another related study reported how the neuroprotective activity of ascorbic acid in matured rats may be its ability to decrease the rate of lipid peroxidation and its higher enzymatic effect following seizures and epileptics state caused by pilocarpine [43]. Shokouhi et al. [44] indicated how ascorbic acid possesses a remarkable impact on decreasing the rate of malondialdehyde formation after nerve injury in rats and may have the potential for healing, and the degree of protection does not depend on the dose.

As a cofactor for the enzyme folate reductase, it participates in the conversion of methemoglobin back into hemoglobin and retains folic acid in its reduced form of tetrahydofolic acid, which is necessary for red blood cell maturation. Ascorbic acid stimulates the initial step in cholesterol metabolism to bile acids through the 7-alpha-hydroxylase enzyme. This function may have importance in the creation of gallstones and the maintenance of normal blood cholesterol levels. It is also involved in tyrosine metabolism, carbohydrate metabolism, synthesis of proteins, resistance to infections, and cellular respiration [45].

Ascorbic acid is also an important co-factor for lysyl hydroxylase and prolyl hydroxylase, enzymes essential for collagen biosynthesis, the most abundant extracellular protein. Ascorbic acid is a required element for the fusion of hydroxyproline and hydroxylysine in collagen. Hydroxyproline acts to hold the triple helix of collagen. The absence of hydroxyproline can lead to destabilization of the collagen structure that is not produced at normal rates from the cells. Hydroxylysine is necessary for the process of development of intermolecular crosslinks in collagen. Furthermore, certain carbohydrate remnants are connected to collagen glycosidically via hydroxylysine, an activity that could be essential in the control of crosslink production [46, 47]. Normally, ascorbic acid is considered to control collagen formation via its action on prolyl hydroxylation.

Ascorbic acid supplementation has been found to improve components of the human immune system such as antimicrobial and natural killer cell activities, lymphocyte proliferation, chemotaxis, and delayed-type hypersensitivity. Many cells of the immune system such as phagocytes and t-cells can accumulate ascorbic acid as they are needed to perform their task [48]. Recently, the importance of ascorbic acid in Tet-mediated DNA demethylation has been demonstrated to restrain the growth of acute myeloid leukemia [49], to promote homeostasis of hemopoietic stem cells [50], and to boost the lineage stability of regulatory T (Treg) cells [51]. Thus, ascorbic acid deficiency can result in decreased resistance against certain pathogens, while a higher level enhances several immune system parameters. Ascorbic acid concentrations in the plasma and leukocytes rapidly decline during infections and stress.

6. Ascorbic acid and cancer

Cancer is a broad term that describes the disease that results when cellular changes cause the uncontrolled growth and division of cells [52]. Presently, cancer has been identified as a complicated disorder that involves the modification of gene articulation, supports cell survival and acceleration, and can be altered by genomic and epigenomic components [53]. Genomic components are defined by the modifications at the sites of the genes, aiding mutations, while epigenomic components adapt to changes that do not reverse the chain of DNA bases except their form by histone modification, methylations in DNA bases, and nucleosome alteration [54, 55].

Cancer cells function within an advanced level of oxidative stress, as a result of elevated standard degree of ROS, oncogenic alteration, and metabolic deterioration [56].

Redox imbalance induces cell damage caused by lipid peroxidation generating derangement and loss of function and integrity of the cell membrane, as well as DNA damage, promoting genomic instability and cell proliferation, thereby increasing the somatic mutations and neoplastic transformation [57]. It is estimated that in 2030, the global burden of cancer will be 21.4 million new cases and 13.2 million deaths, mainly due to the growth and aging of the population [58]. The World Health Organization (WHO) states that the consumption of fruits and vegetables can help prevent cancer due to their composition with nutrients such as vitamins, minerals, and fiber [59]. As published in the Dietary Reference Intake (DRI), food containing antioxidants such as ascorbic acid, carotenoids, tocopherol, selenium, and flavonoids are suggested to be consumed because of their antagonistic reaction, as they often have the potential for cancer prevention, suppress degeneration of molecules, and formation of free radicals [60].

The relationship between vitamin C and cancer is still under study and is associated with antioxidant, prooxidant, and gene expression regulator properties [61]. The role of ascorbic acid on tumor advancement relays on whether it is administered orally or intravenously, together with the manifestation and the category of ascorbic acid transporters in the tumor cells. It has been reported that some cancer cells manifest an elevation in the expression of sodium vitamin C transporter 2 (SVCT2) and/or glucose transporter 1 (GLUT1) and absorbs more vitamin C than normal cells for the ascorbic acid transporter’s expression [61]. The high expression of GLUT1 in tumors constitutes the principle of oncological diagnosis based on the positron emission tomography of ¹⁸F-fluorodeoxyglucose.

In treatment, ascorbic acid was selectively toxic to cancer cells in vitro and in vivo [62, 63]. In animal models, ascorbic acid either has anti-cancer activity similar to conventional chemotherapy or synergizes with it [64, 65]. Contrarily, there are no data showing that treatment of cancer with ascorbic acid interferes with chemotherapy. Early-phase clinical trials show that i.v. ascorbic acid at 1 g/kg over 90–120 minutes, two to three times weekly is well tolerated and may enhance chemo sensitivity as well as decrease chemotherapy-related side effects [66, 67, 68]. Agathocleous et al. [50] indicated the role of oral ascorbic acid in delaying cancer development in mice which was based on ascorbate-dependent activation of TET and HIF-PHD enzymes. In these mice, oral vitamin C postponed progression of genetically susceptible transplanted hematopoietic cells to acute myelogenous leukemia. In a similar study in lymphoma xenograft mode, tumor development was decreased by oral vitamin C, which had HIF-1-dependent anti tumorigenic properties [69]. Treatment of cancer to reduce tumor growth in many rodent models was mediated by parenteral ascorbic acid to produce treatment concentrations [62, 65, 70, 71, 72].

Ascorbic acid concentrations in cancer patients are often at deficiency concentrations [73, 74], the basis for which is still unknown. The justification for the above may include generalized cancer wasting, improved consumption of vitamins by cancer cells, or suppression of vitamin C carriers. It is consequently, in accordance to determine whether there is a place for vitamin C as a treatment regimen through rectification of vitamin C deficiency, irrespective of parenteral administration. The outcome of sporadic treatment trials in persons with advanced stages of cancers particularly colon and rectal cancers were negative [75]. Based on recent in vitro evidence, correction of ascorbate deficiency may aid conventional hypomethylating agents, that is, DNA methyltransferase inhibitors, to exert maximal effect [74]. The rationale for the use of high-dose i.v. ascorbate in cancer treatment is not to correct plasma deficiency, but rather to induce an oxidative stress on cancer cells [62] and to ensure adequate delivery of ascorbic acid within tumors for optimal cofactor function [76]. It is therefore important to accept that only parenteral ascorbic acid has yielded results for treatment success in the early phase of human trials in the literature. The use of rodent models in confirmed cancer cases, only parenteral ascorbic acid has proven to be beneficial irrespective of the model. This information gives directions for clinical trials [77].

7. Conclusion

Ascorbic acid is widely regarded as an essential antioxidant in both humans and animals. As an antioxidant, it has two basic functions: first, it reacts and inactivates free radicals in plasma, cytosol, and extracellular fluid compartment of the body. Second, it can regenerate vitamin E in its oxidized form. Those functions are possible because of its ability to donate electron. Studies have reported the use of ascorbic acid in the prevention and treatment of cancer. There are different publications on the antioxidant activity of ascorbic acid through various mechanisms at different dosages such as reduction of cytotoxicity, decreased apoptosis, and preservation of tumor cells from lipid peroxidation, inhibiting cancer cell growth, and secretion of inflammatory cytokines. The briefly discussed antioxidant role of ascorbic acid just represents some abilities of its involvement in physiological activities. More research is needed to understand dose–response variations, as well as its targeting mechanisms of action as an antioxidant and anticancer agent, to aid in the prevention and treatment of cancer, with the goal of improving the quality of life for both patients and the general population.

Acknowledgments

We thank the management of the Federal College of Animal Health and Production Technology, Vom, and Mr. Kenneth Emeribe for his encouragement during the course of this work.

Conflict of interest

The authors declare no conflict of interest.