**2. Vitamin C**

Physiologically, VC is a six-carbon compound similar to glucose, containing two acid-ionizable groups, and it loses hydrogen ions depending on the pH of the environment, forming ascorbate monoanion or dianion by losing hydrogen ions attached to the 2nd and 3rd carbons [4]. VC is reported to be a water-soluble vitamin that is naturally hydrophilic [1]. It is a powerful natural antioxidant that is not synthesized in any tissue, including the brain [5–8]. VC is described as a neutral molecule and is suggested to be a white crystalline solid [4]. Vitamin C (VC) is an essential nutrient with various health-promoting properties. Unlike most mammals, humans lack the ability to synthesize VC endogenously and rely on dietary intake for meeting their physiological requirements [9]. VC is abundant in fruits and vegetables, and its consumption is vital for humans to obtain this essential vitamin [5]. VC plays a crucial role in the synthesis of catecholamines, carnitine, cholesterol, amino acids, and certain peptide hormones. Additionally, it acts as a cofactor in numerous important enzymatic reactions [6, 7, 10]. One of the well-known functions of VC is its involvement in the hydroxylation of lysine and proline residues during collagen synthesis. This process is essential for the proper intracellular folding, export, and accumulation of procollagen, ultimately leading to the formation of mature collagen [2, 6, 7]. VC is reported to be an indispensable electron donor in various biological reactions, including collagen hydroxylation, carnitine biosynthesis, and tyrosine metabolism, making it a fundamental nutrient for humans (**Figure 1**) [12].

VC functions as an electron donor during the generation of ascorbate free radical (AFR). Within the cell, AFR is subsequently reduced back to VC through the activity *The Role of Vitamin C in Preventing Some Neurodegenerative Diseases: Alzheimer's, Parkinson's… DOI: http://dx.doi.org/10.5772/intechopen.112185*

### **Figure 1.** *Chemical structure of vitamin C [11].*

of NADH- and NADPH-dependent reductases. However, if AFR accumulates or becomes concentrated in areas where NADH and NADPH-dependent reductases are not accessible, it can react with VC and dehydroascorbic acid (DHA) to form a different molecule. Furthermore, AFR can undergo a process called dismutation. DHA, on the other hand, can be enzymatically reduced to VC through various cellular mechanisms, including reduction by reduced glutathione (GSH) or enzymatic reduction by thiol transferases or NADPH-dependent reductases. This enzymatic recycling system allows for the regeneration of VC from its oxidized form within the cell. Notably, due to its hydrophilic nature and negative charge at physiological pH, VC can traverse various cell types, including neurons [6]. Vitamin C plays a crucial role in the biochemical functions of redox processes, making it a vital substrate for several key enzymes in the organism's functioning. One of its significant functions is its ability to be oxidized to ascorbyl radical and further to dehydroascorbic acid (DHA), thereby acting as an antioxidant [4]. Deficiency of vitamin C leads to scurvy, a condition characterized by swollen and bleeding gums, as well as the reopening of previously healed wounds. The enediol structure of vitamin C enables it to function as a potent reducing agent in aqueous solutions, readily undergoing reversible oxidation to DHA. It can also undergo irreversible degradation into diketogulonic acid. In addition to its role as an antioxidant, vitamin C is essential for detoxification reactions and combating bacterial infections [2]. Most animals synthesize VC in their livers, providing a regular supply to the body through circulation. However, humans, primates, and several other species, including guinea pigs, have lost this ability due to functional loss mutations in the gene encoding gulonolactone oxidase, the terminal enzyme in the biosynthetic pathway of VC [13]. The physiological intake and storage of VC can be measured by both plasma and leukocyte levels [14]. It is believed that VC protects cellular DNA, proteins, and lipids from oxidative damage by undergoing self-oxidation, neutralizing reactive oxygen species (ROS). In fact, VC can donate two electrons to ROS, rendering them ineffective. Additionally, VC has various beneficial functions such as immune stimulation and anti-inflammatory effects [7]. Diet and food have been reported to play a protective role in cognition. Various diets, B vitamins, VC, vitamin E, omega-3 fatty acids, polyphenols, flavonoids, and caffeine, among other dietary components, further enhance cognition in association with biliş [15]. It has


### **Table 1.**

*Physiological functions of vitamin C [17].*

been reported that foods and functional foods have neuroprotective roles against neurodegeneration. In particular, certain vitamins, minerals, and phytochemicals play a crucial role in exhibiting antioxidant properties by directly scavenging reactive oxygen species (ROS), acting as cofactors to antioxidant enzymes, and regulating genes that control intracellular antioxidant systems [5]. Numerous compounds, including GSH, VC, vitamin E, coenzyme Q10, and others, possessing antioxidant properties, are currently under investigation for their potential to mitigate the impact of ROS-related neurodegenerative processes. Typically, neurodegenerative diseases are characterized by elevated levels of oxidative stress and reduced levels of antioxidant defense markers in the brain and peripheral tissues [16]. The neuroprotective effects of VC have been documented to be effective in various neurodegenerative diseases (**Table 1**) [5].
