# Cell Response to Oxidative Stress in Antarctic Filamentous Fungi
## 5 Oxidative Stress and Antioxidant Defence
### Antioxidants and Antioxidant Defence
#### 5.2.2 Non-Enzymatic Antioxidants  
Natural compounds without enzymatic characteristic include vitamins, flavonoids, carotenoids, glutathione, plant polyphenols, melatonin, bilirubin, curcumin, and polyamines. Some of them are water-soluble, and in most cases, localized in the cytosol or cytoplasmic matrix, while others are fat-soluble and are present in cell membranes. Vitaminsare complex organic compounds essential for the body in minimal quantities. Among these, antioxidant vitamins such as A, E, and C play a crucial role in neutralizing reactive oxygen species and exhibit a diverse array of biological functions. They contribute to cellular protection against genomic alterations by mitigating DNA damage caused by oxidative stress. Vitamin A, also known as retinol, is a fat-soluble vitamin classified within the retinol family. This family comprises 20 distinct compounds characterized by a methyl-substituted cyclohexenyl and a tetraene side chain, which contains four double bonds. Beta-carotene serves as the precursor to vitamin A, undergoing hydrolysis to form retinol in the gastrointestinal system. The mechanism through which vitamin A exerts its antioxidant effects involves the neutralization of peroxyl radicals (LOO\(\bullet\)). This is achieved by the addition of these radicals to the \(\beta\)-ionone ring of retinol, resulting in the formation of a highly conjugated retinol-carbon radical. This intermediate compound is relatively stable and, under typical physiological conditions, does not possess sufficient reactivity to promote further lipid peroxidation [146]. Vitamin C, also known as ascorbic acid, is a water-soluble vitamin that was first isolated in 1928. It serves as an essential trace element necessary for the proper metabolism of aerobic cells [146]. Within the cytosol, vitamin C engages directly with reactive species such as \({}^{\bullet}\)O\({}_{2}\)\({}^{-}\) and HO\(\bullet\), thereby protecting cellular membranes against oxidative damage [125]. The antioxidant mechanism of vitamin C operates through its role as an electron donor, functioning as a reducing agent that can neutralize oxidized compounds. Upon interaction with a free radical, vitamin C donates an electron, resulting in the formation of the ascorbyl radical (AscR), which exhibits greater stability compared to other free radicals. The AscR can subsequently neutralize more reactive radicals, including OH\(\bullet\). In a further reaction, AscR can donate an additional electron to another reactive radical, leading to the production of dihydroracorbic acid. Notably, a single molecule of ascorbic acid has the capacity to neutralize two free radicals. Vitamin E is a group of compounds that includes four tocopherols and four tocotrienols, with alpha- and gamma-tocopherol being the most abundant in human tissues. The antioxidant mechanism of vitamin E operates through its interaction with free radicals, resulting in the formation of a tocopheroxyl radical along with a non-radical by-product [146]. In the cellular context, vitamin E serves several critical functions: it quenches peroxyl radicals produced during lipid peroxidation, protects polyunsaturated fatty acids in cell membranes, assists in the neutralization of singlet oxygen, and participates in reactions with peroxpiritrites. It is recognized as the most potent antioxidant in the lipid environment, and its efficacy is greatly enhanced when combined with other antioxidants such as vitamin C, selenium, and beta-carotene [147]. Carotenoids represent a diverse class of red-orange pigments found in plants. These pigments are characterized as fat-soluble chromophores that utilize a system of conjugated double bonds to absorb light within the wavelength range of 400 to 500 nm [148]. More than 600 different carotenoids have been identified, with six being particularly prominent: alpha-carotene, beta-carotene, lycopene, lutein, beta-cryptoxanthin, and zeaxanthin.