Functions of Vitamin C
1) Antioxidant role- Vitamin C is the major water-soluble antioxidant within the body. The vitamin readily donates electrons to break the chain reaction of lipid peroxidation. The water-soluble properties of vitamin C allow for the quenching of free radicals before they reach the cellular membrane. Tocopherol (Vitamin E) and glutathione also rely on Ascorbic acid (AA) for regeneration back to their active isoforms (Figure). The relationship between AA and glutathione is unique. Vitamin C reduces glutathione back to the active form. Once reduced, glutathione will regenerate vitamin C from its dehydro Ascorbic acid or oxidized state.
Figure–showing the interrelationship of vitamin C, E and Glutathione.
Vitamin C is required as a redox agent, reducing metal ions in many enzymes and removing free radicals. In this capacity, it protects DNA, protein, and vessel walls from damage caused by free radicals.
2) Amino acid Metabolism- Ascorbic acid has been implicated in –
a) Tryptophan metabolism- for hydroxylation of tryptophan to serotonin
b) Tyrosine metabolism- For oxidation of para hydroxy phenyl pyruvate to homogentisic acid.
1) Bone formation-Vitamin C is required for the activity of osteoblasts to form osteoid. the intercellular cementing substance. In vitamin C deficiency without the normal ground substance, the deposition of bone is arrested.The resulting scorbutic bone is weak and fractures easily.
2) Bile acid synthesis- 7-alpha hydroxylase- the enzyme needed for the first step in bile acid synthesis, cholesterol 7-alpha hydroxylase, is dependent upon the presence of vitamin C. Bile acid formation, and hence cholesterol degradation are highly dependent on AA. Some hypothesize that vitamin C may even have a hypocholesterolemic effect.
3) Brain function- Ascorbic acid regulates the activity of some neurons within the brain– Some of these functions include neurotransmitter membrane receptor synthesis, and neurotransmitter dynamics. Indirectly, AA plays important regulatory roles throughout the entire body due to its involvement in the synthesis of hormones, hormone-releasing factors, and neurotransmitters.
1) Collagen formation- Vitamin C is necessary for the triple-helix formation of collagen. Vitamin C dependent Proline and lysine hydroxylases are required for the postsynthetic modification of procollagen to collagen. Deficiency of vitamin C leads to impaired collagen synthesis, causing capillary fragility, poor wound healing, and bony abnormalities in affected adults and children.
2) Carnitine synthesis- Trimethyllysine and γ -butyrobetaine hydroxylases are required for the synthesis of carnitine. Carnitine synthesis prefers to use vitamin C as the reducing agent. Carnitine facilitates the beta-oxidation of fat, through its role of transporting long chain fatty acids from the cytoplasm into the mitochondrial matrix of cardiac and skeletal muscle.
3) Complement synthesis- Vitamin C dependent Proline hydroxylase is also required in formation of C1q component of complement.
4) Clotting- vitamin C has an important role in the coagulation of blood. Vitamin C requiring Aspartate -Β hydroxylase is required for the postsynthetic modification of the precursor of protein C, the vitamin K–dependent protease that hydrolyzes activated factor V in the blood-clotting cascade.
5) Catecholamine synthesis- Dopamine Β-hydroxylase is a copper-containing enzyme involved in the synthesis of the catecholamines, norepinephrine, and epinephrine, from tyrosine in the adrenal medulla and central nervous system. During hydroxylation the Cu+ is oxidized to Cu2+; reduction back to Cu+ specifically requires ascorbate, which is oxidized to monodehydroascorbate. Vitamin C is directly involved in the enzyme activity of these copper dependent mono-oxygenases, which are important in the formation of catecholamines and serotonin.
Drug detoxification- It is a component of many drug-metabolizing enzyme systems, particularly the mixed-function oxidase systems.
Enzymes– Vitamin C is the coenzyme for two groups of hydroxylases. These are copper-containing hydroxylases and the α-ketoglutarate-linked iron-containing hydroxylases. In addition, it has a number of nonenzymic effects as a result of its action as a reducing agent and oxygen radical quencher.
Folic acid metabolism- Ascorbic acid is required for reducing folic acid to its tetrahydrofolate form. Thus it helps in the maturation of red blood cells.
General body growth- Vitamin C has a protective role and because of its role in disease prevention, it stimulates general body growth.
1) Haemoglobin– Vitamin C is helpful in the reconversion of met haemoglobin to haemoglobin.
2) Hormone synthesis vitamin C is also a cofactor in the synthesis of peptide hormones, corticosteroids, and aldosterone.
1) Iron metabolism– The vitamin is an important aid in the absorption and conversion of iron to its storage form. This may contribute to the anemia seen with vitamin C deficiency.
2) Immunity- Vitamin C has been proposed to have pharmacological benefits in preventing cancer, infections, and the common cold. The role of vitamin C in preventing cancer is controversial, but has been studied for cancers of the oral cavity, uterus, esophagus, bladder, and pancreas.
Vitamin C affects several components of the human immune system; for example, vitamin C has been shown to stimulate both the production and function of leukocytes ,especially neutrophils, lymphocytes, and phagocytes. Specific measures of functions stimulated by vitamin C include cellular motility chemotaxis, and phagocytosis. Neutrophils, which attack foreign bacteria and viruses, seem to be the primary cell type stimulated by vitamin C, but lymphocytes and other phagocytes are also affected .
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