Vitamin C, or ascorbic acid, is an essential requirement for various physiological processes because of its reducing activity.
Vitamin C is vital for the normal synthesis of collagen, L-carnitine, catecholamines, and proteins. It acts as a cofactor to the mixed-function oxidase enzymes which catalyze these chemical pathways. The reaction centers of these enzymes contain bound metals, which are maintained in reduced form by vitamin C. It thus sustains enzymatic activity.
Vitamin C catalyzes the post-translational hydroxylation of proline and lysine residues incorporated into various proteins found in supporting tissues such as collagen, osteoid and intercellular cement. Hydroxylation of these residues imparts tensile strength by allowing fibers to cross-link within the protein.
Collagen is one of the important proteins whose formation is catalyzed by vitamin C, and it forms a fundamental part of connective tissue. Vitamin C is thus vital to body growth and wound repair.
Vitamin C donates electrons readily to eight enzymes in the human body. Three of these are involved in the biosynthesis of collagen.
However, an equally important role of vitamin C is its antioxidant role. It is a powerful reducing agent, and therefore readily takes part in redox reactions, shifting between the two forms ascorbic acid and dehydroascorbic acid. It is responsible for the non-enzymatic regeneration of other antioxidant molecules which take part in various physiological processes, such as alpha-tocopherol (vitamin E), in addition, it is known to protect glutathione from oxidation.
Thus vitamin C is the primary water-soluble antioxidant in the body. It protects cell membranes, DNA, cell proteins and lipids from the oxidizing effects of free radicals and reactive oxygen species. These are not only by-products of normal cellular metabolism, but are also overproduced during immune activation or inflammation, in response to exposure to biological or chemical toxins or pollutants.
A third function of vitamin C is its role in immune regulation. Vitamin C stimulates phagocytosis as well as antibody formation.
In addition, vitamin C enhances the absorption of non-heme iron, which is the type present in plant foods, in the intestine. It does this by reducing ferric iron to the ferrous state, which is better absorbed.
Bile acid synthesis
Vitamin C stimulates the initial step in cholesterol metabolism to bile acids, via the 7-alpha-hydroxylase enzyme. This function may have importance in the formation of gallstones and the maintenance of normal blood cholesterol levels.
Vitamin C is also essential for the synthesis of serotonin, during which it hydroxylates the amine tryptophan to 5-hydroxy tryptophan.
Reducing activity in other situations
It reduces methemoglobin back to hemoglobin, and also maintains folic acid in the reduced form (tetrahydrofolic acid) which is essential for red cell maturation, by acting as a cofactor for the enzyme folate reductase.
Adrenal steroid synthesis
Vitamin C is present in relatively high concentration in the adrenal cortex, but the levels go down after ACTH stimulation of the gland. This implies that vitamin C has a part to play in the synthesis of adrenal steroids.