Vitamin C or L-ascorbic acid, or simply ascorbate (the anion of ascorbic acid), is an essential nutrient for humans and certain other animal species. Vitamin C refers to a number of vitamers that have vitamin C activity in animals, including ascorbic acid and its salts, and some oxidized forms of the molecule like dehydroascorbic acid. Ascorbate and ascorbic acid are both naturally present in the body when either of these is introduced into cells, since the forms interconvert according to pH.

Vitamin C is a cofactor in at least eight enzymatic reactions, including several collagen synthesis reactions that, when dysfunctional, cause the most severe symptoms of scurvy.[1] In animals, these reactions are especially important in wound-healing and in preventing bleeding from capillaries. Ascorbate may also act as an antioxidant against oxidative stress.[2] However, the fact that the enantiomer D-ascorbate (not found in nature) has identical antioxidant activity to L-ascorbate, yet far less vitamin activity,[3] underscores the fact that most of the function of L-ascorbate as a vitamin relies not on its antioxidant properties, but upon enzymic reactions that are stereospecific. "Ascorbate" without the letter for the enantiomeric form is always presumed to be the chemical L-ascorbate.

Ascorbate (the anion of ascorbic acid) is required for a range of essential metabolic reactions in all animals and plants. It is made internally by almost all organisms; the main exceptions are most bats, all guinea pigs, capybaras, and the Anthropoidea (i.e., Haplorrhini, one of the two major primate suborders, consisting of tarsiers, monkeys, and humans and other apes). Ascorbate is also not synthesized by some species of birds and fish. All species that do not synthesize ascorbate require it in the diet. Deficiency in this vitamin causes the disease scurvy in humans.

The name vitamin C always refers to the L-enantiomer of ascorbic acid and its oxidized forms. The opposite D-enantiomer called D-ascorbate has equal antioxidant power, but is not found in nature, and has no physiological significance. When D-ascorbate is synthesized and given to animals that require vitamin C in the diet, it has been found to have far less vitamin activity than the L-enantiomer.[3] Therefore, unless written otherwise, "ascorbate" and "ascorbic acid" refer in the nutritional literature to L-ascorbate and L-ascorbic acid respectively. This notation will be followed in this article. Similarly, their oxidized derivatives (dehydroascorbate, etc., see below) are all L-enantiomers, and also need not be written with full sterochemical notation here.

Ascorbic acid is a weak sugar acid structurally related to glucose. In biological systems, ascorbic acid can be found only at low pH, but in neutral solutions above pH 5 is predominantly found in the ionized form, ascorbate. All of these molecules have vitamin C activity, therefore, and are used synonymously with vitamin C, unless otherwise specified.

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