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Companion Animals: Vitamin C

Properties and Metabolism

Vitamin C occurs in two forms, namely L-ascorbic acid (reduced form) and dehydro-L-ascorbic acid (oxidized form). Although in nature the vitamin is primarily present as ascorbic acid, both forms are biologically active. The L-isomer of ascorbic acid is biologically active; the D-isomer is not. In nature, the reduced form of ascorbic acid may reversibly oxidize to the dehydroxidized form, the dehydroascorbic acid is irreversibly oxidized to the inactive diketogulonic acid. Since this change takes place readily, vitamin C is very susceptible to destruction through oxidation, which is accelerated by heat and light. Reversible oxidation-reduction of ascorbic acid with dehydroascorbic acid is the most important chemical property of vitamin C and the basis for its known physiological activities and stabilities (Moser and Bendich, 1991). Vitamin C is the least stable and, therefore, most easily destroyed of all the vitamins.

Vitamin C is a white to yellow-tinged crystalline powder. It crystallizes out of water as square or oblong crystals (Illus. 1), slightly soluble in acetone and lower alcohols. A 0.5% solution of ascorbic acid in water is strongly acid with a pH of 3. The vitamin is more stable in an acid than an alkaline medium. A number of chemical substances, such as air pollutants, industrial toxins, heavy metals and tobacco smoke, as well as several pharmacologically active compounds, among them some antidepressants and diuretics, are antagonistic to vitamin C and can lead to increased requirements for the vitamin.

Vitamin C is absorbed in a manner similar to carbohydrates (monosaccharides). Intestinal absorption in vitamin C-dependent animals appears to require a sodium-dependent active transport system. It is assumed that those species that are not scurvy prone do have an absorption mechanism by diffusion (Spencer et al., 1963). Ascorbic acid is readily absorbed when quantities ingested are small, but limited intestinal absorption occurs when excess amounts of ascorbic acid are ingested. Bioavailability of vitamin C in feeds is limited, but apparently 80% to 90% appears to be absorbed (Kallner et al., 1977). Site of absorption in the guinea pig is in the duodenal and proximal small intestine, whereas the rat showed highest absorption in the ileum (Hornig et al., 1984).

In its metabolism, ascorbic acid is first converted to dehydroascorbate by several enzymes or nonenzymatic processes and can then be reduced back to ascorbic acid in cells (Rose et al., 1986). Absorbed vitamin C readily equilibrates with the body pool of the vitamin. No specific binding proteins for ascorbic acid have been reported, and it is suggested that the vitamin is retained by binding to subcellular structures.

Ascorbic acid is widely distributed throughout the tissues, both in animals capable of synthesizing vitamin C as well as in those dependent on an adequate dietary amount of the vitamin. In experimental animals, highest concentrations of vitamin C are found in the pituitary and adrenal glands, and high levels are also found in the liver, spleen, brain and pancreas. Vitamin C also tends to localize around healing wounds. Tissue levels are decreased by virtually all forms of stress, which also stimulates the biosynthesis of the vitamin in those animals capable of synthesis.

Ascorbic acid is excreted mainly in urine, with small amounts in sweat and feces. In guinea pigs, rats, and rabbits, CO2 is the major excretory mechanism for vitamin C. Primates do not normally utilize the CO2 catabolic pathway, with the main loss occurring in the urine. Urinary excretion of vitamin C depends on body stores, intake and renal function.

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