Vitamin C is a vitamin for only a limited number of vertebrate species: humans and the other primates, the guinea pig, bats, the passeriform birds, and most fishes. Most insects and invertebrates are also incapable of ascorbate synthesis. Ascorbate is synthesized as an intermediate in the gulonolactone pathway of glucose metabolism; in those vertebrate species for which ascorbate is a vitamin, one enzyme of the pathway, gulonolactone oxidase, is absent.
The vitamin C deficiency disease, scurvy, has been known for many centuries, and was described in the Ebers papyrus of 1500 B.C. and by Hippocrates. The Crusaders are said to have lost more men through scurvy than were killed in battle; in some of the long voyages of exploration of the fourteenth and fifteenth centuries, upto 90% of the crew died from scurvy. Cartier's expedition to Quebec in 1535 was struck by scurvy; the local native Americans taught him to use infusion of swamp spruce leaves to prevent or cure the condition. Recognition that scurvy was the result of a dietary deficiency came relatively early. James Lind demonstrated in 1757 that orange and lemon juice were protective, and Cook maintained his crew in good health during his circumnavigation of the globe (1772 to 1775) by stopping frequently to take on fresh fruit and vegetables. In 1804, the British Navy decreed a daily ration of lemon or lime juice for all ratings, a requirement that was extended to the merchant navy in 1865.
Ascorbic acid was isolated from cabbage, lemon juice, and adrenal glands by Szent-Gyorgy in 1928, and identified as the antiscorbutic factor by Waugh and King in 1932. Its structure was established by Haworth and coworkers in 1933, and the same year Haworth, in Birmingham, and Reichstein, in Switzerland, succeeded in synthesizing the vitamin.
Ascorbate is a reducing sugar; in addition to its specific role as cofactor for a variety of redox reactions, it also functions as a relatively nonspecific reducing agent. Some of these nonspecific reactions are physiologically important;
others have led to confusion in the literature because in vitro it will enhance the activity of a number of enzymes for which it is not a cofactor.
Ascorbate is synthesized in large amounts in plants. It can reach 20 to 300 mmol per L in chloroplasts, where its function is mainly to remove hydrogen peroxide formed during photosynthesis. Ascorbate-deficient mutant plants are especially sensitive to ozone- and ultraviolet-induced stress (Smirnoff, 2000).
The physiologically important compound is L-ascorbic acid (Figure 13.1). It can undergo oxidation to the monodehydroascorbate free radical and onward to dehydroascorbate, both of which have vitamin activity because they can be reduced to ascorbate. Further oxidation in the presence of oxygen, and especially under alkaline conditions, or in the presence of transition metal ions that undergo reduction, results in the formation of diketogulonic acid (dioxogulonic acid; see Figure 13.2), which has no biological activity.
D-Iso-ascorbic acid (erythorbic acid; see Figure 13.1) also has vitamin activity. In vivo and in cell culture, it has only about 5% of the biological activity of ascorbate, but this seems to be from poor intestinal absorption and tissue uptake. In vitro with purified enzymes, it has the same cofactor activity as ascorbate. Although it is not a naturally occurring compound, erythorbic acid is widely used interchangeably with ascorbic acid, in cured meats and as an antioxidant in a variety of foods.
Ascorbic acid 2-phosphate and triphosphate are more stable to atmospheric oxidation than ascorbate and are used in food processing. They have the same biological activity as ascorbic acid on a molar basis, because they are substrates for intestinal phosphatases. Ascorbic acid 2-sulfate is a metabolite of the vitamin in some species and has little or no biological activity.
Vitamin C is used in food processing as the free acid (E-300), the sodium (E-301) and calcium (E-302) salts, andasascorbylpalmitateorstearate (E-304), a lipid-soluble antioxidant. The palmitate and stearate have low biological activity. Although most of the ascorbate used as a flour improver in bread making is destroyed in baking, a considerable number of otherprocessed foods provide significant amounts of the vitamin because of its use as an antioxidant and in meat curing.
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