Ascorbic acid (vitamin C)

Ascorbic acid also known as vitamin C is a crystalline, optically active, water-soluble, and a strongly reducing substance. It is very much susceptible to atmospheric oxidation. Its oxidation is promoted further by the presence of Cu⁺⁺ or Fe⁺⁺ ions. It is easily destroyed in alkaline medium whereas it is relatively much stable in the acidic medium. Vitamin C is lost during cooking.

It is a hexose derivative.

Its acidic property is due to easy dissociation of two enolic groups. Ascorbic acid is oxidized easily by loss of two hydrogen atoms from the two enolic groups, existing across the double bond forming dehydroascorbic acid and this is a reversible process. Ascorbic acid can exist in two isomeric forms – L- and D. Only the L-form is biologically active. In the biological tissues, glutathione is involved in the oxidation and reduction of ascorbic acid. Glutathione is abundantly present in the animal tissues as well as in the biological fluids. It does not require any enzyme for oxidation-reduction purposes, however, its action is further promoted by the presence of copper or iron ions.

Sources: the richest sources of vitamin C are the citrus fruits such as lemons and oranges. The other rich sources are wale nuts, black current, bilberry, dog rose, plums, guava, grapefruits, strawberries and apples. Lesser important sources are sprouts during germination of seeds, cauliflowers, cabbage, tomatoes and potatoes. Vitamin C is also present in the human and cow milk. For infants, mother’s milk and cow milk are the only sources of vitamin C. If pasteurized milk is taken, vitamin C deficiency must be compensated by giving fruit juice. The meat, fish, eggs and cereals are devoid of vitamin C. animal tissues such as adrenal gland, pituitary, wall of gum, muscles, brain, eye lenses and blood cells have also a fairly good concentration of vitamin C.

Functions: ascorbic and dehydroascorbic acids together form oxidative-reductive system, which can accept and donate electrons and protons.

Ascorbic acid takes part in following processes of biological oxidation: 1) the hydroxylation of tryptophan to 5-hydroxytriptophan during the biosynthesis of serotonine; 2) the hydroxylation of corticosteroids during the biosynthesis of hormones of adrenal cortex from cholesterol; 3) the transformation 3,4dehydrooxyphenylethylamine (DOPhA) to noradrenaline; 4) the reduction of folic acid to the tetrahydrofolic acid; 5) the hydroxylation proline and lysine during biosynthesis of collagen; 6) ascorbic acid can reduce an ions Fe⁺⁺ to an ions Fe⁺⁺⁺ , releasing the absorption of iron from intestine; 7) ascorbic acid can liberate irons from transferrin, that increase to the entering of iron to the tissues; 8) ascorbic acid is involved in the electron transport chain oxidation in the microsomes; 9) ascorbic acid acts as coenzyme for cathepsine and liver esterases; 10) vitamin C significantly reduces symptoms of severity and total intensity of colds in girls but does not benefit cold symptoms in boys at a daily dose of 500mg daily. During colds, especially of catarrh variety, ascorbic acid is transferred out of the leukocytes into plasma where it is rapidly metabolized in both the sexes but this effect is more pronounces in females. There is a significant positive correlation between metabolic utilization of ascorbic acid during colds and intensity of cold symptoms; 11) ascorbic acid leads to hypocholestremic effect in man and guinea-pigs.

Deficiency symptoms Man, monkey and guinea-pigs require vitamin C in the diet as these can not synthesize this vitamin. Deficiency of this vitamin leads to retardation of growth, swelling and pain at joints, because deficiency increases the permeability of capillaries and very small hemorrhages starts: in joints, muscles, gum. Extreme deficiency of ascorbic acid in human subjiects leads to scurvy – the bleeding mouth, the loosing of teeth, because new molecules of collagenes do not synthesize – oxyproline does not synthesize and teeth’s alveolus and all cartilage are dissolved. Healing of wounds are delayed. The scurvy shows a feeling of fatigue, neuromuscular co-ordination is impaired. There are serious changes in the myocardia (edema, hemorrhages), hypoxia of tissues and electrocardiogram, cardiac-ishemia can to occurs.

In infectious diseases, congestive heart failure, renal and hepatic diseases and malignancies, circulatory vitamin C level is found to be low. It is possible that there might be some bearing of these diseases on the vitamin C level.

Requirement the exact vitamin C require ment of human beings is yet to be precisely determined. The recommended dose is 50-100mg per day for the adult, which should be increased during pregnancy, lactation and diseases, surgical operations, infectious diseases, thyrotoxicosis, fever and rheumatic arthritis. The level of the ascorbic acid in leukocytes and platelets is lower in women taking steroid contraceptives, because oral contraceptive steroids increase the break down of ascorbic acid. Estrogenes have been reported to increase the rate of break down of ascorbic acid and lower its tissue levels.