Vitamin C (Ascorbic Acid)

 Vitamin C (Ascorbic Acid)

(Anti scurvy factor)

Introduction

In 1753 Captain Lind of the British Navy showed that the disease Scurvy which was common in sailors could be cured by giving them lemons and oranges. Captain Cook (1772-75) kept the sailors free from scurvy by giving them fresh food. In 1907 scurvy was experimentally produced in guinea-pigs by Holst and Frölich, and effects of vitamin C were studied on them. It was isolated by Szent-Gyorgyi (1928) and synthesized by Reichstein (1933).

Chemistry

Ascorbic acid is an enediol isomer of 2-keto-L-gulonolactone with a configuration similar to that of L-glucose. The natural variety is the laevo form; the dextro form is physiologically inactive. Oxidation of ascorbic acid gives rise to dehydroascorbic acid. This reaction is reversible. Both forms are physiologically active.

Properties

·        white crystalline

·        soluble in water

·        insoluble in fat solvents

·        Heat-labile

·        easily oxidized at 100°C in presence of oxygen

·        Alkali and copper salts help destruction. Hence, cooking and canning destroy it. Slow cooking is more destructive than rapid cooking, even at high temperature.

Distribution

Animal sources

·        Very poor, not significant

Plant sources

·        fresh fruits, mainly citrus fruits- Indian gooseberry, orange, lemon, tomato, pine-apple, papaya, etc

·        fresh vegetables, e.g., cabbage, cauliflower, lettuce, spinach, peppers, beans, etc

·        sprouted pulses

·        germinating grains

·        Potato and seeds are poor, but rich during germination

Storage

It is not much stored in the body. It is stored in minute quantities in adrenal glands. The aqueous and vitreous humors are very rich in it.

Excretion

Most of the ingested vitamin C is promptly excreted, although vitamin C is a high threshold substance. Daily excretion in urine is 30-50 mg.

Biosynthesis of ascorbic acid

Human cannot synthesise or store this vitamin. They presumably lack the enzyme system necessary to convert L-glucoronic acid to ascorbic acid. So they require an external supply to meet their body needs. It is not much stored in the body. Most of the ingested vitamin C is promptly excreted, although vitamin C is a high threshold substance.

Functions

(1) Ascorbic acid regulates oxidation-reduction potential inside the cell probably by acting as a hydrogen carrier.

(2) It helps in the oxidation of p-hydroxyphenyl pyruvic acid to homogentisic acid which is the intermediate product of tyrosine metabolism.

(3) Ascorbic acid is related to carbohydrate metabolism. Deficient production of insulin takes place in ascorbic acid deficiency.

(4) It is essential for the proper functioning of the formative cells of various tissues, such as fibroblasts, osteoblasts, etc.

(5) It is necessary to maintain the normal state of the intercellular substance (mucoprotein and collagen) in different tissues like bone, cartilage, teeth, skin and connective tissue; also the cementing substance of the capillary endothelium.

(6) It helps in the development of the protein matrix, i.e., by synthesizing collagen with special reference to the synthesis of hydroxyproline from a proline precursor. Hydroxyproline is a characteristic constituent of collagen, and it also helps in deposition of calcium and phosphate in the bones.

(7) It plays an important role in wound repair. It lays down connective tissues which help in healing of wounds.

 (8) It takes part in the maturation of red cells. It helps in the mobilization of iron from its storage form ferritin.

(9) Due to the reducing properties of ascorbic acid, it helps in the absorption of iron from the intestine.

(10) It helps in the conversion of folic acid to folinic acid.

(11) This vitamin has some role in enhancing the immunity of the body.

Deficiency signs

(1) Scurvy- Increased fragility of capillaries causing hemorrhages-under the skin periosteum, intestine, kidney, etc. The guns show erosion of the mucous membrane at their margins and due to the increased fragility of the capillaries there is frequent bleeding.

(2) Malformations of bones and teeth. The osteoblasts remain functionless. Many of them are reverted to fibroblasts resulting in irregular depositions of bone salts and decrease in the density of long bones and teeth. The functions of the odontoblasts are impaired. New dentine is not formed and the tissue becomes spongy and porous.

(3) Osteoporosis- Increased brittleness of bones leading to fractures.

(4) Anaemia- as number of red cells and platelets are reduced.

(5) Clotting defects- Delayed blood clotting and clot retraction.

(6) Skin eruptions.

(7) Low immunity- Increased susceptibility to infections.

(8) Healing- Impaired healing of wounds.

(9) Metabolism- Disturbance in carbohydrate metabolism.

(10) Reproductive failure- both in males and females.

Daily requirement

·        Adults	100 mg
·        Children	30 mg
·        Adolescents	80 mg
·        Pregnancy	100 mg
·        Lactation	150 mg