VITAMIN E

VITAMIN E

A. History: The presence of this active principle was first demonstrated in vegetable oils by Evans and Mattill independently in 1920. This was designated as vitamin E or antisterility factor on account of the development of sterility in animals in its absence. In 1936, two compounds with vitamin E activity were isolated from wheat germ oil by Evans and his associates and given the name, α- and β-tocopherol (tokosG = childbirth ; pherosG = to bear; ol = an alcohol). Subsequently, five other tocopherols were obtained from various cereal grains like wheat germ, corn oil, rice etc.

B. Occurrence: The tocopherols are of widespread occurrence in many plant oils such as wheat germ, rice, corn, cottonseed, soybean and peanut but not olive oil. The are also present in small amounts in meat, milk, eggs, leafy plant and some fruits. Fish liver oils, so abundant in vitamin A and D, are devoid of vitamin E. Of all the tocopherols discovered so far, the α-form has the widest distribution and greatest biologic activity. The relative biologic potencies of various tocopherols are :

• α-tocopherol—100

• β-tocopherol—25

• γ-tocopherol—19

The vitamin E content of some oils are presented in Table

C. Structure: Vitamin E is the collective name for a group of closely related lipids called tocopherols. The tocopherols are derivatives of 6-hydroxychroman (also known as tocol) bearing an isoprenoid side chain at carbon 2. The structure of a-tocopherol (C29H50O2) is given in fig The various tocopherols differ from each other in substituents on carbons 5, 7 and 8. These substituents are methyl groups and hydrogen atoms. a-tocopherol contains 3 methyl groups whereas other tocopherols are short one or two methyl groups on the aromatic ring (refer Table ). It is noteworthy that the presence of all 3 methyl groups attached to the benzene ring is necessary for full activity. d-tocopherol has but one methyl group and is almost without activity A slight change in the structure of the tocopherols, for example shortening the side chain, may greatly diminish their physiologic activity.

D. Properties: Vitamin E is a light yellow oil. It is resistant to heat (up to 200°C) and acids but acted upon by alkalies. It is easily but slowly oxidized and is destroyed by UV rays. The tocopherols are excellent antioxidants. They prevent other vitamins presents in food (e.g.,vitaminA) from oxidative destruction. It is found in the nonsaponifiable fraction of the vegetable oils.
E. Metabolism: Tocopherols act as antioxidants, i.e., they can prevent the oxidation of various other easily oxidized substances such as fats and vitamin A. It is for this reason that they are commercially added to foods to retard their spoilage. It may be recalled that vitamin A is essential for reproduction. Whereas the beneficial action of vitamin A is mainly on the ectoderm and endoderm, that of vitamin E is on the mesodermal tissue. But, very likely, vitamin E influences all the 3 germinal layers of the embryo by preventing the too rapid destruction of vitamin A.
Certain substances such as phenols and vitamin C (ascorbic acid) stimulate the antioxidant property of vitamin E.In fact, the biochemical activity of tocopherol lies in its capacity to protect mitochondrial system from inactivation by fat peroxides. Thus, in mitochondria obtained from vitamin E-deficient animals, a marked deterioration in activity is found due to peroxidation of unsaturated fatty acids which are usually present in these particles. Addition of vitamin E prevents this deterioration by acting as antioxidant for peroxidation. It has been observed that tocopherol-deficient muscles (esp., cardiac and skeletal) show a high oxygen uptake. Administration of tocopherol brings down the oxygen consumption to normal. The catabolism of α-tocopherol involves both the oxidative cleavage of the chroman ring to yield quinone or hydroquinone-like compounds and the degradation of the isoprenoids side chain (Simon, 1956)
F. Deficiency: The characteristic symptoms of experimentally-induced vitamin E deficiency vary from animal to animal. In mature female rats, sterility develops because of reabsorption of fetus after conception while in males, the germinal epithelium of the testes degenerates and the spermatozoa become nonmotile. Avitaminosis E in herbivorous animals like rabbits and guinea pigs leads to acute muscular dystrophy (atrophy of muscle fibres), which ultimately results in creatinuria ; young chicks exhibit capillary damage and encephalomalacia ; hen eggs show low hatchability and monkeys reveal
hemolytic anemia. There is, however, little evidence that man is ever short of vitamin E. Finally, as is true for almost all the vitamins, avitaminosis E prevents normal growth. It also sometimes causes degenatation of the renal tubular cells.
G. Human requirements: Vitamin E is not a problem in human nutrition because it is ubiquitous in foods. However, the minimum daily requirement of vitamin E for adults is 30 I.U. for men and 25 I.U. for women. The pregnant and lactating mothers, however, require 30 I.U. daily. For infants and children,. the vitamin E requirement is at the rate of 1 to 1.25 I.U. per kilogram of body weight. One International Unit of dl-α-tocopherol is equivalent to the biologic activity of 1.1 mg of pure compound or 0.67 mg of d-α-tocopherol