VITAMIN A

VITAMIN A

A. History. It was first recognized as an essential nutritional factor by Elmer McCollum in 1915 and then isolated from fish-liver oil by Holmes in 1917. On account of its established role in the visual process, it is often called as antixerophthalmic factor or the “bright eyes” vitamin. It was first synthesized in 1946 by Milas.
B. Occurrence. Liver oils of various fishes are the richest natural sources of vitamin A. Shark and halibut contain maximum amount whereas the cod-liver has lowest amount. Depending upon the species of fish and the time of year of catch, the fish livers contain 2,000 to 100,000 I.U. per gram of vitamin A. The amount present in human liver is much less (500 to 1,000 I.U.per gram). However, polar bear liver is an extremely concentrated source of vitamin A. Other noteworthy sources are butter, milk and eggs and, to a lesser extent, kidney. In its provitamin form (i.e., as carotenes) it is supplied by all pigmented (particularly yellow) vegetables and fruits such as carrots, pumpkins, cantaloupes, turnips, peppers, peas, sweet potatoes, papayas, tomatoes, apricots, peaches, plums, cherries, mangoes etc. Yellow corn is the only cereal containing significant amounts of carotene. New-born infants have low quantities of vitamin A content that is rapidly augmented because colostrum and breast milk furnish large amounts of the vitamin. The milk of well-fed mothers, however, contains sufficient amounts of vitamin A (in ester form) for the infant's need. However, vitamin A is absent from vegetable fats and oils (olive oil, linseed oil, groundnut oil). It is added to margarine during its manufacture from these oils.
The β-carotene content of some items are presented in Table Vitamin A originates in marine algae, and then passes up the food chain to reach the large carnivorous animals. Toxic levels of vitamin A may accumulate in the livers of a wide range of creatures such as Polar bears, seals, porpoises, dolphins, sharks, whales, Arctic foxes and huskies. Even a small meal of southern Australian seal liver, say 80g, may produce illness in man. Most of the foods recommended as source of vitamin A contain well below the toxic levels of vitaminA, but one– Halibut liver oil– contains dangerously high amounts of vitamin A.
As regards the vitamin A contents of polar animals, one can see that, in reality, very little quantities of livers of these animals are required to kill a human being : 30–90 g of polar bear liver or halibut liver, 80–240 g of bearded seal liver and 100-300 g of Antarctic husky liver is enough to kill a human being.
C. Structure. Vitamin A is found in two forms A1 and A2. The carotenoids that give rise to vitamin A in animal body is named as provitamin A. These inculde α-, β- and γ-carotenes and cryptoxanthin. β-carotene is most potent of all these forms. A molecule of β-carotene is made of eight 5-carbon isoprenoid units, linked to form a long chain of 40 carbon atoms with an ionone ring at each end. It is an orange-red hydrocarbon and upon hydrolysis yields 2 moles of vitamin A1 . During hydrolysis, a cleavage occurs at the mid point of the carotene in the polyene chain connecting the 2 β-ionone rings. This conversion usually takes place in livers of fishes and mammals. Vitamin A1 is a complex primary alcohol called retinol, having the empirical formula, C20H29OH. The terminalhydroxyl group is ordinarily esterified. It contains β-ionone ring. Another form of vitamin A, present in fresh-water fishes, is known as vitamin A2. It differs from vitamin A1, which is found in salt-water fishes, in possessing an additional conjugate double bond between carbon atoms 3 and 4 of the β-ionone ring. Its potency is 40% that of vitamin A1.
D. Properties. Ordinarily retinol is a viscid, colourless oil but by careful fractionation it has also been isolated as pale yellowish needles. It gives a characteristic absorption band in ultraviolet (UV) spectrum at 328 mμ. It is soluble in fat and fat solvents but insoluble in water. Loss of vitamin A in cooking, canning and freezing of foodstuffs is small; oxidizing agents, however, destroy it. It is destroyed on exposure to UV light. Vitamin A is relatively unstable in air unless protected by antioxidants including vitamin E.
E. Metabolism. In the tissues, the metabolic transformation of retinol is carried out by enzymes. The dietary β-carotene is split into retinal by an enzyme of the intestine. Retinal is then reduced by another enzyme to retinol which, in turn, is converted to retinyl ester by reacting with a fatty acid like palmitic. The retinyl ester, on the contrary, is enzymatically hydrolyzed t to produce retinol which is re-esterified with palmitic acid to produce retinyl ester. This is absorbed
through the lymphatic system and is stored in the liver. Liver stores vitamin A in large quantities mainly in the form of retinyl ester. But the vitamin A that circulates in the blood is in the form of retinol and is bound to a specific carrier protein called retinol binding protein (RBP). The retinyl ester present in the liver, therefore, has to be converted to retinol before it mixes with the blood. Liver can also successively convert retinol to retinal and retinal to retinoic acid. Retinoic acid is quickly absorbed from the intestine through the portal system and is rapidly excreted back into the intestine through the bile. Vitamin A helps maintain the epithelial cells of the skin and the linings of the digestive, respiratory and genito-urinary systems. These linings play a protective role against cancer-causing agents (or carcinogens), viruses and bacteria and are rendered vulnerable by a deficiency of vitamin A. Vitamin A guards against cancer by protecting cell walls from undesirable oxidation, and scavenging the products of oxidation— free radicals, which are linked to the development of cancer.
F. Deficiency. Vitamin A is perhaps the most important as it affects the various metabolic processes in the body. It has profound effect on epithelial structures, in general. Vitamin A deficiency leads to the onset of many diseases like nyctalopia or night blindness (inability to see in night), xerophthalmia (scaly condition of the delicate membrane covering the eyes), keratomalacia (softening of the cornea), phrynoderma or “toad skin” (hard and horny skin) and stunted growth. Xerophthalmia or xerosis is a major cause of blindness in childhood and is still a major health problem in Far East such as Hong Kong, Jakarta, Manila and Dhaka. The disease affects large number of children but few adults. Xerophthalmia is characterized by drying of the eyes and hence so named (xerosG = dry ; ophthalmosG = eyes). The lacrymal glands become stratified and keratinized and cease to produce tears. This makes the external surface dry and dull. The ulcers develop. The bacteria are not washed away. The eyelids swell and become sticky. This results in frequent exudation of blood, causing severe infection to the eye. If left untreated, blindness results. Incredible, yet true, about 1400 cases of xerophthalmia were reported in 1904 among Japanese children. Besides, during World War I, many xerophthalmic cases were reported in Denmark because of the fact that butterfat was shipped out of that country in huge quantities and people had to live on substitutes with no retinol.
Keratomalacia (keratoG = hair ; malakiaG = softness) is a corneal disease, occurring maximally in pre-school children of 3–4 years. This usually happens suddenly in young children with kwashiorkor esp., after an episode of diarrhea or infection. At first the cornea loses its lustre, undergoes a necrosis and develops a few pin-point ulcers, which later coalesce to form a large, white ulcerative area. The perforation of the cornea may follow sometime, causing the release of aqueous humour and prolapse of the iris. Occasionally, instead of extensive ulceration and perforation of the cornea, the whole eyeball may shrink. Keratomalacia is unfortunately still prevalent on a wide scale in many parts of India (South and Eastern) and Indonesia and some other countries of Asia and Africa. Phrynoderma is a skin lesion and is characterized by follicular hyperkeratosis. In it, the skin esp., on the outer aspects of forearms in the regions of the elbows and of the thighs and buttocks becomes rough and spiky. In severe cases, the trunk is also affected. Also, the damaged epithelial structures in diverse organs such as the eyes, the kidneys or the respiratory tract often become infected. It is for this reason that vitamin A has been called an ‘anti-infection’ vitamin. The vaginal epithelium may become cornified, and epithelial metaplasia of the urinary tract may contribute to pyuria and hematuria. Increased intracranial pressure with wide separation of cranial bones at the sutures may occur.
In the alimentary tract, the deficiency of vitamin A causes damage to the intestinal mucosa, resulting in diarrhea. Vitamin A is an important factor in tooth formation. In its deficiency, there is a defective formation of enamel so that the dentin is exposed. Evidently, sound tooth formation does not occur. Vitamin A deficiency may result in retardation of mental and physical growth and in apathy. Anemia with or without hepatosplenomegally is usually present. Other deficiency diseases that have been attributed to vitamin A are atrophy of the testes and disturbances of the female genital organs.
According to a report, presented at a meeting of the National Seminar on Corneal Disasters held in New Delhi (1981), India accounts for about one-third of the total blind population of the world, that is about 9 million blind people and that every year about 25,000 children go blind in India due to malnutrition. Researches conducted by the National Institute of Nutrition (NIN), Hyderabad (1978), show that about 30% of the blind in India lost their sight before they were 21. NIN studies have also revealed that about 10% of the school children belonging to the poorer socioeconomic groups show signs of vitamin A deficiency. It is however, worth-mentioning that the vitamin A deficiency is mainly a result of ignorance rather than the non-availability of food or of the resources to acquire the food as is the case with protein calorie malnutrition. According to NIN researches, if 40 g of green leafy vegetables are included in the existing diet without any other changes, the child will get the necessary vitamin A.
G. Hypervitaminosis A. Vitamin A is less toxic to man and other animals when it is taken in large doses over a long period of time. The children receiving overdosages of 500,000 units of
vitamin A per day exhibit tender swellings over the bones, limited motion and definite hyperosteoses. Human adults consuming 500,000 units or more show nosebleed, weakness, headache, anorexia and nausea. Continuous excessive intake of the vitamin is dangerous because it results in fragile bones and abnormal fetal development. In the case of plant carotenoids, a dietary oversupply (for example, eating too many carrots on a daily basis) results in carotenosis, a condition readily diagnosed by yellowing of the skin. These toxic effects develop presumably due to the fact that viatmin A, like vitamin D, is not readily excreted and consequently tissue levels may build up dangerous concentrations.
H. Human requirements. The recommended dietary allowance (RDA) of vitamin A is about 5,000 International Unit (I.U.). Growing children, adults and pregnant and lactating mothers require high doses of up to 8,000 I.U. It is also possible that some individuals require more than the minimal requirement due to either faulty absorption or some other reason. For vitamin A, the World Health Organization (WHO) has chosen as one International Unit the biologic activity of 0.000344 mg (0.344 μg) of synthetic vitamin A-acetate, which is equivalent to 0.30 μg of retinol.
In fact, one μg of β-carotene, the provitamin form, gets converted to about 0.167 μg of retinol, the true vitamin form. Another way of putting it is that the retinol equivalent of 1 μg of β-carotene is 0.167. A diet consisting of 1/2 pint of milk, 1 ounce of butter and an adequate amount of green vegetables or carrots daily is sufficient enough to meet minimal requirement. The optimal requirement of this vitamin is met with by taking 1 pint of milk and cod liver oil daily.I. Treatment. For xerophthalmia, the treatment consists in giving 1,500 μg/kg/24 hr of vitamin A orally for 5 days and then continued with intramuscular injection of 7,500 μg of vitamin A in oil daily until recovery occurs.
J. Vitamin A and the vision. George Wald (Nobel Laureate, 1966) of Harvard University
has made major contributions to our understanding of the role of vitamin A in visual process. The retina of the human eye contains 2 types of receptor cells, rods and cones. Animals which have vision only in bright light (“day vision”) like pigeons have only cones while animals which can see in night or dim light (“night vision”) like owls have only rods. Thus, the rods are concerned with seeing at low illumination and the cones are responsible for colour vision.
I. Rod vision. The rods contain a photosensitive visual pigment known as rhodopsin. It is conjugated protein and upon illumination splits into a protein called opsin and a carotenoid called retinene1 . It is actually this reaction which may initiate an enzymic reaction responible for visual mechanism. The retinene1, released by bleaching of rhodopsin, is reduced to vitamin A1 by NADH in the presence of retinene1 reductase, an enzyme present in the retina. Vitamin A1 of mammals is in all-trans form like the retinene liberated by bleaching rhodopsin. The isomerization of all-trans retinene1 and vitamin A1 to Δ”-cis forms is catalyzed by the enzyme retinene isomerase. The major site of this reaction is the liver. Opsin in dark reacts with Δ”-cis retinene1 to regenerate rhodopsin. The series of reactions leading from vitamin A1 to rhodopsin constitutes the major events in ‘dark adaptation’. And the reactions leading to bleaching of rhodopsin constitute what is known as ‘light adaptation’. Whereas the retina of mammals, birds, frogs and marine fishes contain rhodopsin, all freshwater fishes have another visual pigment porphyropsin in their retina. Wald has shown that porphyropsin undergoes the same cyclic changes on bleaching and regeneration as in the case of rhodopsin except that here retinene A2 and vitamin A2 come in picture.
II. Cone vision. According to Young-Helmholtz trichromatic theory, there are present at least 3 closely related pigments in cones. These are iodopsin, chlorolabe and erythrolabe. The last two pigments absorb actively in the green and the red portions of the spectrum respectively. When light strikes the retina, it bleaches one or more of these pigments based upon the quality of light. The pigments are converted to all-trans retinene and the protein, opsin. This reaction produces the nerve impulse which is read as blue, green or red based on the pigment affected.

VITAMIN D

A. History. The first demonstration of the existence of vitamin D was shown by Elmer McCollum in 1922 who found that cod liver oil was effective in preventing rickets, a disease induced in rats by providing low calcium diet. On account of its preventive action on rickets, vitamin D is often called as antirachitic factor. It is also known as ‘sunshine vitamin’ as its provitamin form present in human skin is easily converted to the active form by irradiation with ultraviolet light. At least 10 different compounds are known to have antirachitic properties and are designated as D2, D3 etc, but the two, namely, vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol) are more important. Vitamin D3 was, however, first isolated by Brockmann and others.

B. Occurrence. The best natural sources of vitamin D are the liver oils of many fishes such as cod and halibut. The flesh of oily fishes (e.g., sardine, salmon, herring ) is also excellent source. Egg yolks are fairly good but milk, butter and mushrooms are poor. The diets of infant may contain only small amounts of vitamin D; cow’s milk contains only 0.1 to 1 μg/quart (1 μg = 40 IU). Cereals, vegetables and fruits contain only negligible amounts. Eggs and yolks contain 3 to 10 μg/g. Most marketed cow’s milk is fortified with 10 μg of vitamin D per quart and most commercially–prepared milks for infant formulae are also fortified. Vitamin D2 is of plant origin and is produced commercially by irradiation with ultraviolet light of a provitamin known as ergosterol which is found in plants, especially in ergot (hence so named) and yeast. Vitamin D3,

C. Structure. The transformation of ergosterol (C28H44O) to the active form D2 takes place through a series of intermediate steps illustrated as below :
Ergosterol→Lumisterol→Protachysterol→Tachysterol→Precalciferol→Calciferol→Toxisterol (and)Suprasterols

Similarly, cholecalciferol (C27H44O) is produced from 7-dehydrocholesterol through a seriesof intermediaries as follows :
7-dehydrocholesterol→Lumisterol3→Tachysterol3→Precalciferol3→Cholecalciferol
During the activation of the provitamins, the ring B is cleaved between carbon atom 9 and 10 to produce vitamins D2 and D3 Note that in both the cases, the effect of irradiation is the opening of ring B. It may, however, be noted that the two vitamins, D2 and D3 and also D4 and D5 (in Fig. ), differ only in their side chains attached to C17. Other forms of vitamin D may be obtained by irradiation of other sterols. Vitamin D4, for example, is produced by irradiation of22-dihydroergosterol. Its potency is 50—70% that of vitamin D2.

However, researches conducted by DeLuca et al (1968) indicate that the biologically active form of vitamin D3, present in animals such as rat, has a slightly different structure. It is identified as 25-hydrocholesterol (in Fig.). It is a more polar compound and has an additional OH group at C25. It is one and a half times more potent than the vitamin in curing ricket. Also, it stimulates bone metabolism and intestinal calcium transport more rapidly than the vitamin. It is synthesized in the liver and is then converted into 1,25-dihydroxycholecalciferol [1,25(OH)2D3] in the kidneys.
Cholecalciferol and its derivatives are seco-steroids, i.e., ring A is not rigidly fused to ring B. Because of its conformational mobility, ring A of a seco-steroid exists in two equilibrated conformers. Now as the in vitro mode of action of vitamin D is understood, it has been proposed that 1,25(OH)2D3 is actually a hormone and not a vitamin as it fits the traditional description of a hormone. 1,25(OH)2D3 is produced in the kidneys (organ) and transported by the blood to intestinal mucosa and bone (target tissues), where it functions in the processes for the absorption, reabsorption and mobilization of calcium and phosphate ions. In conjunction with parathormone and calcitonin, it has a major role in homeostasis of Ca and P in the body’s fluids and tissues. Now because a hormone-receptor protein has been identified for 1,25(OH)2D3, the status of vitamin D as a hormone is well established.
D. Properties. Vitamin D is a white and almost odourless crystalline substance, soluble in fat and fat solvents. It is fairly heat resistant and also relatively resistant to oxidation. It is not affected by acids and alkalies.

E. Metabolism. The provitamin D3 can be synthesized within the human body so that it may, in fact, not be required in the diet. This may, henceforth, not be treated as a vitamin. In the past when man lived mainly outdoors and with minimum clothing, there was no hindrance for the penetration of ultraviolet light from the sun to convert it into the active form. In the far northern
areas, however, the amount of light is not adequate for conversion and as such fish liver oils serve as excellent source of vitamin D in these areas. The increased need of this vitamin is usually felt in growth and in pregnancy to provide for the needs of the foetus.
Vitamin D plays an important role in calcification of bones and teeth. It encourages the absorption, into the blood, of calcium salts and phosphates. Calcium passage across duodenum occurs mainly by diffusion and active transport of Ca2+ occurs across the ileal mucosa. Both these processes are related in dificiency of vitamin D. The subsequent release of bound calcium is also Conten markedly stimulated by vitamin D but only in the presence of parathyroid hormone. On the whole, the function of vitamin D is to cause increased absorption, longer retention and better utilization of calcium and phosphorus in the body. There is considerable difference in the potency of these 2 common forms of vitamin D. For example, vitamin D2 is a powerful antirachitic agent for man and for the rat but not for the chicken. Vitamin D3, on the contrary, is much more potent for the chicken than either for man or
the rat.
F. Deficiency. The most characteristic symptom of vitamin D deficiency is the childhood disease known as rickets. Deficiency of it in human adults leads to osteomalacia, a condition that might also be termed “adult rickets”. Rickets (derived from an old English word, wrickken = to twist) is primarily a disease of growing bones. In it, the deposition of inorganic materials on the matrix of bones (i.e., calcification) fails to occur, although matrix formation continues. Rickets is unusual below the age of 3 months (mo). It may occur in older children with malabsorption. Clinical manifestations of rickets in children usually manifest in the first year or in the second year. One of the early signs of rickets is craniotabes, which is due to thickening of the outer table of the skull and is detected by pressing firmly over the occiput or posterior parietal bones. A ping-pong ball like sensation will be felt. Craniotabes near the suture line may, sometimes, be present in normal premature infants. Costochondral junctions become prominent to give appearance of a beaded ribs, the rachitic rosary. Thickening of the wrists and ankles are other early evidences of osseous changes. Increased sweating, especially around the head, may also be present.

Signs of advanced rickets are easily identified. These are listed below :
1. Head : Craniotabes may obliterate before the end of the 1st year, although the rachitic process continues. The softness of the skull may result in flattening and, at times, permanent asymmetry of the head. The anterior fontenel is larger than normal; its closure may be delayed until after the 2nd year of life. The central parts of the parietal and frontal bones are usually thickened, forming prominences or bosses, which give the head a box-like appearance (catput quadratum).
2. Thorax : The sides of the thorax become flattened, and the longitudinal grooves develop posterior to the rosary. The sternum with its adjacent cartilage projects forward leading to protruding chest (pigeonbreast). Along the lower border of the chest develops a horizontal depression (Harrison groove), which corresponds with the costal insertions of the diaphragm.
3. Spinal cloumn : Moderate degree of lateral curvature (scoliosis) is common and a kyphosis (=increased convexity in the region of thoracic spine) may appear in the dorsolumbar region when sitting. Lordosis (=forward curvature of the lumbar spine) may be seen in the erect position.
4. Pelvis : The pelvic entrance is narrowed by a forward projection of the promontory; the exit, by a forward displacment of the caudal part of the sacrum and coccyx. In the female, these changes, if they become permanent, add to the hazards of childbirth and necessitate caesarean section.
5. Extremities : The epiphyseal enlargement at the wrists and ankles becomes more noticeable. Bending of the softened shafts of the femur, tibia and fibula results in bowlegs (knockknees) ; the femur and the tibia may also acquire an anterior convexity. Coxa vara is sometimes the result of rickets. Deformities of the spine, pelvis and legs results in reduced stature, rachitic dwarfism.
6. Ligaments : Relaxation of ligaments helps to produce deformities and partly accounts for knock-knees, weak ankles, kyphosis and scoliosis.
7. Muscles : The muscles are poorly developed and lack tone. As a result, the rachitic children are late in standing and walking. Abdomen becomes protuberant (potbelly) because of marked hypotonia of abdominal wall muscles, visceroptosis and lumbar lordosis.
8. Sense organ : Avitaminosis D in early infancy results in bilateral lamellar cataracts.
9. Dentition : Eruption of temporary teeth may be delayed in rachitic children. The first tooth in such babies appears between 6th and 9th month, at which time it has appeared in half of the normal babies. In deficiency of vitamin D, the formation of teeth becomes defective and leads to the development of dental caries. Chemical analysis of the bones of rachitic children reveals the presence of low inorganic and high organic and water contents in them. The ratio of calcium to phosphorus (C/P), however, remains constant. In the blood serum, there is usually a normal content of calcium but the phosphate content is reduced (1.5–3.5 mg/dL), against a normal value of 4.5–6.5 mg/dL in healthy infants. Vitamin D deficiency is also accompanied by generalized aminoaciduria, a decrease of citrate in bone and its increased urinary excretion, decreased ability of the kidneys to make an acid urine, phosphaturia, and, occasionally, mellituria. The parathyroid glands hypertophy in rickets. Rickets in itself is not a fatal disease but complications and intercurrent infections such as pneumonia, tuberculosis, and enteritis are more likely to cause death in rachitic children than in normal children. Rickets is most prevalent where climate or custom prevents individuals from exposure to sun, where by checking vitamin D production by irradiation of the skin. In the seventeenth century. this disease was so common in England that it used to be known as “English disease”. A study conducted by the Indian Social Institute, New Delhi (1981) shows that about 168 children per 1,000 of live births die of rickets in India. In osteomalacia (osteonG = bone ; malakiaG = softness), the action of bones is essentially like that in rickets. However, the bones become softer than the rachitic bones and the C/P ratio does not remain constant. The loss of calcium is greater than that of phosphorus and there is a relative gain in magnesium content. The disease is prevalent in India, China and Arab, particularly among women because of the custom that keeps them indoor and also prevents them from exposure to sun. This is particularly true of Bedouin Arab women who are clothed so that only their eyes are exposed to sunlight. The serum calcium is reduced, sometimes to such an extent that tetany develops. Vitamin D3 deficiency also leads to a disease called idiopathic steatorrhea or celiac disease. Like osteomalacia, the disease is characterized by demineralization of the bones which may result in deformities or dwarfism. In fact, celiac disease is indirectly a vitamin D deficiency because the primary abnormality appears to be, in part, a fatty diarrhea. The fat is not absorbed in the intestine and is passed out in stool along with calcium salts and vitamin D.
G. Hypervitaminosis D. Overdosing of calciferol to the children and adults as well produces demineralization of bone. Serum concentrations of both calcium and phosphate are greatly raised, resulting in metabolic calcification of many soft tissues and the formation of renal calculi. The latter disorder may block the renal tubules causing hydronephrosis. Contents The use of very high doses of vitamin D is not danger-free, however. The toxic effects caused by excess dosage include anorexia, nausea, polyuria, weakness, headache etc. The toxicity is due to the diminished excretion of this vitamin, rather than its storage in the liver. The water-soluble vitamins, on the contrary, if given in excess pass out immediately in the urine and are henceforth nontoxic. Much of the whole milk available in urban areas and evaporated milk are fortified with vitamin D concentrate so that 1 quart of fresh, whole milk or a cane of evaporated milk contains the required amount (i.e., 10 μg).
H. Human requirements. Vitamin D requirement is greatly influenced by the amount of
ultraviolet light to which the individual is exposed. Half an hour of direct sunlight on the cheeks
of a baby each day is sufficient to generate the minimal daily requirement of vitamin D. For adults also, exposure to sunlight for 30 minutes a day is believed to satisfy the daily requirement (about 10 μg or 400 IU) of vitamin D. As effective UV rays do not penetrate glass windows, exposure to sun through window glass is of little importance. Smoke also hinders the progress of these rays and as such city sunshine is not much beneficial. It is for these and some other reasons that vitamin D should be included in the diet. This is particularly true for older people. The recommended daily allowance of vitamin D is 400 I.U. for infants, pregnant women and lactating mothers. For adults, 400 units are adequate. One International Unit is defined as the biologic activity of 0.025 μg of pure crystalline vitamin D3.

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

VITAMIN K

VITAMIN K

A. History. Henrick Dam (1929), a Danish investigator, found that newly-hatched chicks, fed on artificial diets, develop hemorrhage, a fatal disease characterized by prolonged blood-clotting period. The term vitamin K (K for Danish koagulations) was then proposed by Dam (1934) himself to designate the active factor which cured or prevented this disease. On account of its blood-clotting power, it is also called as antihemorrhagic factor or coagulation vitamin. Of the 2 naturally-occurring forms of this vitamin, vitamin K1 was first isolated by Dametal from alfalfa in 1939 and the other
form, vitamin K2 from fish meal by Doisy et al, also in 1939.
B. Occurrence. Vitamin K1 occurs in green vegetables like spinach, alfalfa, cabbage etc.
Fruits and cereals are poor sources. Vitamin K2 is found in some intestinal bacteria. A rich source
of K2 is putrefied fish meal. Their relative biologic potencies are :

• vitamin K1—100

• vitamin K2—80

C. Structure. Chemically, the two forms of vitamin K (in Fig.) are derivatives of quinones and differ from each other in the composition of their side chain present at carbon 3 of the naphthoquinone ring. It is a phytol radical in vitamin K1 (C31H46O2) and a difarnesyl radical in vitamin K2 (C41H56O2). Vitamin K1, found in plants, has 4 isoprene units in its side chain whereas vitamin K2, found in animals, contains in its side 6 isoprene units, each with a double bond.
Analogues— The various analogues of naphthoquinone, however, have also been shown to possess vitamin K activity for animals. This is due to their structural resemblance. The common examples are menadione and phthiocol (in Fig.).
Menadione, which is sometimes referred to as vitamin K3, is twice as potent as vitamin K1. It is soluble in oil, sparingly so in water and is not oxidized in air when protected from light. Its diphosphate ester is water-soluble and is widely used clinically.
D. Properties. Vitamin K1 is a yellow viscid oil but vitamin K2 is a yellowish crystalline
solid. It is sensitive to light and is, therefore, kept in dark bottles. It is destroyed by irradiation,
strong acids, alkalies and oxidizing agents.
E. Metabolism. Vitamin K plays an essential role in the biosynthesis of prothrombin— a
blood plasma protein needed in the process of blood clotting and produced in liver. The process
of blood coagulation may be summarized as below.
[Note the role of vitamin K in the synthesis of prothrombin which is a precursor of thrombin, the latter has dual function of (a) hydrolysing fibrinogen to fibrin and (b) activating fibrinase of FSF, which brings about
clotting.]
The formation of the blood clot (refer Fig. ) is caused by the enzymic hydrolysis of the soluble plasma protein, fibrinogen to the insoluble protein, fibrin and fibrinopeptides. This transformation is catalyzed by an enzyme, thrombin. Thrombin itself is not present in the blood but is produced from its precursor, prothrombin in the presence of Ca2+ ions and another protein called thromboplastin ( = thrombokinase). In the next step, fibrin forms soft, fibrous networks (or soft clots). Then in the presence of Ca2+, thrombin activates fibrinase, an enzyme precursor found in blood plasma. Fibrinase is also known as fibrin-stabilizing factor (FSF). Finally, fibrous networks of fibrin link with each other under the influence of activated FSF to produce cross-linked fibrin (or hard clots).
The vitamins K are fat-soluble and are absorbed only in the presence of bile. As a result, the absorption occurs in the upper portion of the small intestine where bile salts are present. The avitaminosis K may occur where bile is prevented from entering the intestinal tract. This is true for most of the fat-soluble vitamins but is, in particular, important in the case of vitamin K because of its blood clotting action.
Like vitamin E and coenzyme Q, the vitamin K has also been ascribed a role in electron transport system (ETS) and oxidative phosphorylation in mitochondria. The vitamin K1 and K2 both activate electron transport in the succinate oxidase of cardiac muscle preparations that have been made inactive by treating with isooctane. The bacterial extracts or liver mitochondria, when Antagonists irradiated, require vitamin K for oxidative phosphorylation. This suggests a possible role of vitamin K in oxidative phosphorylation. The specific site of action is believed to occur between NADH and cytochrome b. It has been suggested that a phosphate ester of vitamin K, upon oxidation, transfers phosphate to ADP to form ATP.
Antagonists— Two antagonists of vitamin K are dicumarol and warfarin (inFig.) ; both antagonists prevent blood clotting. Dicumarol was first isolated from mouldy clover hay. It is often given to patients, who have suffered heart attacks caused by blood clots, as as preventive measure against further clotting in the blood vessels. Warfarin (name derived from the initials of the Wisconsin alumni Research Foundation, which sponsored the research on the compound) is a synthetic analogue of vitamin K. It is extremely poisonous to rats, causing death by internal bleeding. Ironically, this potent rodenticide is also a valuable anticoagulant drug for the treatment of human patients in whom excessive bleeding is dangerous— surgical patients and victims of coronary thrombosis.
Note that both dicumarol and warfarin lack an isoprenoid side chain.
In 1988, by introducing the antioxidant vitamin E, David Gershon and his colleagues at the Technion–Israel Institute of Technology, have succeeded in reducing cell damage and increasing the life span of nematode worms. The damage body cells sustain is due to oxidation, an underlying mechanism of ageing, which also damages the disposal system. Paradoxically, the oxygen we depend on for life is a source of our age-associated decline in function. Similar researches conducted on humans might shed light on how to intervene and retard ageing in them.
F. Deficiency. Deficiency of vitamin K causes loss of blood-clotting power. The infants
may also show signs of vitamin K deficiency by developing hemorrhage. This disease persists
by the time the bacteria develop in the intestine. Administration of this vitamin to pregnant
mothers before parturition decreases the onset of this disease. In man, however, avitaminosis K
results in steatorrhea with diminished intestinal absorption of lipids.
In general, vitamin K deficiency is rarely found in higher animals as this is provided by food and also synthesized by intestinal bacteria.
G. Human requirements. There is seldom a lack of sufficient vitamin K in human beings. As such, no standard requirement has been set.