VITAMINS Vitamins may be regarded as organic compounds required in the diet in small amounts to perform specific biological functions for normal maintenance of optimum growth and health of the organism.
There are about 15 vitamins, essential for humans. • They are classified as fat soluble and water soluble vitamins. • Fat soluble: A, D, E and K • Water soluble: C and B group. • The B complex vitamins may be sub divided into energy releasing (B1, B2, B6, biotin etc) and hematopoietic (folic acid and B12).
Most of the water soluble vitamins exert the functions through their respective coenzymes while only one fat soluble vitamins (K) has been identified to function as a coenzyme. • As far as human are concerned, it is believed that the normal intestinal bacterial synthesis, and absorption of vitamin K and biotin may be sufficient to meet the body requirements.
Administration of antibiotics often kills the vitamin synthesizing bacteria present in the gut, hence additional consumption of vitamins is recommended. • Generally, vitamins deficiencies are multiple rather than individual with overlapping symptoms. • The term vitamers represents the chemically similar substances that possess qualitatively similar vitamin activity.
Vitamin A • The fat soluble vitamin A, as such is present only in foods of animal origin. However, its provitamins carotenes are found in plants. • The term retinoids is often used to include the natural and synthetic forms of vitamin A. • Retinol, retinal and retinoic acid are regareded as vitamers of vitamin A.
BIOCHEMICAL FUNCTIONS • Vitamin A is necessary for a variety of functions such as vision, proper growth and differentiation, reproduction and maintenance of epithelial cells. • Rhodopsin: (mol. Wt. 35,000) is a conjugated protein present in rods. It contain 11-cis retinal and the protein opsin. The aldehyde group (of retinal) is linked to ε- amino group of lysine.
Rods are involved in the dim light vision whereas cones are responsible for bright light and color vision. • Dark adaptation time: when a person shifts from a bright light to dim light (e.g. entry into a dim cine theatre), rhodopsin stores are depleted and vision is impaired. However, within few minutes, known as dark adaptation time, rhodopsin is resynthesized and vision is improved. Dark adaptation time is increased in Vitamin A deficient individuals.
Retinol and retinoic acid function almost like steroid hormones. They regulate the protein synthesis and thus involved in the cell growth and differentiaition. • Vitamin A is essential to maintain healthy epithelial tissue.This is due to the fact that ratinol and retinoic acid are required to prevent keratin synthesis (responsible for horny surface) • Retinyl phosphate synthesized form rationol is necessary for the synthesis of
Certain glycoprotieins which are required for growth and muscus secretion. • Retinol and retinoic acid are involved in the synthesis of transferrin, the iron transport protein. • Vitamin A is considered to be essential for the maintenance of proper immune system to fight against various infections. • Chelesterol synthesis requires vitamin A. Mevalonate an intermediate in the cholesterol biosynthesis , is diverted for the
Synthesis of coenzyme Q in vitamin A deficiency. It is pertinent to note that the discovery of coenzyme Q was originally made in vitamin A deficient animals. • Carotenoids (most important β-carotene) function as antioxidants and reduce the risk of cancers initiated by free radicals and strong oxidants. β –carotene is found to be beneficial to prevent heart attacks. This is also attributed to the antioxidant property.
Recommended dietary allowance • The RDA of vitamin A for adults is around 1000 retinol equivalents (3500 IU) for man and around 800 retinol equivalents (2500) for woman. • One international unit (IU) equals to 0.3 mg of retinol. • The requirements increases in growing childern, pregnant woman and lactating mothers.
Dietary sources • Animal sources contain preformed vitamin A. The best sources are liver, kidney, egg yolk, milk, cheese, butter. • Fish (cod or shark) liver oils are very rich in vitamin A. • Vegetables sources contain the provitamin A- carotenes. Yellow and dark green vegetables and fruits are good sources of carotenes. E.g. carrots, spinach, amaranthus, pumpkins, mango, papaya etc
Vitamin A deficiency • The deficiency manifestations are related to the eyes, skin and growth. • Deficiency manifestation of the eyes: night blindness (nyctalopia), is one of the earliest symptoms of vitamin A deficiency. Difficult to see in dim light- as dark adaptation time is increased. Prolonged deficiency irreversibly damages a number of visual cells.
Severe deficiency of vitamin A leads to xeropthalmia. This is characterized by dryness in conjuctiva and cornea, keratinization of epithelial cells. • If xeropthalmia persists for a long time, corneal ulceration and degeneration occur. This results in the destruction of cornea, a condition referred to as keratomalacia, causing total blindness.
Effect on Growth: Vitamin A deficiency results in growth retardation due to imperiment in skeletal formation. Effect on Reproduction : The reproductive system is adversely affected in Vitamin A deficiency. Degeneration of germinal epithelium leads to sterility in males. Effect on Skin and epitelial cells : The skins becomes rough and dry. Keratiniza
Of epithelial cells of gastrointestinal tract, urinary tract and respiratory tract is noticed. This leads to increased bacterial infection. Vitamin A deficiency is associated with formation of urinary stones. The plasma level of retinol binding protein is decreased in Vitamin A deficiency .
Hypervitaminosis A • Excessive consumption of vitamin A leads to toxicity. • The symptoms of hypervitaminosis A include dermatitis (drying and redness of skin), enlargement of liver, skeletal decalcification, tenderness of long bones, loss of weight, irritability, loss of hair, joint pains etc.
Vitamin D • Vitamin D is a fat soluble vitamin. It resembles sterol in structure and functions like a hormone. • Vitamin D was isolated by Angus (1931) who named it calciferol.
Chemistry • Ergocalciferol (vitamin D2) is formed from ergosterol and is present on plants. • Cholecalciferol (vitamin D3) is found in animals. Both the sterol are similar in structure except that ergocalciferol has an additional methyl group and a double bond. • Ergocalciferol and cholecalciferol are
Biochemical functions • Calcitriol (1, 25- DHCC) is the biologically active form of vitamin D. • It regulates the plasma level of calcium and phosphate. • Calcitriol acts at 3 different levels (intestine, kidney and bone) to amintain plasma calcium level ( normal 9-11 mg/dl)
Action of calcitriol on the intestine: calcitriol increases the intestinal absorption of calcium and phosphate. • Action of calcitriol on the bone: • Calcitriol stimulates the calcium uptake for deposition as calcium phosphate. Calcitriol is essential for bone formation. • Action of calcitriol on the kidney: • Calcitriol is also involved in mininmizing the excretion of calcium and phosphate through the kidney by decreasing their excretion and enhancing reabsorption.
Vitamin D is a hormone not a vitamin- a justification. • Calcitriol is now considered as an important calcitropic hormone, while cholecalciferol is the prphormone. • Cholecalciferol (vitamin D3) is synthesized in the skin by ultra violet rays of sunlight. • The biologically active form of vitamin D, calcitriol is produced in the kidney. • Calcitriol has target organs- intestine bone and kidney, where it specifically acts.
Calcitrol action action is similar to steroid hormobnes. • Actinomycin D inhibits the action of calcitriol . This support the view that calcitriol excerts its effect on DNA leadind to the synthesis of RNA (transcription). • Cacitriol synthesis is self regulated by a feedback mechanism i.e., calcitriol decreases its own synthesis.
Recommended dietary Allowance • The daily requirements of vitamin D is 400 international units or 10 mg of cholecalciferol.
Dietary sources • Good sources of vitamin d include fatty fish, fish liver oil, egg yolk etc. • Milk is not a good source of vitamin D.
Deficiency symptoms • Insufficient exposure to sunlight and consumption of diet lacking vitamin D results in its deficiency. • Deficiency of vitamin D causes rickets in childern and osteomalacia in adults. • Vitamin d is often called as antirachitic vitamin. • In rickets plasma calcitriol level is decreased and alkaline phosphatase activity is elevated.
Renal rickets • This seen in patients with chronic renal failure. • Renal rickets is mainly due to decreased synthesis of calcitriol in kidney. • It can be treated by the administration of calcitriol.
Hypervitaminosis • Vitamin D is stored mostly in liver and slowly metabolized. • Vitamin D is the most toxic in overdoses. • Toxic effects- demineralization of bone (resorption) and increased calcium absorption from the intestine, hypercalcemia, loss of appetite, nausea, increased thirst, loss of weight.
Vitamin E • Vitamin E (tocopherol) is a naturally occuring antioxidant. • Essential for normal reproduction in many animals, hence known as anti sterility vitamin. • Described as a vitamin in search of a disease.
Chemistry • Vitamin E is the name given to a group of tocopherols and tocotrienols. • About eight tocopherols (vitmin E vitamers) have been identified α, β, gama, sigma etc. • Α- tochopherols is the most active. • The tochopherols are the derivatives of 6-hydroxy chromane (tocol) ring with isoprenoid (3units) side chain. • The antioxidant property is due to chromane ring.
Absorption , transport and storage Vitamin E is absorbed along with fat in the small intestine. Bile salts are necessary for the absorption. In the liver, it is incorporated into lipoproteins (VLDL and LDL) and transported. Vitamin E is stored in adipose tissue, liver and muscle. The normal plasma level of tocopherol in less than 1 mg/dl. Biochemical Functions Most of the functions of vitamin E are related to its antioxidant property.
It prevents the non-enzymatic oxidations of various cell components (e.g unsaturated fatty acids) by molecular oxygen and free radicals such as superoxide (O2) and hydrogen peroxide (H2 O2). The element selenium helps in these function. • Vitamin E is lipohilic in character and is found in association with lipoproteins , fat deposits and cellular membranes. It protects the per oxidation reactions.
Vitamin E acts as a scavenger and gets itself oxidized (to quinone form) by free radicals (r) and spares PUFA. • FUNCTIONS • Vitamin E is essential for the membrane structure and integrity of the cell, hence it is regarded as a membrane antioxidant. • It prevents the peroxidation of poly-unsaturated fatty acids in various tissues and membranes.It protects RBC from hemolysis by oxidizing agent (e.g H2O2).
It is closely associated with reproductive functions and prevents sterility. Vitamin E preserves and maintains germinal epithelium of gonads for proper reproductive function. • It increases the synthesis of heme by enhancing the activity of enzymes aninolevulinic acid (ALA) synthase and ALA dehydratase. • It is required for cellular respiration through electron transport chain (believed to stabilize coenzyme Q).
Vitamin E prevents the oxidation of vitamin A and carotenes. • It is required for proper storage of creatine in skeletal muscle. • Vitamin E is needed for optimal absorption of amino acids from the intestine. • It is involved in proper synthesis of nucleic acids. • Vitamin E protects liver from being damaged by toxic compounds such as carbon tetrachloride.
It works in association with vitamin A , C and B carotene, to delay the onset of cataract. • Vitamin E has been recommended for the prevention of chronic diseases such as cancer and heart diseases.
VITAMIN K Vitamin K is the only fat soluble vitamin with a specific coezyme function. It is required for the production of blood clotting factors, essential for coagulation (in German – Koagulation; hence the name k for this vitamin. CHEMISTRY Vitamin K exists in different forms vitamin K1 (Phylloquinone) is present in plants. Vitamin K2 (menaqquinone) is produced by the
Intestinal bacteria and also found in animals. Vitamin K3 (menadione) is synthetic form. All the three vitamin (k1,k2,k3) are naphthoquinone derivatives. Isoprenoid side chain is present in vitamins K1 and k2. The three vitamins are stable to heat. Their activity is, however, lost by oxidizing agents, irradiation, strong acids and alkalies. Absorption , transport and storage Vitamin k is taken in the diet or synthesized by the intestinal bacteria. Its absorption takes place along with fat (chylomicrons) and is dependent on bile
Salt. Vitamin K is transported along with LDL and is stored mainly in liver and , to a lesser extent, in other tissues. Biochemical functions The functions of vitamin K are concerned with blood clotting process. It brings about the post-translational (after protein biosynthesis in the cell) modification of certain blood clotting factors. The clotting factors II (prothrombin) VII IX and X are synthesized as inactive precursors (zymogens) in the liver. Vitamin K act as a