Vitamin A

Metabolism of Vitamin A

1. Vitamin A Gandham . Rajeev Department of Biochemistry, Akash Institute of Medical Sciences & Research Centre, Devanahalli , Bangalore, Karnataka, India. eMail : [email protected]

2. 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 organisms Hopkins coined the term Accessory factors to the unknown and essential nutrients present in the natural foods

3. Funk (1913) is isolated an active principle (an amine) from rice polishings and , later yeast, which could cure beri-beri in pigeons. He coined the term Vitamine Vital + Amine Earlier identified ones had amino groups. It was later realized that only few of them are Amines. The term Vitamin-continued without the final letter ‘e’

4. Classification All vitamins are broadly divided into two groups according to solubility. Fat-soluble Vitamin-A Vitamin-D Vitamin-E Vitamin-K

5. Water-soluble A) Vitamin-C (Ascorbic acid) B) Vitamin B-Complex group includes: Vitamin - B1 (Thiamine) Vitamin – B2 (Riboflavin) Niacin (Nicotinic Acid) Vitamin – B6 (Pyridoxine) Pantothenic Acid Folic acid Vitamin – B12 ( Cyanocobalamine ) Biotin Α - Lipoic acid

6. Vitamins Fat Soluble Water Soluble - Vitamin-A -Vitamin-D -Vitamin-E -Vitamin-K Non B - Complex Vitamin-C B-Complex Energy-releasing Hemopoietic -Thiamine (B1) -Riboflavin (B2) -Niacin (B3) -Pyridoxine (B6) -Biotin (B7) - Pantothenic Acid (B5) -Folic Acid (B9) -Vitamin B12 ( Cyanocobalamin

7. Vitamin A Vitamin A is a fat soluble Vitamin. Present only in foods of animal origin Its provitamins carotenes are found in plants Chemistry: Retinol, retinal and retinoic acid are termed as vitamers of Vitamin A

8. Retinol (vitamin A alcohol) It is a primary alcohol containing β -ionone ring The side chain has two isoprenoid units, four double bonds and one hydroxyl group Retinols present in animal tissues as retinyl ester with long chain fatty acids CH3 CH3 CH2OH CH3 β -Ionone CH3 CH3 Retinol

9. Retinal (Vitamin A aldehyde) This is an aldehyde form obtained by the oxidation of retinol. Retinal and retinol are interconvertible CH3 CH3 CHO CH3 β -Ionone CH3 CH3 Retinal

10. Retinoic acid (vitamin A acid) This is produced by the oxidation of retinal Retinoic acid cannot give rise to the formation of retinal or retinol CH3 CH3 COOH CH3 β -Ionone CH3 CH3

11. β - Carotene (provitamin A) This is present in plant foods It is cleaved in the intestine to produce two moles of retinal; but it may produce only one in biological system CH3 CH3 β - Carotene CH3 β -Ionone CH3 CH3 CH3 CH3 CH3 CH3 H3C

12. All the compounds with vitamin A activity are referred as retinoids They are poly-isoprenoid compounds having beta-ionone ring system The retinal may be reduced to retinol by retinal reductase and it is reversible Retinal is oxidized to retinoic acid , which cannot be converted to the other forms Retinol (alcohol) Retinal (aldehyde) Retinoic acid Reductase NAD+ NADH + H+

13. Absorption of vitamin A Dietary retinyl esters are hydrolyzed by pancreatic or intestinal brush border hydrolases, releasing retinol and free fatty acids β- Carotene is cleaved by di-oxygenase of intestinal cells to release 2 moles of retinal Retinal is reduced to retinol by an NADH or NADPH dependent retinal reductase present in intestinal mucosa

14. In the intestinal mucosal cells, retinol is reesterified to long chain fatty acids, incorporated into chylomicrons and transferred to the lymph Intestine is the major site of absorption Absorption is along with other fats and requires bile salts In biliary tract obstruction and steatorrhoea, vitamin A absorption is reduced The retinol esters of chylomicrons are taken up by the liver and stored (As retinol palmitate)

15. Transport from liver to tissues: Vitamin A is released from the liver as retinol Zn is essential for retinol metabolism Retinol is transported in the circulation by the retinol binding protein(RBP) in association with pre-albumin One molecule of RBP binds one molecule of retinol The retinol-RBP complex binds to specific receptors on the cell membrane of peripheral tissue and enters the cells

16. Many cells of target tissues contain a cellular retinol-binding protein (CRBP) that carries retinol to the nucleus and binds to the chromatin (DNA) Retinol exerts its function in a manner to that of a steroid hormone Retinoic acid is mainly transported in the blood by binding to albumin Small amounts of retinoic acid in the blood is also transported in combination with apo -retinol binding protein

17. Intestinal cell β - Carotene Retinal Retinol Retinyl esters Chylomicrons All-trans-retinol Retinyl palmitate (stored) Retinol Retinol-RBP Target cell Retinol Retinoic acid Nuclear receptor m-RNA Specific proteins Cell differentiation Retina All- transretinol All-trans retinal Visual Cycle Diet β - carotene Retinylesters Retinol FFA

18. Biochemical functions Rods and cones The retina of the eye possesses two types of cells – rods and cones The human eye has about 10 million rods and 5 million cones The rods are in the periphery while cones are at the centre of retina Rods are involved in dim light vision Cones are responsible for bright light and colour vision The number of rods is more in cats, mice and owls

19. Vitamin A and Vision( Wald’s visual cycle) Rhodopsin (mol.wt.35,000) is a conjugated protein present in rods It contains 11-cis-retinal and the protein opsin The aldehyde group (of retinal) is linked to ε –amino group of lysine(of opsin ) When light falls on retina, 11-cis-retinal is isomerised to all-trans-retinal This leads to a conformational change in opsin

20. Responsible for the generation of nerve impulse The all-trans retinal is isomerized to 11-cis-retinal by retinal isomerase (retinal epithelium) This combines with opsin to regenerate rhodopsin and complete the visual cycle Most of the all-trans retinal is transported to liver and converted to all-trans retinol by alcohol dehydrogenase

21. The all-trans retinol is undergoes isomerization to 11-cis retinol which is oxidized to 11-cis retinal to participate in the visual cycle

22. Rhodopsin (11-cis-retinal+opsin) Opsin All-trans-retinal All-trans-retinol NAD+ NADH + H+ ADH (liver) 11-cis-retinol 11-cis-retinal NAD NADH + H+ ADH(liver) Isomerase (liver) Retinal isomerase Wald’s visual cycle Light

23. Dark adaptation mechanism: When a person shifts from a bright light to a dim light, rhodopsin stores are depleted and vision is impaired After few minutes rhodopsin is resynthesized and vision is improved Called as dark adaptation and is increased in Vitamin-A deficiency

24. Bleaching of rhodopsin When exposed to light, the color of rhodopsin changes from red to yellow by a process known as bleaching Bleaching occurs in a few milliseconds and many unstable intermediates are formed during this process Rhodopsin Prelumirhodopsin Lumirhodopsin All-trans-retinal + Opsin metarhodopsin II Metarhodopsin I

25. Visual cascade and cGMP When light strikes the retina, a number of biochemical changes leading to membrane hyperpolarization occur resulting in genesis of nerve impulse When a photon (from light) is absorbed by rhodopsin , metarhodopsin II is produced The protein Transducin is activated by metarhodopsin II

26. Involves the exchange of GTP for GDP on inactive transducin The activated transducin activates cyclic GMP phosphodiesterase This enzyme degrades cGMP in rod cells A rapid decrease in cGMP closes Na+ channels in the membrane of the rod cells This results in hyperpolarization which is an excitatory response transmitted through the neuron network to the visual cortex of brain

27. Cones are for color vision Cones are responsible for vision in bright light as well as color vision They contain the photosensitive protein, conopsin There are three types of cones, each is characterized by a different conopsin , that is maximally sensitive to either - blue ( cyanopsin ), green ( iodopsin ), red ( porphyropsin )

28. In cones, 11-cis-retinal is the chromoprotein Reduction in number of cones or cone proteins, will lead to color blindness One eye contains about 6 million cones

29. Other biochemical functions of vitamin A Retinol and retinoic acid function like steroid hormones They regulate protein synthesis and involved in cell growth and differentiation Vitamin A is essential to healthy epithelial tissue Vitamin A is considered to be essential for maintenance of proper immune system

30. Role of vitamin A in Reproduction Active form: The active form of vitamin A which is involved in reproduction is retinol Mechanism: retinol binds to specific intracellular receptor Retinol receptor complex binds to DNA and regulates the expression of genes required for reproductive function

31. Role in growth and Differentiation Active form: The active form of vitamin A involved in growth and differentiation is retinoic acid Mechanism: retinoic acid (present as either all trans-retinoic acid or 9 cis -retinoic acid) binds to specific cellular retinoic acid binding protein (receptor) Retinoic acid receptor complex binds to DNA and regulate the expression of genes required for growth and differentiation

32. In cancer treatment all-trans retinoic acid has been shown to cause differentiation of tumors, and has a potential for the treatment of cancer All-trans retinoic acid also induces apoptosis (programmed cell death) of cancer cells

33. Role in maintenance of epithelial integrity and glycoprotein synthesis Active form: Retinol is involved in the maintenance of epithelial integrity and glycoprotein synthesis Retinol prevents the excess keratin synthesis Retinyl phosphate formed from retinol is required for glycoprotein synthesis Glycosyl retinyl phosphate acts as donor of carbohydrates for synthesis of glyco -proteins and GAGs

34. Collagen breakdown: Retinoic acid inhibits the enzyme collagenase and thus prevents the breakdown of collagen Role of β - C arotene as an antioxidant: The antioxidant effect of beta-carotene is due to the stabilization of peroxide free radicals within the conjugated alkyl structure of beta-carotene

35. Significance: The antioxidant properties of beta –carotene is partly responsible for its anticancer activity , protective effect against coronary heart disease , and prevention of cataract formation

36. Recommended dietary allowance(RDA) The daily requirement of vitamin A is expressed as retinol equivalents (RE) rather than International Units (IU) 1 retinol equivalent = 1 μ g retinol = 6 μ g beta-carotene = 12 μ g other carotinoids Children = 400 – 600 μ g /day Men = 750 – 1000 μ g /day

37. Women = 750 μ g /day Pregnancy = 1000 μ g /day or 1 mg/day Dietary sources of vitamin A: Animal sources: Include milk, butter, cream, cheese, egg yolk and liver Fish liver oils ( cod liver oil and shark liver oil ) are very rich sources of the vitamin A Vegetable sources contain yellow pigment beta-carotene

38. Yellow and dark green vegetables and fruits are good sources of carotenes e.g. carrots, spinach, pumpkins, mango, papaya etc. + Deficiency of vitamin A: Visual acuity is diminished in dim light (nyctalopia or night blindness) The dark adaptation time is increased Xerophthalmia The conjunctiva becomes dry, thick and wrinkled

39. The conjunctiva gets keratinized and loses its normal transparency Dryness spreads to cornea It becomes glazy and lusterless due to keratinization of corneal epithelium Bitot’s spots: These are seen as greyish -white triangular plaques firmly adherent to the conjunctiva in certain areas

40. Keratomalacia : When the xerophthalmia persists for a long time, it progress to keratomalacia (softening of cornea) There is degeneration of corneal epithelium which may get vascularised Later, corneal opacities develop Bacterial infection leads to corneal ulceration, perforation of cornea and total blindness

41. Other deficiency manifestations Effect on growth: Vitamin A deficiency results in growth retardation due to impairment in skeletal formation On reproduction: The reproductive system is adversely affected in vitamin A deficiency Degeneration of germinal epithelium leads to sterility in males

42. Effect on skin and epithelial cells: The skin becomes rough and dry Keratinization of epithelial cells of GIT, urinary tract and respiratory tract Vitamin A deficiency is associated with formation of urinary stones Hypervitaminosis : Excessive consumption of vitamin A leads to toxicity

43. Symptoms: Dermatitis (drying and redness of skin), enlargement of liver , skeletal decalcification , tenderness of long bones, loss of weight, irritability, loss of hair, joint pains Normal range : 20 -50 μ g/dl

44. References Harper’s Biochemistry 25 th Edition. Fundamentals of Clinical Chemistry by Tietz . Text Book of Medical Biochemistry-A R Aroor . Text Book of Biochemistry-DM Vasudevan Text Book of Biochemistry-MN Chatterjea Text Book of Biochemistry- Dr.U.Satyanarana