vision biochemistry, role of vitamin A and xerophthalmia

vision biochemistry

1. Vision biochemistry, role of vitamin a & xerophthalmia Sabina Poudel B. Optometry Institute of Medicine

2. Presentation layout 1) Introduction to vitamins & their classification 2) Vitamin A - forms of vitamin A - sources -biochemical functions 3) Role of vitamin A in eye 4) Vision biochemistry 5) Vitamin A deficiency 6) xerophthalmia

3. VITAMINS Potent organic compounds required in the diet in small amounts for optimum growth and health of the organism Not used for energy but for utilization of other nutrients like carbohydrates, proteins and fats Most vitamins are not made in the body, they must be supplied by the diet Most vitamins act as co-enzymes

4. Classification of vitamins 1) Fat soluble vitamins Vitamin A Vitamin D Vitamin E Vitamin K 2) Water soluble vitamins Vitamin C Vitamin B complex - energy releasing: B1, B 2 , B 3 , B 6 , B 7 , pantothenic acid. - hematopoietic: folic acid, B 12

5. Vitamin A is a broad term for a number of similar compounds First recognized fat soluble vitamin Two forms : a) Preformed vitamin A: retinoids ( from animals) b) Provitamin A: carotenoids (predominantly beta- carotene from plants)

6. Preformed vitamin a: Retinoids Active or usable form Four categories of retinoids : a) retinol b) retinal c) retinoic acid d) retinyl esters All retinoids are absorbed as retinol. Sources: animal products like liver, fish , fish oils , milk , eggs etc. Liver is richest source

7. Structure of different forms of vitamin A Unsaturated organic compounds All forms of vitamin A have a beta-ionone ring to which an isoprenoid chain is attached, called a retinyl group

8. Structure of different forms of vitamins A

9. Retinol : Vitamin A alcohol Pure alcohol form unstable Present in animal tissues as retinyl ester with long chain fatty acid. Retinal : Vitamin A aldehyde Obtained by oxidation of retinol Previously known as retinene Retinal and retinol interconvertible

10. Retinoic acid: Vitamin A acid. Produced by oxidation of retinal Can not give rise to formation of retinal or retinol

11. Provitamin A: Carotenoids Precursor of vitamin A Predominantly beta carotene Body has to convert it into active vitamin A after consumption Sources: plant products like carrot , green leafy vegetables , papaya , mango , bringal .

12. Types of carotenes a) α carotenes: yields 1 molecule of vit . A b) β carotenes: yields 2 molecules of vit . A c) γ carotenes: yields 1 molecule of vit . A

13. Biochemical functions of vitamin A Vision in dim light Necessary for maintenance of normal epithelium: Synthesis of goblet cells in epithelial tissue which secrete mucous having antimicrobial component. Embryonic development & reproduction: During fetal development, retinoic acid allows for development of lungs , hearts, eyes and ears & regulates expression of growth hormone gene.

14. Acts as anti-oxidant: carotenoids oxidize free radicals and prevent free radical cellular damage that can lead to cancer and other diseases. Immune function: -ensures working mucosal cells, membranes and epithelial layers: body’s first line of defence -aids in development of lymphocytes and white blood cells

15. Role of vitamin A in eye Formation of rhodopsin - used in night vision Maintenance of healthy cornea and conjunctival cells

16. Can be discussed under: 1. Vitamin A absorption and storage 2. Transport from liver to eye 3. Synthesis of visual pigments 4. Light induced changes in visual pigments

17. 1. Vitamin A absorption and storage dietary vitamin A (carotenes in plant food & retinol in animal food) reaches blood stream through intestinal lymphatics in intestine vitamin A is reesterified most retinol reaches liver (90% stored as well) retinol bound to retinol-binding protein(stable form)

18. RPE is second only to liver in its concentration to vitamin A Aldehyde and alcohol form of vitamin A is membranolytic so are stored as esters in RPE RPE acquires vitamin A by three ways: - from circulation - release during bleaching of rhodopsin and return via the regeneration process. -via phagocytosis of shed photoreceptor outer segment discs.

19. 90 % of the retinoids can be absorbed Carotenoids: -absorbed intact - absorption rate much slower - intestinal cells convert carotenoids to retinoids

20. 2. Transport from liver to eye retinol-protein complex enters circulation becomes attached to the specific receptors present on the basal surfaces of the retinal pigment epithelial ( RPE ) cells RBP is left outside & retinol only enters RPE

21. In Cornea and conjunctiva: Possible routes of vit A transfer to cornea and conjunctiva: 1) migration of holo -RBP from blood capillaries in limbus region 2) direct uptake of vit A from tear fluid 3) transfer of vit A from aqueous humour In tear: concentration of retinol 0.1 μ mol / litre ( 5% of plasma) In aqueous: retinol barely detectable

22. 3. Synthesis of visual pigments Retinol remains unchanged in RPE cells Retinol enters into the outer segments of photoreceptors Retinol oxidation Retinene ( 11- cis retinal ) retinene reductase Retinene combines with the protein opsin to form rhodopsin NAD oxidative system (present in RPE) supports the reaction of rhodopsin formation .

23. Fig:Utilization of vitamin A for synthesis of Rhodopsin

24. 4.Light induced changes in visual pigments Light falling on retina absorbed by photoreceptors Photochemical changes in outer segments of photoreceptors initiate electrical changes Light induced changes as studied in rods : I. Rhodopsin bleaching II. Rhodopsin regeneration III. Visual cycle

25. I. Rhodopsin bleaching & regeneration Rhodopsin : opsin (protein) + retinene ( vit A aldehyde) Light absorbed by rhodopsin converts its 11-cis retinal into all-trans retinal Formation of many intermediates

26. The all-trans retinal can no longer remain in combination with the opsin Separation of opsin and all-trans retinal This process of separation: photodecomposition Rhodopsin is said to be bleached by light

27. M etarhodopsin II: activated rhodopsin , acts as an enzyme to activate transducin molecules Transducin: a GTP/GDP exchange protein present in inactive form bound to GDP in membranes of discs and cell membranes of rods Activated transducin activates phosphodiesterase (PDE) PDE catalyses conversion of cGMP to GMP

28. P hototransduction Process of converting light energy into electrical signals Occurs in the photoreceptors

29. Membrane potential of photoreceptors Both rods and cones slightly depolarized relative to a typical neuron Rather than manifesting a resting membrane potential of -70 mV, the potential is about -50mV

30. In dark: inner segment of the photoreceptor continually pump Na+ from inside to outside negative potential on the inside of entire cell Na + channels present in the cell membrane of photoreceptor outer segment are kept open by cGMP

31. Na + from the extracellular fluid flows inside the outer segment through these pores This is called dark current, producing a slight depolarization

32. In Light: When light strikes the photoreceptors: amount of cGMP is reduced so some of Na + channels are closed results in hyperpolarization Thus excitation of photoreceptors cause increased negativity of the membrane potential (hyperpolarization) rather than the decreased negativity(depolarization )

33. Under dark condition Photoreceptor depolarized Continuously release neurotransmitter glutamate Under light stimulation Photoreceptor hyperpolarized Reduction in release of glutamate

34. The number of sodium channels located in the rod outer segment is limited, constraining the potential magnitude of rod hyperpolarization When about only 10 percent of a rod’s rhodopsin is bleached, a critical number of Na + channels are closed and further bleaching of rhodopsin does not result in further hyperpolarization

35. Phototransduction cascade

36. 39 Incident Light Change in Opsin Configuration Retinene 1 changed to All-trans form α -Subunit separates Transducin (G α) is activated ACTIVATION CASCADE Incident Light Retinene 1 changed to All-trans form

37. 40 Subunit activates cGMP PDE Reduced cytoplasmic cGMP Hyperpolarisation≈ -70 mV Converts cGMP to 5 ’ -GMP Closure of leaky Na + Channels “Switching off” Reduced cytoplasmic cGMP Converts cGMP to 5 ’ -GMP

38. Decrease intracellular Ca 2+ Electrical signal down the neural pathway Decrease Glutamate release D epolarization (rod and cone On Center bipolar cells) H yperpolarization ( cone Off surround bipolar cells)

39. II. Rhodopsin regeneration All-trans retinal enters into the chromophore pool existing in photoreceptor outer segment and RPE cells 1) All-trans retinal may be further reduced to retinol by alcohol dehydrogenase , then esterified to re-enter the systemic circulation 2) All-trans retinal isomerized to 11-cis retinal by retinal isomerase enzyme 11-cis retinal in outer segments of photoreceptors reunites with opsin to form rhodopsin

40. III. Visual cycle Under constant light stimulation: Photoreceptor Bleaching = Photoreceptor Regeneration equilibrium between the photodecomposition and regeneration of visual pigments: visual cycle

41. Summarising …..

42. Recommended Dietary Allowance(RDA) of Vit A (US Food & Drug Administration) Groups RDA (IU) Infants 1500 Children (< 4yrs) 2500 Children (> 4 yrs) 5000 Lactating or Pregnant women 8000 IU= International Unit 1 IU= 0.3 μ g of retinol

43. Vitamin A Deficiency Dietary deficiency of Vitamin A Diarrhoea; Gastroenteritis; Parasites Not enough Vitamin A stored in the liver Low Vitamin A levels in the blood Night Blindness FOOD INTESTINE LIVER BLOOD EYE Anorexia from other diseases Protein Energy Malnutrition Poor intestinal absorption Not enough Retinol Binding Protein synthesis Xerophthalmia

44. 5/28/2016 48 Risk factors PEM Measles, Chickenpox, High Fevers Bronchopneumonia, Tuberculosis, Diphtheria Gastroenteritis, Dysentery, Worm Infestations

45. xerophthalmia

46. Xerophthalmia General term applied to all the ocular manifestations of impaired vitamin A metabolism, from night blindness through complete corneal destruction Xeros – dry ophthalmia – eye literally means “ dry eye “ conventionally xerophthalmia has become synonymous with vitamin A deficiency.

47. Is leading preventable cause of blindness in children throughout the world 30 % of the world’s blindness is due to vitamin A deficiency In Nepal: 0.9 % bilateral blindness due to nutritional corneal ulceration Every day one child dies and one child goes blind of xerophthalmia

48. Classification of xerophthalmia ( WHO classification 1982) XN (Night blindness) X1A ( Conjunctival xerosis ) X1B ( Bitot’s spots) X2 ( Corneal xerosis ) X3A ( Corneal ulceration/ Keratomalacia affecting less than one third corneal surface) X3B ( Corneal ulceration/ Keratomalacia affecting more than one third corneal surface) XS ( Corneal scars) XF ( Xerophthalmic fundus) Biochemical criterion: Plasma vitamin A < 0.35 μ mol /L

49. Night blindness Earliest symptom Difficulty to see in dusky and dark environment.

50. Insufficient vitamin A Insufficient rhodopsin production Lack of functioning of rod cells Brain doesn’t receive enough signals Night blindness

51. Conjunctival xerosis Keratinizing metaplasia of conjunctiva. Loss of goblet cells and mucus. Lusterless , wrinkled and pigmentation of bulbar conjunctiva

52. Bitot’s spots Extension of xerotic process seen in stage X1A Raised, silvery white, foamy, triangular patch of keratinised epithelium Situated on bulbar conjunctiva in the inter- palpebral area

53. Corneal xerosis Earliest change in cornea is punctate keratopathy which begins at lower nasal quadrant Haziness and granular pebbly dryness Involved cornea lacks lusture

54. Corneal ulceration/ keratomalacia Infiltration of corneal stroma giving bluish hazy appearance. Characteristic liquefactive process called colliquative necrosis. At first excavation of ulcer in central part corneal perforation (as disease advances) iris prolapse and even loss of vitreous and extrusion of lens. When this process involves whole cornea keratomalacia

55. Corneal scars Healing of stromal defects results in corneal scars

56. Xerophthalmic fundus Characterized by typical seed like , raised, whitish lesions scattered uniformly over the part of the fundus at the level of optic disc Disappears within 2-4 months of vit A therapy

57. ERG changes Normal a-wave, absent or reduced b-wave Later rest of visual cell degenerates accompanied by disappearance of rest of ERG.

58. Eye changes 1) Conjunctiva X1A X1B warning sign +XN 2) Cornea X2 X3A medical + ophthalmological X3B emergency XS 3) Retina XN first sign of xerophthalmia XF

59. conjunctiva Contains mucosal cells Secretes mucus Dryness , infection prevented Healthy corneal cells, no infection

60. Lack of vitamin A Mucus forming cells deteriorate since vit A is needed for proper epithelial cell maintenance No mucus or tears to protect eyes Dryness, bacterial invasion Hardening of epithelial cells Blindness

61. Systemic associations Dry skin and hair Increased incidence of ear, sinus, respiratory, urinary, and digestive infections Inability to gain weight Nervous disorders Skin sores

62. The Recommended Doses of Vitamin A Prophylactic Schedule: Dose By Mouth mg I.U. All Children ( above 1 yr ) 110 200,000 Every 4 -6 months Newborns (at birth) 50,000 Mothers 110 200,000 Just after giving birth For children less than 1 year of age, reduce dose by one half

63. Treatment Schedule: If the children is severely ill with gastroenteritis or unable to swallow, the first dose should be intramuscular injection water soluble Vitamin A Emergency Treatment of Children with Xerophthalmia or Corneal Ulcers Day / Week Dose By Mouth mg I.U. Day 1 110 200,000 Day 2 110 200,000 Day 14 (2-4 Weeks) 110 200,000 The Recommended Doses of Vitamin A:...

64. Children under age of 1 yr and children of any age who weigh less than 8 kg treated with half dose i.e. 100,000 I.U Women of reproductive age, pregnant or not: a) having XN, X1A and X1B treated with daily dose of 10,000 I.U. of vit A orally for 2 weeks. b) for corneal xerophthalmia : full dose schedule recommended

65. Treatment of Xerophthalmia MEDICAL Vitamin A Massive Dosing Oral: Day 1, 2, 14 - 200,000 I.U. Injection: 1 st Day Only - 100,000 I.U. Water Miscible No oily preparation If not available: Give food rich in Vitamin A Supportive Therapy Fluids Proteins Control of Infection Deworming

66. Treatment of Xerophthalmia MEDICAL Eye Antibiotcs, Mydiatrics Pad specially in X3A, X3B Avoid Exposure: Antibiotic Ointment Methyl Cellulose Drops SURGERY Conjunctivoplasty Keratoplasty Prophylactic Optical REHABILITATION

67. Prevention of Vitamin A deficiency Breastfeeding Vitamin A supplementation Food fortification Promotion of vitamin A-rich diets

68. 5/28/2016 73 National vitamin A program Begun in 1993 AD from 8 district, since 2003-75 district Annual cost: US$1.7 million Children :6 months - 5 years (3.5 Million) Twice a year :April 18 & 19 ( Baisakh 10 & 11) October 18 & 19 ( Kartik 6 & 7) Coverage : 75 districts - 85% children FCHV : 48,000 Reduction of Child Mortality by 20-25 thousand (28%)

69. Reference Biochemistry – U. Satyanarayan Anatomy and physiology of eye- A.K. Khurana Visual perception- Steven H . Schwartz Comprehensive ophthalmology- A.K. Khurana Internet sources