VISION BIOCHEMISTRY,

ROLE OF VITAMIN A &

XEROPHTHALMIA Sabina Poudel B. Optometry Institute of Medicine

Presentation layout

1) Introduction to vitamins & their classification2) Vitamin A - forms of vitamin A - sources -biochemical functions3) Role of vitamin A in eye4) Vision biochemistry5) Vitamin A deficiency6) xerophthalmia

VITAMINSPotent 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

Classification of vitamins1) Fat soluble vitamins Vitamin A Vitamin D Vitamin E Vitamin K

2) Water soluble vitamins Vitamin CVitamin B complex - energy releasing: B1, B2, B3, B6, B7, pantothenic acid. - hematopoietic: folic acid, B12

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)

Preformed vitamin a: RetinoidsActive or usable formFour categories of retinoids: a) retinol b) retinal c) retinoic acid d) retinyl estersAll retinoids are absorbed as retinol.

Sources: animal products like liver, fish , fish oils , milk , eggs etc.

Liver is richest source

Structure of different forms of vitamin A

Unsaturated organic compoundsAll forms of vitamin A have a beta-ionone ring to which an isoprenoid chain is attached, called a retinyl group

Structure of different forms of vitamins A

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

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

Provitamin A: CarotenoidsPrecursor 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.

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

Biochemical functions of vitamin AVision 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.

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

Role of vitamin A in eyeFormation of rhodopsin - used in night visionMaintenance of healthy cornea and conjunctival cells

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

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)

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.

90 % of the retinoids can be absorbed

Carotenoids: -absorbed intact - absorption rate much slower - intestinal cells convert carotenoids to retinoids

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

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 μmol/litre( 5% of plasma)In aqueous: retinol barely detectable

3. Synthesis of visual pigmentsRetinol remains unchanged in RPE cellsRetinol enters into the outer segments of photoreceptors Retinol oxidation Retinene ( 11- cis retinal ) retinene reductase

Retinene combines with the protein opsin to form rhodopsinNAD oxidative system (present in RPE) supports the reaction of rhodopsin formation .

Fig:Utilization of vitamin A for synthesis of Rhodopsin

4.Light induced changes in visual pigments

Light falling on retina absorbed by photoreceptorsPhotochemical 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

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

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

Metarhodopsin 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

Phototransduction • Process of converting light energy into electrical signals• Occurs in the photoreceptors

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

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

Na+ from the extracellular fluid flows inside the outer segment through these pores

This is called dark current, producing a slight depolarization

In Light:When light strikes the photoreceptors:amount of cGMP is reduced so some of Na+ channels are closedresults in hyperpolarization

Thus excitation of photoreceptors cause increased negativity of the membrane potential (hyperpolarization) rather than the decreased negativity(depolarization)

Under dark condition• Photoreceptor depolarized• Continuously release neurotransmitter glutamate

Under light stimulation• Photoreceptor hyperpolarized• Reduction in release of glutamate

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

Phototransduction cascade

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Incident Light

Change in Opsin Configuration

Retinene1 changed toAll-trans form

α-Subunit separatesTransducin (Gα)

is activated

ACTIVATIONCASCADE

Incident Light

Retinene1 changed toAll-trans form

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Subunit activates cGMP PDE

Reduced cytoplasmic cGMP

Hyperpolarisation≈ -70 mV

Converts cGMP to 5’-GMP

Closure of leakyNa+ Channels

“Switching off”

Reduced cytoplasmic cGMP

Converts cGMP to 5’-GMP

Decrease intracellular Ca2+

Electrical signal down the neural pathway

Decrease Glutamate release

Depolarization (rod and cone On Center bipolar cells)

Hyperpolarization (cone Off surround

bipolar cells)

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

III. Visual cycle Under constant light stimulation: Photoreceptor Bleaching = Photoreceptor Regeneration

equilibrium between the photodecomposition and regeneration of visual pigments: visual cycle

Summarising…..

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 Unit1 IU= 0.3 μg of retinol

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

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Risk factors• PEM• Measles, Chickenpox, High Fevers• Bronchopneumonia, Tuberculosis, Diphtheria• Gastroenteritis, Dysentery, Worm Infestations

XEROPHTHALMIA

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.

• 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

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 μmol/L

Night blindness• Earliest symptom

• Difficulty to see in dusky and dark environment.

Insufficient vitamin A

Insufficient rhodopsin production

Lack of functioning of rod cells

Brain doesn’t receive enough signals

Night blindness

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

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

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

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

Corneal scars• Healing of stromal defects results in corneal scars

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

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

disappearance of rest of ERG.

Eye changes1) Conjunctiva X1A X1B warning sign +XN 2) Cornea X2 X3A medical + ophthalmological X3B emergency XS3) Retina XN first sign of xerophthalmia XF

conjunctivaContains mucosal

cells

Secretes mucus

Dryness , infection

prevented

Healthy corneal cells, no infection

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 cellsBlindness

Systemic associations

• Dry skin and hair

• Increased incidence of ear, sinus, respiratory, urinary, and digestive infections

• Inability to gain weight

• Nervous disorders

• Skin sores

The Recommended Doses of Vitamin A

• Prophylactic Schedule:Dose By Mouthmg I.U.

• All Children( above 1 yr) 110 200,000Every 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

• 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 Mouthmg I.U.

Day 1 110 200,000Day 2 110 200,000Day 14(2-4 Weeks)

110 200,000

The Recommended Doses of Vitamin A:...

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

Treatment of Xerophthalmia

A. MEDICAL1. Vitamin A Massive Dosing

• Oral: Day 1, 2, 14 - 200,000 I.U.• Injection: 1st Day Only - 100,000 I.U.

Water MiscibleNo oily preparation

If not available: Give food rich in Vitamin A

2. Supportive Therapy• Fluids• Proteins• Control of Infection• Deworming

Treatment of Xerophthalmia

A. MEDICAL3. Eye

• Antibiotcs, Mydiatrics• Pad specially in X3A, X3B• Avoid Exposure: Antibiotic Ointment• Methyl Cellulose Drops

B. SURGERY1. Conjunctivoplasty2. Keratoplasty

a. Prophylacticb. Optical

C. REHABILITATION

Prevention of Vitamin A deficiency

• Breastfeeding

• Vitamin A supplementation

• Food fortification

• Promotion of vitamin A-rich diets

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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%)

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