Download MBBS Biochemistry PPT 88 Vitamin A Lecture Notes

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Common structural unit

A trimethyl cyclohexenyl ring

An all trans configured chain with four double bonds.

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Retinoic acid

TYPES

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Retinol - when R is -CH2OH

Retinal - when R is -CHO

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Retinoic acid ?when R is -COOH
FORMS OF VITAMIN A

VITAMIN A1 ? found in major species of animals.

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VITAMIN A2- found in fresh water fish.

NEOVITAMIN A- stereoisomer of vitamin A1

COMPARISON

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VIT A1

VIT A2

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1.

Found in major species of

animals

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1.

Found in freshwater

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fish

2.

Only one double bond in beta

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ionone ring.

2.

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Two double bonds in

beta ionone ring

3.

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More potent

3.

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Activity 40% of A1

4.

Can be obtained through

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carotenes

4.

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Carotenes can not give

rise to vitamin A2
DIETARY SOURCES

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Animal sources- liver oil,butter,milk,

cheese,egg- yolk,

Plant sources- tomatoes,carrots,green yellow

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vegetables,spinach and fruits like

mangoes,papayas,corn,sweet potatoes

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Recommended Daily

Allowance

v Adult male- 3000 IU

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v Adult female- 3000 IU

v Recommended daily allowance is 6000IU

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v 1 IU=0.3 micro gms of Retinol
ABSORPTION, STORAGE &

TRANSPORT

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ABSORBED IN SMALL INTESTINE
FREE retinol is absorbed
Retinol is transported in association

with a specific retinol binding protein .

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Stored as retinyl ester (normally as

retinyl palmitate)

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Retinoic acid is transported in

association with a specific retinoic

acid binding protein .

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ROLE IN VISION

Overall mechanism through which it works is known as

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Wald's visual cycle /rhodopsin cycle.

Rods contain ?photosensitive pigment called rhodopsin

or visual purple.

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Visual purple ?opsin


retinine/retinal or

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retinaldehyde


Bathorhodopsin

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RHODOPSIN

Light

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Lumirhodopsin

Metarhodopsin-I

Dark

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Metarhodopsin-II

Isomerase

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opsin

opsin

Cis-retinal

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Trans-retinal

Blue light

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Retinene reductase

Retinene reductase

Isomerase

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Trans-retinol

Cis-retinol

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Blue light

WALD'S VISUAL CYCLE

EPITHELIAIZATON

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Epithelial structures show gross

structural changes in deficiency

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Skin-dry,rough& scaly
Cornea-epithilium keratinised,opaque
Bitot's spot
Lacrimal glands-similar changes

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leading to dryness of

conjunctiva&cornea(xerophthalmia)

Respiratory tract ?keratinzation-

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susceptibility to infection.
BONE AND TEETH FORMATION

Deficiency results in slowing of endochondral bone

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formation and reduced osteoblastic activity.

Thinning of enamel and deposit of chalky deposit on the

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surface of the teeth.

ROLE IN REPRODUCTION

EXPERIMENTS ON RATS-

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Deficient male-ill developed testes, immature sperms

Deficient female-unable to carry pregnancy to full term

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.

ROLE IN METABOLISM-

Protein synthesis

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DNA metabolism
Beta-Carotene as an

antioxidant and anticancer

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In addition to its antioxidant property, Beta- Carotene

also has been shown to have anticancer action.

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It increases the number of receptors on WBC for a

molecule known as "major histocompatibility complex

I".

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OCULAR MENIFASTATIONS- VITAMIN A

DEFICIENCY

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Night blindness/ Nyctalopia-first symptom

Conjunctival xerosis-first clinical sign

Bitot's spot-frequently bilateral

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Corneal xerosis-cornea dry & nonwettable

Keratomalacia-ulceration of cornea.

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Extra ocular manifestations

vitamin A deficiency

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Follicular hyperkeratosis

Anorexia

Growth retardation

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Increased respiratory & intestinal infection
HYPER VITAMINOSIS A

Alteration of skin

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Hepatic dysfunction
Headache
Drowsiness
CHRONIC EFFECTS-

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Roughening of skin,desquamation

Irritability

Coarsening of skin&falling of hair

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Anorexia

Wt loss

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THERAPEUTIC USES OF

RETINOIC ACID

Oral leukoplakia

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Promyelocytic leukaemia
MCQ

Worldwide, the most common vitamin

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deficiency is that of

(A) Ascorbic acid (B) Folic acid

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(C) Vitamin A (D) Vitamin D

Vitamin A or retinal is a

(A) Steroid

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(B) Polyisoprenoid compound containing a

cyclohexenyl ring

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(C) Benzoquinone derivative

(D) 6-Hydroxychromane
-Carotene, precursor of vitamin A, is

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oxidatively cleaved by

(A) -Carotene dioxygenase

(B) Oxygenase

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(C) Hydroxylase

(D) Transferase

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The international unit of vitamin A is

equivalent to the activity caused by

(A) 0.3 g of Vitamin A alcohol

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(B) 0.344 g of Vitamin A alcohol

(C) 0.6 g of Vitamin A alcohol

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(D) 1.0 g of Vitamin A alcohol
Retinol is transported in blood bound to

(A) Aporetinol binding protein

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(B) 2-Globulin

(C) -Globulin

(D) Albumin

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Deficiency of Vitamin A causes

(A) Xeropthalmia

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(B) Hypoprothrombinemia

(C) Megaloblastic anemia

(D) Pernicious anemia

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Retinal is a component of

(A) Iodopsin (B) Rhodopsin

(C) Cardiolipin (D) Glycoproteins

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An important function of vitamin A is

(A) To act as coenzyme for a few enzymes

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(B) To play an integral role in protein synthesis

(C) To prevent hemorrhages

(D) To maintain the integrity of epithelial tissue

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Conversion of all-trans-retinal into alltrans-

retinol requires

(A) NAD (B) NADH

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(C) NADP (D) NADPH

Two molecules of vitamin A can be formed

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from 1 molecule of

(A) -Carotene (B) -Carotene

(C) -Carotene (D) All of these

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