vitamin a Flashcards
structure of vitamin a
Vitamin A contains a single 6-membered ring to which
is attached an 11-carbon side chain.
Vitamin A is an alcohol (retinol), but can be converted
into an aldehyde (retinal), or an acid (retinoic acid).
Active Form of vitamin A
Vitamin A consists of three biologically active molecules
which are collectively known as retinoids.
1. Retinol: Primary alcohol (CH2OH) containing
form
2. Retinal: Aldehyde (CHO) containing form
3. Retinoic acid: Carboxyl (COOH) containing form
* Each of these compounds are derived from the plant
precursor molecule, β-carotene (a member of a
family of molecules known as carotenoids).
* β-carotene which consists of two molecules of retinal
linked at their aldehyde ends is also referred to as
the provitamin form of vitamin A.
* The retinol and retinal are interconverted by enzyme
retinal aldehyde reductase. The retinoic acid is
formed by oxidation of retinal. The retinoic acid
connot be reduced to either retinol or retinal.
Absorption, Transport and Storage of vitamin A
Ingested β-carotene is cleaved in the intestine by
β-carotene dioxygenase to yield retinal. Retinal is
reduced to retinol by retinaldehyde reductase, an
NADPH requiring enzyme within the intestine.
* Retinol is esterified with palmitic acid incorporated
into chylomicrons together with dietary lipid and
delivered to the liver for storage.
* Transport of retinol from the liver to extrahepatic
tissues, occurs by binding of retinol to retinol binding
protein (RBP).
* Transport of retinoic acid is accomplished by binding
to albumin.
Sources of vitamin A
- The richest dietary sources of vitamin A are fish liver
oils (cod liver oil). Animal livers are also rich sources
but meat is rather low in vitamin A. - Other good sources are milk and dairy products, darkgreen leaves, such as spinach and yellow and red
fruits and vegetables, such as carrots, tomatoes, and
Figure 7.17: Structure of vitamin A, retinol peaches.
Functions of Vitamin A
- Vitamin A is required for a variety of functions such
as vision, cell differentiation and growth, mucus
secretion, maintenance of epithelial cells, etc. - The role of vitamin A in vision has been known
through the studies of G Wald, who received the
Nobel prize in 1943 for this work. - Different forms of the vitamin have different
functions.
– Retinal and retinol are involved in vision.
– Retinoic acid is involved in cellular differentiation
and metabolic processes.
– β-carotene is involved in antioxidant function.
Role of Vitamin A in Vision
The cyclic events occur in the process of vision, known
as rhodopsin cycle or Wald’s visual cycle.
Both rod and cone cells of retina contain a photoreceptor
pigment in their membrane and vitamin A is a
component of these pigments. Rhodopsin or visual
purple, the visual pigment of rod cells in the retina
consists of 11-cis-retinal bound to protein opsin.
* When rhodopsin absorbs light, the 11-cis-retinal is
converted to all-trans retinal.
* The isomerization is associated with a conformational change in the protein opsin.
* Conformational changes in opsin generates a nerve
impulse that is transmitted by the optic nerve to the brain.
* This is followed by dissociation of the all-trans retinal
from opsin.
* The all-trans retinal is immediately isomerized by
retinal isomerase to 11-cis-retinal.
* This combines with opsin to regenerate rhodopsin
and complete the visual cycle.
The conversion of all-trans retinal to 11-cis-retinal is
incomplete and therefore remaining all-trans retinal
which is not converted to 11-cis-retinal is converted to
all-trans retinol by alcohol dehydrogenase and is stored
in the liver. When needed, retinol re-enters the circulation and is taken up by the retina, where it is converted
back to 11-cis-retinal which combines with opsin again
to form rhodopsin.
Dark adaptation time: When a person enters from
bright light to dark there is difficulty in seeing due to
depletion of rhodopsin, but after few minutes the vision
improves. During these few minutes, rhodopsin is
resynthesized and vision is improved. The time taken
for regeneration of rhodopsin is known as dark
adaptation time. Dark adaptation time is increased in
vitamin A deficient individuals.
Color Vision function of vitamin A
While vision in dim light is mediated by rhodopsin
of the rod cells, color vision is mediated by three
different retinal containing pigments in the cone
cells, the three pigments are called porphyropsin,
iodopsin and cyanopsin and are sensitive to the three
essential colors: red, green and blue respectively. All
these pigments consist of 11-cis-retinal bound to
protein opsin.
* Thus, when light strikes the retina, it bleaches one
or more of these pigments, depending on the color
quality of the light. The pigments are converted to
all-trans retinal, and the protein moiety opsin is
released as in the case of rhodopsin. This reaction
gives rise to the nerve impulse that is read out in the
brain as color:
– Red if porphyropsin is split
– Green if iodopsin is split
– Blue if cyanopsin is split.
* If mixtures of the three are converted, the color read
out in the brain depends on the proportions of the
three split.
Cellular Differentiation and Metabolic Effect of vitamin A
- Retinoic acid is an important regulator of gene
expression especially during growth and development. Retinoic acid is essential for normal gene
expression during embryonic development such as cell differentiation in spermatogenesis and in the
differentiation of epithelial cells. - Retinoic acids exert a number of metabolic effects on
tissues. These include:
– Control of biosynthesis of membrane glycoproteins
and glycosaminoglycans (mucopolysaccharide)
necessary for mucus secretion. The normal mucus
secretion maintains the epithelial surface moist and
prevents keratinization of epithelial cell.
– Control of biosynthesis of cholesterol.
Antioxidant Function
β-carotene is an antioxidant and may play a role in
trapping peroxy free radicals in tissues.
* The antioxidant property of lipid soluble vitamin A
may account for its possible anticancer activity.
* High levels of dietary carotenoids have been associated
with a decreased risk of cardiovascular disease.
Nutritional Requirements of vitamin A
The RDA of vitamin A for adults is 800–1000 retinol
equivalents. (1 retinol equivalent = 1 μg retinol = 6 μg
β-carotene).
Deficiency Manifestation
Clinically, degenerative changes in eyes and skin are
observed with vitamin A deficiency.
Effect on Vision (vitamin A deficiency)
Night blindness (nyctalopia)
* Night blindness is one of the earliest symptoms of
vitamin A deficiency. This is characterized by loss of
vision in night (in dim or poor light) since dark
adaptation time is increased. Prolonged deficiency of
vitamin A leads to an irreversible loss of visual cells.
* Severe vitamin A deficiency causes dryness of cornea
and conjuctiva, a clinical condition termed as
xerophthalmia (dry eyes).
* If this situation prolongs, keratinization and ulceration
of cornea takes place.This results in destruction of
cornea. The cornea becomes totally opaque resulting
in permanent loss of vision (blindness), a clinical
condition termed as keratomalacia. Xerophthalmia and
keratomalacia are commonly observed in children.
* White opaque spots develop on either side of cornea
in vitamin A deficiency are known as Bitot’s spot.
Effect on Skin and Epithelial Cells (vitamin A deficiency)
- Vitamin A deficiency causes keratinization of
epithelial cells of skin which leads to keratosis of hair
follicles, and dry, rough and scaly skin. - Keratinization of epithelial cells of respiratory,
urinary tract makes them susceptible to infections.
Other Symptoms of Vitamin A Deficiency
- Failure of growth in children.
- Faulty bone modelling producing thick cancellous
(spongy) bones instead of thinner and more compact
ones. - Abnormalities of reproduction, including degeneration of the testes, abortion or the production of
malformed offspring.
Therapeutic Use of Vitamin A
- The use of retinoic acid preparations, in the treatment
of psoriasis, acne and several other skin diseases, is
related to its involvement with epithelial cell
differentiation and integrity - Some precancerous lesions seem to respond to
treatment with carotenoids.
