New Vitamin A

PERAN VITAMIN A PADA
PENGLIHATAN
DAN FUNGSINYA SEBAGAI
ANTIOKSIDAN
dr. Rahila, M.Biomed
INTRODUCTIONS
• The soluble vitamin A, as such is present only foods of animal origin.
• The provitamins carotenes are found in plants
• It is recorded in the history that Hippocrates (about 500 B.C) cured
night blindness.
• He prescribed to the patiesnts ox liver (in honey), which is now known
to contain high quantity of vitamin A.
VITAMIN A
• Active forms:
• Retinol
• Retinaldehyde (retinal)
• Retinoic acid
Structure of vitamin A
• Retinol: a primary alcohol containing β-ionone
ring with unsaturated side chain, retinol is found
in animal tissues as a retinyl ester with long
chain fatty acids
• Retinal: This is aldehyde derived from the
oxidation of retinol. Retinal and retinol can
readily be interconverted
• Retinoic acid: this is acid derived from the
oxidation of retinal. Retinoic cid cannot be
reduced in the body, and therefore cannot give
rise to either retinal or retinol.
• β-carotene: found in plant foods, which can be
oxidatively cleaved in the intestines to yield two
molecules of retinal. In humans, the conversion
is inefficient, hence β-carotene possesses about
one-twelfth vitamin A activity compared to that
of retinol.
Absorption, Storage and Transport
• Dietary retinyl esters are hydrolysed by pancreatic or intestinal brush
border hydrolases in the intestine, releasing retinol and free fatty
acids.
• Carotenes are hydrolysed by β-carotene 15-15’-dioxygenase of
intestinal cells to release 2 moles of retinal which is reduced to
retinol.
• Retinol derived from esters and from the cleavage and reduction of
carotenes is reesterified to long-chain fatty acids in the intestinal
mucosa, incorporated into chylomicrons and transferred to the lymph
• Retinyl esters of chylomicrons are taken up by the liver and stored.
Absorption, Storage and Transport
• When needed, retinol is released from the liver and transported to
extrahepatic tissues by the plasma retinolbinding protein (RBP).
• The retinol–RBP complex attaches to specific receptors on the surface
of the cells of peripheral tissues, and enter the cells.
• Many cells of target tissues contain a cellular retinolbinding protein
that carries retinol to the nucleus and binds to chromatin (DNA)
where the retinol acts in a manner analogous to that of steroid
hormones.
BIOCHEMICAL FUNCTIONS
• 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 whereas cones are responsible
for bright light and colour vision.
• Vitamin A is a component of the visual pigments of rod and cone cells
• Rod cells  Rhodopsin (11-cis retinal + opsin)
• Cone cells  porphyropsin (red), iodopsin (green) and cyanopsin
(blue)  all these pigments are retinal-opsin complexes
Visual cycle
• Rhodopsin : 11-cis retinal + opsin
• The primary event in visual cycle, on exposure to light, is the
isomerization of 11-cis retinal to all-trans retinal.
• This lead to a conformational change in opsin which is responsible for
the generation of nerve impulse.
• The all-trans retinal is immediately isomerized by retinal isomerase (of
retinal epithelium) to 11-cis retinal
• 11-cis retinal combines with opsin  rhodopsin  visual cycle
Other biochemical functions
• Retinol and retinoic acid : almost like steroid hormones  regulate the protein synthesis 
involved in the cell growth and differentiation
• Vit A is essential to maintain healthy epithelial tissue  retinol and retinoic acid are required to
prevent keratin synthesis (responsible for horny surfaces)
• Retinyl phosphate  synthesis of certain glycoproteins and mucopolysaccharides  growth an
dmucus secretion
• Retinol is necessary for normal reproduction. It acts like a hormones and regulates gene
expression
• Vit A is considered to be essential for the maintenance of proper immune system to fight
various infections
• Vit A is required for synthesis of cholesterol
• Caretoids (most importan β-carotene)  as antioxidants and reduce the risk of cancers initiated
by free radicals and strong oxidants.
Recommended Dietary Allowance
(RDA)
• The daily requirement of vit A is expressed as retinol equivalents (RE)
rather than IU
• 1 retinol equivalent = 1 µg retinol
= 6 µg β-carotene
= 12 µg other carotene
= 3.33 IU vit A activity from retinol
= 10 IU of vit A activity from β-carotene
• RDA of vit A for adults : 1000 RE (3500 IU) for man
800 RE (2500 IU) for woman
• The requireman increases in pregnancy and lactating mothers
Dietary Sources
• Animal sources contain (preformed) vit A
• Liver, kidney, egg, yolk, mil, cheese, butter
• Fish (cod or shark) liver oils are very rich in vit A
• Vegetable sources contain provitamin A-carotenes.

• Yellow and dark green veg and fruits e.g carrots, spinach, pumpkins, mango,
papaya etc
Vitamin A Deficiency
• May be due to inadequate dietary intake, impaired intestinal absorption, reduced storage in
liver and chronic alcoholism
• The deficiency symptoms are not immediate, since the hepatic stores can meet the body
requirements for sometime (2-4 months)
• The deficiency manifestations are related to the eyes, skin and growth
• Deficiency manifestation of the eyes: Night Blindness (nyctalopia) is oen of the earliest
symptom of vit A def.
• Prolonged def irreversibly damages a number of visual cells
• Severe def of vit A  xerophtalmia : dryness in conjunctiva and cornea, and keratinization
of epithelial cells. In certain area of conjunctiva : white triangular palques (Bitot’s spots)
• If xerophtalmia persist for a long time corneal ulceration and degeneration  destruction
of cornea (keratomalacia)  total blindness
• Effect on growth : growth retardation due to impairment of skeletal
formation
• Effect on reproduction : degeneration of germinal epithelium 
sterility in males
• Effect on skin and epithelial cells : the skin becomes rough and dry
• Keratinization of epithelial cells of GI tract, urinary tract and respiratory tract
increased bacterial infection
• Vit A def is associated with formation of urinary stones
• The plasma level of RBP is decreased in vit A def
Hypervitaminosis A
• Excessive consumption of vit A  toxicity
• Total serum vit A level is elevated in hypervitaminosis A (normal 20-50
µd/dl).
• Free retinol or retinol bound to plasma lipoprotein is actually harmful
to the body
• A toxicosis symptoms appear only after retinol binding capacity of
RBP exceeds
HYPERVITAMINOSIS A
• Alteration of skin
• Hepatic dysfunction
• Headache
• Drowsiness
• CHRONIC EFFECTS-
• Roughening of skin,desquamation
• Irritability
• Coarsening of skin & falling of hair
• Anorexia
• Weight loss
β-carotene
•  consumption of β-carotene is associated with  incidence of heart
attack, skin and lung cancers  antioxidant role
• Ingestion of high doses of β-carotene for long periods are not toxic
Vitamin A as an Antioxidants
• Carotenoids are known to be very efficient physical and chemical quenchers of
singlet oxygen (O2), as well as potent scavengers of other reactive oxygen species
(ROS)
• Carotenoids and some of their metabolites are suggested to play a protective role
in a number of ROS-mediated disorders, such as, i.e., cardiovascular diseases,
several types of cancer or neurological, as well as photosensitive or eye-related
disorders.
• One of the most characteristic features of Crts is their strong coloration, which is
a consequence of light absorption stemming from the presence of an extensive
system of conjugated double bonds.
• The presence of such a conjugated chain is crucial for the proper functioning of
Crts, which is essentially light absorption in photosynthetic organisms and
(photo)protection in all living organisms.
• Carotenoids are also suggested to participate in:
• the stimulation of the immune system;
• the modulation of intracellular signaling pathways (gap junction communication)
• the regulation of the cell cycle and apoptosis;
• the modulation of growth factors;
• cell differentiation; and
• the modulation of various types of receptors or adhesion molecules and many
other physiologically significant processes
THANK YOU

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PERAN VITAMIN A PADA

• Vegetable sources contain provitamin A-carotenes.

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