Vitamin A

Vitamin A
A pale-yellow alcohol, soluble in fat but not in water.
In pure form, it is readily destroyed by oxidation and
light, which may cause losses during storage.
Source. Vitamin A is found in all animal tissues,
although it is particularly concentrated in the liver.
There are two different dietary sources for the vitamin:
animal sources which contain vitamin A itself,
mostly in the formof retinyl esters, and plant sources
which contain carotenoids that are converted to
vitamin A in animal tissues such as the absorptive
cells in the intestine. The most vitamin A–enriched
animal food source is fish liver oil. Plant carotenoids
are found in green and yellow fruits and vegetables
such as carrots, apricots, asparagus, broccoli, and
green leafy vegetables. Some fresh-water fish contain
vitamin A2, which differs slightly from vitamin A in
structure.

Bioassay. The vitamin A activity of carotenoids
varies with their chemical structure, with β-carotene
being the most potent. One international unit (IU)
of vitamin A has been set at 0. 3 microgram of vitamin
A or 0. 6 μg of β-carotene. This is somewhat
confusing since the efficiency of conversion of
β-carotene to vitamin A becomes greater in a deficiency
state. Although biological assays are sometimes
used, vitamin A and carotene are usually determined
by spectrophotometric techniques.

Physiological activities. In vitamin A deficiency,
the epithelial tissues of many organs are affected.
Growth failure occurs, and young animals can
suffer from neurological symptoms resulting from
pressures on the central nervous system. Changes
occur in the skin, mouth, respiratory tract, urogenital
tract, and some glands. Vitamin A deficiency is also
strongly associated with depressed immune function
and higher morbidity and mortality due to infectious
diseases such as diarrhea, measles, and respiratory
infections. A severe manifestation of vitamin A deficiency
is night blindness and inflammation of the
eyes (xerophthalmia), followed by irreversible blindness.
The symptoms seen in vitamin A deficiency
reflect the multiple roles of this compound in animals.
These roles are fulfilled by two compounds
that are synthesized from vitamin A in the body: vitamin
A aldehyde (retinaldehyde), which is critical
for vision, and vitamin A acid (retinoic acid), which
controls many physiological functions in both the
embryo and the adult. The similar chemical structures
of vitamin A, retinaldehyde, and retinoic acid
are shown at right.

Retinaldehyde. Retinaldehyde has a critical role in
sight. This compound binds to a protein termed
opsin to generate the visual pigment rhodopsin in
the retina. Rhodopsin is the “visual antenna” that
responds to light by sending signals to the brain.
Following absorption of light, rhodopsin releases
the bound vitamin A compound. Resynthesis of
rhodopsin by providing fresh molecules of retinaldehyde
is therefore essential for normal vision.

Retinoic acid. Retinoic acid is the active vitamin A
metabolite that is critical for many processes, such as
cell growth and differentiation, in the embryo and in
the adult. Retinoic acid exerts its effects by modulating
the expression of many genes. This activity is due
to the ability of the compound to control the activities
of proteins known as retinoid nuclear receptors.
When activated by retinoic acid, these proteins bind
to deoxyribonucleic acid (DNA) and regulate gene
transcription. Retinoic acid is currently used in treatment
of various pathologies such as skin disorders
Vitamin A: all-trans -retinol
11-cis -Retinaldehyde
All-trans -retinoic acid
9-cis -Retinoic acid
OH
O
O
OH
O OH
and certain types of cancer. See DEOXYRIBONUCLEIC
ACID (DNA).