Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

698
Chapter | 23 Vitamins
A
B
C
D
Retinyl esters
Retinol
Retinoids and Carotenoids
15
1
6 7 9 11 13 CH 2
OH
3 5
CHO
CHO
COOH
E
F
G
H
Retinal
COOH
CH 2 OOC(CH 2 ) 7 CH 3
Retinoic Acid
their isolated states) found in many fruits and vegetables
(Stahl and Sies, 2005). To act as a provitamin A, a carotenoid
must contain a β -ionone structure (i.e., the ring
structure shown in Figure 23-2 containing a single double
bond and three methyl groups). The carotenoids represent
an unusual class of biological pigments. Carotenoids are
rich in conjugated double bonds and are designed to interact
with light. Green plants are the main sources of carotenoids
in the diet of most animals.
In plants and prokaryotes, carotenoids serve as mediators
of photo-energy-related processes by capturing energy
from light (Stahl and Sies, 2005). Carotenoids are also
readily destroyed by intense light, particularly UV light.
From a chemical perspective, this is important given the
wide range of functions involving carotenoids and vitamin
A. Carotenoids can also quench singlet oxygen and may
act as both antioxidants and prooxidants. The resulting
products of such reactions may also have unwanted side
effects, a problem that is not often appreciated.
When hays are stored for long periods (e.g., a year or
more), the carotenoid content may be markedly reduced
or modified because of chemical or physical (UV light)
oxidation. Moreover, in plants, carotenoids occur in association
with chloroplasts complexed with protein and other
lipids and provide the main source of provitamin A for animals.
In nonruminant animals, poor digestion of complex
organelle structures, such as chloroplasts, in turn may lead
to poor digestibility of carotenoid components.
Grains, with some exceptions (e.g., corn) are minor
sources of provitamin A. Among the legume grains,
O
C
O
HO
Interrelationships between dietary and cellular retinoids
-Carotene
COOH
O
OH
O
H
FIGURE 23-2 Structures of retinoids and carotenoids related to vitamin
A. The structures are for (A) retinol, (B) retinal or retinaldehyde, (C) retinoic
acid [all-trans], (D) retinoic acid [11- cis ], (E) retinoic acid [13- cis ],
(F) retinyl ester [palmitate], (G) retinoyl β -glucuronide, and (H) β -carotene.
β -Carotene is a precursor to retinal, which in turn may be reduced to
retinol or irreversibly oxidized to retinoic acid. In animal cells, retinol is
“ stored ” as retinyl ester.
all-trans-Lutein
HO
all-trans-Zeaxanthin
HO
all-trans-Lycopene
FIGURE 23-3 Structures of carotenoids without vitamin A activity.
Lutein is found in green leafy vegetables and is employed as an antioxidant
and for blue light absorption. Lutein covalently bound to one or
more fatty acids is present in some fruits and flowers, notably marigolds.
As a pigment, lutein and other xanthophylls (e.g., zeaxanthin) are used as
natural colorants (e.g., in chicken feed to provide the yellow skin color).
Lutein is also found to be present in a concentrated area of the macula, a
small area of the retina responsible for central vision. As a consequence,
there is interest in lutein and diseases of the eye, such as age-related macular
degeneration. Lycopene is a bright red carotenoid pigment found in
tomatoes and other red fruits.
chickpeas, green and black grams are the best sources of
provitamin A. The richest source of carotenoid is red palm
oil, which contains 500 μ g of mixed α - and β -carotene per
milliliter. Of the carotenoids, six are known to be biologically
important: α -carotene, lycopene, lutein, zeaxanthin,
cryptoxanthin, and β -carotene (e.g., because of its role
as a precursor to vitamin A). The structures of lycopene,
lutein, and zeaxanthin are shown in Figure 23-3 . The following
sections focus mostly on β -carotene and vitamin A
followed by short descriptions for the other carotenoids,
which serve as important biofactors, although with no specific
or known vitamin functions.
3 . Metabolism
Following ingestion, retinyl esters in animal products are
hydrolyzed to retinol by pancreatic hydrolases (esterases)
or lipid hydrolases localized on the surface of the brush
border of intestinal cells ( Harrison, 2005 ). Bile and dietary
lipids facilitate the absorption process, as retinyl esters must
be a part of a lipid micelle to be absorbed. The micellar
structures enhance fusion into the microvillus of intestinal
cells. Similarly, lipid micelles enhance the uptake of
carotenoids into intestinal cells. The bioavailability and
digestion of vitamin A and carotenoids are affected by the
overall nutritional status and the integrity of the intestinal
microvillus. Absorption of physiological doses of vitamin
A in most animals is 70% to 90%, but the efficiency
of absorption for carotenoids added to diets is 40% to
60%, depending on the type of carotenoid. Carotenoids
contained in plant chloroplasts, however, are often poorly
OH
OH