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

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Chapter | 23 Vitamins
plant tissues and give them color ( Fig. 23-3 ). Carotenoid
pigments attach themselves to proteins or fats and can produce
blue, green, purple, or brown pigments in addition to
yellow, orange, and red. If an animal’s skin or feather color
comes from carotenoids and it is not available in food, some
or all of the color fades. For example, many birds develop
bright red, orange, or yellow carotenoid pigmentation that
they use presumably to attract mates. Because animals often
obtain several different carotenoids from plant and animal
food sources, it is possible that these pigments are accumulated
at different levels, which results in the ultimate color
expression of individual animals. As an example, when
finches are fed a lutein-zeaxanthin mix, proportionally more
zeaxanthin was found than lutein than occurred in the diet
(i.e., there is preferential accumulation in the body). In fish,
pigmentation is influenced by diet and sex. Presumably,
males absorb/retain more pigments than females. Often
consumers of various products (notably egg yolk, eggshell,
broiler skin, and salmon flesh) prefer a specific type and
degree of coloration. Although some birds can be sexed by
visual inspection of their genitalia, mating resulting in sexassociated
color phenotypes is becoming more in use. The
genetic markers involved affect the color of the plumage and
the cloning of genes involved in pigmentation offers the prospect
of deciphering the genetic control of animal pigmentation
and modifying it to meet specific pigmentation needs
(Castaneda et al., 2005 ; Johnson et al., 2000 ).
Regarding specific carotenoids, α -carotene is one of the
most abundant carotenoids in the diet and can be converted to
vitamin A, but with only one-half the activity as β-carotene
(contains only one β-ionone ring in contrast to two for
β -carotene). Other differences in biological activity have
also been reported. The α -carotene is a better inhibitor
toward certain growth factors (e.g., N-myc activity) than
β -carotene. N-myc is in the oncogene family of growth
factors. Because of its abundance, α -carotene is also an
excellent biomarker of intake of fruits and vegetables ( Stahl
and Sies, 2005 ). Another carotenoid, lycopene, is a red pigment
found in fruits and vegetables. In human epidemiological
studies, its consumption in modest amounts is weakly
associated with a reduced risk of certain cancers. Lutein and
zeaxanthin are carotenoids found in green, leafy vegetables
and algae and have been considered recently for potential
benefits to sight and vision, particularly a decrease in the
risk of cataracts. Cryptoxanthin has even been reported to
decrease bone loss in ovariectomized rodents. Thus, there
are a wide range of health effects, which may have nutritionally
and pharmacological potential ( Stahl and Sies, 2005 ).
B . Vitamin D
1 . Introduction
Sir Edward Mellanby in 1921 reported the induction of rickets
in dogs through dietary manipulation. He discovered that
the disease could be corrected with cod liver oil. McCollum
in 1922 reported the curative factor in cod liver oil was not
vitamin A and appeared to be another fat-soluble substance.
This substance was later identified as vitamin D, based on
the ability to inactivate the vitamin A factor in cod liver
by mild oxidation with the retention of antirachitic activity
( Goldblith and Joslyn, 1964 ).
2 . Sources, Functions, and Metabolism of Vitamin D
The D vitamins are a family of 9,10-secosteroids that differ
only in the structure of the side chain attached to carbon-
17. The two forms of vitamin D significant in veterinary
medicine are ergocalciferol (vitamin D 2 ) and cholecalciferol
(vitamin D 3 ). The differences in the side chain result
in the vitamins having disparate potencies with some species
of animal and differing in toxicity when consumed
in large amounts. These two forms of vitamin D are produced
in a two-step reaction when their respective sterols
ergocalciferol and 7-dehydrocholesterol absorb ultraviolet
radiation and undergo photolysis, which is then followed
by thermal isomerization ( Fig. 23-9 ). Excessive
ultraviolet radiation of the sterols produces inactive compounds.
Under most instances, animals can synthesize
sufficient quantities of cholecalciferol if they receive adequate
exposure to ultraviolet light of wavelength 280 to
320 nm ( Hendy and Goltzman, 2005 ; Hendy et al., 2006 ;
Xue et al., 2005 ). This is particularly true when the calcium
and phosphorus requirements of the animal are met.
As vitamin D is produced at one site and acts at other sites
including bone and intestine, it fulfills the definition of a
prohormone.
In most animals, 7-dehydrocholesterol is abundant in
skin, being the ultimate precursor for cholesterol, which
is synthesized from acetate. However, the skin of cats and
dogs and possibly other carnivores contains only small
quantities of 7-dehydrocholesterol, which does not permit
adequate synthesis of vitamin D. These animals are solely
dependent on the diet for this vitamin. With the exception
of animal products, most natural foods contain low vitamin
D activity. Fish, in particular saltwater fish, such as sardines,
salmon and herring, and fish liver oils contain significant
to large quantities of vitamin D. Many plants also
contain hydroxylated ergosterol derivatives, some of which
have potent vitamin D activities ( Wasserman, 1975 ).
Initially, it was speculated that vitamin D might serve as
an enzymatic cofactor for reactions that served to maintain
calcium and phosphorus (as phosphate). When isotopes of
calcium became available, it was soon appreciated that there
was considerable lag between the administration of vitamin
D and its effect on calcium-related metabolism. This
lag was shown to be due to the conversion of vitamin D
to an active form. Investigations throughout the 1960s
and 1970s led to the sequence of events that is outlined
in Figure 23-9 . For example, the kidneys were identified