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

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Chapter | 23 Vitamins
When biotin-containing carboxylases are degraded,
biotin is released as biocytin ( Fig. 23-20 ). Biocytinase is
an important liver enzyme that catalyzes the cleavage of
the peptide linkage between biotin and lysine to release
free biotin for reutilization. The biotin requirement in animals
is relatively low (i.e., in the microgram per kilogram
of diet range). Furthermore, biotin can also be produced by
gut microflora and the biotin that is covalently attached to
enzymes is reutilized.
Nevertheless, there can be nutritional problems associated
with biotin status. Biotin and biocytin have affinity
for certain proteins, particularly avidin in egg white. The
use of raw eggs can cause biotin deficiency because of
the association of biotin with avidin in uncooked eggs. The
response in fur-bearing animals to ingestion of significant
quantities of raw egg white has been described as “ egg
white injury. ” Native (nondenatured) avidin in eggs causes
egg white injury because it binds tightly to biotin, preventing
its absorption.
The relationship of biotin to avidin is important, particularly
to industries that utilize fur-bearing animals for profit.
It was subsequently found that egg white injury could be
cured by a liver factor that was first called protective factor
X and later determined to be biotin. Because biotin cured
the skin disorder of egg white injury, it was called vitamin
H (for haut , the German word for skin). Conditions that
may increase biotin requirements in pregnancy, lactation,
and therapies are the use of anticonvulsants or exposure
to high concentrations of lipoic acid. Spontaneous biotin
deficiency occurs rarely in animals because biotin is well
distributed among foodstuffs, and a good part, if not all, of
the requirement for the vitamin is met by microbial synthesis
in the gut. As noted, the deficiency can, however, be
induced by the inclusion of unheated (raw) egg white in
the diet ( Zempleni, 2005 ). For most monogastric animals,
50 to 100μg of biotin per 1000 kcal or 0.2 to 0.4 mg per
kilogram of diet is probably sufficient.
Biotin deficiency leads to impaired gluconeogenesis
and impaired fat metabolism. Alopecia and dermatitis are
characteristics of biotin deficiency in most animals and
birds. Biotin deficiencies can also cause severe metabolic
acidosis. The inability to carry out fat metabolism markedly
affects the dermis in biotin-deficient animals. Unless
there is an inborn error or genetic polymorphism involving
one of the carboxylase enzymes, the likelihood of a biotinrelated
metabolic compromise or deficiency is low, except
when uncooked egg white is the major protein source.
Biotin turnover and requirements can be estimated on
the basis of (1) concentrations of biotin and metabolites
in body fluids, (2) activities of biotin-dependent carboxylases,
and (3) the urinary excretion of organic acids that
are formed at increased rates if carboxylase activities are
reduced. Urinary excretion of biotin and its metabolite,
bisnorbiotin, activities of propionyl-CoA carboxylase and
beta-methylcrotonyl-CoA carboxylase in lymphocytes,
and urinary excretion of 3-hydroxyisovaleric acid are good
indicators of marginal biotin deficiency.
2 . Folic Acid and Vitamin B 12
a . Introduction
Knowledge regarding folic acid and B 12 evolved from
efforts to better understand macrocytic anemias and certain
degenerative neurological disorders ( Scott, 1994 ). Combe,
the Scottish physician, recognized in the early 1800s that a
certain form of macrocytic anemia appears related to a disorder
of the digestive organs. In classic studies by Minot
and Murphy, Castle, and others, it became clearer that
the disorder was associated with gastric secretions and in
some cases could be reversed by consuming raw or lightly
cooked liver. Through careful clinical investigations and
inferences, Castle postulated the existence of an intrinsic
factor in gastric juice, which appeared to combine with a
dietary extrinsic factor to modulate the severity of the anemia
( Goldblith and Joslyn, 1964 ).
In parallel studies, folic acid was also associated with
macrocytic anemia. Large-scale efforts by a number of pharmaceutical
companies throughout the 1940s and 1950s and
careful clinical and basic studies at academic institutions
eventually led to the isolation of folic acid and vitamin B 12 .
b . Chemistry and Functions
The structures for folic acid and vitamin B 12 are given in
Figures 23-21 and 23-22. Folic acid is part of a family of
compounds with a pteridine moiety. In the case of folic
acid, the pteridine moiety is associated with aminobenzoic
acid and glutamyl residues conjugated by a methylene
bridge to para-aminobenzoic acid, which in turn is joined
to glutamyl residues by a peptide linkage. Figure 23-21
presents an overview of one-carbon transfers involving
tetrahydrofolate (THF) coenzymes and their metabolic origins.
The reactions include the generation and utilization
of formaldehyde and formimino groups in the synthesis of
pyridine nucleotides, interconversion of some amino acids,
and eventual reduction of the methylene form of THF to
methyl THF to facilitate the conversion of homocysteine to
methionine ( Goldblith and Joslyn, 1964 ; Scott, 1994 ).
To set the stage for these conversions, folic acid must
be in its completely reduced state. The reductions occur
at positions 5, 6, 7, and 8 to form a tetrahydrofolic acid
(THFA) ( Fig. 23-21 ). The reduction brings the nitrogen
at positions 5 and 10 closer together and changes electrochemical
properties of both nitrogens, which facilitates the
formation of the various THFA single carbon derivatives
that are involved in the metabolic conversions shown in
Figure 23-21 . The formyl, methanyl , and methylene forms
are utilized for purine synthesis and important steps in thymidylate
(i.e., DNA-related) synthesis. These reactions are
therefore of obvious importance and are essential to cell