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

V. Vitamin-Like Compounds
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4 . Taurine
Although taurine is not generally considered a vitamin, the
requirement for taurine by some animals is of the similar
order as choline. Taurine, 2-aminoethanesulfonic acid,
is present in all animal tissues and is one of the principal
free amino acids. Some tissues such as the retina, olfactory
bulb, and granulocytes have particularly high concentrations
of taurine. Many animals use taurine or glycine as a
conjugate for the bile acids. Some of the taurine excreted in
the bile is returned to the liver in the enterohepatic circulation
(MacDonald et al., 1984). Most animals can synthesize
adequate amounts of taurine from the oxidation of cysteine;
however, some animals, particularly domesticated and
wild felids, and human infants do not synthesize adequate
amounts of taurine. Dogs normally synthesize adequate
amounts of taurine; however, when they are given low-sulfur
amino acid diets, taurine may become limiting. Taurine deficiency
occurs in cats when the diet does not contain adequate
amounts of the amino acid. Defective synthesis in cats
is a result of low activity of two enzymes in the synthetic
pathway: cysteine dioxygenase and cysteine sulfinic acid
decarboxylase and an obligatory requirement for taurine to
conjugate bile acids. A wide array of clinical signs has been
described in taurine-deficient cats including central retinal
degeneration, reversible dilated cardiomyopathy, reproductive
failure in queens, teratogenic defects, and abnormal
brain development in kittens (MacDonald et al., 1984).
Dietary concentrations of taurine required to maintain
adequate levels in plasma and whole blood of cats are a
function of type of diet, which affects the degree of microbial
degradation that occurs in the enterohepatic circulation.
For most expanded diets 1g taurine/kg is adequate,
but canned diets may require concentrations up to 2.5 g
taurine/kg dry matter.
Plasma and whole blood concentrations of 40 and
300 μ M of taurine appear to be adequate in cats for reproduction,
which is the most demanding physiological state
for taurine (MacDonald et al., 1984).
B . Other Vitamin-Like Compounds
The following compounds are highlighted because of their
known role as coenzymes in prokaryotes and potential role
as probiotics (growth- promoting substances) in higher animals.
These compounds include queuosine coenzyme Q,
pteridines (other than folic acid), such as biopterin and the
pteridine cofactor for the Mo-Fe flavoproteins, lipoic acid,
and pyrroloquinoline quinone (PQQ).
1 . Queuosine
Queuosine is included because it represents a known and novel
product arising from a microbe-host interaction. Queuine is
the nucleoside base, which is modified to queuosine.
Queuosine resembles guanidine and is preferably utilized
in some t-RNAs. The importance of this interaction has yet
to be fully understood. Germ-free animals survive without
a source of queuine or queuosine ( Farkas, 1980 ).
2 . Coenzyme Q
Although claims have been made for a nutrition requirement
for coenzyme Q, more work is needed to fully clarify
a true nutritional role for this compound. Ubiquinone
or coenzyme Q is found in mitochondria. Coenzyme Q
is structurally similar to vitamins E and K. As a quinone,
coenzyme Q is ideally suited to interact with cytochromes
to affect the flow of electrons in the mitochondrial respiratory
chain. Coenzyme Q can be synthesized and is easily
absorbed from the intestine by the same route as other fatsoluble
vitamins. However, there is no known requirement
for coenzyme Q in higher animals.
Of the lipophilic substances with redox cycling capacity,
the ubiquinones (coenzyme Q) are a group of ubiquitous
2,3-dimethoxy-5-methyl benzoquinones substituted at the
position 6 with terpenoid chains of varying lengths. In mitochondria,
coenzyme Q causes two electron processes and
helps initiate two single electron transfers through semiquinone
intermediates. Coenzyme Q is found mainly in the
mitochondrial intermembrane. Although there is no apparent
dietary requirement, coenzyme Q is present in food and
promoted for various putative health benefits. Coenzyme
Q that is absorbed from the intestine is transported by the
same transport system as vitamin E and vitamin K.
3 . Pteridines
In animals, tetrahydrobiopterin (commonly abbreviated
BH 4 ) is an important redox cofactor, best known for its
role at the catalytic site for phenylalanine and tyrosine
hydroxylases. Tetrahydrobiopterin is made in sufficient
quantities from pathways related to guanine synthesis. A
related cofactor is the molybdenum cofactor, also in the
pterin family, a cofactor for xanthine oxidase and aldehyde
oxidase (important to purine metabolism) and sulfite oxidase
(important in sulfur amino acid metabolism; see the
Molybdenum section in Chapter 22 ) .
4 . Lipoic Acid
Lipoic acid (LA) is made in the liver of most animals. This
coenzyme is linked by amide linkage to lysyl residues
within transacetylases ( Fuchs and Zimmer, 1997 ). Lipoyl
moieties functions in the transfer of electrons and activated
acyl groups from the thiazole-moiety of thiamin pyrophosphate
to CoASH. In this process, the disulfide bond is broken
and dihydrolipoyl transiently generated. Reoxidation
is required to reinitiate this cycle. Although most reactions
in biological systems may be described as nucleophilic in