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

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Chapter | 23 Vitamins
occur in the various steps of food processing and preservation.
Canning, with its elevated temperatures and often
alkaline conditions, can result in very low recoveries of
thiamin. Thiamin can also be destroyed enzymatically by
thiaminases, which are abundant in the flesh of some fish,
particularly spoiled fish, and bacteria associated with fermentation
processes. Thiamin deficiency has been observed
in fish-eating birds, seals, dolphins, even other fish ( Ceh
et al., 1964 ; Cowey et al. , 1975 ; Evans, 1975 ; Geraci, 1972,
1974 ; Murai and Andrews, 1978 ; Rigdon and Drager, 1955 ;
Vimokesant et al. , 1982 ; White, 1970 ), when spoiled or
uncooked fish has been routinely fed. Thiaminase activity
is strikingly high, particularly in tuna and sardines (i.e., the
destruction of mg quantities of thiamin per hour per gram
of fish muscle). Thiamin deficiency has also been reported
in foxes fed uncooked fish products and in cats given both
fresh fish and canned cat food that has suffered excessive
processing losses. Naturally occurring thiamin deficiency
has also been reported in cats given diets that have been
preserved by sodium metabisulfite that degrades the thiamin
in the diet ( Donoghue and Langenberg, 1994 ). A novel
case of thiamin deficiency in fish has reported for which
consumption of shad was proposed as the mechanism.
Shad contain high concentrations of thiaminase, which
was inferred to be the mechanism. So-called early mortality
syndrome is a noninfectious disease affecting lake trout
and other salmonids associated with thiamin deficiency. It
is characterized by loss of equilibrium, hyperexcitability,
anorexia, and eventually death.
In herbivores, thiamin deficiency can occur from the
ingestion of bracken fern ( Pteridium aquillnimum ) or nardoo
( Marsilea drumen ). In both herbivores and monogastric
animals, the most predominant characteristic of thiamin
deficiency is polioencephalomalacia, primarily of the deep
cordial gray matter, periventricular gray matter, and alteration
in the vestibular and lateral geniculate nuclei (Frye
et al., 1991). A relationship between excessive production
of hydrogen sulfide in the rumen of cattle and sheep and
polioencephalomalacia has recently been demonstrated.
The availability of thiamin in foodstuffs is comprised of
high levels of tannins. As a general requirement, animals
should receive from 4 to 10 mg of thiamin per kilogram
of dry food ( Committee on Animal Nutrition, 2001a ;
Donoghue and Langenberg, 1994 ).
d . Determination of Thiamin Status
Traditionally the erythrocyte transketolase saturation test,
which is a measure of the stimulation of the transketolase
reaction, has been used to assess thiamin status. A stimulation
of greater than 16% has been taken as a thiamin
deficiency. A more sensitive test, however, is the measurement
of thiamin-phosphorylated esters in plasma; the level
of phosphate esters declines in plasma before any change
occurs in erythrocyte transketolase values. A thiamin loading
test, which measures the urinary excretion of thiamin
following an oral dose of thiamin, has also been used.
However, this test lacks sensitivity compared to measuring
metabolites in plasma.
2 . Pyridoxine
a . Introduction
Vitamin B 6 is a collective term for pyridoxine, pyridoxal,
and pyridoxamine ( Fig. 23-18 ). Pyridoxine is most
abundant in plants, and pyridoxal and pyridoxamine are
most abundant in animal tissues ( Coburn, 1996 ). Each
can be converted to the other. The active form pyridoxal
(B 6 cofactor form) is phosphorylated. When pyridoxal-
5-phosphate is in excess, it is converted to pyridoxic acid
(-CHO → -COOH), which is then excreted.
Vitamin B 6 is essential in reactions important to amino
acid metabolism and glycogen hydrolysis. The major types
of reactions involving amino acids fall into three general
categories. The most common of these is the transaminase
reaction. Transaminations are essential to the interconversion
of amino acids to corresponding W A α-keto acids.
The transamination mechanism also applies for reactions
important to producing racemic amino acid mixtures,
for example, the conversion of L-alanine to D-alanine,
and α ,β-additions or elimination reactions. Examples of
α ,β-elimination reactions are the conversion of serine to
pyruvic acid and the conversion of homocysteine plus serine
to cystathionine. The basic feature of a transaminationtype
mechanism involves electron withdrawal from the
α -carbon resulting in a proton liberation that sets the stage
for substitution and additions reactions ( Fig. 23-18 ).
The second most common reaction involves electron
withdrawal from the α -carbon and carboxylic acid group
carbon. This facilitates decarboxylation. Examples of
decarboxylation reactions include the conversion of tyrosine
to tyramine, 5-hydroxytryptophan to serotonin, histidine
to histamine, and glutamate to gamma-aminobutyric
acid (GABA). The convulsions associated with vitamin
B 6 deficiency are attributed to insufficient activity of PLPdependent
L-glutamate decarboxylase leading to a deficit
of the inhibitory neurotransmitter GABA.
A third type of reaction involves electron withdrawal
from the α , β -carbons of amino acids. This sets the stage
for hydride condensations or aldol reactions. A good
example of an aldol reaction is the conversion of serine to
glycine with the transfer of the β -carbon (as formaldehyde)
to another vitamin cofactor, tetrahydrofolic acid. An excellent
example of a hydride condensation is the formation of
α -aminolevulinic acid, the first step in heme biosynthesis
( Bender, 1994 ; Coburn, 1996 ).
Regarding glycogen, vitamin B 6 (as pyridoxal 5 -phosphate)
is a cofactor for glycogen phosphorylase ( Helmreich, 1992 ).
Glycogen phosphorylase catalyzes the hydrolysis of ether
bonds in glycogen to form 6-phosphoglucose. Ether bonds
are best catalyzed through acid-mediated mechanisms.