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

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Chapter | 23 Vitamins
phylloquinones are transported to the liver by chylomicrons
and intestinal VLDL particles and from liver by VLDL and
LDL. From studies of vitamin K clearance, it was appreciated
that the total pool of vitamin K in the body is replaced
rapidly, within hours to days in contrast to the slower turnover
of the other fat-soluble vitamins (weeks to months).
The mechanism of action for vitamin K became much
clearer after it was demonstrated that the formation of
γ -carboxyglutamic acid residues (GLA) in thrombin and
other proteinases associated with the blood-clotting cascade
was vitamin K dependent. The formation of GLA residues
is a key in that they serve as calcium-binding sites in the
proforms of proteinases associated with blood coagulation.
Calcium binding is a requisite for their eventual activation.
In this regard, vitamin K serves as cofactor for microsomal
carboxylases, which are responsible for GLA formation. The
vitamin K-dependent carboxylase utilizes oxygen and bicarbonate
as substrates. The reaction only occurs if glutamic
acid is a part of a polypeptide with the correct sequence for
specificity. Only the reduced form of vitamin K serves as a
cofactor, which led to an appreciation that a reductase system
was necessary for vitamin K regeneration and that one of the
intermediate forms was a vitamin K epoxide. As this pathway
was resolved, it next was apparent that many of the vitamin K
antagonists functioned as inhibitors of reductases important
for vitamin K generation ( Suttie, 2007 ). The rate of carboxylation
is mainly controlled by the level of reduced vitamin K
available for the reactions, whereas the dissociation rate constant
depends on both the propeptide and the Gla domain
of the substrate. In addition, there are allosteric effects that
increase the rate of dissociation of the fully carboxylated
substrates. Carboxylation requires the abstraction of a proton
from the 4-carbon of glutamate by reduced vitamin K and
results in the conversion of vitamin K to vitamin K epoxide.
The vitamin K epoxide must be recycled to vitamin K before
it can be reused, a reaction catalyzed by the enzyme vitamin
K epoxide reductase.
Specifically, vitamin K provides important control of
blood coagulation by regulating the activities of factor
VIIIa (FVIIIa) and factor Va (FVa), cofactors in the activation
of factor X and prothrombin, respectively. The system
comprises membrane-bound and circulating proteins that
assemble into multimolecular complexes on cell surfaces.
Vitamin K-dependent protein C, the key component of the
system, circulates in blood as zymogen to an anticoagulant
serine protease. It is activated on the surface of endothelial
cells by thrombin bound to the membrane protein
thrombomodulin. An endothelial protein C receptor further
stimulates the protein C activation. Moreover, activated
protein C together with another protein, cofactor protein S,
can also slow coagulation by degrading FVIIIa and FVa on
the surface of negatively charged phospholipid membranes
providing a level of reversible control ( Suttie, 2007 ).
GLA residues are also found in bone proteins. The
GLA-containing proteins in bone (osteocalcins) appear
to be involved in the regulation of new bone growth and
formation. The presence of GLA protein in bone helps to
explain why administration of the vitamin K antagonist at
levels that cause hemorrhagic diseases also may result in
bone defects, particularly in neonates. The mineralization
disorders are characterized by complete fusion of the proximal
tibia growth plate and cessation of longitudinal bone
growth ( Suttie, 2007 ).
3 . Nutritional Requirements
The establishment of the dietary requirement for many animals
has been difficult, in part because of (1) the short halflife
of vitamin K, (2) the fact that large amounts of vitamin K
may be synthesized by intestinal bacteria, and (3) the extent
to which different animal species practice coprophagy. Birds
tend to have relatively high requirements for vitamin K;
thus, chickens are often used as experimental animals in
vitamin K studies ( Stafford, 2005 ; Suttie, 2007 ). Recent
work suggests that the vitamin K requirement depends on
the relative content of vitamin K epoxide reductase activity.
A low level of epoxide reductase activity can increase the
requirement for vitamin K. Ruminal microorganisms synthesize
large amounts of vitamin K; thus, ruminants do not
need an external source for this reason.
Assessments of nutritional requirements suggest that
small animals should obtain approximately 500 to 1000 μ g
of phylloquinone per kilogram diet. Oxidized squalene and
high intakes of vitamin E may act as vitamin K antagonists.
Insufficient vitamin K can also occur with antibiotic
treatment, treatment with coccidiostatic drugs, or long-term
parenteral hyperalimentation without vitamin K supplements.
Poultry and swine diets are regularly supplemented
with menadione, but the need to supplement the diet of
other species is questionable. Few hazards have been
attributed to long-term ingestion of vitamin K in amounts
of 1 to 10 mg per kilogram diet of phylloquinone. However,
menadione in amounts corresponding to 10 to 100mg per
kilogram of diet may act as a prooxidant, and high dietary
concentrations produce hemolysis. Phylloquinone (vitamin
K 1 ) rather than menadione should be used parenterally to
treat animals that have ingested warfarin or other anticoagulants.
Menadione being water soluble, at high concentrations
it can promote hemolysis. Like many quinones it may
act as a prooxidant and initiate free-radical formation.
IV . WATER-SOLUBLE VITAMINS
We have chosen to organize the discussion of water-soluble
vitamins based on physiological function. Most vitamins
serve eventually as enzymatic cofactors. For example, niacin,
riboflavin, and ascorbic acid serve primarily as redox
cofactors. The roles of thiamin, pyridoxine (vitamin B 6 ),
and pantothenic acid (as a component of coenzyme A)