CLINICAL PHARMACOLOGY
Vitamin B12 is essential to growth, cell reproduction,
hematopoiesis, and nucleoprotein and myelin synthesis.
Cyanocobalamin is quantitatively and rapidly absorbed
from intramuscular and subcutaneous sites of injection;
the plasma level of the compound reaches its peak within
1 hour after intramuscular injection. Absorbed vitamin
B12 is transported via specific B12 binding proteins,
transcobalamin I and II to the various tissues. The liver
is the main organ for vitamin B12 storage.
Within 48 hours after injection of 100 or 1000 mcg of
vitamin B12, 50 to 98% of the injected dose may appear
in the urine. The major portion is excreted within the
first eight hours. Intravenous administration results
in even more rapid excretion with little opportunity for
liver storage.
Gastrointestinal absorption of vitamin B12 depends on
the presence of sufficient intrinsic factor and calcium
ions. Intrinsic factor deficiency causes pernicious anemia,
which may be associated with subacute combined degeneration
of the spinal cord. Prompt parenteral administration of
vitamin B 12 prevents progression of neurologic damage.
The average diet supplies about 5 to 15 mcg/day of vitamin
B12 in a protein-bound form that is available for absorption
after normal digestion. Vitamin B12 is not present in
foods of plant origin, but is abundant in foods of animal
origin. In people with normal absorption, deficiencies
have been reported only in strict vegetarians who consume
no products of animal origin (including no milk products
or eggs). Vitamin B12 is bound to intrinsic factor during
transit through the stomach; separation occurs in the
terminal ileum in the presence of calcium, and vitamin
B12 enters the mucosal cell for absorption. It is then
transported by the transcobalamin binding proteins. A
small amount (approximately 1% of the total amount ingested)
is absorbed by simple diffusion, but this mechanism is
adequate only with very large doses. Oral absorption is
considered too undependable to rely on in patients with
pernicious anemia or other conditions resulting in malabsorption
of vitamin B12.
Cyanocobalamin is the most widely used form of vitamin
B12, and has hematopoietic activity apparently identical
to that of the antianemia factor in purified liver extract.
Hydroxycobalamin is equally as effective as cyanocobalamin,
and they share the cobalamin molecular structure.
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