| Cobalamin is more commonly known
as vitamin B12. Vitamin B12 is composed of a complex tetrapyrrol
ring structure (corrin ring) and a cobalt ion in the center. Vitamin
B12 is synthesized exclusively by microorganisms and is found
in the liver of animals bound to protein as methycobalamin or
5'-deoxyadenosylcobalamin. The vitamin must be hydrolyzed from
protein in order to be active. Hydrolysis occurs in the stomach
by gastric acids or the intestines by trypsin digestion following
consumption of animal meat. The vitamin is then bound by intrinsic
factor, a protein secreted by parietal cells of the stomach, and
carried to the ileum where it is absorbed. Following absorption
the vitamin is transported to the liver in the blood bound to
transcobalamin II.
There are only two clinically significant reactions
in the body that require vitamin B12 as a cofactor. During the
catabolism of fatty acids with an odd number of carbon atoms and
the amino acids valine, isoleucine and threonine the resultant
propionyl-CoA is converted to succinyl-CoA for oxidation in the
TCA cycle. One of the enzymes in this pathway, methylmalonyl-CoA
mutase, requires vitamin B12 as a cofactor in the conversion of
methylmalonyl-CoA to succinyl-CoA. The 5'-deoxyadenosine derivative
of cobalamin is required for this reaction.
The second reaction requiring vitamin B12 catalyzes
the conversion of homocysteine to methionine and is catalyzed
by methionine synthase. This reaction results in the transfer
of the methyl group from N5-methyltetrahydrofolate to hydroxycobalamin
generating tetrahydrofolate (THF) and methylcobalamin during the
process of the conversion.
Clinical Significances of B-12 Deficiency
The liver can store up to six years worth of vitamin
B12, hence deficiencies in this vitamin are rare. Pernicious anemia
is a megaloblastic anemia resulting from vitamin B12 deficiency
that develops as a result a lack of intrinsic factor in the stomach
leading to malabsorption of the vitamin. The anemia results from
impaired DNA synthesis due to a block in purine and thymidine
biosynthesis. The block in nucleotide biosynthesis is a consequence
of the effect of vitamin B12 on folate metabolism. When vitamin
B12 is deficient essentially all of the folate becomes trapped
as the N5-methylTHF derivative as a result of the loss of functional
methionine synthase. This trapping prevents the synthesis of other
THF derivatives required for the purine and thymidine nucleotide
biosynthesis pathways.
Neurological complications also are associated with
vitamin B12 deficiency and result from a progressive demyelination
of nerve cells. The demyelination is thought to result from the
increase in methylmalonyl-CoA that result from vitamin B12 deficiency.
Methylmalonyl-CoA is a competitive inhibitor of malonyl-CoA in
fatty acid biosynthesis as well as being able to substitute for
malonyl-CoA in any fatty acid biosynthesis that may occur. Since
the myelin sheath is in continual flux the methylmalonyl-CoA-induced
inhibition of fatty acid synthesis results in the eventual destruction
of the sheath. The incorporation methylmalonyl-CoA into fatty
acid biosynthesis results in branched-chain fatty acids being
produced that may severely alter the architecture of the normal
membrane structure of nerve cells |