Cobalamin (vitamin B12) is a cyclic tetrapyrrol that contains a corrin ring with a cobalt atom in the center. Cobalamin is
actually made up of a group of compounds and is exclusively derived from bacterial sources. The biologically active forms
of this vitamin are methylcobalamin (required for methyl-group transfers) and adenosylcobalamin (required for adenosyl-group
transfers), but there are other molecules that belong to this group of vitamins, such as hydroxocobalamin or cyanocobalamin.
Cyanocobalamin, does not occur naturally, but is manufactured by bacterial fermentation and cyanide incorporation for treatment
of cobalamin deficiency. Cobalamin has important functions in amino acid metabolism and DNA synthesis.
Cobalamin is an essential cofactor for several enzyme systems in mammalian species. The first enzyme system, methylmalonyl-CoA
mutase, is located in the mitochondria and plays a crucial role in the transformation of propionyl-CoA to succinyl-CoA. Thus,
cobalamin plays a major role in the metabolism of several amino acids.
Cobalamin is also important in the transformation of the sulfur-containing amino acids methionine and cysteine. Homocysteine
is an intermediary amino acid that is being formed from methionine and is not found in the diet. The transformation of homocysteine
to methionine is linked to another metabolically crucial process, the generation of the biologically active tetrahydrofolate
from N5 -methyltetrahydrofolate. Simplistically, the cobalamin-dependant enzyme methionine synthase transfers a methyl group
from N5 -methyltetrahydrofolate to homocysteine, which results in tetrahydrofolate and methionine. Thus, this enzyme not only
plays a role in the transformation of sulfur-containing amino acids, but may be even more important in the generation of the
biologically active tetrahydrofolate, which is involved in the synthesis of both purines and pyrimidines.
Dietary cobalamin is tightly bound to dietary animal-derived protein. In the stomach, dietary protein is partially digested
by pepsin and HCl and cobalamin is being released. However, cobalamin immediately binds to a transporter protein called haptocorrin
or R-protein. Haptocorrin is mostly synthesized and secreted by the gastric mucosa. Haptocorrin in turn is digested by pancreatic
proteases in the small intestine. Free cobalamin binds to intrinsic factor. In humans, intrinsic factor is mostly synthesized
and secreted by the parietal cells of the gastric mucosa, but there is good evidence that in dogs and cats most of intrinsic
factor is synthesized and secreted by pancreatic acinar cells. Cobalamin/intrinsic factor complexes are being absorbed by
a complex receptor in the microvillus pits of the apical brush border membrane of the ileal enterocytes. Thus, the absorption
of cobalamin is an extremely complex system that relies on a multitude of factors and processes. As cobalamin is being absorbed
into the intestinal epithelial cells, it dissociates from intrinsic factor and free cobalamin is released into the circulation,
where most of it binds to yet another protein, transcobalamin II. The main storage compartments for cobalamin in the body
are the liver and the kidney, which maintain serum cobalamin concentrations by releasing cobalamin when needed.
Cobalamin deficiency is quite common in humans. Several causes of cobalamin deficiency have been described in humans, including
hereditary causes that are due to mutations of the genes encoding carrier proteins, dietary insufficiency, postsurgical malabsorption
after gastrectomy or ileal resections, food-cobalamin malabsorption, and idiopathic cobalamin deficiency. Dietary insufficiency
is quite uncommon as a cause of cobalamin deficiency and mainly occurs in elderly people, who are already malnourished and
consume an all vegetarian diet. Food-cobalamin malabsorption is a term that combines any cause of cobalamin deficiency that
is not due to dietary deficiency, such as exocrine pancreatic insufficiency (EPI), intestinal lymphoma or tuberculosis, celiac
disease or Crohn's disease.
In dogs and cats the most common causes of cobalamin deficiency are chronic and severe small intestinal disease and EPI. In
addition, hereditary cobalamin deficiency has been described in various dog breeds. A recent study has shown that 82% of dogs
with EPI were cobalamin deficient. Similar studies in cats have shown that most, if not all, cats with EPI are cobalamin deficient.
While cobalamin deficiency can occur in humans with EPI it is certainly not as prominent of a feature as it is in dogs and
cats, which can be explained by the differences in cobalamin absorption between species. As discussed above, intrinsic factor
in humans is mostly supplied through the gastric mucosa, while in dogs and cats it is mostly supplied by pancreatic acinar
cells. The lack of pancreatic proteases and the alteration of the small intestinal microbiota may also play a role, but appear
to be less important than the lack of intrinsic factor.