deoxyadenosine group, a methyl group or a CN- group in the commercially
available form in vitamin tablets. Dietary cobalamin is absorbed in the Co3+
oxidation state and must be reduced by intracellular reductases to the Co+ form
for use.
Dietary cobalamin is absorbed from animal food sources by a multistage
process shown in Figure 42-2. Cobalamin absorption requires the presence of
a protein (the intrinsic factor, IF) secreted from the parietal cells of the stom-
ach to bind cobalamin and aid in its absorption in the ileum. The protein is
released into the ileum while the cobalamin is transported to the blood stream
where it binds specialized serum proteins, the transcobalamins (TC), which
transport it to other tissues such as liver where cobalamin can be stored (usu-
ally several milligrams are present in liver). In the absence of the intrinsic factor
Figure 42-2. Absorption, transport and storage of vitamin B12. IF = intrinsic
factor, a glycoprotein secreted by gastric parietal cells; TC = transcobalamins,
blood proteins that carry cobalamin to the liver.
(Reproduced, with permission,
from D.B. Marks, et al.
Basic Medical Biochemistry: A Clinical Approach.
Philadelphia: Lippincott Williams & Wilkins, 1996:619.)
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