F IG U R E 3 8 -1 8
Absorption, transport, and cellular uptake of vitamin B
in humans. IF, Intrinsic factor; TCII, transcobalamin II; circles
in the membranes of the ileal mucosal cell and peripheral tissues represent transport molecules for IF/B
of carbohydrate. Their isoelectric points range from
2.3 to 5.0 owing to the presence of 6-18 sialic acid
residues per molecule. Because of the low pi values,
these proteins have high anodic mobility during
electrophoresis at alkaline pH; hence, their
designation as “R” (for rapidly migrating) proteins.
Cobalophilins bind one cobalamin per molecule.
They also bind corrinoids that have modifications in
the nucleotide or corrinoid regions of the molecule
and so may be important for excretion of inactive
forms of the vitamin via bile.
The primary structures of several cobalophilins are
similar. A single cobalophilin gene is suggested by
the discovery of a family with congenital absence of
cobalophilin from cerebrospinal fluid, gastric juice,
granulocytes, saliva, and serum.
The serum cobalophilins, transcobalamin I and III
(TCI and TCIII), are immunologically identical and
may differ only in their carbohydrate content. TCI
moves as an
] -globulin in standard serum protein
electrophoresis, while TCIII moves as an a
owing to its lower sialic acid content. Plasma TCIII