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chapter38

Vitamin Metabolism

F IG U R E 3 8 -1 8

Absorption, transport, and cellular uptake of vitamin B

12

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

[2

and TCII/B

12

,

respectively.

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

a

] -globulin in standard serum protein

electrophoresis, while TCIII moves as an a

2

-globulin

owing to its lower sialic acid content. Plasma TCIII