Transcobalamin metabolism

Metabolism of cobalamins in mammalian cells. Note the compartmentalization of the synthesis of the two coenzymes adenosylcobalamin (AdoCbl) synthesis occurs in mitochondria, whereas that of methylcobalamin (MeCbl) occurs in cytoplasm. TCII, Transcobalamin II OH-Cbl, hydroxocobalamin T, transport protein for TCII-OHCbl complex FH4, MeFH4, tetrahydrofolate and methyltetrahydrofolate, respectively Cbl, a cobalamin in which the ligand occupying the sixth coordination position of the cobalt is not known. The numerical superscripts adjacent to some of the cobalamins indicate the oxidation state of the cobalt ion. Cobalamins in the +1, +2, and +3 oxidation states are also known as Bn, B 2, and Bi2j, respectively. 

FIGURE 53-6 Interrelationships and metabolic roles of vitamin and folic acid. See text for explanation and Figure 53-9 for structures of the various folate coenzymes. FIGLU, formiminoglutamic acid, which arises from die catabolism of histidine Tell, transcobalamin II CH3H4PteGlUj, mediyltetrahydrofolate. 

Vitamin B12 is synthesized in large quantities by the intestinal flora, particularly in ruminants. The exact amount of vitamin B12 required by the normal human is not known. The absorption of vitamin B12 from the gastrointestinal tract is dependent on the presence of a gastric mucoprotein called intrinsic factor. Calcium ions seem to be necessary for the interaction of vitamin B12 with this intrinsic factor. Vitamin B12, which is absorbed only in the ileum, is stored in the liver. There are two transport proteins for vitamin Bj2 transcobalamin I and II, the latter being physiologically more important. Vitamin B12 plays an important role in the metabolism of functional groups with one carbon atom such as the methyl group... 

Orning, L., Rian, A., Campbell, A., Brady, J., Fedosov, S.N., Bramlage, B., Thompson, K., and Quadros, E.V., 2006. Characterization of a monoclonal antibody with specificity for holo-transcobalamin. Nutrition Metabolism. 3 3. 

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