A Review of Vitamin B12 in Dermatology

Jennifer Brescoll; Steven Daveluy

Disclosures

Am J Clin Dermatol. 2015;16(1):27-33. 

In This Article

Biochemistry of Cobalamin

Vitamin B12 exists in many forms in the body, two of which are biologically active coenzymes: methylcobalamin and adenosylcobalamin. Methylcobalamin is a coenzyme with methionine synthase, a key enzyme in the folic aciddependent synthesis of pyrimidines and purines. Adenosylcobalamin is involved in the enzymatic degradation of fatty acids by methylmalonyl CoA mutase. These enzymes are needed for normal function of bone marrow and the central nervous system.[1]

Cobalamin has a complex uptake and metabolism that requires several coenzymes (Fig. 1). When cobalamin is ingested from food sources, it is first released from the proteins in food by pepsin in the stomach. It then binds to haptocorrin, which is found in saliva and protects cobalamin from the acidic environment of the stomach. Haptocorrin is then degraded in the duodenum by digestive proteases and the free cobalamin binds to intrinsic factor. Intrinsic factor is needed for cobalamin to be absorbed by receptors on the enterocytes in the terminal ileum and to protect it from catabolism by intestinal bacteria. Cobalamin is then transported into the portal circulation and taken up by the liver. The majority of cobalamin in circulation is bound to haptocorrin, with a smaller percentage bound to transcobalamin II, and a negligible amount of circulating free cobalamin.[2] It is taken up into tissues, mainly the liver, via transcobalamin receptors, which are located on endothelial cells and not hepatocytes themselves. The liver stores enough cobalamin to last several years before symptoms of deficiency present.[3]

Figure 1.

Cobalamin's complex metabolism: after cobalamin is ingested and goes to the stomach, it is released from proteins in food by pepsin and then binds to haptocorrin, which protects it from the acidic stomach environment. In the duodenum, haptocorrin is degraded by digestive proteases, and free cobalamin binds to intrinsic factor, which helps the absorption of cobalamin by receptors on enterocytes in the terminal ileum. It is then taken up into the portal circulation and absorbed by the liver via transcobalamin receptors on endothelial cells. C cobalamin, H haptocorrin, IF intrinsic factor, TCII transcobalamin

Due to the complexity of the metabolism and absorption of cobalamin, there are many opportunities for defects to occur, which can result in deficiency or imbalance.

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