Cobalamin (Vitamin B12)

COBALAMIN (VITAMIN B₁₂)

Vitamin B₁₂ is required in humans for two essential enzymatic reactions: the remethylation of homocysteine (Hcy) to methionine and the isomerization of methylmalonyl coenzyme A (CoA), which is produced during the degradation of some amino acids (isoleucine, valine, threonine, and methionine) and fatty acids (FAs) with odd numbers of carbon atoms (Figure 28.5). When cobalamin is deficient, unusual (branched) FAs accumulate and become incorporated into cell membranes, including those of the central nervous system (CNS). This may account for some of the neurologic manifestations of vitamin B₁₂ deficiency. [Note: Folic acid (as N⁵-methyl THF) is also required in the remethylation of Hcy. Therefore, deficiency of B₁₂ or folate results in elevated Hcy levels.]

Figure 28.5 Reactions requiring coenzyme forms of vitamin B12. CoA = coenzyme A.

A. Structure of cobalamin and its coenzyme forms

Cobalamin contains a corrin ring system that resembles the porphyrin ring of heme, and but differs in that two of the pyrrole rings are linked directly rather than through a methene bridge. Cobalt is held in the center of the corrin ring by four coordination bonds with the nitrogens of the pyrrole groups. The remaining coordination bonds of the cobalt are with the nitrogen of 5,6-dimethylbenzimidazole and with cyanide in commercial preparations of the vitamin in the form of cyanocobalamin (Figure 28.6). The physiologic coenzyme forms of cobalamin are 5 - deoxyadenosylcobalamin and methylcobalamin, in which cyanide is replaced with 5-deoxyadenosine or a methyl group, respectively (see Figure 28.6).

Figure 28.6 Structure of vitamin B₁₂ (cyanocobalamin) and its coenzyme forms (methylcobalamin and 5 -deoxyadenosylcobalamin).

B. Distribution of cobalamin

Vitamin B₁₂ is synthesized only by microorganisms, and it is not present in plants. Animals obtain the vitamin preformed from their natural bacterial flora or by eating foods derived from other animals. Cobalamin is present in appreciable amounts in liver, red meat, fish, eggs, dairy products, and fortified cereals.

C. Folate trap hypothesis

The effects of cobalamin deficiency are most pronounced in rapidly dividing cells, such as the erythropoietic tissue of bone marrow and the mucosal cells of the intestine. Such tissues need both the N⁵,N¹⁰-methylene and N10-formyl forms of THF for the synthesis of nucleotides required for DNA replication. However, in vitamin B₁₂ deficiency, the utilization of the N⁵-methyl form of THF in the B₁₂-dependent methylation of homocysteine to methionine is impaired. Because the methylated form cannot be converted directly to other forms of THF, folate is trapped in the N⁵-methyl form, which accumulates. The levels of the other forms decrease. Thus, cobalamin deficiency leads to a deficiency of the THF forms needed in purine and TMP synthesis, resulting in the symptoms of megaloblastic anemia.

D. Clinical indications for vitamin B₁₂

In contrast to other water-soluble vitamins, significant amounts (2–5 mg) of vitamin B₁₂ are stored in the body. As a result, it may take several years for the clinical symptoms of B₁₂ deficiency to develop as a result of decreased intake of the vitamin. [Note: Deficiency happens much more quickly if absorption is impaired (see below).] B₁₂ deficiency can be determined by the level of methylmalonic acid in blood, which is elevated in individuals with low intake or decreased absorption of the vitamin.

Figure 28.7 Absorption of vitamin B₁₂. IF = intrinsic factor.

1. Pernicious anemia: Vitamin B₁₂ deficiency is most commonly seen in patients who fail to absorb the vitamin from the intestine. B₁₂ is released from food in the acidic environment of the stomach. [Note: Malabsorption of cobalamin in the elderly is most often due to reduced secretion of gastric acid (achlorhydria).] Free B₁₂ then binds a glycoprotein (R-protein), and the complex moves into the intestine. B₁₂ is released from the R-protein by pancreatic enzymes and binds another glycoprotein, intrinsic factor (IF). The cobalamin–IF complex travels through the intestine and binds to specific receptors on the surface of mucosal cells in the ileum. The cobalamin is transported into the mucosal cell and, subsequently, into the general circulation, where it is carried by its binding protein (transcobalamin). B₁₂ is taken up and stored in the liver, primarily. It is released into bile and efficiently reabsorbed in the ileum. Severe malabsorption of vitamin B₁₂ leads to pernicious anemia. This disease is most commonly a result of an autoimmune destruction of the gastric parietal cells that are responsible for the synthesis of IF (lack of IF prevents B₁₂ absorption). [Note: Patients who have had a partial or total gastrectomy become IF deficient and, therefore, B₁₂ deficient.] Individuals with cobalamin deficiency are usually anemic, and they show neuropsychiatric symptoms later, as the disease develops. The CNS effects are irreversible and occur by mechanisms that appear to be different from those described for megaloblastic anemia. Pernicious anemia requires life-long treatment with either high-dose oral B₁₂ or intramuscular injection of cyanocobalamin. [Note: Supplementation works even in the absence of IF because approximately 1% of B₁₂ uptake is by IF-independent diffusion.]

Folic acid supplementation can partially reverse the hematologic abnormalities of B₁₂ deficiency and, therefore, can mask a cobalamin deficiency. Thus, to prevent the CNS effects of B₁₂ deficiency, therapy for megaloblastic anemia is initiated with both vitamin B₁₂ and folic acid until the cause of the anemia can be determined.