Vitamin B12 (cobalamin) serves as a cofactor for several essential biochemical reactions in humans. Deficiency of vitamin B12 leads to megaloblastic anemia (Table 33–2), gastrointestinal symptoms, and neurologic abnormalities. Although deficiency of vitamin B12 due to an inadequate supply in the diet is unusual, deficiency of B12 in adults—especially older adults—due to inadequate absorptionof dietary vitamin B12 is a relatively common and easily treated disorder.
Vitamin B12 consists of a porphyrin-like ring with a central cobalt atom attached to a nucleotide. Various organic groups may be covalently bound to the cobalt atom, forming different cobalamins. Deoxyadenosylcobalamin and methylcobalamin are the active forms of the vitamin in humans. Cyanocobalamin and hydroxo-cobalamin (both available for therapeutic use) and other cobala-mins found in food sources are converted to the active forms. The ultimate source of vitamin B12 is from microbial synthesis; the vitamin is not synthesized by animals or plants. The chief dietary source of vitamin B12 is microbially derived vitamin B12 in meat (especially liver), eggs, and dairy products. Vitamin B12 is some-times called extrinsic factor to differentiate it from intrinsicfactor, a protein secreted by the stomach that is required forgastrointestinal uptake of dietary vitamin B12.
The average American diet contains 5–30 mcg of vitamin B12 daily, 1–5 mcg of which is usually absorbed. The vitamin is avidly stored, primarily in the liver, with an average adult having a total vitamin B12 storage pool of 3000–5000 mcg. Only trace amounts of vitamin B12 are normally lost in urine and stool. Because the normal daily requirements of vitamin B12 are only about 2 mcg, it would take about 5 years for all of the stored vitamin B12 to be exhausted and for megaloblastic anemia to develop if B12 absorp-tion were stopped. Vitamin B12 is absorbed after it complexes with intrinsic factor, a glycoprotein secreted by the parietal cells of the gastric mucosa. Intrinsic factor combines with the vitamin B12 that is liberated from dietary sources in the stomach and duode-num, and the intrinsic factor-vitamin B12 complex is subsequently absorbed in the distal ileum by a highly selective receptor-mediated transport system. Vitamin B12 deficiency in humans most often results from malabsorption of vitamin B12 due either to lack of intrinsic factor or to loss or malfunction of the absorp-tive mechanism in the distal ileum. Nutritional deficiency is rare but may be seen in strict vegetarians after many years without meat, eggs, or dairy products.
Once absorbed, vitamin B12 is transported to the various cells of the body bound to a family of specialized glycoproteins, transcoba-lamin I, II, and III. Excess vitamin B12 is stored in the liver.
Two essential enzymatic reactions in humans require vitamin B12 (Figure 33–2). In one, methylcobalamin serves as an intermediate in the transfer of a methyl group from N5-methyltetrahydrofolate to homocysteine, forming methionine (Figure 33–2A; Figure 33–3). Without vitamin B12, conversion of the major dietary and storage folate—N5-methyltetrahydrofolate—to tetrahydrofolate, the precursor of folate cofactors, cannot occur. As a result, vitamin B12 deficiency leads to deficiency of folate cofactors necessary for several biochemical reactions involving the transfer of one-carbon
groups. In particular, the depletion of tetrahydrofolate prevents synthesis of adequate supplies of the deoxythymidylate (dTMP) and purines required for DNA synthesis in rapidly dividing cells, as shown in Figure 33–3. The accumulation of folate as N 5-methyltetrahydrofolate and the associated depletion of tetra-hydrofolate cofactors in vitamin B12 deficiency have been referred to as the “methylfolate trap.” This is the biochemical step whereby vitamin B12 and folic acid metabolism are linked, and it explains why the megaloblastic anemia of vitamin B12 deficiency can be partially corrected by ingestion of large amounts of folic acid. Folic acid can be reduced to dihydrofolate by the enzyme dihydro-folate reductase (Figure 33–3, section 3) and thereby serve as a source of the tetrahydrofolate required for synthesis of the purines and dTMP required for DNA synthesis.
Vitamin B12 deficiency causes the accumulation of homo-cysteine due to reduced formation of methylcobalamin, which is required for the conversion of homocysteine to methionine (Figure 33–3, section 1). The increase in serum homocysteine can be used to help establish a diagnosis of vitamin B12 deficiency (Table 33–2).
There is evidence from observational studies that elevated serum homocysteine increases the risk of atherosclerotic cardiovascular disease. However, randomized clinical trials have not shown a definitive reduction in cardiovascular events (myocardial infarction, stroke) in patients receiving vitamin supplementation that lowers serum homocysteine.
The other reaction that requires vitamin B12 is isomerization of methylmalonyl-CoA to succinyl-CoA by the enzyme methylmalonyl-CoA mutase (Figure 33–2B). In vitamin B12 deficiency, this con-version cannot take place and the substrate, methylmalonyl-CoA, as well as methylmalonic acid accumulate. The increase in serum and urine concentrations of methylmalonic acid can be used to support a diagnosis of vitamin B12 deficiency (Table 33–2). In the past, it was thought that abnormal accumulation of methylmalonyl-CoA causes the neurologic manifestations of vitamin B12 defi-ciency. However, newer evidence instead implicates the disruption of the methionine synthesis pathway as the cause of neurologic problems. Whatever the biochemical explanation for neurologic damage, the important point is that administration of folic acid in the setting of vitamin B12 deficiency will not prevent neurologic manifestations even though it will largely correct the anemia caused by the vitamin B12 deficiency.
Vitamin B12 is used to treat or prevent deficiency. The most char-acteristic clinical manifestation of vitamin B12 deficiency is mega-loblastic, macrocytic anemia (Table 33–2), often with associated mild or moderate leukopenia or thrombocytopenia (or both), and a characteristic hypercellular bone marrow with an accumulation of megaloblastic erythroid and other precursor cells. The neuro-logic syndrome associated with vitamin B12 deficiency usually begins with paresthesias in peripheral nerves and weakness and progresses to spasticity, ataxia, and other central nervous system dysfunctions. Correction of vitamin B12 deficiency arrests the progression of neurologic disease, but it may not fully reverse neurologic symptoms that have been present for several months. Although most patients with neurologic abnormalities caused by vitamin B12 deficiency have megaloblastic anemia when first seen, occasional patients have few if any hematologic abnormalities.
Once a diagnosis of megaloblastic anemia is made, it must be determined whether vitamin B12 or folic acid deficiency is the cause. (Other causes of megaloblastic anemia are very rare.) This can usually be accomplished by measuring serum levels of the vitamins. The Schilling test, which measures absorption and uri-nary excretion of radioactively labeled vitamin B12, can be used to further define the mechanism of vitamin B12 malabsorption when this is found to be the cause of the megaloblastic anemia.
The most common causes of vitamin B12 deficiency are perni-cious anemia, partial or total gastrectomy, and conditions that affect the distal ileum, such as malabsorption syndromes, inflammatory bowel disease, or small bowel resection.
Pernicious anemia results from defective secretion of intrinsicfactor by the gastric mucosal cells. Patients with pernicious anemia have gastric atrophy and fail to secrete intrinsic factor (as well as hydrochloric acid). The Schilling test shows diminished absorp-tion of radioactively labeled vitamin B12, which is corrected when intrinsic factor is administered with radioactive B12, since the vitamin can then be normally absorbed.
Vitamin B12 deficiency also occurs when the region of the dis-tal ileum that absorbs the vitamin B12-intrinsic factor complex is damaged, as when the ileum is involved with inflammatory bowel disease or when the ileum is surgically resected. In these situations, radioactively labeled vitamin B12 is not absorbed in the Schilling test, even when intrinsic factor is added. Rare cases of vitamin B12 deficiency in children have been found to be secondary to con-genital deficiency of intrinsic factor or to defects of the receptor sites for vitamin B12-intrinsic factor complex located in the distal ileum.
Almost all cases of vitamin B12 deficiency are caused by malab-sorption of the vitamin; therefore, parenteral injections of vitamin B12 are required for therapy. For patients with potentially revers-ible diseases, the underlying disease should be treated after initial treatment with parenteral vitamin B12. Most patients, however, do not have curable deficiency syndromes and require lifelong treat-ment with vitamin B12.
Vitamin B12 for parenteral injection is available as cyanocoba-lamin or hydroxocobalamin. Hydroxocobalamin is preferred because it is more highly protein-bound and therefore remains longer in the circulation. Initial therapy should consist of 100– 1000 mcg of vitamin B12 intramuscularly daily or every other day for 1–2 weeks to replenish body stores. Maintenance therapy con-sists of 100–1000 mcg intramuscularly once a month for life. If neurologic abnormalities are present, maintenance therapy injec-tions should be given every 1–2 weeks for 6 months before switch-ing to monthly injections. Oral vitamin B12-intrinsic factor mixtures and liver extracts should not be used to treat vitamin B12 deficiency; however, oral doses of 1000 mcg of vitamin B12 daily are usually sufficient to treat patients with pernicious anemia who refuse or cannot tolerate the injections. After pernicious anemia is in remission following parenteral vitamin B12 therapy, the vitamin can be administered intranasally as a spray or gel.