Vitamin B12 refers to a group of compounds called cobalamins that are available in the human body in a variety of mostly interconvertible forms. Together with folate, cobalamins are essential cofactors required for DNA synthesis in cells where chromosomal replication and division are occurring—most notably the bone marrow and myeloid cells. As a cofactor, cobalamins are essential for two cellular reactions: *the mitochondrial methylmalonyl-CoA mutase conversion of methylmalonic acid (MMA) to succinate, which links lipid and carbohydrate metabolism, and *the activation of methionine synthase, which is the rate-limiting step in the synthesis of methionine from homocysteine and 5-methyltetrahydrofolate. Cobalamins are characterized by a porphyrin-like corrin nucleus that contains a single cobalt atom bound to a benzimidazolyl nucleotide and a variable residue (R) group. The variable R group gives rise to the four most commonly known cobalamins: CNCbl, methylcobalamin, 5-deoxyadenosylcobalamin, and<unk> . In the serum,<unk>  and cyanocobalamin are believed to function as storage or transport forms of the molecule, whereas methylcobalamin and 5-deoxyadenosylcobalamin are the active forms of the coenzyme required for cell growth and replication. Cyanocobalamin is usually converted to<unk>  in the serum, whereas<unk>  is converted to either methylcobalamin or 5-deoxyadenosyl cobalamin. Cobalamins circulate bound to serum proteins called transcobalamins (TC) and haptocorrins.<unk>  has a higher affinity to the TC II transport protein than cyanocobalamin, or 5-deoxyadenosylcobalamin. From a biochemical point of view, two essential enzymatic reactions require vitamin B12 (cobalamin). Intracellular vitamin B12 is maintained in two active coenzymes, methylcobalamin and 5-deoxyadenosylcobalamin. In the face of vitamin B12 deficiency, conversion of methylmalonyl-CoA to succinyl-CoA cannot take place, which results in accumulation of methylmalonyl-CoA and aberrant fatty acid synthesis. In the other enzymatic reaction, methylcobalamin supports the methionine synthase reaction, which is essential for normal metabolism of folate. The folate-cobalamin interaction is pivotal for normal synthesis of purines and pyrimidines and the transfer of the methyl group to cobalamin is essential for the adequate supply of tetrahydrofolate, the substrate for metabolic steps that require folate. In a state of vitamin B12 deficiency, the cell responds by redirecting folate metabolic pathways to supply increasing amounts of methyltetrahydrofolate. The resulting elevated concentrations of homocysteine and MMA are often found in patients with low serum vitamin B12 and can usually be lowered with successful vitamin B12 replacement therapy. However, elevated MMA and homocysteine concentrations may persist in patients with cobalamin concentrations between 200 and 350 pg/mL. Supplementation with vitamin B12 during conditions of deficiency restores the intracellular level of cobalamin and maintains a sufficient level of the two active coenzymes: methylcobalamin and deoxyadenosylcobalamin.
Hydroxocobalamin
Cyanocobalamin
Sodium thiosulfate 
Dimethyl methylphosphonate
Acetylcysteine
Fomepizole
Penicillamine
Zinc pyrithione