This was because hydroxocobalamin produced by bacteria was changed to cyanocobalamin during purification in activated charcoal columns after separation from the bacterial cultures (because cyanide is naturally present in activated charcoal). Cyanocobalamin is the form in most pharmaceutical preparations because adding cyanide stabilizes the molecule.The total world production of vitamin B12, by four companies (the French Sanofi-Aventis and three Chinese companies) in 2008 was 35 tonnes. Metabolism
The two bioactive forms of vitamin B12 are methylcobalamin in cytosol and adenosylcobalamin in mitochondria. Multivitamins often contain cyanocobalamin, which is presumably converted to bioactive forms in the body. Both methylcobalamin and adenosylcobalamin are commercially available as supplement pills. The MMACHC gene product catalyzes the decyanation of cyanocobalamin as well as the dealkylation of alkylcobalamins including methylcobalamin and adenosylcobalamin. This function has also been attributed to cobalamin reductases. The MMACHC gene product and cobalamin reductases enable the interconversion of cyano- and alkylcobalamins.Cyanocobalamin is added to fortify nutrition, including baby milk powder, breakfast cereals and energy drinks for humans, also animal feed for poultry, swine and fish. Vitamin B12 becomes inactive due to hydrogen cyanide and nitric oxide in cigarette smoke. Vitamin B12 also becomes inactive due to nitrous oxide N2O commonly known as laughing gas, used for anaesthesia and as a recreational drug. Vitamin B12 becomes inactive due to microwaving or other forms of heating. In the cytosol
Methylcobalamin and 5-methyltetrahydrofolate are needed by methionine synthase in the methionine cycle to transfer a methyl group from 5-methyltetrahydrofolate to homocysteine, thereby generating tetrahydrofolate (THF) and methionine, which is used to make SAMe.
SAMe is the universal methyl donor and is used for DNA methylation and to make phospholipid membranes, choline, sphingomyelin, acetylcholine, and other neurotransmitters. In mitochondria
The enzymes that use B12 as a built-in cofactor are methylmalonyl-CoA mutase (PDB 4REQ) and methionine synthase (PDB 1Q8J).The metabolism of propionyl-CoA occurs in the mitochondria and requires Vitamin B12 (as adenosylcobalamin) to make succinyl-CoA. When the conversion of propionyl-CoA to succinyl-CoA in the mitochondria fails due to Vitamin B12 deficiency, elevated blood levels of methylmalonic acid (MMA) occur. Thus, elevated blood levels of homocysteine and MMA may both be indicators of vitamin B12 deficiency. Adenosylcobalamin is needed as cofactor in methylmalonyl-CoA mutase—MUT enzyme. Processing of cholesterol and protein gives propionyl-CoA that is converted to methylmalonyl-CoA, which is used by MUT enzyme to make succinyl-CoA. Vitamin B12 is needed to prevent anemia, since making porphyrin and heme in mitochondria for producing hemoglobin in red blood cells depends on succinyl-CoA made by vitamin B12. Absorption and transport
Inadequate absorption of vitamin B12 may be related to coeliac disease. Intestinal absorption of vitamin B12 requires successively three different protein molecules: haptocorrin, intrinsic factor and transcobalamin II. See also
Methylcobalamin
Hydroxocobalamin
Adenosylcobalamin
Cobalamin biosynthesis

References
External links
"Cyanocobalamin". Drug Information Portal. U.S. National Library of Medicine.