The two bioactive forms of vitamin are methylcobalamin in cytosol and adenosylcobalamin in mitochondria. Multivitamins often contain<unk> , 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<unk>  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.<unk>  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 becomes inactive due to hydrogen cyanide and nitric oxide in cigarette smoke. Vitamin also becomes inactive due to nitrous oxide commonly known as laughing gas, used for anaesthesia and as a recreational drug. Vitamin 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 Vitamin adenosylcobalamin in mitochondrion—cholesterol and protein metabolism The enzymes that use 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 (as adenosylcobalamin) to make succinyl-CoA. When the conversion of propionyl-CoA to succinyl-CoA in the mitochondria fails due to Vitamin deficiency, elevated blood levels of methylmalonic acid (MMA) occur. Thus, elevated blood levels of homocysteine and MMA may both be indicators of vitamin 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 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 . Absorption and transport Inadequate absorption of vitamin may be related to coeliac disease. Intestinal absorption of vitamin requires successively three different protein molecules: haptocorrin, intrinsic factor and transcobalamin II.
Cyanocobalamin
Hydroxocobalamin
Sodium thiosulfate 
Dimethyl methylphosphonate
Acetylcysteine
Selenium disulfide
Disufenton sodium
Fomepizole