Hydroxocobalamin

Hydroxocobalamin is a form of vitamin B12, a water-soluble corrinoid vitamin essential for human metabolism. It consists of a corrin ring with a central cobalt atom bound to a hydroxyl ligand. It is one of several cobalamin vitamers, alongside cyanocobalamin, methylcobalamin, and adenosylcobalamin. In humans, cobalamins act as cofactors for two key enzymes: methionine synthase, which converts homocysteine to methionine (methylcobalamin is the cofactor in this reaction), and methylmalonyl-CoA mutase, which converts methylmalonyl-CoA to succinyl-CoA (adenosylcobalamin is the cofactor in this reaction). Hydroxocobalamin itself serves largely as a storage and transport form that is converted intracellularly to the active cofactors.

Hydroxocobalamin is produced by gut bacteria and obtained from animal-derived foods as part of dietary vitamin B12. Deficiency can cause megaloblastic anemia and neuropathy.

In medicine, hydroxocobalamin is used as a vitamin B12 replacement therapy and, in high-dose parenteral form, as an antidote for cyanide poisoning. For cyanide exposure, it binds cyanide to form cyanocobalamin, which is excreted in urine. Typical treatment uses an initial 5-gram intravenous dose, sometimes repeated, with a maximum cumulative dose around 10 grams.

Common adverse effects are generally mild and include transient hypertension, headache, nausea, and reddish discoloration of the skin and urine. Hypersensitivity reactions are rare. Hydroxocobalamin is usually preferred when rapid correction of B12 deficiency is desired or when cyanide poisoning is encountered, and it is supplied in brands such as Cyanokit.