Hydroxocobalamin: Is not a form normally found in the human body, primarily produced by bacteria but can be converted in the body to useable, coenzyme forms of B12, methylcobalamin and adenysylcobalamin, the only forms able to cross the blood brain barrier. Interestingly, Hydroxocobalamin attaches itself to cyanide and is therefore used for smoke inhalation victims If glutathione is low the conversion from hydroxocobalamin will be blocked and methylcobalamin would be the preferred form of B12. Link to research.
Cyanocobalamin: Is the cheapest and most stable form of B12 and has a long shelf life but needs to have the cyanide molecule taken off, so has an extra conversion before being converted to methyl or adenyslcobalamin. With a certain eye condition, (Lebers hereditary optic neuropathy) cyanocobalamin should not be used.
Methylcobalamin: This is the most natural form of B12 and needs no converting .It is already in its “ready to use” form and is a more expensive form. Methylcobalamin is the active coenzyme form necessary for any biological activity. It also is the least stable with the shortest shelf life, converting back to hydroxycobalamin if not stored correctly. Methylcobalamin is sometimes referred to as “active B12” as it is in a form ready to be used by the cells.
Adenosylcobalamin: This is also an “active “ B12 form, making this coenzyme immediately available for use by the body. It is required for an enzyme known as MCM (Methylmalonyl Coenzyme A mutase), which resides in the mitochondria and is needed to make succinyl CoA to produce energy in the citric acid cycle. In humans adenyslcobalamin is found mainly in the tissues especially the liver. It is the primary form of B12 found in non-human animals.
Stability of B12 All forms of B12 are stable when protected from light. Light exposure cleaves the cyanide with the production of hydroxycobalamin. The B12s have an optimal stability at a PH4.00-4.5, even at higher temperatures. In the presence of acid or alkaline mediator the presence of reducing agents such as ascorbic acid the vitamin is destroyed to a greater extent. It is therefore advisable that B12 is not taken with fruit juice.
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