MeCbl

MeCbl, or methylcobalamin, is one of the two physiologically active coenzyme forms of vitamin B12 in humans, the other being adenosylcobalamin (AdoCbl). It is a cobalt-containing corrin compound with a methyl group bound to the cobalt center, making it a methylcobamide.

In human metabolism, MeCbl acts as a cofactor for methionine synthase in the cytosol. It transfers its methyl group from 5-methyltetrahydrofolate to homocysteine, producing methionine and regenerating tetrahydrofolate. This remethylation is essential for the synthesis of S-adenosylmethionine, a universal methyl donor, and for proper folate and amino acid metabolism. During turnover, the cobalt cycles between oxidation states as the enzyme regenerates MeCbl.

Dietary vitamin B12 is obtained from animal-derived foods and some fortified products. In the body, various forms—including MeCbl—can be derived from or converted to one another; MeCbl itself is also supplied directly as a supplement or prescription. Some products promote MeCbl specifically for B12 deficiency or neuropathic symptoms, though evidence supporting superiority to other B12 forms is mixed. B12 deficiency can lead to elevated homocysteine, megaloblastic anemia, and neurologic deficits; diagnosis relies on low B12 status with markers such as methylmalonic acid and homocysteine.

MeCbl is distinct from adenosylcobalamin, which serves as the mitochondrial cofactor for the mutase reaction converting methylmalonyl-CoA to succinyl-CoA. In human cells, MeCbl predominates in cytosolic reactions, while AdoCbl operates in mitochondria.