Methylcobalamin

Methylcobalamin, also known as MeCbl, is a form of vitamin B12 that functions as a cofactor in humans. It contains a methyl group bound to a cobalt center and is one of the two biologically active coenzyme forms of B12, the other being adenosylcobalamin. Methylcobalamin participates in one-carbon metabolism alongside other B12-dependent processes.

In the methionine cycle, methylcobalamin acts as a cofactor for methionine synthase, catalyzing the transfer of a methyl group from 5-methyl-tetrahydrofolate to homocysteine to form methionine. This reaction regenerates methionine and supports the synthesis of S-adenosylmethionine (SAM), a key methyl donor for numerous reactions including DNA, protein, and lipid methylation. The cobalt center of the molecule cycles through different oxidation states during turnover.

MeCbl is found in animal-derived foods and is available as a dietary supplement, sometimes marketed as a preferred form of B12. In nutrition, cyanocobalamin or hydroxocobalamin are also common; the body can convert these to methylcobalamin in cells. Some supplements and injections specifically use methylcobalamin, but efficacy in correcting deficiency is generally similar when dosed appropriately.

Clinically, methylcobalamin is used to treat vitamin B12 deficiency due to pernicious anemia, gastritis, or malabsorption and can help restore hematologic function and nerve health. It has been studied for neuropathic conditions and diabetic neuropathy, with mixed evidence. Safety is generally good; excess B12 has low toxicity, but high-dose therapy should be guided by a clinician.