Methylcobalamin is vitamin B12 that has a methyl group (-CH3) attached to it. This typically occurs once vitamin B12 is absorbed into the cell and accepts the methyl group from methyltetrahydrofolate (the active form of folate). Once B12 is 'methylated' by folate, it transfers this single carbon methyl group to homocysteine which converts it into the amino acid, methionine. This takes place in the cytoplasm of the cell.
