Methionine (Met) is an essential amino acid because mammals cannot synthesize homocysteine. Betaine-homocysteine methyltransferase (BHMT) and BHMT-2 methylate homocysteine to form methionine using betaine and S-methylmethionine, respectively. Betaine is produced de novo from choline and is also found in the diet, whereas S-methylmethionine is only made in plants and fungi and so must be obtained from the diet. These enzyme activities are only be detected in the liver of adult rodents, but in adult humans and pigs, these activities are found in both the liver and kidney cortex.
