Consideration of betaine and one-carbon sources of N5-methyltetrahydrofolate for use in homocystinuria and neural tube defects.

A major focus in attempts to ameliorate homocystinuria and neural tube defects is supplementation of the diet with B vitamins. The metabolic defect in these cases may be due in part to a deficiency of methyl groups. B vitamin supplementation supports the need for enzyme cofactors but cannot provide substrate in the form of methyl groups. l-Methionine is an essential amino acid and is required for protein synthesis, but it also plays a unique role in metabolism as S-adenosylmethionine, which is the primary methyl donor in metabolism. The observation that l-homocysteine, which is produced in the metabolism of l-methionine, is remethylated 2-4 times before it is destroyed is key to understanding the possibility of a methyl group deficiency. This suggests that the requirement for methyl groups (ie, S-adenosylmethionine) may be 2-4 times that for methionine in support of protein synthesis. l-Homocysteine can be remethylated to form l-methionine by betaine or N(5)-methyltetrahydrofolate. Betaine and one-carbon sources that lead to the production of N(5)-methyltetrahydrofolate and the remethylation of l-homocysteine to form l-methionine should be considered along with B vitamin supplementation in the treatment of homocystinuria and neural tube defects.