Vitamin B9 Functions

One-carbon metabolism

The only function of folate coenzymes in the body appears to be mediating the transfer of one-carbon units. Folate coenzymes act as acceptors and donors of one-carbon units in a variety of reactions critical to the metabolism of nucleic acids and amino acids.

Nucleic acid metabolism: Folate coenzymes play a vital role in DNA metabolism through two different pathways.1) The synthesis of DNA from its precursors is dependent on folate coenzymes. 2) A folate coenzyme is required for the synthesis of methionine, and methionine is required for the synthesis of S-adenosylmethionine (SAM). SAM is a methyl group (one-carbon unit) donor used in many biological methylation reactions, including the methylation of a number of sites within DNA and RNA. Methylation of DNA may be important in cancer prevention.

Amino acid metabolism: Folate coenzymes are required for the metabolism of several important amino acids. The synthesis of methionine from homocysteine requires a folate coenzyme as well as a vitamin B₁₂ dependent enzyme. Thus, folate deficiency can result in decreased synthesis of methionine and a build up of homocysteine. Increased levels of homocysteine may be a risk factor for heart disease, as well as several other chronic diseases.

Nutrient interactions

Vitamin B₁₂ and vitamin B₆: The metabolism of homocysteine, an intermediate in the metabolism of sulphur-containing amino acids, provides an example of the interrelationships among nutrients necessary to optimize physiological function and health. Healthy individuals utilize two different pathways to metabolize homocysteine. One pathway (methionine synthase) results in the synthesis of methionine from homocysteine, and is dependent on a folate coenzyme and a vitamin B₁₂ dependent enzyme. The other pathway converts homocysteine to another amino acid, cysteine, and requires two vitamin B₆ dependent enzymes. Thus, the amount of homocysteine in the blood is regulated by three vitamins: folic acid, vitamin B₁₂, and vitamin B₆.