Wednesday 15 October 2003
Definition: Folic acid and folate (the anion form) are forms of the water-soluble vitamin B9. These occur naturally in food and can also be taken as supplements. Folate gets its name from the Latin word folium ("leaf").
Folates are carriers of one-carbon units and are metabolized by 5,10-methylenetetrahydrofolate reductase (MTHFR) and other enzymes that use riboflavin (vitamin B2), cobalamin (vitamin B12), or pyridoxin (vitamin B6) as cofactors. Folate acts as a methyl donor for methionine synthesis via homocysteine (Hcy) remethylation. Folate also acts a donor of one-carbon groups for synthesis of thymidine and purines, the building blocks of DNA.
Folic acid is an inactive water-soluble B vitamin that is absorbed in the proximal small intestine via a carrier-mediated mechanism involving reduced folate carrier (RFC).
Marginal body stores and inadequate dietary intake contribute to folate deficiency throughout the world. Folates are essential cofactors in nucleic acid synthesis; the conversion of 5-methyltetrahydrofolate to tetrahydrofolate requires vitamin B12. Therefore, deficiency of either folate or vitamin B12 results in megaloblastic anemia.
Rapidly dividing cells in the fetus are especially vulnerable to folate deficiency. The requirement for folate, similar to that of pyridoxine, is increased in pregnancy. Poor diet during the first trimester of pregnancy has been shown to be associated with an increased incidence of neural tube defects in the fetus. Folate supplements have been shown to decrease the risk of neural tube defects.
Since neural tube defects occur within the first few weeks post-conception, folic acid must be taken by all women who expect to be pregnant.
As with vitamins B6 and B12, low plasma folate is associated with high levels of plasma homocysteine. Although elevations in homocysteine predispose to atherosclerosis, as with other vitamins that affect homocysteine levels, there is no clear evidence that folate supplements reduce atherosclerotic disease.
Folate is found in whole-wheat flour, beans, nuts, liver, and green leafy vegetables. It is heat labile and depleted in cooked and processed foods. In the United States, it is estimated that 15% to 20% of adults have low serum folate.
In developing countries that rely on diets based on corn with few fresh vegetables, folate deficiency is more common. Even in those people with adequate diets, oral contraceptives, anticonvulsants, ethanol, and cigarette smoking interfere with folate absorption and metabolism. Chronic diseases such as intestinal malabsorption and metastatic cancer are also associated with folate deficiency.
Combined folate and vitamin B12 deficiency has been postulated to contribute to the development of colon cancer. The following mechanisms have been proposed: (1) altered DNA methylation; (2) accumulation of cells in S phase with increased susceptibility in induction of DNA damage; and (3) perturbations of nucleotide pools that impair DNA synthesis and repair.
The consequences of vitamin B12 deficiency and pernicious anemia are described in Chapter 13. In contrast to folate deficiency, vitamin B12 deficiency is associated with myelin degeneration in both sensory and motor pathways of the spinal cord
Once entered into the bloodstream, folate is transported into cells mainly through the folate receptor (FR-α, -β and -γ) and through the RFC.
Methionine synthesis - Once entered into the cell, folate acts as a methyl donor for methionine synthesis via homocysteine (Hcy) remethylation. Methionine is the single most important methyl donor for the methylation of DNA and tRNA.
Synthesis of thymidine and purines - Folate also acts a donor of one-carbon groups for synthesis of thymidine and purines, the building blocks of DNA.
Folate (folic acid or vitamin B9) and homocysteine metabolic cycles are closely related and involve over 25 proteins.
Key enzymes involved in folate and homocysteine metabolism are:
the 5,10-methylene-tetrahydrofolate reductase (MTHFR)
the trifunctional enzyme methyleneTHF dehydrogenase/formylTHF synthase/methenylTHF cyclohydrolase (MTHFD)
the methionine synthase (MTR)
the methionine synthase reductase (MTRR).
DNA biosynthesis and cell division
Folate is necessary for the production and maintenance of new cells. This is especially important during periods of rapid cell division and growth such as infancy and pregnancy.
Folate is needed to synthesize DNA bases (most notably thymine, but also purine bases) needed for DNA replication.
In the form of a series of tetrahydrofolate (THF) compounds, folate derivatives are substrates in a number of single-carbon-transfer reactions, and also are involved in the synthesis of dTMP (2′-deoxythymidine-5′-phosphate) from dUMP (2′-deoxyuridine-5′-phosphate).
It is a substrate for an important reaction that involves cobalamin (vitamin B12) and it is necessary for the synthesis of DNA, required for all dividing cells.
The pathway leading to the formation of tetrahydrofolate (FH4) begins when folate (F) is reduced to dihydrofolate (DHF) (FH2), which is then reduced to THF.
Dihydrofolate reductase catalyses the last step.
Niacin (Vitamin B3) in the form of NADPH is a necessary cofactor for both steps of the synthesis.
Methylene-THF (CH2FH4) is formed from THF by the addition of methylene groups from one of three carbon donors: formaldehyde, serine, or glycine.
Methyl tetrahydrofolate (CH3-THF) can be made from methylene-THF by reduction of the methylene group with NADPH. It is important to note that Vitamin B12 is the only acceptor of methyl-THF.
There is also only one acceptor for methyl-B12 which is homocysteine in a reaction catalyzed by homocysteine methyltransferase.
This is important because a defect in homocysteine methyltransferase or a defeciency of B12 can lead to a methyl-trap of THF and a subsequent deficiency.
Thus, a deficiency in vitamin B12 (cobalamin) can generate a large pool of methyl-THF that is unable to undergo reactions and will mimic folate deficiency.
Another form of THF, formyl-THF or folinic acid) results from oxidation of methylene-THF or is formed from formate donating formyl group to THF. Finally, histidine can donate a single carbon to THF to form methenyl-THF.
In other words:
F → DHF → THF → CH2-THF Formyl-THF ↔ Methynl-THF ↔ Methylene-THF → Methyl-THF
Thus folate deficiency hinders DNA synthesis and cell division, affecting most notably bone marrow and cancer, both of which participate in rapid cell division.
RNA transcription, and subsequent protein synthesis, are less affected by folate deficiency as the mRNA can be recycled and used again (as opposed to DNA synthesis where a new genomic copy must be created).
Since folate deficiency limits cell division, erythropoiesis, production of red blood cells (RBCs) is hindered and leads to megaloblastic anemia which is characterized by large immature RBCs.
This pathology results from persistently thwarted attempts at normal DNA replication, DNA repair, and cell division and produces abnormally large cells (megaloblasts) with abundant cytoplasm capable of RNA and protein synthesis but with clumping and fragmentation of nuclear chromatin.
Some of these large cells, although immature, are released early from the marrow in an attempt to compensate for the anemia caused by lack of RBCs.
Both adults and children need folate to make normal RBCs and prevent anemia.
susceptibility to neural tube defects (NTDs)
Deficiency of folate in pregnant women has been implicated in neural tube defects (NTDs) and so many cereals sold in developed countries are enriched with folate to avoid such complications.
The intracellular intake of folate through FR-α is critically important for embryogenesis as demonstrated in mouse where functional knockout of folate-binding protein 1 (Folp1, ortholog of FR-α) leads to exencephaly and is embryonically lethal.
Interestingly, a recent study showed that some mothers with an NTD pregnancy produce autoantibodies that bind to folate receptors on the placental membrane and therefore block the binding of folic acid.
Very few variations in FR-α and FR-β have been identified and none was found to be associated with an increased NTD risk.
On the other hand, a prevalent polymorphism (80A→G) in RFC-1 has been demonstrated as a genetic risk factor for NTDs especially when maternal folate status is low.
A number of drugs interfere with the biosynthesis of folic acid and THF. Among them are the dihydrofolate reductase inhibitors such as trimethoprim, pyrimethamine and methotrexate; the sulfonamides (competitive inhibitors of para-aminobenzoic acid in the reactions of dihydropteroate synthetase).