Role of Folate and Folic Acid

Role of folate and folic acid in human metabolic processes

Folates accept one-carbon units from donor molecules and passes them on via various biosynthetic reactions. In their reduced form cellular folates function conjugated to a polyglutamate chain. These folates are a mixture of unsubstituted polyglutamyl tetrahydrofolates and various substituted one-carbon forms of tetrahydrofolate (e.g., 10- formyl, 5,10-methylene, and 5-methyl). The reduced forms of the vitamin, particularly the unsubstituted dihydro and tetrahydro forms, are unstable chemically. They are easily split between the C-9 and N-10 bond to yield a substituted pteridine and p-aminobenzoylglutamate, which have no biologic activity. Substituting a carbon group at N-5 or N-10 decreases the tendency of the molecule to split; however, the substituted forms are also susceptible to oxidative chemical rearrangements and, consequently, loss of activity. The folates found in food consist of a mixture of reduced folate polyglutamates.

Although natural folates rapidly lose activity in foods over periods of days or weeks, folic acid (e.g., in fortified foods) is almost completely stable for months or even years. The chemical lability of all naturally occurring folates results in a significant loss of biochemical activity during harvesting, storage, processing, and preparation. Half or even three-quarters of initial folate activity may be lost during these processes. This is in contrast to the stability of the synthetic form of this vitamin, folic acid. In this form the pteridine (2-amino-4- hydroxypteridine) ring is not reduced, rendering it very resistant to chemical oxidation. However, folic acid is reduced in cells by the enzyme dihydrofolate reductase to the di- and tetrahydro forms. This takes place within the intestinal mucosal cells, and 5-methyltetrahydrofolate is released into the plasma.

Natural folates found in foods are all conjugated to a polyglutamyl chain containing different numbers of glutamic acids depending on the type of food. This polyglutamyl chain is removed in the brush border of the mucosal cells by the enzyme folate conjugase, and folate monoglutamate is subsequently absorbed. The primary form of folate entering human circulation from the intestinal cells is 5-methyltetrahydrofolate monoglutamate. This process is, however, limited in capacity. If enough folic acid is given orally, unaltered folic acid appears in the circulation, is taken up by cells, and is reduced by dihydrofolate reductase to tetrahydrofolate

The bio-availability of natural folates is affected by the removal of the polyglutamate chain by the intestinal conjugase. This process is apparently not complete, thereby reducing the bio-availability of natural folates by as much as 25–50 percent. In contrast, synthetic folic acid appears to have a bio-availability of close to 100 percent. The low bio-availability and – more importantly – the poor chemical stability of the natural folates has a profound influence on the development of nutrient recommendations. This is particularly true if some of the dietary intake is in the synthetic form, folic acid, which is much more stable and bio-available. Food fortification of breakfast cereals, flour, etc. can add significant amounts of folic acid to the diet.

Functional folates have one-carbon groups derived from several metabolic precursors (e.g., serine, N-formino-L-glutamate, folate, etc.). With 10-formyltetrahydrofolate the formyl group is incorporated sequentially into C-2 and C-8 of the purine ring during its biosynthesis. Likewise the conversion of deoxyuridylate (a precursor to RNA) into thymidylate (a precursor to DNA) is catalysed by thymidylate synthase, which requires 5,10-methylenetetrahydrofofate.

Alternatively 5,10-methylenetetrahydrofolate can be channelled to the methylation cycle. This cycle has two functions. It ensures that the cell always has an adequate supply of S-adenosylmethionine, an activated form of methionine, which acts as a methyl donor to a wide range of methyltransferases. These enzymes methylate a wide range of substrates including lipids, hormones, DNA, proteins, etc. One such important methylation is that of myelin basic protein, which acts as insulation for nerves cells. When the methylation cycle is interrupted as it is during vitamin B12 deficiency, one of the clinical consequences is the demyelination of nerve resulting in a neuropathy which leads to ataxia, paralysis, and, if untreated, ultimately death. Other important methyltransferase enzymes down-regulate DNA and suppress cell division.

In the liver the methylation cycle also serves to degrade methionine. Methionine is an essential amino acid in Humans and is present in the diet of people in developed countries at about 60 percent over that required for protein synthesis and other uses. The excess methionine is degraded via the methylation cycle to homo-cysteine, which can either be catabolised to sulfate and pyruvate (with the latter being used for energy) or remethylated to methionine. The need to maintain intracellular S-adenosylmethionine levels is related to the amount of methionine metabolised via homo-cysteine.

The DNA and methylation cycles both regenerate tetrahydrofolate. However, there is a considerable amount of catabolism of folate and a small loss of folate via excretion from the urine, skin, and bile. There is a need to replenish the body’s folate content by uptake from the diet. If there is inadequate dietary folate, the activity of both the DNA and the methylation cycles will be reduced. A decrease in the former will reduce DNA biosynthesis and thereby reduce cell division. Although this will be seen in all dividing cells, the deficiency will be most obvious in cells that are rapidly dividing, for example, in a decrease in red cell production, producing anaemia. Other cells derived from bone marrow also decrease, leading to leucopenia and thrombocytopenia. Likewise there is a reduction in cell division in the lining of the gut. 

Taken together, this reduction in the DNA cycle results in an increased susceptibility to infection, a decrease in blood coagulation, and secondary malabsorption. In folate deficiency, the flux through the methylation cycle is decreased but the DNA cycle may be more sensitive. The most obvious expression of the decrease in the methylation cycle is an elevation in plasma homo-cysteine. This is due to a decreased availability of new methyl groups provided as 5-methyltetrahydrofolate, necessary for the remethylation of plasma homo-cysteine. 

Previously it was believed that a rise in plasma homo-cysteine was nothing more than a biochemical marker of possible folate deficiency. However, there is increasing evidence that elevations in plasma homo-cysteine are implicated in the aetiology of cardiovascular disease. This moderate elevation of plasma homo-cysteine occurs in subjects with a folate status previously considered adequate.

Interruption of the methylation cycle resulting from impaired folate status or deceased vitamin B12 or vitamin B6 status may have serious long-term risks. Such interruption, as seen in vitamin B12 deficiency (e.g., pernicious anaemia), causes a very characteristic demyelination and neuropathy known as subacute combined degeneration of the spinal cord and peripheral nerves. If untreated, this leads to ataxia, paralysis, and ultimately death. Such neuropathy is not usually associated with folate deficiency but is seen if folate deficiency is very severe and prolonged. 

The explanation may lie in the well-established ability of nerve tissue to concentrate folate to a level of about five times that in the plasma. This may ensure that nerve tissue has an adequate level of folate when folate being provided to the rapidly dividing cells of the marrow has been severely compromised for a prolonged period. The resultant anaemia will thus inevitably present clinically earlier than the neuropathy.

References:

Scott, J.M. & Weir, D.G. 1994. Folate/vitamin B12 interrelationships. Essays in Biochemistry, 28: 63-72.

Blakley, R. 1969. The biochemistry of folic acid and related pteridines. North Holland Research Monographs Frontiers of Biology. Vol. 13, Editors H. Newbergen and E.L. Taton. Amsterdam. North Holland Publishing Company.

Kelly, P., McPartlin, J., Goggins, S., Weir, D.G. & Scott J.M. 1997. Unmetabolised folic acid in serum: acute studies in subjects consuming fortified food and supplements. Amer. J. Clin Nut., 69:1790-1795.

Gregory, J.F. 1997. Bio-availability of folate. Eur. J. Clin. Nutr., 51: 554-559.

Cuskelly, C.J., McNulty, H. & Scott, J.M. 1996. Effect of increasing dietary folate on red-cell folate: implications for prevention of neural tube defects. Lancet, 347:657-659.

McPartlin, J., Halligan, A., Scott, J.M., Darling, M. & Weir, D.G. 1993 Accelerated folate breakdown in pregnancy. Lancet, 341:148-149.

Scott, J.M. & Weir , D.G. 1996. Homo-cysteine and cardiovascular disease. Q. J. Med., 89: 561-563.

Wald, N.J., Watt, H.C., Law, M.R., Weir, D.G., McPartlin, J. & Scott, J.M. 1998.

Homo-cysteine and ischaemic heart disease: results of a prospective study with

implications on prevention. Arch. Internal Med., 158: 862-867.