2008/9 Schools Wikipedia Selection. Related subjects: Chemical compounds; Health and medicine
|IUPAC name||(S)-2-(4-((2-amino-4-hydroxypteridin-6-yl)methylamino)benzamido)pentanedioic acid|
|Other names||N-[4(2-Amino-4-hydroxy pteridin-6-ylmethylamino) benzoyl]-L(+)-glutamic acid; pteroyl-L-glutamic acid; Vitamin B9; Vitamin M; Folacin|
|Molar mass||441.403 g/mol|
|Appearance||yellow-orange crystalline powder|
250 °C (523 K), decomp.
|Solubility in water||0.0016 mg/ml (25 °C)|
|Acidity (pKa)||1st: 2.3, 2nd: 8.3|
|Main hazards||non-toxic, non-flammable|
|Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)
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").
A key observation by researcher Lucy Wills in 1931 led to the identification of folate as the nutrient needed to prevent anaemia during pregnancy. Dr. Wills demonstrated that anaemia could be reversed with brewer's yeast. Folate was identified as the corrective substance in brewer's yeast in the late 1930s and was extracted from spinach leaves in 1941. It was first synthesized in 1946 by Yellapragada Subbarao.
Biological roles of folate
- DNA 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. 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 anaemia 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 anaemia caused by lack of RBCs. Both adults and children need folate to make normal RBCs and prevent anaemia. Deficiency of folate in pregnant women has been implicated in neural tube defects and so many cereals sold in developed countries are enriched with folate to avoid such complications.
- Biochemistry of DNA base and amino acid production
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 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. 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. If 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 B12 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 → DHF2 → THF → CH2-THF Formyl-THF <--> Methynl-THF <--> Methylene-THF --> Methyl-THF
Overview of drugs that interfere with folate reactions 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).
- Recommended Daily Allowance (RDA)
|400 µg||400 µg||600 µg||500 µg|
|1 µg of food folate = 0.6 µg folic acid from supplements and fortified foods|
The National Health and Nutrition Examination Survey (NHANES III 1988-91) and the Continuing Survey of Food Intakes by Individuals (1994-96 CSFII) indicated that most adults did not consume adequate folate. However, the folic acid fortification program in the United States has increased folic acid content of commonly eaten foods such as cereals and grains, and as a result diets of most adults now provide recommended amounts of folate equivalents.
Folic acid is very important for all women who may become pregnant. Adequate folate intake during the periconceptional period, the time just before and just after a woman becomes pregnant, helps protect against a number of congenital malformations including neural tube defects. Neural tube defects result in malformations of the spine ( spina bifida), skull, and brain ( anencephaly). The risk of neural tube defects is significantly reduced when supplemental folic acid is consumed in addition to a healthy diet prior to and during the first month following conception. Women who could become pregnant are advised to eat foods fortified with folic acid or take supplements in addition to eating folate-rich foods to reduce the risk of some serious birth defects. The most notable birth defects that occur from folate deficiency are neural tube defects. Taking 400 micrograms of synthetic folic acid daily from fortified foods and/or supplements has been suggested. The Recommended Dietary Allowance (RDA) for folate equivalents for pregnant women is 600-800 micrograms, twice the normal RDA of 400 micrograms for women who are not pregnant.
Recent research has shown that it is also very important for men who are planning on fathering children, reducing birth defect risks.
Folic acid supplements and masking of B12 deficiency
There has been concern about the interaction between vitamin B12 and folic acid.Folic acid supplements can correct the anaemia associated with vitamin B12 deficiency. Unfortunately, folic acid will not correct changes in the nervous system that result from vitamin B12 deficiency. Permanent nerve damage could theoretically occur if vitamin B12 deficiency is not treated. Therefore, intake of supplemental folic acid should not exceed 1000 micrograms (1000 µg or 1 mg) per day to prevent folic acid from masking symptoms of vitamin B12 deficiency. In fact, to date the evidence that such masking actually occurs is scarce, and there is no evidence that folic acid fortification in Canada or the US has increased the prevalence of vitamin B12 deficiency or its consequences.
However one recent study has demonstrated that high folic or folate levels when combined with low B12 levels are associated with significant cognitive impairment among the elderly. If the observed relationship for seniors between folic acid intake, B12 levels, and cognitive impairment is replicated and confirmed, this is likely to re-open the debate on folic acid fortification in food, even though public health policies tend generally to support the developmental needs of infants and children over slight risks to other population groups.
In any case, it is important for older adults to be aware of the relationship between folic acid and vitamin B12 because they are at greater risk of having a vitamin B12 deficiency. If you are 50 years of age or older, ask your physician to check your B12 status before you take a supplement that contains folic acid.
Health risk of too much folic acid
The risk of toxicity from folic acid is low. The Institute of Medicine has established a tolerable upper intake level (UL) for folate of 1 mg for adult men and women, and a UL of 800 µg for pregnant and lactating (breast-feeding) women less than 18 years of age. Supplemental folic acid should not exceed the UL to prevent folic acid from masking symptoms of vitamin B12 deficiency.
Research suggests high levels of folic acid can interfere with some antimalarial treatments.
A 10000-patient study at Tufts University in 2007 concluded that excess folic acid worsens the effects of B12 deficiency and in fact may affect the absorption of B12.
Some current issues and controversies about folate
Dietary fortification of folic acid
Since the discovery of the link between insufficient folic acid and neural tube defects (NTDs), governments and health organizations worldwide have made recommendations concerning folic acid supplementation for women intending to become pregnant. For example, the United States Public Health Service (see External links) recommends an extra 0.4 mg/day, which can be taken as a pill. However, many researchers believe that supplementation in this way can never work effectively enough since about half of all pregnancies in the U.S. are unplanned and not all women will comply with the recommendation.
This has led to the introduction in many countries of fortification, where folic acid is added to flour with the intention of everyone benefiting from the associated rise in blood folate levels. This is controversial, with issues having been raised concerning individual liberty, and the masking effect of folate fortification on pernicious anaemia (vitamin B12 deficiency). However, almost all American countries now fortify their flour, along with a number of Middle Eastern countries and Indonesia. Mongolia and a number of ex-Soviet republics are amongst those having widespread voluntary fortification; about five more countries (including Morocco, the first African country) have agreed but not yet implemented fortification. In the UK the Food Standards Agency has recommended fortification. To date, no EU country has yet mandated fortification. Australia and New Zealand have jointly agreed to fortification. Australia will fortify all flour, whereas New Zealand will fortify bread (excluding organic and unleavened varieties) from 18 September 2009. The requirement is 0.135mg of folate per 100g of bread.
In 1996, the United States Food and Drug Administration (FDA) published regulations requiring the addition of folic acid to enriched breads, cereals, flours, corn meals, pastas, rice, and other grain products. This ruling took effect 1998- 01-01, and was specifically targeted to reduce the risk of neural tube birth defects in newborns. There are concerns that the amount of folate added is insufficient . In October 2006, the Australian press claimed that U.S. regulations requiring fortification of grain products were being interpreted as disallowing fortification in non-grain products, specifically Vegemite (an Australian yeast extract containing folate). The FDA later said the report was inaccurate, and no ban or other action was being taken against Vegemite.
Since the folic acid fortification program took effect, fortified foods have become a major source of folic acid in the American diet. The Centers for Disease Control and Prevention in Atlanta, Georgia used data from 23 birth defect registries that cover about half of United States births and extrapolated their findings to the rest of the country. This data indicates that since the addition of folic acid in grain-based foods as mandated by the Food and Drug Administration, the rate of neural tube defects dropped by 25% in the United States.
Although folic acid does reduce the risk of birth defects, it is only one part of the picture and should not be considered a cure. Even women taking daily folic acid supplements have been known to have children with neural tube defects.
Adequate concentrations of folate, vitamin B12, or vitamin B6 may decrease the circulating level of homocysteine, an amino acid normally found in blood. There is evidence that an elevated homocysteine level is an independent risk factor for heart disease and stroke. The evidence suggests that high levels of homocysteine may damage coronary arteries or make it easier for blood clotting cells called platelets to clump together and form a clot. However, there is currently no evidence available to suggest that lowering homocysteine with vitamins will reduce risk of heart disease. Clinical intervention trials are needed to determine whether supplementation with folic acid, vitamin B12 or vitamin B6 can lower the risk of developing coronary heart disease. The NORVIT trial suggests that folic acid supplementation may do more harm than good.
As of 2006, studies have shown that giving folic acid to reduce levels of homocysteine does not result in clinical benefit. One of these studies suggests that folic acid in combination with B12 may even increase some cardiovascular risks.
However a 2005 study found that 5 mg. of folate daily over a three-week period reduced pulse pressure by 4.7 mmHg. compared with a placebo, and concluded that
- Folic acid is a safe and effective supplement that targets large artery stiffness and may prevent isolated systolic hypertension.
Also, as a result of new research, "heart experts" at Johns Hopkins Medical Centre reported in March 2008 in favour of therapeutic folate, although they cautioned that it was premature for those at risk of heart attack to self-medicate.
Folic acid appears to reduce the risk of stroke. The reviews indicate only that in some individuals the risk of stroke appears to be reduced, but a definite recommendation regarding supplementation beyond the current recommended daily allowance has not been established for stroke prevention. Observed stroke reduction is consistent with the reduction in pulse pressure produced by folate supplementation of 5 mg per day, since hypertension is a key risk factor for stroke.
The association between folate and cancer appears to be complex. It has been suggested that folate may help prevent cancer, as it is involved in the synthesis, repair, and functioning of DNA, and a deficiency of folate may result in damage to DNA that may lead to cancer. Some investigations have proposed that good levels of folic acid may be related to lower risk of esophagus, stomach and ovarian cancer, but benefices of folic acid against cancer may depend on when it is taken and on individual conditions. In addition folic acid may not be helpful, and could even be damaging, in people who already are suffering from cancer or from a precancerous condition. Conversely, it has been suggested that excess folate may promote tumor initiation. Diets high in folate are associated with decreased risk of colorectal cancer; some studies show an association which is stronger for folate from foods alone than for folate from foods and supplements, while other studies find that folate from supplements is more effective due to greater bioavailability and a 2007 randomized clinical trial found that folate supplements did not reduce the risk of colorectal adenomas. A 2006 prospective study of 81,922 Swedish adults found that diets high in folate from foods, but not from supplements, were associated with a reduced risk of pancreatic cancer. Most epidemiologic studies suggest that diets high in folate are associated with decreased risk of breast cancer, but results are not uniformly consistent: one large cancer screening trial reported a potential harmful effect of high folate intake on breast cancer risk, suggesting that routine folate supplementation should not be recommended as a breast cancer preventive, but a 2007 Swedish prospective study found that a high folate intake was associated with a lower incidence of postmenopausal breast cancer. It is very difficult to affirm how each nutrient or nutrient combination affects a person’s risk of cancer. People whose diets are rich in vegetables and low in animal fat and meat have lower risks for some of the most frequent types of cancer. Taking a variety of healthful foods, with most of them coming from plant sources, is a better solution than taking vitaminic supplements. So the authorities are not really sure if this will work for cancer or not, (or the age at which it is safe to start taking folate supplements) but hopefully this will all become clear in the light of research now underway.
Folate is important for cells and tissues that rapidly divide. Cancer cells divide rapidly, and drugs that interfere with folate metabolism are used to treat cancer. The antifolate methotrexate is a drug often used to treat cancer because it inhibits the production of the active form of THF from the inactive dihydrofolate (DHF). Unfortunately, methotrexate can be toxic, producing side effects such as inflammation in the digestive tract that make it difficult to eat normally.
Folinic acid, under the drug name leucovorin, is a form of folate (formyl-THF) that can help "rescue" or reverse the toxic effects of methotrexate. Folinic acid is not the same as folic acid. Folic acid supplements have little established role in cancer chemotherapy. There have been cases of severe adverse effects of accidental substitution of folic acid for folinic acid in patients receiving methotrexate cancer chemotherapy. It is important for anyone receiving methotrexate to follow medical advice on the use of folic or folinic acid supplements. The supplement of folinic acid in patients undergoing methotrexate treatment is to give non rapidly dividing cells enough folate to maintain normal cell functions. The amount of folate given will be depleted by rapidly dividing cells (cancer) very fast and so will not negate the effects of methotrexate. Low dose methotrexate is used to treat a wide variety of non-cancerous diseases such as rheumatoid arthritis, lupus, scleroderma, psoriasis, asthma, sarcoidoisis, primary biliary cirrhosis, and inflammatory bowel disease. Low doses of methotrexate can deplete folate stores and cause side effects that are similar to folate deficiency. Both high folate diets and supplemental folic acid may help reduce the toxic side effects of low dose methotrexate without decreasing its effectiveness. Anyone taking low dose methotrexate for the health problems listed above should consult with a physician about the need for a folic acid supplement. While the role in folate as a cancer treatment is well established its long term effectiveness is diminished by cellular response. In response to decreased THF the cell begins to transcribe more DHF reductase, the enzyme that reduces DHF to THF. Because methotrexate is a competitive inhibitor of DHF reductase increased concentrations of DHF reductase can overcome the drugs inhibition.
Some evidence links low levels of folate with depression. There is some limited evidence from randomised controlled trials that using folic acid in addition to antidepressant medication may have benefits. Researchers at the University of York and Hull York Medical School have confirmed a link between depression and low levels of folate in a research study involving 15,315 . However, the evidence is probably too limited at present for this to be a routine treatment recommendation.
Memory and mental agility
In a 3-year trial on 818 people over the age of 50, short-term memory, mental agility and verbal fluency were all found to be better among people who took 800 micrograms of folic acid daily—twice the current RDA—than those who took placebo. The study was reported in The Lancet on 19 January 2007.
Folate is necessary for fertility in both men and women. In men, it contributes to spermatogenesis. In women, on the other hand, it contributes to oocyte maturation, implantation, placentation, in addition to the general effects of folic acid and pregnancy. Therefore, it is necessary to receive sufficient amounts through the diet, in order to avoid subfertility.
Induction of acute renal failure
Folic acid is used in extremely high doses to induce Acute renal failure in murine models. The exact method through which folic acid induces kidney injury in such massive dose is unknown, however it is characterized by the appearance of folic acid crystals in renal tubules and acute tubular necrosis. This method of renal injury is also linked to increased expression of Tumor necrosis factor-alpha. The dose of folic acid used to induce renal injury is usually around 250mg of folic acid per kg of body weight—approximately 120 times the FDA's daily recommended intake of 0.4mg. The folic acid is usually administered in a vehicle of 0.3mmol/L of sodium bicarbonate.