Folate , formerly known as folacin , is the generic term for both naturally occurring food folate and folic acid , the fully oxidised monoglutamate form of the vitamin that is used in dietary supplements and fortified foods . Folic acid consists of a p-aminobenzoic molecule linked to a pteridine ring and one molecule of glutamic acid . Food folates , which exist in various forms , contain additional glutamate residues , making them polyglutamates

Folate functions as a coenzyme or cosubstrate in single-carbon transfers in the synthesis of nucleic acids ( DNA and RNA ) and metabolism of amino acids . One of the most important folatedependent reactions is the conversion of homocysteine to methionine in the synthesis of S-adenosyl-methionine , an important methyl donor . Another folate-dependent reaction , the methylation of deoxyuridylate to thymidylate in the formation of DNA , is required for proper cell division . An impairment of this reaction initiates a process that can lead to megaloblastic anemia , one of the hallmarks of folate deficiency .

An elevated homocysteine level has been associated with an increased risk of cardiovascular disease . Folate and other B vitamins are involved in homocysteine metabolism and researchers have hypothesised that they reduce cardiovascular disease risk by lowering homocysteine levels .

Although folic acid ( and vitamin B12 ) supplements lower homocysteine levels , research indicates that these supplements do not actually decrease the risk of cardiovascular disease , although they might provide protection from stroke . The Heart Outcomes Prevention Evaluation ( HOPE ) 2 study , for example , recruited 5 522 patients aged 55 years or older with vascular disease or diabetes from some countries - including the United States and Canada - that had a folic acid fortification programme and some countries that did not . Patients received 2 500mcg folic acid plus 50mg vitamin B6 and 1mg vitamin B12 or placebo for an average of five years . Compared with placebo , treatment with B vitamins significantly decreased homocysteine levels but did not reduce the risk of death from cardiovascular causes or myocardial infarction .

Supplementation did , however , significantly reduce the risk of stroke . In the Women ’ s Antioxidant and Folic Acid Cardiovascular Study , US women at high risk of cardiovascular

disease who took daily supplements containing 2500mcg folic acid , 1mg vitamin B12 , and 50mg vitamin B6 for 7.3 years did not have a reduced risk of major cardiovascular events , even though the supplements lowered their homocysteine levels .

The authors of a 2012 meta-analysis of 19 randomised controlled trials that included 47 921 participants concluded that B-vitamin supplementation has no effect on the risk of cardiovascular disease , myocardial infarction , coronary heart disease , or cardiovascular death , although it does reduce the risk of stroke by 12 %. It is not possible to evaluate the impact of folic acid alone from these trials , but little evidence shows that supplemental folic acid with or without vitamin B12 and vitamin B6 can help reduce the risk or severity of cardiovascular disease . B-vitamin supplementation does , however , appear to have a protective effect on stroke .

HYPERHOMOCYSTEINEMIA The demonstration of a concentrationdependent relationship between plasma homocysteine levels and cardiovascular disease risk has generated interest in characterising to what extent lowering homocysteine levels may improve vascular disease-associated morbidity and mortality . Elevated levels of circulating homocysteine increase the risk for developing atherothrombotic coronary artery disease ( CAD ), peripheral vascular disease , myocardial infarction ( MI ), and stroke .

Data in support of this conclusion , however , are derived largely from population-based observational and cross-sectional analyses . Nevertheless , these data , together with the characterisation of cellular mechanisms by which homocysteine promotes oxidant stress-induced vascular dysfunction , have provided ample evidence to support clinical trials of homocysteine lowering with B-vitamins as a novel therapeutic approach to patients with vascular disease .

FOLATE CYCLES Intracellularly , 5-methylTHF functions as a methyl donor for homocysteine remethylation ( Blom et al 2006 ). The resulting THF can directly be converted into 5,10-methyleneTHF by the action of serine hydroxymethyltransferase ( SHMT ). SHMT is a vitamin B6- dependent enzyme that uses serine as a one-carbon donor . In humans , SHMT has a cytosolic as well as a mitochondrial isoform

( Garrow et al 1993 ). Conversion of THF into 5,10-methyleneTHF , via 10-formylTHF and 5,10-methenylTHF , is catalysed by the trifunctional enzyme methylenetetrahydrofolate dehydrogenase ( MTHFD1 ) that has formyltetrahydrofolate synthetase , methenyltetrahydrofolate cyclohydrolase and methylenetetrahydrofolate dehydrogenase activities ( Hum et al 1988 ).

The 10-formylTHF can donate one-carbon groups for purines biosynthesis , whereas 5,10-methylenetetrahydrofolate can be used as a cofactor for the conversion of dUMP into dTMP . The latter reaction is catalysed by the thymidylate synthase ( TYMS ) enzyme and produces dihydrofolate ( DHF ), which requires subsequent reduction back to THF by the action of dihydrofolate reductase ( DHFR ). In addition to being a cosubstrate for dTMP synthesis , 5,10-methyleneTHF can also be reduced to 5-methylTHF by the riboflavin ( vitamin B2 ) -dependent enzyme methylenetetrahydrofolate reductase ( MTHFR ), which competes for 5,10-methyleneTHF with TYMS . The function of the MTHFR enzyme is of great importance for the regulation of available 5-methylTHF for homocysteine remethylation .

FOLATE UPTAKE AND TRANSPORT

The water-soluble B-vitamin folate is an essential vitamin for humans and is obtained from the diet , especially from fruits and vegetables . According to chemical nomenclature , the difference between folate and folic acid is just one proton . However , the term folic acid is in general applied to the synthetic form of this B-vitamin , which is also the most stable form ( Blom et al . 2006 ; Pitkin 2007 ).

Dietary folates predominantly exist as polyglutamates , which have to be hydrolysed to monoglutamates in order to be transported . The enzyme folylpoly-γ-glutamate carboxypeptidase ( FGCP ), that is anchored to the intestinal apical brush border and is encoded by the glutamate carboxypeptidase II ( GCPII ) gene , is responsible for this hydrolysis in the gut .

Monoglutamylated folates are subsequently absorbed in the duodenum and upper part of the jejunum by the high-affinity protoncoupled folate receptor PCFT1 ( Qiu et al 2006 ). 5-MethylTHF is the main

form of folate circulating in the plasma and can be transported into the cell by means of carrier- or receptormediated transport . Folate receptor α ( FR-α ) is a glycosylphosphatidylinositol-linked glycoprotein with a high affinity for the monoglutamate 5-methylTHF and is expressed in a limited number of epithelial cells , predominantly in the proximal tubules of the kidney , the choroid plexus and the placenta .

The other folate receptors , β and γ , have a much lower affinity for 5- methylTHF than FR-α . Carriermediated transport occurs via the ubiquitously expressed reduced folate carrier ( RFC ) whose affinity for 5-methylTHF is also considerably lower than that of the FR-α . The elucidation of the role of PCFT1in cellular uptake and transport requires further research .

KEY POINTS

1 . Epidemiologic reports have established that elevated levels of homocysteine are an independent risk factor for atherothrombotic cardiovascular disease and stroke .

2 . Oxidation of homocysteine promotes reactive oxygen species formation that may impair vascular function via decreased bioavailable nitric oxide levels , depletion of nitric oxide synthase cofactors , or impairment of normal antioxidant enzyme function .

3 . Homocysteine overexcitation of N-methyl-D-aspartate receptors in the central nervous system may promote reactive oxygen species formation to induce synaptic failure in brain tissue .

4 . Randomised clinical trials have shown that oral supplementation with the combination of folic acid , B6 and B12 vitmains substantially lowers circulating homocysteine levels , but does not appear to improve outcome in the secondary prevention of cardiovascular disease or dementia .

5 . The diverse biologic effects of folic acid include indirectly increasing asymmetrical dimethylarginine ( ADMA ) levels , a molecule associated with impaired vascular function .

References available on request .

34 MAY 2017 | MEDICAL CHRONICLE