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