Folate methylene-tetrahydrofolate reductase

Kaye JM, Stanton KG, McCann VJ, Vasikaran VB, Taylors RR, van Bockxmeer FM. 2002. Homocysteine, folate, methylene tetrahydrofolate reductase genotype and vascular morbidity in diabetic subjects. Clin Sci 102 631-637. 

Supplements of 400 Ig/d of folate begun before conception result in a significant reduction in the incidence of neural mbe defects as found in spina bifida. Elevated blood homocysteine is an associated risk factor for atherosclerosis, thrombosis, and hypertension. The condition is due to impaired abihty to form methyl-tetrahydrofolate by methylene-tetrahydrofolate reductase, causing functional folate deficiency and resulting in failure to remethylate homocysteine to methionine. People with the causative abnormal variant of methylene-tetrahydrofolate reductase do not develop hyperhomocysteinemia if they have a relatively high intake of folate, but it is not yet known whether this affects the incidence of cardiovascular disease. 

Fig. 14.1 Cellular pathway of methotrexate. ABCBl, ABCCl-4, ABC transporters ADA, adenosine deaminase ADP, adenosine diphosphate AICAR, aminoimidazole carboxamide ribonucleotide AMP, adenosine monophosphate ATIC, AICAR transformylase ATP, adenosine triphosphate SjlO-CH -THF, 5,10-methylene tetrahydrofolate 5-CHj-THF, 5-methyl tetrahydro-folate DHFR, dihydrofolate reductase dTMP, deoxythymidine monophosphate dUMP, deoxy-uridine monophosphate FAICAR, 10-formyl AICAR FH, dihydrofolate FPGS, folylpolyglutamyl synthase GGH, y-glutamyl hydrolase IMP, inosine monophosphate MTHFR, methylene tetrahydrofolate reductase MTR, methyl tetrahydrofolate reductase MTX-PG, methotrexate polyglutamate RFCl, reduced folate carrier 1 TYMS, thymidylate synthase. Italicized genes have been targets of pharmacogenetic analyses in studies published so far. (Reproduced from ref. 73 by permission of John Wiley and Sons Inc.)...

CFD is further associated with the following inherited metabolic disorders 5,10-methylen-tetrahydrofolate reductase (MTHFR) deficiency [7], 3-phos-phoglycerate dehydrogenase (PGDH) deficiency [8], dihydropteridine reductase (DHPR) deficiency [9], as well as with Rett syndrome [10], and Aicardi-Gou res Syndrome [11]. Furthermore, folate deficiency may be associated with congenital folate malabsorption, severe malnutrition, and formiminotransferase deficiency. 

Riboflavin (vitamin B2) Folate cycle reduction of 5,10-methyltetrahydrofolate cofactor for methylene-tetrahydrofolate reductase... 

In the folate coenzymes, the pteridine ring is fully reduced to tetrahydro-folate, although the oxidized form, dihydrofolate, is an important metabolic intermediate. In the reactions of thymidylate synthetase (Section 10.3.3) and methylene tetrahydrofolate reductase (Section 10.3.2.1), the pteridine ring has a redox role in the reaction. The folate coenzymes are conjugated with up to six additional glutamate residues, finked by y-glutamyl peptide bonds. 

Methylene-Tetrahydrofolate Reductase The reduction of methylene-tetrahydrofolate to methyl-tetrahydrofolate, shown in Figure 10.7, is catalyzed hy methylene-tetrahydrofolate reductase, a flavin adenine dinucleotide-dependent enzyme during the reaction, the pteridine ring of the substrate is oxidized to dihydrofolate, then reduced to tetrahydrofolate by the flavin, which is reduced by nicotinamide adenine dinucleotide phosphate (NADPH Matthews and Daubner, 1982). The reaction is irreversible under physiological conditions, and methyl-tetrahydrofolate - which is the main form of folate taken up into tissues (Section 10.2.2) - can only be utilized after demethylation catalyzed by methionine synthetase (Section 10.3.4). 

This functional deficiency of folate is exacerbated by the associated low concentrations of methionine and S-adenosyl methioitine, although most tissues (apart from the central nervous system) also have betaine-homocysteine methyltransferase that may be adequate to maintain tissue pools of methionine. Under normal conditions S-adenosyl methioitine inhibits methylene-tetrahydrofolate reductase and prevents the formation of further methyl-tetrahydrofolate. Relief of this inhibition results in increased reduction of one-carbon substituted tetrahydrofolates to methyl-tetrahydrofolate. 

Administration of diphenylhydantoin leads to decreased activity of methylene tetrahydrofolate reductase and an increased rate of oxidation of formyl tetrahydrofolate (increased oxidation of formate and histidine), with a fall in methylene- and methyl-tetrahydrofolate - the reverse of the effect of the methyl folate trap (Billings, 1984a, 1984b). 

In experimental animals and with isolated tissue preparations and organ cultures, the test can be refined by measuring the production of G02 from [ C]histidine in the presence and absence of added methionine. If the impairment of histidine metabolism is the result of primary folate deficiency, the addition of methionine wUl have no effect. By contrast, if the problem is trapping of folate as methyl-tetrahydrofolate, the addition of methionine will restore normal histidine oxidation as a result of restoring the inhibition of methylene-tetrahydrofolate reductase by S-adenosylmethionine and restoring the activity of 10-formyl-tetrahydrofolate dehydrogenase, thus permitting more normal folate metabolism (Section 10.3.4.1). 

People with the abnormal variant of methylene-tetrahydrofolate reductase do not develop hyperhomocysteinaemia if they have a relatively high intake of folate. This seems to be due to the methylation of folate in the intestinal mucosa during absorption intestinal mucosal cells have a rapid turnover (section 4.1), and therefore it is not important that methylene-tetrahydrofolate reductase is less stable than normal — there is still an adequate activity of the enzyme in the intestinal mucosa to maintain absorption of methyl-tetrahydrofolate. 

Folate supplements of 400 Ig/day reduce the incidence of spina bifida and neural tube defect about 1% of pregnant women are at risk. Similar supplements lower plasma homocysteine in people with the abnormal variant of methylene tetrahydrofolate reductase (section 11.3.3) between 10% and 20% of the population are homozygous for the abnormal gene. 

Riboflavin in the form of FAD is an essential coenzyme for 5,10-methylene tetrahydrofolate reductase, a key enzyme of the folate pathway, which catalyzes the interconversion of 5,10-methylene-tetrahydrofolate and 5-methyltetrahydrofolate. Of the known single nucleotide polymorphisms affecting this enzyme, the best known are the C699T and A1298C variants. The former confer thermolability and potentially reduced enzyme activity in the TT homozygote. Marginal riboflavin status may, in some situations, be associated with increased plasma homocysteine levels (possibly predictive of increased... 

The other major class of antimalarials are the folate synthesis antagonists. There is a considerable difference in the drug sensitivity and affinity of dihydrofolate reductase enzyme (DHFR) between humans and the Plasmodium parasite. The parasite can therefore be eliminated successfully without excessive toxic effects to the human host. DHFR inhibitors block the reaction that transforms deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP) at the end of the pyrimidine-synthetic pathway. This reaction, a methylation, requires N °-methylene-tetrahydrofolate as a carbon carrier, which is oxidized to dihydrofolate. If the dihydrofolate cannot then be reduced back to tetrahydrofolate (THF), this essential step in DNA synthesis will come to a standstill. 

Methylation of homocysteine by 5-methyltetrahydrofolate-homocysteine methyl reductase depends on an adequate supply of 5-methyltetrahydrofoIate. The unmethylated folate is recycled in a cobalamin-dependent pathway, by remethylation to 5,10-methylene-tetrahydrofolate, and subsequent reduction to 5-methyltetrahydrofolate. The transferase enzyme, also named 5,10-methyltretrahydrofolate reductase catalyzes the whole cycle [3,91]. S-adenosylmethionine and 5-methyltetrahydrofolate are the most important methyl unit donors in biological system. S-adenosylmethionine is reported to regulate methylation and transsulfuration pathways in the homocysteine metabolism [3,91]. 

For thymidylate synthase in animals, plants, and many but not all bacteria, methylene tetrahydrofolate serves not only as a Cl transponder but also as a reductive agent the dihydrofolate resulting from that reaction needs to be recycled by the salvage action of dihydrofolate reductase, the most intensely studied enzyme in the family of folate-cofactor processing enzymes. 

The methyl donor is methylene-tetrahydrofolate the reaction involves reduction of the one-carbon fragment to a methyl group at the expense of the folate, which is oxidized to dihydrofoiate. Dihydrofoiate is then reduced to tetrahydrofolate by dihydrofoiate reductase. 

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