Methyltetrahydrofolate-reductase

Determination of the effective functioning of particular enzymes or metabolic pathways potentially may be useful in demonstrating adequacy of provision. Enzymes in plasma that may be helpful in this regard are glutathione peroxidase as an index of selenium status, and red cell enzymes, such as transketolase (thiamine), glutathione reductase (riboflavin) or transaminase (pyridoxine), or glutathione peroxidase (selenium) are all widely used. Methyltetrahydrofolate reductase is involved in metabolism of homocysteine, hence assessment of plasma homocysteine is a useful measure of... 

Fig. 17.5 Effect of nitric oxide on the synthesis of methionine and S-adenosylmethionine and methylation reactions. NO inhibits methyltetrahydrofolate reductase (MTR). This results in a decrease in tetrahydrofolate (FH4) and methionine. Additional reduction in the FH4 level may occur by the NO-induced oxidation of ferritin, a compound that inhibits the proteasomal degradation of FH4. NO affects SAM synthesis not only by inducing a decrease in methionine synthesis but also by directly inhibiting the liver-specific methyl-thioadenosyltransferase I/III (MATI/III) isozymes. The fall in SAM level cannot be fully compensated by an increase in the extrahepatic isozyme MATH, since this enzyme is inhibited by its reaction product. The reduction in homocysteine utilization for methionine synthesis may result in homocysteine accumulation. This probably does not lead to a consistent rise in cystathionine and reduced glutathione synthesis, dne to a reduced stabilization of cystathionine P-synthase (CBS) by SAM. Consequently, an inciea.se in SAH, associated with a decrease in the SAM/SAH ratio, inhibits methyltransferases (MT) and DNA methylation. The reduction in SAM level may decrease IicBa activation, thus favoring NF-kB activity...

Methylenetetrahydrofolate reductase (MTHFR) catalyzes the NAD(P)H-dependent reduction of 5,10-methylenetetrahydrofolate (CH2-THF) to 5-methyltetrahydrofolate (CH3-THF). CH3-THF then serves as a methyl donor for the synthesis of methionine. The MTHFR proteins and genes from mammalian liver and E. coli have been characterized,12"15 and MTHFR genes have been identified in S. cerevisiae16 and other organisms. The MTHFR of E. coli (MetF) is a homotetramer of 33-kDa subunits that prefers NADH as reductant,12 whereas mammalian MTHFRs are homodimers of 77-kDa subunits that prefer NADPH and are allosterically inhibited by AdoMet.13,14 Mammalian MTHFRs have a two-domain structure the amino-terminal domain shows 30% sequence identity to E. coli MetF, and is catalytic the carboxyterminal domain has been implicated in AdoMet-mediated inhibition of enzyme activity.13,14... 

The best characterized B 12-dependent methyltransferases is methionine synthase (Figure 15.11) from E. coli, which catalyses the transfer of a methyl group from methyltetrahydrofolate to homocysteine to form methionine and tetrahydrofolate. During the catalytic cycle, B12 cycles between CH3-Co(in) and Co(I). However, from time to time, Co(I) undergoes oxidative inactivation to Co(II), which requires reductive activation. During this process, the methyl donor is S-adenosylmethionine (AdoMet) and the electron donor is flavodoxin (Fid) in E. coli, or methionine synthase reductase (MSR) in humans. Methionine synthase... 

Methylenetetrahydrofolate reductase (NADPH) [EC 1.5.1.20] is an FAD-dependent enzyme that catalyzes the reaction of 5-methyltetrahydrofolate with NADP to produce 5,10-methylenetetrahydrofolate and NADPH. 5,10-Methylenetetrahydrofolate reductase (FADH2) [EC 1.7.99.5] is an FAD-dependent enzyme that catalyzes the reaction of 5-methyltetrahydrofolate with an acceptor to produce 5,10-methylenetetrahydrofolate and the reduced acceptor. 

Methylentetrahydrofolate reductase (MTHFR) is another key enzyme of 5-FU metabolism, alternating 5-FU sensitivity indirectly by folate pool variations. MTHFR plays an important role in the action of 5-FU, an inhibitor of TS, by converting 5,10-methylenetetrahydrofolate, a substrate of TS, to 5-methyltetrahydrofolate (19). 

Methotrexate inhibits folate metabolism by preventing methylenetetrahydrofolate reductase from converting 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate thus inhibiting thymidylate synthase conversion of dUMP to dTMP. DNA replication is effectively decreased by the diminution of dTMP availability. As shown in Fig. 2, multiple enzymes mediate the folate cycle. Thus, genetic variation in these enzymes may... 

The loss of a methyl group from AdoMet in each of the reactions yields S-ad-enosylhomocysteine (AdoHcy) and this is subsequently hydrolysed to adenosine and Hey by AdoHcy-hydrolase. Hey sits at a metabolic branch point and can be remethylated to methionine by way of two reactions. One is the 5-methyltetrahydrofo-late dependent reaction catalysed by methionine synthase, which itself is reductively methylated by cobalamin (vitamin B12) and AdoMet, requiring methionine synthase reductase. 5-Methyltetrahydrofolate is generated from 5,10-methylenetetrahydrofo-late (MTHF) by MTHF reductase. The second remethylation reaction is catalysed by betaine methyltransferase, which is restricted to the liver, kidney and brain, while methionine synthase is widely distributed. 

Figure 21-2. Metabolism of homocysteine. BHMT, betaineihomocysteine methyl-transferase CBS, cystathionine P-synthase Cob, cobalamin CTH, cystathionine y-lyase DHF, dihydrofolate DMG, dimethylglycine FAD, flavin adenine dinucleotide MAT, methionine adenosyltransferase 5-MTHF, 5-methyltetrahydrofolate 5,10-MTHF, 5,10-methylenetetrahydrofolate MTHFR, methylenetetrahydrofolate reductase MS, methionine synthase MTRR, methionine synthase reductase MTs, methyl transferases PLE pyridoxal phosphate SAH, S-adenosylhomocysteine SAHH, SAH hydrolase SAM, 5-adenosylmethionine SHMT, serine hydroxymethyltransferase THF, tetrahydrofolate Zn, zinc.

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

Fig. 6 Methyl trap hypothesis 5,10-Methylenetetrahydrofolate is reduced to 5-methyltetiahy-drofolate in an irreversible reaction. When vitamin Bn is deficient, methyl groups are trapped as 5-methyltetrahydrofolate, resulting in decreased substrates for DNA synthesis and neural lipid methylation. MTHFR, methylenetetrahydrofolate reductase DHFR, dihydrofolate reductase MS, Methionine synthase TS, thymidylate synthase SAM, S-adenosyl-methionine dUMP, deoxyuridine 5 -monophosphate dTTP, deoxythymidine 5 -monophosphate...

Serine-hydroxymethyl transferase, methylenetetrahydrofolate reductase, and methyltetrahydrofolate-homocysteine methyltransferase, mechanism of biological methylation with 90CRV1275. 

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]. 

Figure 8 Extended folate metabolism, including compartmentation. MTHFR, methylenetetrahydrofolate reductase SHMT, serine hydroxymethyltransferase BHMT, betaine homocysteine methyltransferase, MAT, methionine adenosyltransferase SAH-hydrolase, S-adenosylhomocysteine hydrolase MT, methyltransferase CBS, cystathionine /i-synthase SAM, S-adenosylmethionine SAH, S-aden-osylhomocysteine THF, tetrahydrofolate and 5-MeTHF, 5-methyltetrahydrofolate. (Reproduced from Van der Put etal. (2001) Folate, homocysteine and neural tube defects An overview. Experimental Biology and Medicine 226 243-270.)...

Methylenetetrahydrofijlate reductase activity was found to be 20% or less than that present in controls (M14). The enzyme is required for the reduction of methylenetetrahydrofolate to methyltetrahydrofolate however, the lack of methyltetrahydrofolate did not result in any anemia and the blood picture has been normal in the patients so far described. 

Deficiency of either vitamin Bj or folate decreases the synthesis of methionine and SAM, thereby interfering with protein biosynthesis, a number of methylation reactions, and the synthesis of polyamines. In addition, the cell responds to the deficiency by redirecting folate metabolic pathways to supply increasing amounts of methyltetrahydrofolate this tends to preserve essential methylation reactions at the expense of nucleic acid synthesis. With vitamin Bj deficiency, methylenetetrahydro-folate reductase activity increases, directing available intracellular folates into the methyltetrahydrofolate pool (not shown in Figure 53-6). The methyltetrahydrofolate then is trapped by the lack of sufficient vitamin Bj to accept and transfer methyl groups, and subsequent steps in folate metabolism... 

The (3R)- and (3S)-[3- Hi,3- Hi]serines 60, Hj, = H, H = H, and 60, H = H, Hj = H, respectively, were also used with the coupled enzymes serine hydroxymethyltransferase and methylenetetrahydrofolate reductase (EC 1.1.1.171) (100). Enzymic reduction of the labeled samples of the intermediate 5,10-methylenetetrahydrofolic acid 56b gave samples of 5-methyltetrahydrofolic acid 91 (Scheme 26). These were degraded to acetate by a sequence that involved one inversion of configuration, and assay of the acetates showed that the overall stereochemistry of the reduction was as in Scheme 26 (100). 

Homocysteine lies at a metabolic crossroad it may condense with serine to form cystathionine, or it may undergo remethylation, thereby conserving methionine. There are two pathways for remethylation in humans. In one, betaine provides the methyl groups, while in the other 5-methyltetrahydrofolate is the methyl donor. This latter reaction is catalyzed by a Bj -containing enzyme, 5-methyltetrahydrofolate homocysteine methyltransferase. Two defects in this latter mechanism may account for the inability to carry out remethylation. In one of them, patients are unable to synthesize or accumulate methylcobalamin, while others cannot produce the second cofactor, 5 -methyltetrahydrofolate, because of adefect in 5,10-methylenetrahydrofolate reductase. 

Fig. 20.3 Pathway of methionine metabolism. The numbers represent the following enzymes or sequences (1) methionine adenosyltransferase (2) S-adenosylmethionine-dependent transmethylation reactions (3) glycine methyltransferase (4) S-adenosylhomocysteine hydrolase (5) betaine-homocysteine methyltransferase (6) 5-methyltetrahydrofolate homocysteine methyltransferase (7) serine hydroxymethyltransferase (8) 5,10-methylenetetrahydrofolate reductase (9) S-adenosylmethionine decarboxylase (10) spermidine and spermine synthases (11) methylthio-adenosine phosphorylase (12) conversion of methylthioribose to methionine (13) cystathionine P-synthase (14) cystathionine y-lyase (15) cysteine dioxygenase (16) cysteine suplhinate decarboxylase (17) hypotaurine NAD oxidoreductase (18) cysteine sulphintite a-oxoglutarate aminotransferase (19) sulfine oxidase. MeCbl = methylcobalamin PLP = pyridoxal phosphate...

Methyltetrahydrofolate (5-MTHF). The circulating form of folic acid in humans. 5-MTHF is produced by 5,10-methylenetetrahydrofolate via the action of MTFH reductase (MTHFR). 5-MTHF scavenges peroxynitrites, the main BH4 oxidant, and helps to BH4 regeneration inside the human vascular wall. It is considered the key mediator of folic acid s vascular effects (as in the presence of the C677T mutation in MTHFR gene that reduces enzyme s activity almost by half). 

L-Met 7 and SAM 1, respectively. A similar increase in metabolic flux toward SAM 1 was achieved by recombinant expression of a chimeric gene for methylenetetrahy-drofolate reductase in Saccharomyces cerevisiae [80], which increases availability of the co-substrate of Met synthase, N -methyltetrahydrofolate (Figure 18.4). 

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... 

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