5,10-Methylene-tetrahydrofolates

A second relevant enzyme to 5-FU metabolism is thymidylate synthetase (TYMS). If this enzyme is complexed with 5-FU metabolites along with 5,10-methylene-tetrahydrofolate, it cannot maintain a thymidine-5 -monophosphate pool required for DNA replication and repair. A tandem repeat polymorphism in the 5 -promoter region of the TYMS gene can increase enzyme expression (85,86). Tumors carrying the repeats have higher enzyme expression, resulting in lower response to chemotherapy compared to wild type (87). 

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.)...

He isoleucine, Kyn kynurenine, Leu leucine, Lys lysine, Met methionine, MTHFR 5,10-methylene tetrahydrofolate reductase, Orn ornithine, p plasma, P5C pyrroline-5-carboxylic acid, PEA phosphoethanolamine, Phe phenylalanine, P-Hyl O-phosphohydroxylysine, Pip pipecolic acid, Pro proline, Sacch saccharopine, Sar sarcosine, Ser serine,... 

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. 

MTHF (Fig. 6.3.1) is synthesized biologically from 5,10-methylene-tetrahydrofolic acid through the action of MTE1FR. Commercially, it is available as a calcium salt. The 5MTHF calcium salt is soluble as 500 mg in 100 ml water. A 1 mM solution... 

Dimethylglycine and sarcosine dehydrogenases in the catabolism of choline (Section 14.2.1). In these reactions, a methyl group in the substrate is oxidized by FAD, then the intermediate adduct undergoes hydrolysis to release formaldehyde, which reacts with tetrahydrofolate to form 5,10-methylene tetrahydrofolate. 

The possible relationship of the methane fermentation with the more conventional examples of one-carbon metabolism as catalyzed by folate and vitamin B12 cofactors has been long apparent. 5-Methyl tetrahydro-folate, 5,10-methylene tetrahydrofolate, and methyl vitamin B12 are converted to methane by cell-free extracts of M. barkeri 32) and M. omeli-anskii (33). The involvement of vitamin B12 is further implicated by its high cellular level in methane bacteria and by the isolation of B12-containing proteins in extracts of M. barkeri 30) which stimulate methane evolution from methyl vitamin B12. The components and pathways that can be demonstrated in cell-free M. barkeri extracts 32) are listed below. 

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]. 

Metabolic Role. Riboflavin coenzymes are required for most oxidations of carbon-carbon bonds (Fig. 8.29). Examples include the oxidation of succinyl CoA to fumarate in the Krebs cycle and introduction of a,jS-unsaturation in /3-oxidation of fatty acids. Riboflavin is also required for the metabolism of other vitamins, including the reduction of 5,10-methylene tetrahydrofolate to 5-methyl tetrahydrofolate (Fig. 8.49), and interconversion of pyridoxine-pyridoxal phos-phate-pyridoxamine (Fig. 8.33). Because oxi-dation/reductions that use FAD or FMN as the coenzyme constitute a two-step process, some flavin coenzyme systems contain more than one FAD or FMN. 

Fig. 6. Chemical structures of inhibitors of thymidylate synthase designed based on modifications of 5,10-methylene tetrahydrofolate (a) ZD1694, (b) OSI1843U89. (c) CB3717 (34).

Thymidylate synthase catalyzes the conversion of 2 -deoxyuridine 5 -phosphate to thymidine 5 -phosphate, and it is the sole source of this essential component of DNA. The reaction proceeds by transfer of a one-carbon fragment from the cofactor, 5,10-methylene tetrahydrofolate (CH2H4folate), of the... 

The reaction catalyzed by thymidylate synthase is the only one known in the cell in which tetrahydrofolate is not regenerated. Dihyrofolate reductase, thus, plays an essential role in the ultimate regeneration of 5,10-methylene tetrahydrofolate. This enzyme, which can be inhibited by the drug methotrexate, is a target of some anticancer treatments. 

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