Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:1.5.7.1 (methylenetetrahydrofolate reductase)
 2,116 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The true intracellular substrates for folate-dependent enzymes are folylpolyglutamates. We have used measurements of the Ki values of folylpolyglutamate dead end inhibitors to assess the relative affinities of folate-dependent enzymes for folate derivatives of different polyglutamate chain lengths. Studies of four enzymes from pig liver, methylenetetrahydrofolate reductase, serine hydroxymethyltransferase, methylenetetrahydrofolate dehydrogenase and thymidylate synthase, have indicated that folylpolyglutamate inhibitors are bound 3-500 fold more tightly than the corresponding monoglutamates. The individual enzymes differ in their selectivity for polyglutamate vs. monoglutamate inhibitors, and in the chain length associated with the greatest affinity of enzyme for inhibitor. We have also examined the effect of polyglutamate chain length on the catalytic parameters associated with folate substrates. Two enzymes, methylenetetrahydrofolate reductase and serine hydroxymethyltransferase, show decreases in Km values for folylpolyglutamate substrates. Methylenetetrahydrofolate dehydrogenase shows no detectable differences in the catalytic parameters of polyglutamate vs. monoglutamate substrates and no change in the order of substrate addition or product release. Thymidylate synthase shows small effects of Km and Vmax values, but the order of addition of substrates and of release of products is reversed with polyglutamate as compared with monoglutamate substrates. Our studies with thymidylate synthase from L. casei have shown that the bacterial enzyme also exhibits a greatly increased affinity for polyglutamate vs. monoglutamate derivatives of folic acid, and that reversal in the order of substrate addition and product release also occurs with polyglutamate as compared with monoglutamate substrates. We have also studied the polyglutamate specificity of methionine synthase, which is responsible for the conversion of CH3-H4PteGlu1 into H4PteGlu1. This reaction is required for the incorporation of plasma folate into the cellular folate pool, because methyltetrahydrofolate is a poor substrate for folylpolyglutamate synthetase. Our studies demonstrate that CH3-H4PteGlu6, and suggest that incorporation of plasma CH3-H4PteGlu1 will only occur when methylenetetrahydrofolate reductase is inhibited by adenosylmethionine and cellular pools of CH3-H4PteGlu6 are at very low levels.
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PMID:Folylpolyglutamates as substrates and inhibitors of folate-dependent enzymes. 244 77

Most mammalian cells receive exogenous folate from the bloodstream in the form of 5-methyltetrahydropteroylmonoglutamate (CH3-H4PteGlu1). Because this folate derivative is a very poor substrate for folylpolyglutamate synthetase, the enzyme that adds glutamyl residues to intracellular folates, CH3-H4PteGlu1 must first be converted to tetrahydropteroylmonoglutamate (H4PteGlu1), 10-formyltetrahydropteroylmonoglutamate (CHO-H4PteGlu1), or dihydrofolate (H2folate), which are excellent substrates for folylpolyglutamate synthetase. Polyglutamylation is required both for retention of intracellular folates and for efficacy of folates as substrates for most folate-dependent enzymes. Two enzymes are known that will react with CH3-H4PteGlu1 in vitro, methylenetetrahydrofolate reductase and methyltetrahydrofolate-homocysteine methyltransferase (cobalamin-dependent methionine synthase). These studies were performed to assess the possibility that methylenetetrahydrofolate reductase might catalyze the conversion of CH3-H4PteGlu1 to CH2-H4PteGlu1. CH2-H4PteGlu1 is readily converted to CHO-H4PteGlu1 by the action of methylenetetrahydrofolate dehydrogenase/methenyltetrahydrofolate cyclohydrolase, and these enzyme activities show very little preference for folypolyglutamate substrates as compared with folylmonoglutamates. We conclude from in vitro studies of the enzyme that methylenetetrahydrofolate reductase cannot convert CH3-H4PteGlu1 to CH2-H4PteGlu1 under physiological conditions and that uptake and retention of folate will be dependent on methionine synthase activity.
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PMID:Examination of the role of methylenetetrahydrofolate reductase in incorporation of methyltetrahydrofolate into cellular metabolism. 333 80

The studies discussed in this review support the view that biochemical and clinical symptoms common to both folate and vitamin B12 deficiency are due to the induction of a functional folate deficiency, which in turn is induced by cobalamin deprivation. The interrelationship between these two vitamins is best explained by the methyl trap hypothesis stating that vitamin B12 deficiency can lead to lowered levels of methionine synthetase, which results in a functional folate deficiency by trapping an increased proportion of folate as the 5-methyl derivative. In addition, as 5-methyl-H4PteGlu is a poor substrate for folylpolyglutamate synthetase, there is a decreased synthesis of folylpolyglutamates and consequently a decreased retention of folates by tissues. The real folate deficiency that ensues because of decreased tissue folate levels is probably as important physiologically as the functional deficiency caused by the methyl trap. The sparing effect of methionine can be explained by adenosylmethionine inhibition of methylenetetrahydrofolate reductase, which would prevent the buildup of 5-methyl-H4PteGlun. A deficiency in vitamin B12 would not, in itself, be sufficient to cause a disturbance in folate metabolism. The deficiency would have to result in lowered methyltransferase levels before any such disturbance would be manifest.
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PMID:Vitamin B12-folate interrelationships. 392 46

4-Amino-4-deoxy-5,8,10-trideazapteroyl-d,l-4'-methyleneglutamic acid (CH-1504) is the prototype of a potentially therapeutically more selective class of antifolates for rheumatoid arthritis treatment. This class is characterized by retention of dihydrofolate reductase (DHFR; EC 1.5.1.3) as their locus of action and transport by the reduced folate carrier (RFC; SLC19A1), but their lack of metabolism by known pathways of antifolate (e.g., methotrexate (MTX)) metabolism. Five new CH-1504 analogs (CHL-001-CHL-005) were synthesized and diastereomers of CH-1504 itself were obtained by preparative chiral HPLC; all were characterized biochemically. The analogs are not metabolized by aldehyde oxidase (EC 1.2.3.1), carboxypeptidase G2 (EC 3.4.17.11), or (excepting CHL-003) folylpolyglutamate synthetase (EC 6.3.2.17) and thus, unlike MTX, are "metabolism-blocked". All analogs are potent DHFR inhibitors; several are nearly as potent as MTX or CH-1504. Each analog uses the RFC for transport, although with varying apparent affinities. In contrast, each weakly inhibits other enzymes of folate metabolism relevant to rheumatoid arthritis therapy (thymidylate synthase (EC 2.1.1.45), two formyltransferases of purine biosynthesis (EC 2.1.2.2 and EC 2.1.2.3), and 5,10-methylenetetrahydrofolate reductase (EC 1.5.1.20)). Biochemical characterization showed one 4'-diastereomer of racemic CH-1504 was significantly more active than the other. Based on literature data concerning the effect of d- and l-glutamic acid substitution on antifolate activity, it is likely that the diastereomer containing l-4'-methylene-glutamic acid is the more active. Because of concern about potential pharmacokinetic and biochemical effects of d-4'-methylene-glutamic acid-containing species, these data suggest that future analogs should contain only l-4'-methylene-glutamic acid. Overall, these data provide several interesting new leads for preclinical development.
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PMID:Metabolism-blocked antifolates as potential anti-rheumatoid arthritis agents: 4-amino-4-deoxy-5,8,10-trideazapteroyl-d,l-4'-methyleneglutamic acid (CH-1504) and its analogs. 1917 54