Polyglutamyl derivatives of tetrahydrofolate as substrates for Lactobacillus casei thymidylate synthase

Effects of antisense-based folypoly-gamma-glutamate synthetase down-regulation on reduced folates and cellular proliferation in CCRF-CEM cells

The effect of down-regulation of folylpoly-gamma-glutamate synthetase (FPGS) activity on intracellular reduced folate accumulation and cellular proliferation was examined, using an inducible antisense expression system in the human T-lymphoblastic leukemia cell line CCRF-CEM. FPGS catalyzes the addition of gamma-glutamyl residues to natural folates and classical antifolates, which results in their enhanced cellular retention and increased cytotoxicity. As such, this enzyme has become a focus as a potential anticancer drug target. However, direct evidence to support this concept has been elusive. Hence, a study was undertaken using an antisense-based expression system to down-regulate FPGS activity. This inducible expression system was used to demonstrate that lower FPGS activity can lead to substantially lower intracellular folate content, which coincides with suppression of thymidylate synthesis and inhibition of cellular proliferation.

Tetrahydrohomofolate polyglutamates as inhibitors of thymidylate synthase and glycinamide ribonucleotide formyltransferase in Lactobacillus casei

In order to determine the mechanism for the effects of homofolates on growth of Lactobacillus casei, polyglutamated derivatives of homofolate (HPteGlu), dihydrohomofolate and tetrahydrohomofolate (H4HPteGlu) were synthesized and tested as inhibitors of folate-requiring enzymes. The following L. casei enzymes were examined: thymidylate synthase (TS), glycinamide ribonucleotide formyltransferase (GARFT), aminoimidazolecarboxamide ribonucleotide formyltransferase, serine hydroxymethyltransferase and dihydrofolate reductase. Polyglutamates of (6R,S)-H4HPteGlu are potent inhibitors of TS and GARFT. For example, the IC50 values of (6R,S)-H4HPteGlu6 are 0.7 microM for TS and 0.3 microM for GARFT. By contrast, the value for HPteGlu6 is greater than 10 microM for both TS and GARFT. Inhibition of TS and GARFT by (6R,S)-H4HPteGlu derivatives increases with polyglutamate chain length. For TS, the Glu5 and Glu6 derivatives of (6R,S)-H4HPteGlu are 20 and 30 times more potent than the monoglutamate, respectively. For GARFT, the Glu2-6 derivatives are 2-3 times more potent than Glu1. Inhibition of TS and GARFT by (6R,S)-H4HPteGlu polyglutamates is almost entirely due to the unnatural (6R) diastereomer at C-6. Homofolate derivatives are only weak inhibitors of aminoimidazolecarboxamide ribonucleotide formyltransferase, serine hydroxymethyltransferase, and dihydrofolate reductase. We conclude that both TS and GARFT are potential targets of (6R)-H4HPteGlu polyglutamates.

Deoxyuridylate effects on thymidylate synthase-5-fluorodeoxyuridylate-folate ternary complex formation

The competitive basis and specificity of deoxyuridylate (dUMP)-mediated decreases in thymidylate synthase-5'-fluorodeoxyuridylate-folate (TS-FdUMP-folate) ternary complex formation at low concentrations of folates were investigated using charcoal isolation of protein-bound [3H]FuUMP ligand. Reaction conditions used 0.02 microM TS (Lactobacillus casei) and 0.10 microM [3H]FdUMP incubated for 10 min at 37 degrees and pH 7.4. Decreases in counts below control (C) values in dUMP-added samples (S) were expressed as C/S ratios. At CH2--H4PteGlu1 or H4PteGlu1 concentrations below 10 microM, highly linear relationships were found to exist between C/S value and dUMP concentrations, expressed as dUMP/FdUMP ratios. For H4PteGlu1, maximal C/S values for dUMP interference occurred at the lowest H4PteGlu1 concentrations, approaching the value of the TS-FdUMP binary complex. The efficiency of ternary complex formation by H4PteGlu1 was 28 +/- 5% of CH2--H4PteGlu1 values at concentrations below 1.0 microM. The protective effect of increasing H4PteGlu1 against dUMP interference resulted in a linear relationship between the logarithm of H4PteGlu1 concentration and the slope of dUMP interference (C/S vs dUMP/FdUMP). In contrast, the results with CH2--H4PteGlu1 were biphasic. At concentrations of CH2--H4PteGlu1 lower than 0.5 microM, C/S values were greater than those for binary complex alone, a result related to CH2--H4PteGlu1 consumption based on [5-3H]dUMP tritium-release studies. At concentrations of CH2--H4PteGlu1 above 1.0 microM, however, dUMP interference was nearly abolished. Kinetic analysis of the data suggests that this effect of the 5,10-methylene moiety may result in part from positive allosteric effects of first site TS-FdUMP-CH2--H4PteGlu1 ternary complex binding on acceleration of second site binding, in addition to slowed rates of dissociation. Other folylmonoglutamates showed relatively poor TS-[3H]FdUMP-folate complex formation: at 500 microM folate, as a percentage of CH2--H4PteGlu1 values, these were 29.6% for dihydrofolate, 7.5% for 5-CH3--H4PteGlu1, 3.0% for CH = H4PteGlu1, 1.6% for folic acid, 1.1% for 5-CHO--H4PteGlu1 (leucovorin) and 0.9% for 10-CHO--H4PteGlu1. Inhibitory effects by dUMP were consistent with binary complex effects alone for these folates. Study of methotrexate, as the monoglutamate and the hexaglutamate, suggested that ternary complexes with dUMP are favored over those with FdUMP at high concentrations of the antifolate. Our results indicate that activation of leucovorin to over 0.5 microM in intracellular CH2--H4PteGlu1 equivalents may be a requirement for achieving complete TS inhibition by FdUMP in the presence of excess conce

Effect of polyglutamylation of 5,10-methylenetetrahydrofolate on the binding of 5-fluoro-2'-deoxyuridylate to thymidylate synthase purified from a human colon adenocarcinoma xenograft

CH2-H4PteGlu and H4PteGlu exist in human colon adenocarcinoma xenografts predominantly in the form of polyglutamate species at concentrations of less than 3 microM. The interaction of polyglutamates of [6R]CH2-H4PteGlu in the formation and stability of [6-3H]FdUMP-thymidylate synthase-CH2-H4PteGlun ternary complexes has therefore been examined using enzyme purified from a human colon adenocarcinoma xenograft. Dissociation of these complexes was first-order and was dependent upon the concentration of folate. [6R]CH2-H4PteGlu3-6 (0.9 to 1.6 microM) were greater than 200-fold and [6R]CH2-H4PteGlu2 (18.2 microM) was 18-fold more effective than [6R]CH2-H4PteGlu1 (335 microM) at stabilizing ternary complexes for a T1/2 for dissociation of 100 min. Polyglutamylation of CH2-H4PteGlu also increased the affinity of binding of [6-3H]FdUMP to thymidylate synthase as determined by Scatchard analysis at folate concentrations of 10 microM, where the Kd in the presence of [6R]CH2-H4PteGlu1 was in the order of 4.0 x 10(-8) M, and for [6R]CH2-H4PteGlu3-5 was between 3.7 and 5.5 x 10(-9) M. To examine whether this effect was due to differences in the rates at which [6-3H]FdUMP was bound (kon) or dissociated (koff) from the enzyme, the apparent rate of [6-3H]FdUMP binding was determined in the presence of [6R]CH2H4PteGlu1, [6R]CH2-H4PteGlu3 and [6R]CH2-H4PteGlu4. The kon values were similar and were in the range of 1.7 to 2.3 x 10(6) M-1 min-1 for 10 or 20 microM folate concentrations. Differences in binding affinity determined for [6R]CH2-H4PteGlu1 and longer polyglutamate forms of [6R]CH2-H4PteGlu were thus due to differences in koff. The Vmax for the initial velocity of [6-3H]FdUMP binding was achieved at 10 microM folate. Consequently, at concentrations of CH2-H4PteGlu polyglutamates present in tumors, inhibition of thymidylate synthase by FdUMP in vivo would be expected to be transient, based upon the concentration of [6R]CH2-H4PteGlun required for maximal formation and stability of the covalent ternary complex. It would be advantageous for modulation of CH2-H4PteGlun pools to increase the concentrations of the longer polyglutamate species (n greater than or equal to 3) to maximize the interaction between FdUMP, thymidylate synthase and CH2-H4PteGlu.

Folylpolyglutamate synthesis and role in the regulation of one-carbon metabolism

The physiological importance of folylpolyglutamates is now well established. These derivatives are the intracellular substrates and regulators of one-carbon metabolism, and their synthesis is required for normal folate retention by tissues. Over the last few years, a considerable amount of information has been obtained on the mechanism by which these compounds are synthesized, on how this synthesis is regulated, and on the effects of the polyglutamate chain on the interaction of folate substrates and inhibitors with folate-dependent enzymes. Many regulatory implications have been suggested by these studies, but the physiological relevance of some of these observations remains to be explored. Folates in mammalian tissues are metabolized to polyglutamates of chain lengths considerably longer than that required for folate retention, but the metabolic advantages of this are not entirely clear. Several in vivo model systems have been developed to explore the functioning of specific folylpolyglutamate chain lengths in metabolic cycles of one-carbon metabolism, and these are likely to shed further light on this point. The role of folate-binding proteins in folate transport, the metabolic role of glutamylhydrolases, and the role of folylpolyglutamates in putative multifunctional protein complexes are also areas that are being actively pursued at present and are likely to produce new insights in the future. Recent studies on the retention of antifolates by cells and on their substrate efficacy for folylpolyglutamate synthetases have also suggested mechanisms for the differential cytotoxicity of these agents for different tissues.

Characteristics of thymidylate synthase purified from a human colon adenocarcinoma

Thymidylate synthase has been purified greater than 4000-fold from a human colon adenocarcinoma maintained as a xenograft in immune-deprived mice. In this disease, the enzyme is an important target for the cytotoxic action of 5-fluorouracil, which is influenced by the reduced folate substrate CH2-H4PteGlu. Due to the importance of this interaction, and the existence in cells of folate species as polyglutamyl forms, the interaction of folylpolyglutamates with thymidylate synthase was examined. Polyglutamates of PteGlu were used as inhibitors, and the interaction of CH2-H4PteGlu polyglutamates as substrates or in an inhibitory ternary complex were also examined. Using PteGlu1-7, Ki values were determined. A maximal 125-fold decrease in Ki was observed between PteGlu1 and PteGlu4; further addition of up to three glutamyl residues did not result in an additional decrease in Ki. Despite the increased binding affinity of folypolyglutamates for this enzyme, no change in the Km values for either dUMP (3.6 microM) or CH2-H4PteGlu (4.3 microM) were detected when polyglutamates of [6R]CH2-H4PteGlu were used as substrates. Product inhibition studies demonstrated competitive inhibition between dTMP and dUMP in the presence of CH2-H4PteGlu5. In addition, CH2-H4PteGlu4 stabilized an inhibitory ternary complex formed between FdUMP, thymidylate synthase, and CH2-H4PteGlu4. Thus the data do not support a change in the order of substrate binding and product release upon polyglutamylation of CH2-H4PteGlu reported for non-human mammalian enzyme. This is the first study to characterize kinetically thymidylate synthase from a human colon adenocarcinoma.

Overview: rational basis for development of fluoropyrimidine/5-formyltetrahydrofolate combination chemotherapy

Fluorodeoxyuridylate (FdUMP) and thymidylate synthase (TS) are one of the better understood systems of drug-target interaction in cancer chemotherapy. Isolation and characterization of TS (initially from Lactobacillus casei and later from a variety of other sources), cloning and sequencing of the gene, determination of the 3-D structure of the enzyme by X-ray diffraction, and elucidation of the structure of both the catalytic intermediate and the enzyme-inhibitor complex have revealed critical parameters of the target at the molecular level. Potentiation of FdUMP binding by 5,10-methylenetetrahydrofolate (CH2-FH4), discovered at the enzymatic level, has been exploited to increase the clinical effectiveness of fluoropyrimidines. CH2-FH4 can be generated from folate, 5-methyltetrahydrofolate, or 5-formyltetrahydrofolate (citrovorum factor, CF); the latter is the compound of choice for therapeutic regimens. Transformation of CF to CH2-FH4 can occur via two pathways: (a) CF----5,10-methyltetrahydrofolate----CH2-FH4; or (b) CF----tetrahydrofolate----CH2-FH4. The relative importance of these pathways in various cells is not yet clear. The role of CH2-FH4 in FdUMP toxicity, and its central position in folate coenzyme-dependent C1 metabolism, emphasize the need for development of methods to quantitate intracellular levels of this compound.

Rat hepatic uroporphyrinogen III co-synthase. Purification and evidence for a bound folate coenzyme participating in the biosynthesis of uroporphyrinogen III

Rat hepatic uroporphyrinogen III co-synthase was isolated and purified 73-fold with a 13% yield by (NH4)2SO4 fractionation and sequential chromatography on DEAE-Sephacel, Sephadex G-100 (superfine grade) and folate-AH-Sepharose 4B. The purified co-synthase has an Mr of approx. 42 000, and is resolved into two bands, each possessing co-synthase activity, by polyacrylamide-gel electrophoresis. A factor was dissociated from the purified co-synthase. Results of both microbiological and competitive protein-binding assays suggest that it is a pteroylpolyglutamate. The isolated pteroylpolyglutamate factor was co-eluted with authentic N5-methyltetrahydropteroylheptaglutamate on DEAE-Sephacel. Uroporphyrinogen III is formed by cosynthase-free preparations of uroporphyrinogen I synthase in the presence of tetrahydropteroylglutamate. Tetrahydropeteroylheptaglutamate is also able to direct the formation of equivalent amounts of uroporphyrinogen III at a concentration approximately one-hundredth that of tetrahydropteroylmonoglutamate. These results suggest that a reduced pteroylpolyglutamate factor is associated with rat hepatic uroporphyrinogen III co-synthase, and that this may function as a coenzyme for the biosynthesis of uroporphyrinogen III.

Methotrexate analogues—XVII

A new analogue of methotrexate was synthesized from 4-amino-4-deoxy-N10-methylpteroic acid and D,L-homocysteic acid. The product (mAPA-HCysA) was bound tightly to L1210 mouse leukemia dihydrofolate reductase (IC50 = 1 nM), inhibited L1210 cell proliferation in culture (IC50 = 0.3 microM), and prolonged the survival of L1210 leukemic mice (98% increase in lifespan at 120 mg/kg, qdx9). Studies on the interaction of mAPA-HCysA with partially purified mouse liver folyl polyglutamate synthetase revealed that mAPA-HCysA was not a substrate. Hence, the increased dose of mAPA-HCysA required to inhibit tumor growth in vitro and in vivo relative to methotrexate may reflect, in part, the inability of this compound to form non-effluxing polyglutamates. Folyl polyglutamate synthetase was competitively inhibited by mAPA-HCysA (K1 = 190 +/- 70 microM) when folate was the variable substrate. Thus, mAPA-HCysA is the first known compound to inhibit both mammalian dihydrofolate reductase and mammalian folyl polyglutamate synthetase.

Molecular basis for the interaction of polyglutamates of folic acid and its analogs with dihydrofolate reductase

Fluorimetric titration has been used to measure the dissociation constants for the complexes of folate, pteroyltriglutamate and pteroylheptaglutamate with dihydrofolate reductase purified from Lactobacillus casei, Streptococcus faecium (isoenzyme 2) and bovine liver. Effects of pH, temperature, salt concentration and second ligands have been examined. The method is shown to be unsuitable for methotrexate complexes. The polyglutamates do not bind more tightly than folate to the S. faecium reductase under any conditions examined, but bind somewhat more tightly than folate to the L. casei reductase at low pH (less than 7) and to the bovine liver enzyme at pH 7-9. Increasing concentrations of KC1 decrease the binding of all three ligands to the L. casei and bovine liver enzymes. Increasing pH markedly raises the dissociation constants for all complexes of the L. casei reductase, but has only slight effects on the complexes of the S. faecium reductase. Complexes of the bovine enzyme are affected to an intermediate degree by pH, but the folate complex is affected much more than those of the polyglutamates. Model building studies have been performed with a three-dimensional model of the complex of L. casei reductase with NADPH and methotrexate. Additional glutamyl groups were added in gamma-linkage to the glutamate moiety of the complexed methotrexate. A proposed mode of binding of the pteroyl polyglutamates is discussed and sequence comparisons are used to predict residues that might be involved in polyglutamate binding by reductase from other sources.

Regulation of folylpoly-gamma-glutamate synthesis in bacteria: in vivo and in vitro synthesis of pteroylpoly-gamma-glutamates by Lactobacillus casei and Streptococcus faecalis

Lactobacillus casei and Streptococcus faecalis accumulated labeled folic acid and metabolized this compound to poly-gamma-glutamates of chain lengths of up to 11 and 5, respectively. Octa- and nonaglutamates predominated in L. casei, and tetraglutamates predominated in S. faecalis. The most effective monoglutamate substrates for the L. casei and S. faecalis folylpoly-gamma-glutamate (folylpolyglutamate) synthetases were methylene- and formyltetrahydrofolate, respectively. Methylenetetrahydropteroylpoly-gamma-glutamates were the preferred poly-gamma-glutamate substrates for both enzymes and, in each case, the highest activity was observed with the diglutamate substrate. The final distribution of folylpolyglutamates in these bacteria appeared to reflect the ability of folates with various glutamate chain lengths to act as substrates for the bacterial folylpolyglutamate synthetases. The proportions of individual folylpolyglutamates were markedly affected by culturing the bacteria in medium containing adenine, whereas thymine was without effect. Adenine did not affect the level of folylpolyglutamate synthetase in either organism but caused a large increase in the proportion of intracellular folates containing one-carbon units at the oxidation level of formate, folates which are substrates for enzymes involved in purine biosynthesis. The folates with shorter glutamate chain lengths in bacteria cultured in the presence of adenine resulted from primary regulation of the de novo purine biosynthetic pathway, regulation which caused an accumulation of formyltetrahydropteroyl-poly-gamma-glutamates (folate derivatives that are ineffective substrates for folylpolyglutamate synthetases), and did not result from regulation of folylpolyglutamate synthetase per se.

An hypothesis on the role of pteroylpolyglutamate derivatives as coenzymes

Evidence is assembled supporting the view that the poly-gamma-Glu chain of pteroyl poly-gamma-Glu derivatives plays a role in coordinating the activities of enzymes involved in the sequential metabolism of single carbon units.

Interaction of tetrahydropteroylpolyglutamates with two folate-dependent multifunctional enzymes

The naturally occurring pteroylpolyglutamate derivatives are substrates for the folate-mediated reactions in cells, including the reactions catalyzed by two multifunctional folate dependent enzymes in eucaryotes. The appropriate derivatives of tetrahydropteroyl (glutamate)n where n = 1, 3, 5, or 7 were used to determine the specificity for, and kinetic advantages of the extra glutamyl residues with two multifunctional proteins from pig liver: methylenetetrahydrofolate dehydrogenase-methenyltetrahydrofolate cyclohydrolase-formyltetrahydrofolate synthetase, and formiminotransferase-formininotetrahydrofolate cyclodeaminase. Specificity for the polyglutamate derivatives ranged from 10- to 70-fold as indicated from Km values or from the ability to inhibit the five different enzyme activities. With the sequential activities of the transferase-deaminase enzyme, it was demonstrated that when the tetrahydropteroyl pentaglutamate is used as a substrate, the intermediate formimino-compound does not accumulate in the medium. That this kinetic observation is due to preferential transfer of the pentaglutamate- but not monoglutamate intermediate from transferase to deaminase sites without its release from the enzyme molecule was supported by three types of experiments. Chemical modification to yield monofunctional derivatives of the transferase-deaminase affected the kinetics of the recombined activities only with the pentaglutamate substrate, causing a lag in the appearance of final product. Inhibition studies demonstrated that the deaminase activity could preferentially be inhibited only with the monoglutamate substrate. The deaminase activity with the monoglutamate substrate was increased by providing elevated formiminotetrahydrofolate in the assay mixture; no effect was observed when the reaction was carried out with pentaglutamate. Preliminary binding studies indicate a single folate site per subunit of the octameric enzyme, suggesting a type of combined transferase-deaminase site.

Studies on identifying the binding sites of folate and its derivatives in Lactobacillus casei thymidylate synthase

It was shown that folate and its derivatives have a profound effect on stabilizing thymidylate synthase in vitro and in vivo, as a consequence of ternary formation between the folate, dUMP, or FdUMP, and the synthase. The degree to which complex formation is affected can be revealed qualitatively by circular dichroism and quantitatively by equilibrium dialysis using the Lactobacillus casei synthase. In contrast to the pteroylmonoglutamates, the pteroylpolyglutamates bind to thymidylate synthase in the absence of dUMP, but even their binding affinity is increased greatly by this nucleotide or its analogues. Similarly, treatment of the synthase with carboxypeptidase A prevents the binding of the pteroylmonoglutamates and reduces the binding of the polyglutamates without affecting dUMP binding. The latter does not protect against carboxypeptidase inactivation but does potentiate the protective effect of the pteroylpolyglutamates. To determine the region of the synthase involved in the binding of the glutamate residues, Pte[14C]GluGlu6 was activated by a water soluble carbodiimide in the presence and absence of dUMP. This folate derivative behaved as a competitive inhibitor of 5,10-CH2H4PteGlu, in contrast to methotrexate which was non-competitive. Separation of the five cyanogen bromide peptides from the L. casei synthase revealed 80% of the radioactivity to be associated with CNBr-2 and about 15% with CNBr-4. Chymotrypsin treatment of CNBr-2 yielded two 14C-labeled peaks on high performance liquid chromatography, with the slower migrating one being separated further into two peaks by Bio-gel P2 chromatography. All three peptides came from the same region of CNBr-2, encompassing residues 47-61 of the enzyme. From these studies it would appear that the residues most probably involved in the fixation of PteGlu7 are lysines 50 and 58. In contrast, methotrexate appeared to bind to another region of CNBr-2.

Evaluation of folylpolyglutamates by electrophoretic separation of fluorodeoxyuridylate-thymidylate synthase-methylenetetrahydrofolate complexes

The chain length of specific reduced folylpolyglutamates has been estimated by incorporation of the 5,10-methylenetetrahydrofolate form of the cofactor into a stable ternary complex with L. casei thymidylate synthase and tritiated fluorodeoxyuridylate followed by electrophoretic separation based on charge differences in complexed polyglutamates. The method is also applicable to tetrahydrofolate polyglutamates after conversion to the active cofactor form by introduction of formaldehyde. The method can be used to analyze less than one pmole of folylpolyglutamate and can be applied to evaluation of tissue polyglutamates or to monitor relevant enzyme catalyzed reactions.

The interaction of pteroylpolyglutamates with calf thymus thymidylate synthase

Calf thymus thymidylate synthase was purified to homogeneity using a pteroyltetraglutamyllysine Sepharose affinity column. Inhibition of the purified enzyme by folate or methotrexate was enhanced by addition of gamma-Glu residues. Tetrahydrofolate, tetrahydropteroyltriglutamate and tetrahydropteroylheptaglutamate, all having the natural configuration at C-6, all showed similar Km values near 15 microM. Thus, although calf thymus thymidylate synthase showed a higher affinity for pteroylpolyglutamates than pteroylmonoglutamate, this was not reflected in lowered Km values with the corresponding tetrahydropteroylpolyglutamate substrates.

Folylpoly-gamma-glutamate synthetases: properties and regulation

The substrate specificities of folylpolyglutamate synthetases from Corynebacterium species, Lactobacillus casei, Streptococcus faecalis, and Chinese hamster ovary (CHO) cells are described. The purified Corynebacterium enzyme catalyzes the addition of glutamate moieties to reduced pteroylmonoglutamates, and to 5,10-methylenetetrahydropteroyldi- and triglutamates. The purified protein also possesses a separate dihydrofolate synthetase activity. Binding of pteroate substrates to dihydrofolate synthetase impairs folate binding to folylpolyglutamate synthetase. The in vivo distribution of folylpolyglutamates in bacteria and mammalian cells, which differs from source to source, appears to be a reflection of the ability of folylpolyglutamates to act as substrates for folylpolyglutamate synthetases from different sources. Bacterial synthetases use a variety of pteroylmonoglutamates as their preferred monoglutamate substrate, but use 5,10-methylenetetrahydropteroyl polyglutamates as their preferred, and sometimes only, polyglutamate substrates. Mono- and polyglutamyl forms of tetrahydrofolate are the preferred substrates of the CHO cell synthetase. Products of one carbon metabolism do not affect the in vivo distribution of folylpolyglutamates in CHO cells. Although adenine changes the distribution of folylpolyglutamates in bacteria, this effect is mediated by changing the substrate availability for the synthetase reaction, and not by regulating the levels of the synthetase per se.

Further studies stereospecificity at carbon 6 for membrane transport of tetrahydrofolates. Diastereoisomers of 5-methyltetrahydrofolates as competitive inhibitors of transport of methotrexate in L1210 cells

The unnatural d diastereoisomer at carbon 6 of 5-methyltetrahydrofolate was only slightly less effective than the natural 1 diastereoisomer as a competitive inhibitor of the carrier-mediated membrane transport of [3H]methotrexate into L1210 murine leukemia cells. The apparent Ki for a mixture containing equal amounts of both natural and unnatural diastereoisomers was not significantly different from that found for the unnatural form. These results show that the reduced folate carrier system in these cells has a strong affinity for the unnatural stereoisomer, a finding in contrast to that obtained with the corresponding diastereoisomer of 5-formyltetrahydrofolate.

Cell cycle patterns of thymidylate synthetase and 5,10-methylenetetrahydrofolate polyglutamates in cultured mouse hepatoma cells

The regulation of thymidylate synthetase activity was investigated throughout the first cell cycle after release from an isoleucine block in synchronous cultures of mouse hepatoma (Hepa) cells. Activity in cell extracts increased with the onset of S phase and the increased activity was attributed to a parallel increase in enzyme concentration as determined by titration with tritiated fluorodeoxyuridylate. The polyglutamate chain length of reduced folate cofactors, which could also influence thymidylate synthetase activity, was unchanged.

Pteroylpolyglutamates

Reduced derivatives of the vitamin pteroylglutamic acid (folic acid) are essential coenzymes for the biosynthesis of purine nucleotides, methionine, thymidylate and for many other enzyme catalyzed reactions involving the transfer, oxidation and reduction of single carbon units. Pteroylglutamic acid is found in tissues in the form of poly-gamma-glutamyl derivatives of varying chain length. The present review covers the detection, distribution, synthesis, degradation, coenzyme function and inhibitory activities of pteroyl-gamma-glutamates. The biosynthesis and inhibitory activities of poly-gamma-glutamyl derivatives of methotrexate, an analog of pteroylglutamic acid having antitumor activity, are also considered. An hypothesis on the coenzymatic role of pteroylpoly-gamma-glutamates in the coordination of sequential enzymatic steps in the metabolism of single carbon units is presented.