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PubMed2000

Sensitivity to 5,10-dideazatetrahydrofolate is fully conserved in a murine leukemia cell line highly resistant to methotrexate due to impaired transport mediated by the reduced folate carrier.

A murine leukemia cell line was identified that is highly resistant to methotrexate (MTX), due to impaired transport, but fully sensitive to 5,10-dideazatetrahydrofolate (DDATHF). A valine-to-methionine substitution at amino acid 104 in the reduced folate carrier (RFC1) explains this disparity in drug resistance. Transfection of the V104M cDNA into an RFC1-deficient cell line markedly increased DDATHF influx (32x) but only modestly increased influx of MTX and 5-formyltetrahydrofolate (4- and 6-fold, respectively). The growth inhibition or growth requirements for these folates fell by factors of 18, 2, and 4, respectively, in the transfectant. Preservation of DDATHF influx in cells with V104M RFC1 resulted in even greater preservation (60%) of the exchangeable drug level. Another major element in the preservation of DDATHF activity was the impact of the mutated carrier on cellular folate pools. For folic acid, folate pools were essentially unchanged but DDATHF polyglutamate levels decreased in lines that express the V104M carrier. However, with 5-formyltetrahydrofolate as the growth source, there was a marked decrease in folate pools in the lines carrying the mutated carrier, and DDATHF polyglutamate levels were unchanged. Hence, DDATHF activity was preserved in cells with V104M RFC1 due to (a) relative conservation of DDATHF transport, and (b) depletion of cellular THF cofactors with diminishing folate cofactor competition at folylpolyglutamate synthetase and possibly glycinamide ribonucleotide formyltransferase. Hence, resistance to one antifolate, in this case MTX, because of a loss of RFC1 transport activity need not exclude the subsequent utility of another antifolate that uses the same carrier.

Amino Acids↗

PubMed1975

Identification of a folate binder in hog kidney.

A macromolecular binder of folic acid (pteroylglutamic acid) and folic acid derivatives has been identified in extracts of hog kidney. With partially purified preparations, binding of [3H]pteroylglutamate was competed for by unlabeled pteroylglutamate, 5-methyltetrahydrofolic acid and its triglutamate derivative, by tetra- and dihydrofolic acid, and by N-10-formyltetrahydrofolic acid. The partially purified extract did not bine [3H]methotrexate nor could methotrexate or 5-formyltetrahydrofolic acid compete for [3H]folic acid-binding sites. The rate of binding of pterolyglutamate at 37 degrees was approximately 3%/s, was independent of pteroylglutamate concentration, and was essentially irreversible between pH 6.0 and 9.0. Below pH 6.0 binding was reversible, and at pH 3.5 the folic acid-binder complex completely disassociated. Based upon Sephadex gel filtration, the molecular weight of the folate-binder complex is 35,000 to 40,000. Binding activity was unaffected by pretreatment with ribonuclease or deoxyribonuclease but was completely destroyed by trypsin. The initial, unfractionated extract showed gamma-glutamyl carboxypeptidase (conjugase) activity which was lost in subsequent steps of purification of the folate binder.

Animals↗

PubMed1987

Evidence for direct inhibition of de novo purine synthesis in human MCF-7 breast cells as a principal mode of metabolic inhibition by methotrexate.

We have investigated the role of dihydrofolate (H2PteGlu) accumulation in the inhibition of de novo purine synthesis by methotrexate (MTX) in human MCF-7 breast cancer cells. Previous studies have shown that cytotoxic concentrations of MTX that inhibit dihydrofolate reductase produce only minimal depletion of the reduced folate cofactor, 10-formyltetrahydrofolate, required for purine synthesis. At the same time, de novo purine synthesis is totally inhibited. In these studies, we show that 10 microM MTX causes inhibition of purine synthesis at the step of phosphoribosylaminoimidazolecarboxamide (AICAR) transformylase, as reflected in a 2-3-fold expansion of the intracellular AICAR pool. The inhibition of purine synthesis coincides with the rapid intracellular accumulation of H2PteGlu, a known inhibitor of AICAR transformylase. When the generation of H2PteGlu is blocked by pretreatment with 50 microM 5-fluorodeoxyuridine (FdUrd), an inhibitor of thymidylate synthase, MTX no longer causes inhibition of purine synthesis. Intermediate levels of H2PteGlu produced in the presence of lower (0.1-10 microM) concentrations of FdUrd led to proportional inhibition of purine biosynthesis, and the exogenous addition of H2PteGlu to breast cells in culture re-established the block in purine synthesis in the presence of FdUrd and MTX. The early phases of inhibition of purine biosynthesis could be ascribed only to H2PteGlu accumulation. MTX polyglutamates, also known to inhibit AICAR transformylase, were present in breast cells only after 6 h of incubation with the parent compounds and were not formed in cells preincubated with FdUrd. The lipid-soluble antifolate trimetrexate, which does not form polyglutamates, produced modest 10-formyltetrahydrofolate depletion, but caused marked H2PteGlu accumulation and a parallel inhibition of purine biosynthesis. This evidence leads to the conclusion that MTX and the lipid-soluble analog trimetrexate cause inhibition of purine biosynthesis through the accumulation of H2PteGlu behind the blocked dihydrofolate reductase reaction.

Acyltransferases↗

PubMed1988

Isolation and characterization of the Saccharomyces cerevisiae MIS1 gene encoding mitochondrial C1-tetrahydrofolate synthase.

C1-Tetrahydrofolate synthase is a trifunctional polypeptide found in eukaryotic organisms that catalyzes 10-formyltetrahydrofolate synthetase (EC 6.3.4.3), 5,10-methenyltetrahydrofolate cyclohydrolase (EC 3.5.4.9), and 5,10-methylenetetrahydrofolate dehydrogenase (EC 1.5.1.5) activities. In Saccharomyces cerevisiae, C1-tetrahydrofolate synthase is found in both the cytoplasm and the mitochondria. The gene encoding yeast mitochondrial C1-tetrahydrofolate synthase was isolated using synthetic oligonucleotide probes based on the amino-terminal sequence of the purified protein. Hybridization analysis shows that the gene (designated MIS1) has a single copy in the yeast genome. The predicted amino acid sequence of mitochondrial C1-tetrahydrofolate synthase shares 71% identity with yeast C1-tetrahydrofolate synthase and shares 39% identity with clostridial 10-formyltetrahydrofolate synthetase. Chromosomal deletions of the mitochondrial C1-tetrahydrofolate synthase gene were generated using the cloned MIS1 gene. Mutant strains which lack a functional MIS1 gene are viable and can grow in medium containing a nonfermentable carbon source. In fact, deletion of the MIS1 locus has no detectable effect on cell growth.

Amino Acid Sequence↗

PubMed1989

Mammalian glycinamide ribonucleotide transformylase. Kinetic mechanism and associated de novo purine biosynthetic activities.

Glycinamide ribonucleotide transformylase catalyzes the conversion of glycinamide ribonucleotide and 10-formyltetrahydrofolate to formylglycinamide ribonucleotide and tetrahydrofolate. The enzyme purified from the murine lymphoma cell line L5178Y also catalyzes two other de novo purine biosynthetic activities, glycinamide ribonucleotide synthetase and aminoimidazole ribonucleotide synthetase. The transformylase reaction shows a 1:1 stoichiometry for substrate utilization and an optimum rate between pH 7.9 and 8.3. Initial velocity and dead-end inhibition patterns indicate that the kinetic mechanism of the transformylation reaction is ordered-sequential, with 10-formyltetrahydrofolate binding first. alpha, beta-Hydroxyacetamide ribonucleotide (alpha, beta-N-(hydroxyacetyl)-D-ribofuranosylamine) is shown to be an inhibitor of the transformylase, competitive against glycinamide ribonucleotide.

Acyltransferases↗

PubMed1988

Pharmacokinetics of leucovorin calcium after intravenous, intramuscular, and oral administration.

The pharmacokinetics of leucovorin was evaluated after intravenous, intramuscular, and oral administration in a randomized crossover study of 37 healthy men. A single 25-mg dose of leucovorin calcium was administered intravenously, intramuscularly, or orally to the subjects. Blood samples were obtained immediately before and at 13 time points up to 24 hours after the leucovorin dose. The three treatment phases were separated by one-week intervals. Bioavailability was assessed by measuring over 24 hours the blood concentrations of total folates, the parent compound 5-formyltetrahydrofolate, and the metabolite 5-methyltetrahydrofolate, using differential microbiologic assays with Lactobacillus casei and Streptococcus faecalis. Both intravenous and intramuscular administration produced rapid increases in serum concentrations of biologically active folates; these rises were sustained over time and were still detectable at 24 hours after drug administration. The bioavailability of intravenous and intramuscular doses was comparable based on area under the serum concentration-time curve, although for intramuscular administration, the peak concentration was lower and the time to peak concentration was longer. The initial rise in serum folate with intravenous and intramuscular dosing represented 5-formyltetrahydrofolate; this fell concomitantly with the appearance of 5-methyltetrahydrofolate. Oral leucovorin was 92% bioavailable compared with intravenous administration and produced a predictably different pattern of circulating folates, 5-methyltetrahydrofolate being the predominant form. Terminal elimination half-life, apparent volume of distribution, and clearance of total folate were not significantly different among the three treatments.(ABSTRACT TRUNCATED AT 250 WORDS)

Administration, Oral↗

PubMed1988

The activities of the NAD-dependent methylenetetrahydrofolate dehydrogenase-methenyltetrahydrofolate cyclohydrolase from ascites tumor cells are kinetically independent.

The bifunctional NAD-dependent methylenetetrahydrofolate dehydrogenase-methenyltetrahydrofolate cyclohydrolase from ascites tumor cells has very different kinetic properties from the larger NADP-dependent methylenetetrahydrofolate dehydrogenase-methenyltetrahydrofolate cyclohydrolase-formyltetrahydrofolate synthetase present in all mammalian cells. The NAD-dependent dehydrogenase is unique in that it requires formation of a magnesium.enzyme complex to allow addition of the first substrate, NAD+. It catalyzes an equilibrium ordered kinetic mechanism that has methylenetetrahydrofolate as the last reactant to add and NADH as the last product released. The NADP-dependent dehydrogenase has the same order of addition of substrates, but NADPH is released prior to methenyltetrahydrofolate. The dehydrogenase-cyclohydrolase activities of both enzymes channel methenyltetrahydropteroylglutamate intermediates with the same efficiency which is unaffected by the number of glutamyl residues in the methylenetetrahydrofolate substrate. However, the cyclohydrolase activity of the bifunctional protein is kinetically independent of its dehydrogenase activity, as supported by its lack of inhibition by NAD+, whereas NADP+ strongly inhibits that of the NADP-dependent enzyme. This difference is further demonstrated by the observation that conversion of formyltetrahydrofolate to methylenetetrahydrofolate in the presence of reduced pyridine nucleotide is catalyzed readily only by the bifunctional enzyme.

Aminohydrolases↗

PubMed1985

Extent of the requirement for folate transport by L1210 cells for growth and leukemogenesis in vivo.

In these studies the extent of the requirement for 5-methyltetrahydrofolate by L1210 cells for growth and leukemogenesis in vivo was addressed from the aspect of its cellular membrane transport. Growth characteristics and leukemogenesis in vivo were determined for parental and methotrexate-resistant L1210 cell variants with reduced capacity for folate coenzyme transport inward. These variants exhibited 6-, 16-, and 100-fold reductions compared to parental cells in influx Vmax for the high-affinity system transporting 5-substituted reduced folates and methotrexate. They also exhibited reduced saturability for methotrexate influx (3-fold higher Km), but not for influx of 5-formyltetrahydrofolate or 5-methyltetrahydrofolate. The reduced influx capacity in these variants correlated with their increased requirement for reduced folates during growth in vitro and with the ability of the variants to proliferate and develop leukemia in vivo. Lack of growth potential in vivo for one variant appears to reflect the inability for net intracellular accumulation of reduced folate per se, since growth of this variant could be restored by treatment of mice with folic acid, but not with 5-methyltetrahydrofolate or 5-formyltetrahydrofolate, and following reversion to a more transport-proficient phenotype.

Animals↗

PubMed1984

Effects of leucovorin and methylcobalamin with N2O anesthesia.

Results of the deoxyuridine suppression test, a good marker for defining biochemical megaloblastosis caused by deficiency of folate and vitamin B12, became abnormal (greater than 10%) after 6 hours of administration of nitrous oxide anesthesia. 5-Formyltetrahydrofolate and methylcobalamin administration during nitrous oxide anesthesia have no remarkable effect on the correction of deoxyuridine suppression test values. On the other hand, 5-formyltetrahydrofolate and methylcobalamin administrated at the end of nitrous oxide anesthesia corrected the abnormal deoxyuridine suppression test values nearly to normal range within 1 hour.

Bone Marrow↗

PubMed1994

Increased expression and genomic organization of a folate-binding protein homologous to the human placental isoform in L1210 murine leukemia cell lines with a defective reduced folate carrier.

This laboratory previously described an L1210 leukemia cell line (MTXrA) selected for resistance to methotrexate by virtue of impaired transport. In this line, the reduced folate carrier had unchanged affinity for methotrexate, was present at the cell surface in usual quantity, but did not deliver drug into the cell, indicative of a functional defect in the translocation process. In this study, we further characterize this cell line along with a subline (F2-MTXrA) selected for growth in low levels of folic acid. This subline demonstrates continued high resistance to methotrexate and very low influx of [3H]methotrexate and 5-[3H]formyltetrahydrofolate, indicating the persistence of the defect in the reduced folate carrier. Both MTXrA and F2-MTXrA are shown to overexpress FBP2, the murine homolog of a folate-binding protein initially isolated from human placenta. Compared with parent L1210 cells, Northern analysis revealed FBP2 expression to be elevated 40-fold in the MTXrA line and 500-fold in F2-MTXrA. The large increase in FBP2 expression in the F2-MTXrA line correlates with a 10-fold increase in [3H]folic acid membrane surface binding and a 1000-fold decrease in the folic acid growth requirement compared with parental L1210 cells. Also, there are 20- and 500-fold decreases in the 5-formyltetrahydrofolate growth requirement compared with parent L1210 and MTXrA cells, respectively. Finally, the genomic organization of the FBP2 locus is presented. The results of Northern analyses using probes specific to FBP2 5'-untranslated sequences or to a splice junction within this region suggest that the up-regulated FBP2-specific message in F2-MTXrA utilizes 5'-noncoding sequences distinct from those used in the message encoded in L1210 cell lines with low level FBP2 expression. The MTXrA cells provide an example of a line selected for primary resistance to methotrexate that also exhibits concomitant increased expression of a folate-binding protein. Further overexpression of this folate-binding protein (which has homology to that initially identified in placenta) provides cells with the ability to meet cellular folate needs in a folate-deprived environment.

Animals↗

PubMed1993

5,10-Dideazatetrahydrofolic acid (DDATHF) transport in CCRF-CEM and MA104 cell lines.

5,10-Dideazatetrahydrolic acid (DDATHF) is representative of a new class of antifolates acting through inhibition of de novo purine synthesis. We report here the transport characteristics of the diastereomers of DDATHF, which differ in configuration at C6, and comparison studies with other folate and antifolate analogs. (6R)-DDATHF showed high affinity for the influx system of CCRF-CEM cells with a Km of 1.07 microM and an influx Vmax of 4.04 pmol/min/10(7) cells. Comparative studies with methotrexate yielded an influx Km of 4.98 microM and a Vmax of 6.64 pmol/min/10(7) cells, and with 5-formyltetrahydrofolate an influx Km of 2.18 microM and a Vmax of 6.84 pmol/min/10(7) cells. Uptake of (6R)-DDATHF was competitively inhibited by (6S)-DDATHF, methotrexate (MTX), and 5-formyltetrahydrofolate, all with Ki values similar to their influx Km. The (6S)-DDATHF diastereomer had an influx Km of 1.04 microM, similar to that of (6R)-DDATHF; however, the Vmax of 1.72 pmol/min/10(7) cells was 2.3-fold lower than for (6R)-DDATHF. The transport properties of DDATHF were also studied in a mutant cell line (CEM/MTX), resistant to MTX based on impaired drug transport. In this system (6R)-DDATHF showed an influx Km of 1.49 microM and a decreased influx Vmax of 0.60 pmol/min/10(7) cells. A similar effect was shown for MTX (Km of 7.48 microM, Vmax of 1.02 pmol/min/10(7) cells). The number of binding sites in CCRF-CEM cells was similar for (6R)-DDATHF, (6S)-DDATHF, and MTX, 0.74, 0.71, and 0.76 pmol/10(7) cells, respectively. These values were slightly higher in the CEM/MTX cell line (1.07 and 1.09 pmol/10(7) cells for (6R)-DDATHF and MTX, respectively). Treatment of CCRF-CEM cells with either the N-hydroxysuccinimide ester of MTX or the corresponding N-hydroxysuccinimide ester of (6R)-DDATHF caused substantial inhibition (> 90%) of the influx of (6R)-[3H]DDATHF and [3H]MTX, respectively. These results suggest strongly that DDATHF and MTX share a common influx mechanism through the reduced folate transport system. The internalization of DDATHF by monkey kidney epithelial MA104 cells, which express a high affinity folate receptor, was also studied. Competitive binding studies using purified folate receptor and radiolabeled 5-methyltetrahydrofolate showed that (6S)- and (6R)-DDATHF both had I50 values lower than 5-methyltetrahydrofolate (12 nM). Further studies indicate that both DDATHF isomers are actively intracellularly concentrated through this route and are also rapidly converted to high chain length polyglutamates. Transport via this system was inhibited in folate-depleted cells by 10 nM folic acid.(ABSTRACT TRUNCATED AT 400 WORDS)

Biological Transport↗

PubMed1976

Folate metabolism in mammalian cells in culture. I Partial characterization of the folate derivatives present in L1210 mouse leukemia cells.

Chromatographic procedures were developed which permit the separation of the monoglutamylfolic acid derivatives from each other and from their polyglutamyl congeners. Using these methods, the cellular folate derivatives in logarithmically growing L1210 cells were found to consist entirely of polyglutamyl derivatives. When ascorbate extracts of these cells were digested with hog kidney gamma-glutamylcarboxypeptidase and then chromatographed on DEAE-cellulose columns, the cellular folates eluted coincident with tetrahydrofolate, 5-formyltetrahydrofolate, 10-formyltetrahydrofolate, and 5-methyltetrahydrofolate, with minimal amounts associated with folate (approximately 0.8%) or dihydrofolate (approximately 0.4%). When these experiments were repeated on cells previously depleted of folates which were then grown on 0.4 muM pteroylglutamic acid (a concentration insufficient to maintain an optimal growth rate) more than 95% of the folates were again found in polyglutamyl form. These results imply that the polyglutamyl derivatives found are not storage forms of enzymatically active cofactor, but rather are the actual cofactor forms used by these mammalian cells.

Animals↗

PubMed1981

Involvement of tetrahydrofolate cofactors in de novo purine ribonucleotide synthesis by adult Brugia pahangi and Dirofilaria immitis.

Adult Brugia pahangi in vitro, unlike mouse leukemia L1210 cells, converted 5-methyltetrahydrofolate (CH3FH4) directly to 5,10-methylenetetrahydrofolate and thence to other FH4 cofactors. The excreted CO2 that was derived from CH3FH4 was due to the presence within the filariae of 10-formyltetrahydrofolate dehydrogenase (EC 1.5.1.6) which catalyzes the deformylation of 10-formyl-tetrahydrofolate. Adult B. pahangi and Dirofilaria immitis, incubated in a purine-free medium containing [5-14C]CH3FH4 as the only form of folate, synthesized purine ribonucleotides radiolabeled at positions 2 and 8 of the purine ring. Presumably, 10-formyl[14C]FH4 donated Carbon 2 during the synthesis de novo of the purine ring and 5,10-methenyl[14C]FH4 donated Carbon 8.

Brugia↗

PubMed1988

Effect of methotrexate on the deoxyuridine suppression test: results with different cell preparations.

Recently, the deoxyuridine suppression test was used to detect folate deficiency. Several cell preparations were used in this test, and the sensitivity of the test seems to vary with them. We used whole blood (micro-dU test) and bone marrow cells for the deoxyuridine suppression test, and the effects of methotrexate were compared. [3H]Thymidine incorporation into DNA was highest when the concentration of methotrexate was 1 microM in the whole blood culture and bone marrow culture. Methotrexate concentrations from 100 nM to 1 mM did not affect the results of the micro-dU test. However, the bone marrow deoxyuridine test became abnormal with methotrexate, and the abnormality in test values was corrected by the addition of N5-formyltetrahydrofolate. Differences in the results between the micro-dU test and the bone marrow deoxyuridine suppression test seem to represent specificity of each dU test.

Bone Marrow↗

PubMed1988

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.

Antineoplastic Combined Chemotherapy Protocols↗

PubMed1983

Relationships between carrier-mediated transport of folate compounds by L1210 leukemia cells: evidence for multiplicity of entry routes with different kinetic properties expressed in plasma membrane vesicles.

Various independent kinetic criteria for indicating multiplicity of carrier-mediated entry of folate compounds into L1210 cell plasma membrane vesicles are studied. We find a marked inconsistency between values for influx Km and Ki in reciprocal experiments measuring competition between various folate compounds as well as inconsistent effects of transloading shown for 5-formyltetrahydrofolate influx, but not folic acid influx. These results argue strongly against a one-carrier model for transport of folate compounds. The most straightforward interpretation of our data is that two distinct transport systems mediate entry of folate compounds in L1210 plasma membrane vesicles. If a two-carrier model is correct, then our data indicate that one of the carriers has low capacity and high affinity for folate coenzymes and methotrexate. This system is apparently negligible as a transport route for folic acid. Transtimulation of initial influx by substrates of the low capacity system is obtained following transloading with coenzymes but not by transloading with folic acid. Our data indicate that the second folate transport system postulated by the two-carrier model has a low affinity for all the folate compounds studied. Nevertheless, the putative second system is significant, especially for folic acid transport, because it has a much higher capacity than the first transport system. In contrast to the first system, transloading with any of the folate compounds studied had no effect on initial influx mediated by the second folate transport system. The two systems are also differentially inhibited by pCMBS, DIDS and SITS and the influx Vmax for the high-affinity/low-capacity system was altered in a vesicle preparation derived from a methotrexate resistant L1210 cell line.

Animals↗

PubMed1989

A rapid and specific extraction procedure for folates determination in rat liver and analysis by high-performance liquid chromatography with fluorometric detection.

The native fluorescence of the main biological derivatives of folic acid is maximal at acidic pH. The intensities obtained with 5-methyltetrahydrofolic acid, tetrahydrofolic acid, and formyltetrahydrofolic acid are in the ratio 10/5/1. After separating the metabolites by reversed phase high-performance liquid chromatography, the fluorometric detection limit varied between 0.4 and 20 pmol per injection. Rat liver extract (about 2 g/50 ml of 1% ascorbate solution) could be analyzed after incubation at 37 degrees C overnight, deproteinizing with acetone, and purification and concentration in an ion exchange column. The concentrations obtained varied from 1.27 to 7.96 nmol/g depending on the derivative. Recovery varied from 89 to 113%.

Animals↗

PubMed1982

Folic acid derivatives do not reproduce the neurotoxic effects of kainic acid on chicken retina.

Methyltetrahydrofolic acid, formyltetrahydrofolic acid, folic acid and dihydrofolic acid were injected intravitreally into the eyes of chickens. No short-term or long-term signs of neurotoxicity were observed, even when doses 100-200 times those at which kainic acid produces clear neurotoxic effects were injected. The folic acid derivatives neither inhibited nor potentiated the neurotoxic effects. Thus no support is given to the suggestion that folic acid and its derivatives may act as kainic acid agonists or antagonists, even though the receptors involved in kainic acid-induced neurotoxicity appear to be of the kainic acid rather than quisqualic acid or N-methyl-D-aspartic acid type.

Animals↗