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PubMed1987

Chronic treatment of rats with primidone causes depletion of pteroylpentaglutamates in liver.

Anticonvulsants have been shown to cause folacin deficiency in chronically treated epileptic patients. However, a mechanism for this depletion has not been established. In the present study, the effects of chronic primidone treatment on folates in the rat were investigated. Using a continuously protective relatively nontoxic regimen of oral administration, it was found that primidone (100 mg/kg, twice per day) caused a decrease of pteroylpentaglutamates in the liver to less than half the control value within 1 wk. Total liver folacin concentration decreased by 30% in the first week followed by a slow gradual further decline with continuing treatment. Plasma folacin exhibited essentially the same pattern but no effect was observed on brain folacin concentration. Primidone was not detectable in plasma 12 h after gavage but phenobarbital was detectable. These data are consistent with the hypothesis that the anticonvulsant primidone (and/or phenobarbital) cause folate depletion via interaction with folate metabolism.

Animals↗
PubMed1991

Distribution of folates and modified folates in extremely thermophilic bacteria.

Analyses were made of the structures and levels of folates and modified folates present in extremely thermophilic bacteria. These procedures involved the chemical analysis of products resulting from the oxidative cleavage of the 6-substituted, folatelike tetrahydropterins present in the cells. Air-oxidized cell extracts of extreme thermophiles from two members of the archaebacterial order Thermococcales, Thermococcus celer and Pyrococcus furiosus, contained only 7-methylpterin, indicating that these cells contain a modified folate with a methylated pterin. Cell extracts also contained 6-acetyl-7-methyl-7,8-dihydropterin, another product derived from the oxidative cleavage of a dimethylated folate, demonstrating that both the C-7 and C-9 carbons of the pterin were methylated. Extracts, however, contained neither p-aminobenzoylpolyglutamates nor methaniline, the oxidative cleavage products of folates and methanopterin, respectively, indicating that they contain a previously undescribed C1 carrier(s). On the basis of the level of the 7-methylpterin isolated, the levels of modified folate were 2 to 10 times higher than those typically found in mesophilic bacteria and 10 to 100 times less than the level of methanopterin found in the methanogenic bacteria. Oxidized cell extracts of Sulfolobus spp. of the archaebacterial order Sulfolobales contained only pterin, and, like members of the order Thermococcales, they contained neither-p-aminobenzoylpolyglutamates nor methaniline. Oxidized cell extracts of the extreme thermophiles Pyrobaculum sp. strain H10 and Pyrodictium occultum, from the archaebacterial orders Thermoproteales and Pyrodictiales, respectively, and Thermotoga maritima from the eubacterial order Thermotogales, contained pterin and p-aminobenzoylpolyglutamates, indicating that these cells contained unmodified folates. The levels of p-aminobenzoylpolyglutamates in these archaebacterial cell extracts indicate that the folates were present in the cells at levels 4 to 10 times higher than generally found in those mesophilic eubacteria which do not folates in energy metabolism. The levels and chain lengths of the of p-aminobenzoylpolyglutamates present in Thermotoga maritima were typical of those found in mesophilic eubacteria.

Archaea↗
PubMed1986

Effects of growth rate and methotrexate on folate polyglutamates and folylpolyglutamate hydrolase activity in Krebs ascites cells.

The activity of folylpolyglutamate hydrolase was measured throughout intraperitoneal growth of Krebs ascites cells in mice and after exposure to methotrexate. Hydrolase activity was lowest during the log phase of growth. Methotrexate administered intraperitoneally during log growth caused a dose- and time-dependent increase in hydrolase activity. Modest changes were observed in endogenous folate polyglutamate chain length distributions throughout growth and upon exposure to methotrexate, but these changes could not be correlated with hydrolase activity.

Animals↗
PubMed1992

Inability of leucovorin to rescue a naturally methotrexate-resistant human soft tissue sarcoma cell line from trimetrexate cytotoxicity.

A human lymphoblastoid line (RPMI-1788), a methotrexate-sensitive human fibrosarcoma cell line (HT-1080), and a naturally resistant mixed mesodermal human sarcoma cell line with impaired methotrexate polyglutamylation (HS-42), recently established in our laboratory, were used to compare the ability of leucovorin to prevent trimetrexate cytotoxicity. Growth inhibition and an in situ thymidylate synthesis activity assay showed that inhibitory effects of trimetrexate (1 to 10 microM), 24-h exposure, were prevented by 10 microM leucovorin in the RPMI-1788 and HT-1080 cell lines but not in the HS-42 cell line. Total intracellular reduced folates increased about 2-fold in the three cell lines after exposure to leucovorin (10 microM) for 4 h, and after a 6-hour efflux remained elevated (1.5- and 1.3-fold of control levels) in RPMI-1788 and HT-1080 cells but decreased to 80% of control levels in HS-42 cells. Although uptake of leucovorin and levels of N5,N10-methylenetetrahydrofolate achieved after leucovorin administration were similar in RP-MI-1788 and HS-42 cells, polyglutamylate forms of this coenzyme were less in the HS-42 cells as compared to RPMI-1788 cells. Based on these studies, the combination of trimetrexate with leucovorin should be further investigated as a way to increase the therapeutic index in some patients with soft tissue sarcomas.

Drug Resistance↗
PubMed1986

alpha-Carboxyl-linked glutamates in the folylpolyglutamates of Escherichia coli.

Folate cofactors in most cells contain polyglutamate side chains, which since the late 1940s have been assumed to be linked via their gamma-COOH groups. We report here an investigation of the structure of the polyglutamate chain attached to the folates of Escherichia coli. Folates were extracted from E. coli grown with [7-14C] p-aminobenzoate and cleaved to p-aminobenzoyl polyglutamates of varying chain lengths (pAB(Glu)n) by the method of Foo et al. (Foo, S. K., Cichowicz, D. J., and Shane, B. (1980) Anal. Biochem. 107, 109-115). The pAB(Glu)n derived from E. coli did not co-chromatograph with chemically synthesized pAB(gamma-Glu)n-Glu on several high performance liquid chromatography (HPLC) systems, except for the triglutamate which did elute with pAB(gamma-Glu)2-Glu. E. coli-derived pAB(Glu)3-8 were purified by HPLC on C18 columns eluted with acetonitrile/trifluoroacetic acid, and the structures were determined through mass spectrometry, chiral amino acid analysis, and peptidase digestion experiments. Molecular weight determinations on the methyl ester derivatives of E. coli-derived pAB(Glu)n by liquid secondary ion mass spectrometry and sequence analysis using collision-activated dissociation on a tandem mass spectrometer confirmed the structures as pAB(Glu)3-8. Chiral HPLC of hydrolyzed and dansylated E. coli-derived materials, on a beta-cyclodextrin column, identified the glutamate as the L-enantiomer. pAB(Glu)n were digested with carboxypeptidase Y, which specifically cleaved glutamates linked at their alpha-carboxyls; E. coli-derived pAB(Glu)4-8 (but not synthetic pAB(gamma-Glu1-6-Glu) were sequentially digested to pAB(gamma-Glu)2-Glu. Thus, in E. coli folylpolyglutamates, glutamate residues 4-8 were each linked to the polyglutamate chain at the alpha-carboxyl of the preceding glutamate.

4-Aminobenzoic Acid↗
PubMed1984

Spectroscopic studies of ternary complexes of thymidylate synthetase, deoxyribonucleotides, and folate analogs.

Conformational changes accompanying the formation of binary and tightly bound ternary complexes of thymidylate synthetase and all possible combinations of three folate analogs (N-10-ethyl-quinazoline, folic acid triglutamate, and folic acid) and three deoxyribonucleotides (5-fluoro-2'-deoxyuridylic acid (FdUMP), 2'-deoxyuridylic acid (dUMP), and thymidylic acid (dTMP] were studied by means of ultraviolet difference spectroscopy. The amplitudes of the spectral changes upon ternary complex formation were 2-3-fold greater than those generated by formation of binary enzyme-nucleotide and enzyme-folate analog complexes. Difference spectra of the ternary complexes all showed a major increase in absorbance in the region of 320-340 nm, presumably due to perturbations of the folate analog chromophores, whereas decreases in absorbance occurred over a range of 260-310 nm. N-10-ethyl-quinazoline tended to form the complex with the greatest filtration efficiency on nitrocellulose filters, followed by folic acid triglutamate and folic acid, whereas among the nucleotides, the most stable complexes were formed with FdUMP, followed by dUMP and dTMP. A correlation was observed between the apparent stability of the ternary complex and the magnitude of the absorbance change in its difference spectrum. The formation of the various ternary complexes showed three different categories of rate behavior: 1) very rapid formation of the complex; 2) biphasic formation with a rapid phase and a slow phase requiring up to 90 min for completion; and 3) in the case of the ternary complex formed with enzyme, FdUMP, and folic acid, only a slow phase of binding. The slow formation of the latter complex was accompanied by concomitantly slow changes in the difference spectrum. However, in those cases of biphasic formation of the complexes, almost all of the spectral change occurred rapidly, and very little of it corresponded to the slow phase of complex formation. To accommodate these observations, a model is proposed involving a sequential interaction of the two subunits of thymidylate synthetase.

Deoxyribonucleotides↗
PubMed1983

Effect of oral contraceptives on intestinal folate conjugase activity and folate absorption in rats.

To study whether oral contraceptive agents (OCA) may interfere with intestinal absorption of folate polyglutamates by inhibiting the activity of intestinal folate conjugase, female rats weighing on the average 230 gm received either a stock diet and OCA daily for a 6-week period or the stock diet without OCA. The animals receiving the OCA gained 50% less of their original body weight than the controls. There were no significant differences in the in vivo absorption of either folate monoglutamate or folate pentaglutamate between the controls and the OCA-administered animals. The absorption of folate pentaglutamate was lower than that of folate monoglutamate in both groups of rats, but the difference was not statistically significant (p greater than 0.05). As in the case of the two forms of folate absorption, no significant difference was found in intestinal folate conjugase activity between the two groups of animals. These findings indicate that intestinal folate conjugase activity and the absorption of both folate monoglutamate and pentaglutamate are not altered by oral contraceptive administration to female rats.

Animals↗
PubMed1980

Pteroylpoly(gamma-glutamate) synthesis by Corynebacterium species. Studies on the mechanism of folypoly(gamma-glutamate) synthetase.

The mechanism of action of folylpolyglutamate synthetase from Corynebacterium sp. was investigated using tetrahydrofolate, MgATP, and glutamate as the substrates. Initial velocity, product inhibition, and competitive inhibition studies were consistent with an Ordered Ter Ter mechanism with MgATP binding first to the enzyme, tetrahydrofolate second, and glutamate last. The order of dissociation from the enzyme was ADP, folate, and Pi. This mechanism precludes the sequential addition of glutamate moieties to enzyme-bound folate. The Michaelis constants for (dl)-tetrahydrofolate, MgATP, and glutamate were 2,1 muM, 18 MUM, and 160 muM, respectively. Beta, gamma-Methylene-ATP was a very effective inhibitor of the reaction with an affinity for the enzyme 1 order of magnitude greater than that of ATP.

Adenosine Diphosphate↗
PubMed1983

[Effect of folic acid deficiency on the synthesis of pteroylpolyglutamates].

In order to evaluate the effect of folate deficiency on pteroylpolyglutamate synthesis in rat liver, the hepatic and urinary content of reduced folates, the distribution of pteroylpolyglutamates and the incorporation of i.p. injected 3H folic acid in polyglutamates have been studied. In deficient rats the hepatic content of reduced folates and the urinary amounts of reduced metabolites are lower and do not increase after vitamin treatment. Longer chain pteroylpolyglutamates are severely depleted and their content is not significantly modified by folic acid treatment. The percentage of labelled folic acid incorporated into pteroylglutamates is markedly decreased. The inability of deficient rat liver to synthesize polyglutamates might be ascribed to the decrease of pteroylpolyglutamate synthetase activity, and/or to the lower availability of tetrahydrofolates, the preferred substrates for the enzyme.

Animals↗
PubMed1983

Improved synthetic routes to 5,8-dideazapteroylglutamates amenable to the formation of poly-gamma-L-glutamyl derivatives.

A variety of quinazoline analogues of folic acid (5,8-dideazafolates) are of interest as potential antineoplastic agents, biochemical probes, and/or affinity ligands for the purification of folate requiring enzymes. Chief among these are 5,8-dideazaisopteroylglutamate, 5,8-dideazaisoPteGlu, (IAHQ), a compound with proven activity against the growth of human colon adenocarcinoma cells in vitro, and 10-formyl-5,8-dideazapteroylglutamic acid, which serves as a substrate for glycinamide ribonucleotide transformylase and is also an effective inhibitor of mammalian thymidylate synthase. New methods for preparing these compounds in excellent purity as determined by high performance liquid chromatography (HPLC) have been developed. In each case the carboxyl groups of L-glutamic acid are protected with t-butyl ester groups, since these can subsequently be removed readily using trifluoroacetic acid without decomposition or racemization of the final product. This approach has proven to be of particular value in the formation of gamma-L-glutamyl derivatives of IAHQ containing 1-3 additional glutamyl residues.

Cell Line↗
PubMed1992

Evidence for the induction and release of pancreatic folylpolyglutamate hydrolase by the ingestion of folyl polyglutamates in the rat.

Pteroyl polyglutamate hydrolase (folyl conjugase), which hydrolyses the dietary polyglutamyl folates into simple forms prior to absorption, has been shown to be induced in rat pancreas in response to dietary polyglutamyl folates but not by ingestion of synthetic unconjugated folates. Folate absorption, as measured by the rise in serum folate levels after ingestion, of dietary conjugated folates is impaired in pancreatectomised animals whereas absorption of synthetic simple folate is not. A severe build-up of folyl conjugase is observed in the lumen of control but not in pancreatectomised animals after dietary folate ingestion. These results taken together would suggest that dietary folates are hydrolysed to monoglutamyl forms suitable for absorption in the lumen; the hydrolysis is catalysed by a luminal folyl conjugase of pancreatic origin induced by dietary conjugated folates.

Animals↗
PubMed1989

An improved procedure for the purification of formiminotransferase-cyclodeaminase from pig liver. Kinetics of the transferase activity with tetrahydropteroylpolyglutamates.

Formiminotransferase-cyclodeaminase is stabilized and activated approx. 40% in the presence of low concentrations (equal or less than 0.2%) of Triton X-100, possibly because the average hydrophobicity (1.10 kcal per residue) and the frequency of large non-polar side-chains (0.34) of this protein are both somewhat higher than average. This stabilization enabled us to develop a new purification procedure for the enzyme using chromatography on Matrex Gel Orange A and heparin-Sepharose columns in the presence of Triton X-100. This procedure is easier, much more reproducible, and gives slightly higher yield than the previous method described by Drury, et al. Further investigations of the role of tetrahydropteroylpolyglutamates with formiminotransferase-cyclodeaminase reveal that the use of polyglutamylated folate substrates does not change the mechanism of the transferase reaction, but decreases the K(m) for formininoglutamate, the second substrate, more than 10-fold, bringing it closer to the expected physiological concentration.

Amino Acids↗
PubMed1990

Synthesis of N-[N-(4-deoxy-4-amino-10-methylpteroyl)-4-fluoroglutamyl]- gamma-glutamate, an unusual substrate for folylpoly-gamma-glutamate synthetase and gamma-glutamyl hydrolase.

N-[N-(4-Deoxy-4-amino-10-methylpteroyl)-4-fluoroglutamyl]-ga mma-glutamate has been synthesized and its ability to serve as a substrate for folylpolyglutamate synthetase and gamma-glutamyl hydrolase has been investigated. It was anticipated that this compound would be a substrate for both of these enzymes. Although the title compound proved to be a good substrate for folylpolyglutamate synthetase, hydrolysis catalyzed by gamma-glutamyl hydrolase was unexpectedly slow. These results suggest the use of fluoroglutamate-containing peptides as hydrolase-resistant folates or antifols in a variety of chemotherapeutic regimens.

Animals↗
PubMed1986

Biosynthesis of polyglutamates of folates.

Folylpolyglutamate synthetase (FPGS) catalyzes the synthesis of the poly-gamma-glutamate forms of tetrahydrofolate and its co-enzyme adducts, as well as of the folate-analogue drugs. This paper reviews current knowledge of the preparations of FPGS from mammalian sources (rat, hog, mouse, and beef liver). Kinetic constants for the substrates and activators of FPGS are compared. Tetrahydrofolate and 5-formyltetrahydrofolate are excellent substrates for the enzyme. The Km values for the antifolates and their 7-hydroxy metabolites are much higher than those for the tetrahydrofolates. Aminopterin has higher activity with FPGS than does methotrexate, which partially explains its greater toxicity. 5-Formyltetrahydrofolate, which is used as a rescue agent in high-dose methotrexate-rescue chemotherapy, is a better alternate substrate of FPGS than is methotrexate and therefore is a potent competitive inhibitor of the glutamylation of methotrexate. Thus, low concentrations of the rescue agent prevent formation of cytotoxic polyglutamates of methotrexate. The pathway of the reaction is the addition of a glutamate residue to the terminal gamma-carboxyl of the pteridine substrate. That longer folylpolyglutamates are poorer substrates possibly is a result of this addition pathway. Pteroic acid activates FPGS by lowering the Km value of the pteridine substrate. It also greatly increases the activity of the synthetase at physiological pH values.

Animals↗