Comparison of folylpolyglutamate hydrolases of mouse liver, kidney, muscle and brain

High-performance liquid chromatographic measurement of 5,10-methylenetetrahydrofolate in liver

The folate coenzyme 5,10-methylenetetrahydrofolate is an important folate metabolite which cannot be determined directly by HPLC near neutral pH because it dissociates to formaldehyde and tetrahydrofolate. A method for the determination of 5,10-methylenetetrahydrofolate in liver is described. This method involves (1) determination of liver 5-methyltetrahydrofolate; (2) chemical reduction of liver 5,10-methylenetetrahydrofolate (stabilized at pH 10) to 5-methyltetrahydrofolate; and (3) determination of total liver 5-methyltetrahydrofolate. Subtraction of (1) from (3) gives the concentration of 5,10-methylenetetrahydrofolate in liver.

Effect of consecutive lower-dose cisplatin in enhancement of 5-fluorouracil cytotoxicity in experimental tumor cells in vivo

It is known that cisplatin (CDDP) potentiates the cytotoxicity of 5-fluorouracil (5-FU), and that the biochemical mechanism is an increase in the intracellular reduced folate levels in the tumor cells. We investigated the effect of consecutive administration with lower-dose CDDP on intracellular accumulation of reduced folate and the activity of methionine synthase, a key enzyme in intracellular methionine synthesis. When CDDP (1 mg/kg) was administered i.p. to ascitic Yoshida sarcoma-bearing rats for 4 consecutive days, both the reduced folate levels and methionine synthase activity in the cells significantly increased, as the same as a single 5 mg/kg dose of CDDP. Furthermore, when Yoshida sarcoma-bearing rats were pre-treated with 1 mg/kg CDDP for 5 consecutive days, [14C]L-methionine incorporation into the isolated ascitic cells was significantly inhibited as compared to that in non-treated cells, suggesting that consecutive administration of lower-dose CDDP is capable of inducing the intracellular modulation of reduced folate levels and methionine synthase activity via inhibition of cellular uptake of methionine. In addition, 5-day administration of lower-dose (1 mg/kg) CDDP potentiated the antitumor effect of 5 mg/kg S-1, a new oral preparation of tegafur, given for 7 consecutive days, and this combined effect was almost similar to the antitumor effect of a combination of S-1 and a single conventional dose (5 mg/kg) of CDDP. Consecutive lower-dose CDDP also may be concluded to act as an important modulator of the enhancement of 5-FU cytotoxicity in experimental tumors.

Toxicity induced by a polyglutamated folate analog is attenuated by NAALADase inhibition

Folates have been shown to be neurotoxic and convulsive. Endogenously, folates exist in the brain in a polyglutamated form with 1-7 terminal glutamates (approx. 1 microM). The brain enzyme N-acetylated alpha-linked acidic dipeptidase (NAALADase) has been shown to remove sequentially the gamma-linked glutamates from folic acid polyglutamates. We report that, at high concentrations (300 microM-30 mM), a folic acid hexaglutamate analog is dose-dependently toxic to dissociated rat cortical cultures and that this toxicity is reversed by 2-PMPA, a potent and selective NAALADase inhibitor. These data suggest a new mechanism for folic acid toxicity.

Folates and one-carbon metabolism in plants and fungi

Folate-dependent pathways of one-carbon metabolism are essential for the synthesis of purines, formylmethionyl-tRNA, thymidylate, serine and methionine. These syntheses use a cellular source of one-carbon substituted, tetrahydrofolate polyglutamate derivatives which are the preferred substrates of most folate-dependent enzymes. In the last decade, there have been major advances in the folate biochemistry of animal, bacterial, fungal and plant systems. These have included the refinement of methods for folate isolation and characterization, basic work on key enzymes of folate biosynthesis and the detailed characterization of proteins that catalyze the generation and utilization of one-carbon substituted folates.

gamma-Glutamyl hydrolase from human sarcoma HT-1080 cells: characterization and inhibition by glutamine antagonists

Elevated gamma-glutamyl hydrolase (GGH) activity as a contributing factor in mechanisms of acquired and intrinsic antifolate resistance has been reported for several cultured cell lines. Despite this, little is known about this enzyme, especially the human species. Using the human HT-1080 sarcoma line, we observed the secretion of GGH activity into media during culture (a phenomenon that could be markedly stimulated by exposure to NH4Cl) and an acidic pH optimum for in vitro catalytic activity of the enzyme. These properties are consistent with a lysosomal location for the enzyme. Unlike rodent GGH, preparations of HT-1080 enzyme (purified < or = 2000-fold) displayed exopeptidase activity in cleaving successive end-terminal gamma-glutamyl groups from poly-L-gamma-glutamyl derivatives of folate, methotrexate (MTX), and para-aminobenzoic acid substrates and a marked preference for long-chain polyglutamates (Km values for glu4 versus glu1 derivatives were 17- and 15-fold lower for folate and MTX versions, respectively). Using an in vitro assay screen, several glutamine antagonists [i.e., 6-diazo-5-oxo-norleucine (DON), acivicin, and azaserine] were identified as human GGH inhibitors, with DON being the most potent and displaying time-dependent inhibition. In cell culture experiments, simultaneous exposure of DON (10 microM) and [3H]MTX for 24 hr resulted in modest elevations of the long-chain gamma-glutamyl derivatives of the antifolate for HT-1080 and another human sarcoma line. These compounds may serve as useful lead compounds in the development of specific GGH inhibitors for use in examining the relationship between GGH activity and antifolate action and may potentially be used in clinical combination with antifolates that require polyglutamylation for effective cellular retention.

Impact of schedule on leucovorin potentiation of fluorouracil antitumor activity in dietary folic acid deplete mice

A dietary folic acid depleted mouse model was established and used to evaluate the relationship between elevation of reduced folates after leucovorin (LV) administration and potentiation of fluorouracil (FU) response of an implanted tumor. C3H mouse mammary adenocarcinomas from mice maintained on a folic acid deplete diet had modestly decreased methylenetetrahydrofolate and tetrahydrofolate levels, and were somewhat less responsive to FU alone compared with replete animals. LV administration resulted in a substantial increase in tumor folate by 1 hr that returned to near basal levels by 12 hr. Reduced folates were elevated to a lesser extent in animals on a standard diet. Tumor growth was suppressed approximately 80% when FU was administered to depleted animals 1 hr after LV administration, compared with approximately 50% suppression in control mice. LV administered 12 hr before FU resulted in tumor growth stimulation that was consistent with the pronounced growth stimulation when LV was administered without FU. These results show that dietary folic acid depletion can lead to a more responsive FU/LV model and that administration of LV at an improper time before FU not only can fail to potentiate but also can result in tumor growth stimulation.

Human gamma-glutamyl hydrolase: cloning and characterization of the enzyme expressed in vitro

A cDNA encoding human gamma-glutamyl hydrolase has been identified by searching an expressed sequence tag data base and using rat gamma-glutamyl hydrolase cDNA as the query sequence. The cDNA encodes a 318-amino acid protein of Mr 35,960. The deduced amino acid sequence of human gamma-glutamyl hydrolase shows 67% identity to that of rat gamma-glutamyl hydrolase. In both rat and human the 24 amino acids preceding the N terminus constitute a structural motif that is analogous to a leader or signal sequence. There are four consensus asparagine glycosylation sites in the human sequence, with three of them conserved in the rat enzyme. Expression of both the human and rat cDNA in Escherichia coli produced antigenically related proteins with enzyme activities characteristic of the native human and rat enzymes, respectively, when methotrexate di- or pentaglutamate were used as substrates. With the latter substrate the rat enzyme cleaved the innermost gamma-glutamyl linkage resulting in the sole production of methotrexate as the pteroyl containing product. The human enzyme differed in that it produced methotrexate tetraglutamate initially, followed by the triglutamate, and then the diglutamate and methotrexate. Hence the rat enzyme is an endopeptidase with methotrexate pentaglutamate as substrate, whereas the human enzyme exhibits exopeptidase activity. Another difference is that the expressed rat enzyme is equally active on methotrexate di- and pentaglutamate whereas the human enzyme has severalfold greater activity on methotrexate pentaglutamate compared with the diglutamate. These properties are consistent with the enzymes derived from human and rat sources.

Identification, cloning, and sequencing of a cDNA coding for rat gamma-glutamyl hydrolase

Purified gamma-glutamyl hydrolase secreted from rat H35 hepatoma cells has been characterized as a diffuse band of 55 kDa on SDS-polyacrylamide gel electrophoresis that is converted to bands of 35 and 33 kDa after enzymatic removal of N-linked carbohydrate. Polyclonal antibodies against 55-kDa gamma-glutamyl hydrolase captured the enzyme activity and recognized the glycosylated and both deglycosylated forms of gamma-glutamyl hydrolase. A complete cDNA sequence of gamma-glutamyl hydrolase was obtained using degenerate oligonucleotides derived from peptide sequences, screening of a rat hepatoma cDNA library, and reverse transcription polymerase chain reaction. Based upon the deduced amino acid sequence the peptide component of gamma-glutamyl hydrolase had a molecular weight of 33,400. The results of amino acid analysis of the purified protein agreed with the deduced amino acid sequence in which there are seven potential asparagine-containing glycosylation sites.

Antitumour effects of pure diastereoisomers of 5-formyltetrahydrofolate in hepatic transplants of a rodent colon carcinoma model

The effects of the two diastereoisomers of 5-formyltetrahydrofolate on tumour growth, thymidylate synthase (TS, EC 2.1.1.45) levels, and potentiation of 5-fluorouracil cytotoxicity were studied in an in vivo rat colon carcinoma model, transplanted to liver. The animals were randomized into eight groups, treated with daily i.v. tail vein injections of racemic (d,l)-5-formyltetrahydrofolate (5-CHO-FH4), 15 mg/kg, (1)-5-CHO-FH4 7.5 mg/kg, and (d)-5-CHO-FH4 7.5 mg/kg, 5-fluorouracil (FUra) 30 mg/kg, (d,l)-5-CHO-FH4 15 mg/kg+FUra 30 mg/kg, (l) 5-CHO-FH4 7.5 mg/kg+FUra 30 mg/kg, and (d)-5-CHO-FH4 7.5 mg/kg+FUra 30 mg/kg, and a sham-treated control group. The average tumour size of the groups was equal at the start of treatment. After six days' treatment the average tumour sizes were at laparotomy 3.3 +/- 1.0 g in the (d/l)-5-CHO-FH4 treated group, compared to 2.0 +/- 0.1 g in the FUra treated group and 7.1 +/- 3.1 g in the controls. Natural (l)-5-CHO-FH4 promoted tumour growth (average tumour weight 10.8 +/- 4.0 g), whereas the unnatural (d)-5-CHO-FH4 alone retarded it (average tumour weight 1.2 +/- 0.40 g). (l)-5-CHO-FH4 induced a significant increase in tumour tissue TS levels by [3H]FdUMP radioligand assay (27.5 +/- 8.4 pmol/g tumour tissue) compared to controls (16.8 +/- 6.1 pmol/g tumour tissue). Increases in 5,10-methylenetetrahydrofolate and tetrahydrofolate occurred with FUra alone, with a further statistically significant increase in both folates with the addition of (d)-5-CHO-FH4 to FUra.

The properties of the secreted gamma-glutamyl hydrolases from H35 hepatoma cells

gamma-Glutamyl hydrolase has been partially purified and characterized from conditioned culture medium of H35 hepatoma cells. Evidence for heterogeneity of the enzyme is derived from its elution as three distinct peaks of enzymatic activity when the enzyme is purified by TSK-butyl-Sepharose column chromatography. These three enzyme fractions appear to have identical catalytic properties but, as yet, the basis for their resolution is not understood. A rapid, sensitive and simple assay based on reverse-phase HPLC fluorescent detection with pre-column derivatization using o-phthalaldehyde (OPA) was developed to separate OPA-derivatives of poly-gamma-glutamates and glutamic acid. Using this assay and the standard HPLC assay for pteroylpolyglutamates, the enzyme appears to be an endopeptidase with respect to pteroylpenta-gamma-glutamate (PteGlu5), methotrexate penta-gamma-glutamate (4-NH2-10-CH3PteGlu5) and p-aminobenzoyl-penta-gamma-glutamate (pABAGlu5). The initial products are PteGlu1 (or 4-NH2-10-CH3PteGlu1 or pABAGlu1) and intact tetra-gamma-glutamate, which is subsequently degraded to glutamic acid. When penta-gamma-glutamate is the substrate, the cleavage of the gamma-bonds by the enzyme is less ordered, with the early appearance of mono-, di-, tri- and tetraglutamate. Poly-alpha-glutamate is not a substrate nor are pABA-gamma-Glu5 or penta-gamma-glutamate covalently linked to albumin. 4-NH2-10-CH3PteGlu2 or Glu5 bound to dihydrofolate reductase is not a substrate for the enzyme, offering further evidence that protein-associated poly-gamma-glutamates are poor substrates for gamma-glutamyl hydrolase from H35 hepatoma cells.

Metabolic basis of the synergistic antitumor activities of 5-fluorouracil and cisplatin in rodent tumor models in vivo

The biochemical mechanism of the synergy of 5-fluorouracil (FUra) and cisplatin (CDDP) was studied using transplantable tumors in rodents in vivo. The reduced folate 5,10-methylenetetrahydrofolate (CH2FH4) and its precursor tetrahydrofolate (FH4) are essential cofactors for the formation of a tight ternary complex of thymidylate synthase (TS) and 5-fluoro-2'-deoxyuridine-5'-monophosphate (FdUMP) derived from FUra. Intraperitoneal administration of CDDP (5 mg/kg) inhibited the incorporation of exogenous L-methionine into ascitic tumor cells and increased the levels of CH2FH4 and FH4 in ascitic Yoshida sarcoma and P-388 cells transplanted into rats and mice to levels about 2-3 times those measured in cells from animals that were not treated with CDDP. Preincubation with 10(-6) M FUra in Hanks' medium inhibited [6-3H]-2'-deoxyuridine incorporation into DNA of tumor cells from CDDP-treated rats 3 times more than that into cells from untreated rats, indicating that the inhibition of TS by FdUMP derived from FUra was enhanced in the presence of CH2FH4. Intraperitoneal administration of CDDP on day 1 and continuous infusion of FUra from day 1 to day 6 had synergistic effects in inhibiting tumor growth in Yoshida sarcoma-bearing rats. Oral administration of UFT, a combined form of 1 M tegafur and 4 M uracil, for 7 consecutive days beginning at 24 h after tumor implantation and a single i.p. injection of CDDP on day 1 had a significantly greater effect than did either agent alone. These results suggest that CDDP significantly enhances FUra cytotoxicity by inhibiting intracellular L-methionine metabolism and consequently increasing the reduced folate pool in mammalian tumor models in vivo.

The properties and function of gamma-glutamyl hydrolase and poly-gamma-glutamate

gamma-Glutamyl hydrolase is a ubiquitous enzyme that has the capacity to cleave gamma-glutamyl bonds of cellular folyl- and antifolylpoly-gamma-glutamates. This study has revealed that the enzyme is secreted by primary cultures of rat hepatocytes and by H35 hepatoma cells. It was found that more than 99% of the total enzyme from H35 cells accumulated in the medium after 48 hr incubation with the serum-free medium. The cells were shown to remain intact during the secretion period since lactate dehydrogenase, dihydrofolate reductase and lysosomal hydrolases other than gamma-glutamyl hydrolase were retained within the cell. When PteGlu5 (folylGlu4) is used as a substrate the initial product is PteGlu (folate), and there is no appearance of intermediate chain length pteroyl polyglutamates. Therefore, the secreted and cellular gamma-glutamyl hydrolase from hepatoma cells appears to be an endopeptidase. Polyclonal antibodies to the poly-gamma-glutamate substrates of the enzyme were prepared and characterized. The antibodies recognize the structural differences between alpha- and gamma-glutamyl linkages but appear equally active with PteGlu5 and its analogs such as 4-NH2-10-CH3PteGlu5 and pABAGlu5. The affinity of the antibodies is related to the gamma-glutamyl structure since L-glutamic acid, folate or p-aminobenzoic acid are inactive with the antibodies. Furthermore, poly-gamma-glutamate has lower affinity for the antibodies than the poly-gamma-glutamate derivatives of PteGlu, 4-NH2-10-CH3PteGlu or pABA.

A sensitive radioenzymatic assay for (S)-5-formyltetrahydrofolate

A highly sensitive, radioenzymatic method has been developed for the specific and quantitative estimation of (S)-5-formyltetrahydrofolate. This method is based on enzymatic cycling of the 5-formyl derivative to methylenetetrahydrofolate followed by entrapment into a stable ternary complex with thymidylate synthase and tritiated fluorodeoxyuridylate. Determination of bound radiolabeled ligand permits estimation of the original folate. The initial cycling step is catalyzed by the enzyme, methenyltetrahydrofolate synthetase, which is specific for the (S)-diastereomer of 5-formyltetrahydrofolate and generates a product which can be further cycled to tetrahydrofolate using either 10-formyltetrahydrofolate deacylase or glycinamide ribonucleotide transformylase. Tetrahydrofolate is ultimately converted to the entrapable methylene derivative in the presence of excess formaldehyde. Using this assay recovery of reference (S)-5-formyltetrahydrofolate was linear over the range 0.03-1.9 pmol with an average recovery of 83 +/- 2%. The method has been applied to estimation of plasma (S)-5-formyltetrahydrofolate from a volunteer who had been administered (R,S)-5-formyltetrahydrofolate. Where comparison was possible, estimation of plasma (S)-5-formyltetrahydrofolate by this one step ternary complex-based method yielded results that were very similar to those observed by Straw et al. (Cancer Res., 44, 3114, 1984) who used an HPLC-based method for separation of diastereomeric mixtures of reduced folates and microbiological growth dependence to determine (S)-5-formyltetrahydrofolate.

Human red blood cell-mediated metabolism of leucovorin [(R,S)5-formyltetrahydrofolate]

The ability of human blood in vitro, and partially purified red blood cells, to metabolize leucovorin, or 5-formyltetrahydrofolate, has been examined. A radioenzymatic assay based upon entrapment of 5,10-methylenetetrahydrofolate, and other reduced folates after cycling to this form, into a stable ternary complex with thymidylate synthase and tritiated 5-fluoro-2'-deoxyuridine-5'-monophosphate was used to estimate reduced folate metabolites. Incubation of whole blood samples with (R,S)5-formyltetrahydrofolate resulted in a time- and concentration-dependent extracellular accumulation of the reduced folates, 5-methyltetrahydrofolate, tetrahydrofolate, 10-formyltetrahydrofolate, and 5,10-methylenetetrahydrofolate. While accumulation with time was nonlinear, the tetrahydrofolate pool showed the greatest overall increase in concentration. 5-Methyltetrahydrofolate, which was the only reduced folate detected in plasma prior to introduction of (R,S)5-formyltetrahydrofolate, accumulated more slowly than tetrahydrofolate. 10-Formyltetrahydrofolate and 5,10-methylenetetrahydrofolate accumulated even more slowly but exhibited nonlinear kinetic patterns similar to those of tetrahydrofolate and 5-methyltetrahydrofolate. When blood cells were removed by centrifugation, a complete loss of metabolic activity was observed. Exposure of purified red blood cells to (R,S)5-formyltetrahydrofolate resulted in accumulation of extracellular reduced folates that was similar to that in whole blood samples while partially purified white blood cells exhibited little activity. Metabolism of the (S) diastereomer of 5-formyltetrahydrofolate accounted for essentially all of the observed extracellular accumulation of reduced folates. We propose that red blood cell-mediated metabolism of 5-formyltetrahydrofolate could, in part at least, account for reduced folate accumulation in plasma when leucovorin is administered to humans.

Regulation of drosophila folylpolyglutamates during development

The polyglutamate status of reduced folates during the larval, pupal and adult stages of Drosophila melanogaster development was investigated. The chain length distribution is very similar and is predominantly pentaglutamate. Half-life estimates of the hydrolytic degradation to the monoglutamate showed larva less than pupa less than adult. This raises the possibility that polyglutamate hydrolase may have a role in regulating the total intracellular reduced folate content of the different developmental stages.

Liposome-dependent delivery of pteridine antifolates: a two-compartment growth inhibition assay for evaluating drug leakage and metabolism

We have developed a two-compartment growth inhibition assay that can provide information about leakage, metabolism and delivery of liposome-dependent drugs under cell culture conditions, and at drug concentrations that are relevant to drug delivery. Two cell lines are grown in separate compartments separated from each other by a 0.1 micron polycarbonate membrane. The membrane allows free drugs to diffuse rapidly from one compartment to another, and does not allow liposomes to diffuse through. Liposomes are added to the first compartment, which contains target cells. The extent of leakage caused by these cells is determined by the growth inhibition of non-target cells in the second compartment. We show that methotrexate and methotrexate-gamma-aspartate leak rapidly and almost completely when encapsulated in phosphatidylglycerol/cholesterol (67:33) liposomes. In contrast, there is only 42% leakage when the drugs are encapsulated in distearoylphosphatidylglycerol/cholesterol (67:33) liposomes. We also demonstrate that the target cells (CV1-P) may partially degrade encapsulated methotrexate-gamma-aspartate to methotrexate. Therefore, methotrexate-gamma-aspartate may be a lysosomally cleaved pro-drug of methotrexate.

Accumulation of tetrahydrofolates in human plasma after leucovorin administration

Reduced folates in plasma after i.v. and oral leucovorin administration were estimated by a ternary complex assay based on the incorporation of CH2FH4 into a stable complex with Lactobacillus casei thymidylate synthase and [3H]FdUMP. Each of the reduced folates, CH2FH4, FH4, and 5CH3FH4, could be quantitatively recovered from plasma by this approach even in the presence of high concentrations of the parent compound leucovorin. Examination of the accumulation kinetics of these reduced folates showed that after i.v. administration of 20 mg D,L-leucovorin to a healthy volunteer, FH4 and, to a lesser extent, CH2FH4 accumulated to maximal levels very early (less than 15 min), with a subsequent depletion that had a half-life of approximately 30 min. Accumulation of FH4 reached a peak level that was 12% of the maximal level of 5CH3FH4 achieved and more than 3 times greater than the pretreatment level of this common, circulating reduced folate form. Similar accumulation patterns were observed in a female patient with metastatic colonic cancer who was undergoing methotrexate (MTX)/fluorouracil therapy followed by i.v. leucovorin (15 mg). FH4 also accumulated, but to a lesser extent and over a longer period of time, when the same dose of leucovorin given orally. When several similar doses of leucovorin were given prior to the experimental dose, greater accumulation and duration of the FH4 response was observed. We propose that accumulation of FH4 and CH2FH4 could provide a circulating source of the reduced folate thought to be the active form for both high-dose MTX with leucovorin rescue and enhancement of fluorouracil activity.

Folinic acid modulation of fluorouracil: tissue kinetics of bolus administration

Thymidylate synthase (TS) is the enzyme target of 5-fluorouracil (FUra) that recent laboratory and clinical studies with folinic acid (calcium leucovorin) suggest may mediate important antitumor cytotoxicity. Measurement in carcinoma tissue of parameters related to TS inhibition by 5-fluorodeoxyuridylate (FdUMP), by analogy to hormone receptor analysis, should be useful to determine which patients should receive fluoropyrimidine drug therapy and to evaluate folinic acid requirements. Folinic acid is metabolized to 5,10-methylenetetrahydropteroylglutamine (CH2FH4), which must be present in large excess to effect desired levels of maximal inhibition of TS, by promoting formation and stabilization of TS-FdUMP-CH2FH4 ternary complexes. In patients with metastatic disease, serial biopsies of tumor and normal tissues for studies of pharmacodynamic responses to test-dose FUra or folinic acid are shown to be easily added to routine intraoperative management. A suitable methodologic approach is described and examples given of assays of free TS, FdUMP, dUMP, and CH2FH4 levels after FUra or folinic acid, that may be useful in future studies aimed at improving the cost-effectiveness of FUra-folinic acid combinations.

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.

Mechanisms of cross-resistance to methotrexate and 5-fluorouracil in an A2780 human ovarian carcinoma cell subline resistant to cisplatin

Some properties of the human ovarian carcinoma line A2780 and a subline three times more resistant than the parent line to cisplatin are compared in this report. The rates of uptake and release of cisplatin were similar in the two cell lines. Resistance to cisplatin was associated with: (a) cross-resistance to 5-fluorouracil and methotrexate; (b) a 2.5-fold increase in thymidylate synthase, as measured by both enzyme activity and the capacity to complex 5-fluorodeoxyuridylate; and (c) an increase in the intracellular pools of 5,10-methylenetetrahydrofolate and tetrahydrofolate. These data suggest that cross-resistance to 5-fluorouracil and methotrexate in A2780 cells may be a consequence of increases in their respective target enzymes.

The measurement of gamma-glutamyl hydrolase (conjugase) activity in rat brain

An assay using the artificial substrate, 2,4-diamino-10-methyl-pteroylglutamyl-gamma-glutamate (MTX-G1), was developed to measure gamma-glutamyl hydrolase (conjugase), which hydrolyzes folylpolyglutamates. This assay allows us to: 1) measure conjugase for the first time in rat brain and 2) measure conjugase in a reliable, sensitive and inexpensive manner. The MTX-binding assay results were compared to samples analyzed by HPLC and found to vary by only 13%. The artificial substrate, MTX-G1, had a lower rate of hydrolysis than pteroylglutamyl-gamma-glutamate (Pte-G2), 70.7 +/- 0.64 and 92.6 +/- 0.22 nmoles/hr/mg protein respectively. Conjugase was semi-purified 24 fold in H2O and found to have a pH optimum of 5.0.

Biochemical basis for cisplatin and 5-fluorouracil synergism in human ovarian carcinoma cells

The human ovarian cell line A2780 was exposed to either cisplatin (10 microM) or 5-fluorouracil (5FUra) (5 microM) for 1 hr. Cytotoxicity was less than 14% with either agent alone. Cisplatin (10 microM) and 5FUra (5 microM) in combination for 1 hr caused a 76% reduction in cell growth. Thymidine (dThd, 10 microM), if given concomitantly with the combination of cisplatin and 5FUra, completely protected the tumor cells. A 30-min exposure to cisplatin increased the intracellular pools of 5,10-methylenetetrahydrofolate and tetrahydrofolate 2.5-fold. The capacity of intact cells to form 5-fluorodeoxyuridylate (FdUMP)-thymidylate (dTMP) synthase complex when incubated with fluorodeoxyuridine (FdUrd) was enhanced 2.5-fold when the cells were pretreated with cisplatin. These experiments demonstrate that cisplatin can increase the availability of the reduced folate necessary for tight binding of FdUMP to dTMP synthase, thus enhancing the cytotoxicity of the cisplatin and 5FUra combination.

A radioenzymatic method for the determination of tissue 10-formyltetrahydrofolate

A radioenzymatic method for the determination of tissue 10-formyltetrahydrofolate is described based on the stable ternary complex formed from methylenetetrahydrofolate, tritiated fluorodeoxyuridylate and thymidylate synthase. Tissue extract 10-formyltetrahydrofolate is deacylated with 10-formyltetrahydrofolate deacylase and the tetrahydrofolate formed is converted to methylenetetrahydrofolate using formaldehyde. Mouse tissue 10-formyltetrahydrofolate levels and their stability to extraction procedures are described.

Micellar high-performance liquid chromatographic determination of folylpolyglutamate hydrolase activity

A rapid and sensitive method for the quantitative determination of folylpolyglutamate hydrolase activity in crude tissue extracts was developed. The procedure is based on high-performance liquid chromatographic separation of folate analogue mono- and polyglutamates on a reversed-phase column using sodium dodecyl sulfate in water as the mobile phase. Interfering substances in tissue extracts were removed by gel filtration on centrifugally-eluted mini-columns of Sephadex G-25 prior to incubation of polyglutamate substrate with tissue extract hydrolase. Reactions were terminated by denaturation of the enzyme in sodium dodecyl sulfate, which subsequently served as the micellar solvent system for chromatographic separation of substrate from reaction products.

Effects of methotrexate on folates in Krebs ascites and L1210 murine leukemia cells

Changes in reduced folates upon exposure of Krebs ascites cells and L1210 murine leukemia cells to methotrexate (MTX) have been measured by stoichiometric entrapment of tissue methylenetetrahydrofolate into a stable ternary complex with thymidylate synthase and tritiated 5-fluoro-2'-deoxy-uridine-5'-monophosphate. Tetrahydrofolate and 5-methyltetrahydrofolate were determined after conversion to methylenetetrahydrofolate. In both tumor cell lines, treatment with methotrexate at levels which had little effect on methylenetetrahydrofolate and tetrahydrofolate concentrations resulted in nearly complete elimination of the methyltetrahydrofolate pool. Thus, an initial effect of methotrexate on folate metabolism appears to be on methyltetrahydrofolate.

Determination of mouse liver 5-methyltetrahydrofolate concentration and polyglutamate forms

The concentration and polyglutamate status of 5-methyltetrahydrofolate in mouse liver tissue extracts has been determined by enzymatic conversion to methylenetetrahydrofolate and subsequent entrapment of this cofactor form into a ternary complex with Lactobacillus casei thymidylate synthase and tritiated 5-fluorodeoxyuridylate. 5-Methyltetrahydrofolate was oxidized to methylenetetrahydrofolate using the reverse reaction of methylenetetrahydrofolate reductase with menadione as the ultimate electron acceptor. Reference 5-methyltetrahydrofolate could be quantitatively recovered from tissue extracts by this method. The polyglutamate status of enzymatically converted and complexed tissue 5-methyltetrahydrofolate was determined electrophoretically. Unlabeled 5-fluorodeoxyuridylate was used to remove endogenous methylenetetrahydrofolate prior to enzymatic oxidation of 5-methyltetrahydrofolate and subsequent electrophoretic analysis. In this manner, the 5-methyltetrahydrofolate polyglutamate pool alone could be labeled and visualized. There were no observable differences in the polyglutamate distribution of endogenous methylenetetrahydrofolate versus 5-methyltetrahydrofolate polyglutamates in extracts of normal mouse liver tissue.

Response of methylenetetrahydrofolate levels to methotrexate in Krebs ascites cells

Levels of methylenetetrahydrofolate in Krebs ascites cells subsequent to transplantation and the effects of methotrexate on these levels have been measured. To directly measure methylenetetrahydrofolate in tissue extracts, the cofactor was incorporated into a covalent ternary complex with thymidylate synthase and 3H-labelled fluorodeoxyuridine monophosphate. A 3-4-day lag preceded rapid growth of the tumour cells, and this same kinetic behaviour was observed for methylenetetrahydrofolate levels in the tumour cells. Liver and kidney tissue from the same animals also showed an increase in methylenetetrahydrofolate over the same time period. The impact of methotrexate on methylenetetrahydrofolate in the tumour cells depended upon concentration and the post-transplantation time at which treatment was initiated. Levels of methylenetetrahydrofolate in the tumour cells were most sensitive to the drug at the beginning of the rapid growth phase and were more sensitive to a given level of methotrexate in the presence of phospholipids. A slight but significant increase in methylenetetrahydrofolate occurred in some cases in response to the presence of methotrexate.