Purification and characterization of dihydrofolate reductase from methotrexate-resistant human lymphoblastoid cells

Structure and kinetics assays of recombinant Schistosoma mansoni dihydrofolate reductase

The parasite Schistosoma mansoni possesses all pathways for pyrimidine biosynthesis, in which dihydrofolate reductase (DHFR), thymidylate cycle participants, is essential for nucleotide metabolism to obtain energy and structural nucleic acids. Thus, DHFRs have been widely suggested as therapeutic targets for the treatment of infectious diseases. In this study, we expressed recombinant SmDHFR in a heterologous manner to obtain structural, biochemical and kinetic information. X-ray diffraction of recombinant SmDHFR at 1.95Å resolution showed that the structure exhibited the canonical DHFR fold. Isothermal titration calorimetry was used to determine the kinetic constants for NADP⁺ and dihydrofolate. Moreover, inhibition assays were performed using the commercial folate analogs methotrexate and aminopterin; these analogs are recognized as folate competitors and are used as chemotherapeutic agents in cancer and autoimmune diseases. This study provides information that may prove useful for the future discovery of novel drugs and for understanding these metabolic steps from this pathway of S. mansoni, thus aiding in our understanding of the function of these essential pathways for parasite metabolism.

Synthesis, characterization and target protein binding of drug-conjugated quantum dots in vitro and in living cells

Elucidation of unknown target proteins of a drug is of great importance in understanding cell biology and drug discovery. There have been extensive studies to discover and identify target proteins in the cell. Visualization of targets using drug-conjugated probes has been an important approach to gathering mechanistic information of drug action at the cellular level. As quantum dot (QD) nanocrystals have attracted much attention as a fluorescent probe in the bioimaging area, we prepared drug-conjugated QD to explore the potential of target discovery. As a model drug, we selected a well-known anticancer drug, methotrexate (MTX), which has been known to target dihydrofolate reductase (DHFR) with high affinity binding (K(d) = 0.54 nM). MTX molecules were covalently attached to amino-PEG-polymer-coated QDs. Specific interactions of MTX-conjugated QDs with DHFR were identified using agarose gel electrophoresis and fluorescence microscopy. Cellular uptake of the MTX-conjugated QDs in living CHO cells was investigated with regard to their localization and distribution pattern. MTX-QD was found to be internalized into the cells via caveolae-medicated endocytosis without significant sequestration in endosomes. A colocalization experiment of the MTX-QD conjugate with antiDHFR-TAT-QD also confirmed that MTX-QD binds to the target DHFR. This study showed the potential of the drug-QD conjugate to identify or visualize drug-target interactions in the cell, which is currently of great importance in the area of drug discovery and chemical biology.

Distinct mechanistic activity profile of pralatrexate in comparison to other antifolates in in vitro and in vivo models of human cancers

PURPOSE

This study evaluated mechanistic differences of pralatrexate, methotrexate, and pemetrexed.

METHODS

Inhibition of dihydrofolate reductase (DHFR) was quantified using recombinant human DHFR. Cellular uptake and folylpolyglutamate synthetase (FPGS) activity were determined using radiolabeled pralatrexate, methotrexate, and pemetrexed in NCI-H460 non-small cell lung cancer (NSCLC) cells. The tumor growth inhibition (TGI) was assessed using MV522 and NCI-H460 human NSCLC xenografts.

RESULTS

Apparent K ( i ) values for DHFR inhibition were 45, 26, and >200 nM for pralatrexate, methotrexate, and pemetrexed, respectively. A significantly greater percentage of radiolabeled pralatrexate entered the cells and was polyglutamylatated relative to methotrexate or pemetrexed. In vivo, pralatrexate showed superior anti-tumor activity in both NSCLC models, with more effective dose-dependent TGI in the more rapidly growing NCI-H460 xenografts.

CONCLUSIONS

Pralatrexate demonstrated a distinct mechanistic and anti-tumor activity profile relative to methotrexate and pemetrexed. Pralatrexate exhibited enhanced cellular uptake and increased polyglutamylation, which correlated with increased TGI in NSCLC xenograft models.

Cloning, expression, and characterization of Mycobacterium tuberculosis dihydrofolate reductase

The gene for dihydrofolate reductase of Mycobacterium tuberculosis was amplified by polymerase chain reaction (PCR) from M. tuberculosis H37Rv strain genomic DNA. The protein was expressed in inclusion bodies in high yield in Escherichia coli under the control of the T7 promoter. Active enzyme was obtained by refolding from guanidine HCl and after a single chromatography step the sample was > 99% homogeneous with a specific activity of approximately 15.5 micromol min(-1) mg(-1). Mass spectrometry analysis confirmed the expected mass of 17.6 kDa. Gel filtration of the enzyme indicated that it was a monomer. Steady-state kinetic parameters were determined and the effect of pH and KCl on the enzyme examined. Methotrexate and trimethoprim inhibited the enzyme.

Identification of critical amino acid residues on human dihydrofolate reductase protein that mediate RNA recognition

Previous studies have shown that human dihydrofolate reductase (DHFR) acts as an RNA-binding protein, in which it binds to its own mRNA and, in so doing, results in translational repression. In this study, we used RNA gel mobility shift and nitrocellulose filter-binding assays to further investigate the specificity of the interaction between human DHFR protein and human DHFR mRNA. Site-directed mutagenesis was used to identify the critical amino acid residues on DHFR protein required for RNA recognition. Human His-Tag DHFR protein specifically binds to human DHFR mRNA, while unrelated proteins including thymidylate synthase, p53 and glutathione-S-transferase were unable to form a ribonucleoprotein complex with DHFR mRNA. The Cys6 residue is essential for RNA recognition, as mutation at this amino acid with either an alanine (C6A) or serine (C6S) residue almost completely abrogated RNA-binding activity. Neither one of the cysteine mutant proteins was able to repress the in vitro translation of human DHFR mRNA. Mutations at amino acids Ile7, Arg28 and Phe34, significantly reduced RNA-binding activity. An RNA footprinting analysis identified three different RNA sequences, bound to DHFR protein, ranging in size from 16 to 45 nt, while a UV cross-linking analysis isolated an approximately 16 nt RNA sequence bound to DHFR. These studies begin to identify the critical amino acid residues on human DHFR that mediate RNA binding either through forming direct contact points with RNA or through maintaining the protein in an optimal structure that allows for the critical RNA-binding domain to be accessible.

Isolation of rat dihydrofolate reductase gene and characterization of recombinant enzyme

While assays of many antifolate inhibitors for dihydrofolate reductase (DHFR) have been performed using rat DHFR as a target, neither the sequence nor the structure of rat DHFR is known. Here, we report the isolation of the rat DHFR gene through screening of a rat liver cDNA library. The rat liver DHFR gene has an open reading frame of 561 bp encoding a protein of 187 amino acids. Comparisons of the rat enzyme with those from other species indicate a high level of conservation at the primary sequence level and more so for the amino acid residues comprising the active site of the enzyme. Expression of the rat DHFR gene in bacteria produced a recombinant protein with high enzymatic activity. The recombinant protein also paralleled the human enzyme with respect to the inhibition by most of the antifolates tested with PT652 and PT653 showing a reversal in their patterns. Our results indicated that rat DHFR can be used as a model to study antifolate compounds as potential drug candidates. However, variations between rat and human DHFR enzymes, coupled with unique features in the inhibitors, could lead to the observed differences in enzyme sensitivity and selectivity.

Design, synthesis, computational prediction, and biological evaluation of ester soft drugs as inhibitors of dihydrofolate reductase from Pneumocystis carinii

A series of lipophilic soft drugs structurally related to the nonclassical dihydrofolate reductase (DHFR) inhibitors trimetrexate and piritrexim have been designed, synthesized, and evaluated in DHFR assays, with special emphasis on the inhibition of P. carinii DHFR. The best inhibitors, encompassing an ester bond in the bridge connecting the two aromatic systems, were approximately 10 times less potent than trimetrexate and piritrexim. The metabolites were designed to be poor inhibitors. Furthermore, molecular dynamics simulations of three ligands in complex with DHFR from Pneumocystis carinii and from the human enzyme were conducted in order to better understand the factors determining the selectivity. A correct ranking of the relative inhibition of DHFR was achieved utilizing the linear interaction energy method. The soft drugs are intended for local administration. One representative ester was selected for a pharmacokinetic study in rats where it was found to undergo fast metabolic degradation to the predicted inactive metabolites.

Antimycobacterial activities of 2,4-diamino-5-deazapteridine derivatives and effects on mycobacterial dihydrofolate reductase

Development of new antimycobacterial agents for Mycobacterium avium complex (MAC) infections is important particularly for persons coinfected with human immunodeficiency virus. The objectives of this study were to evaluate the in vitro activity of 2, 4-diamino-5-methyl-5-deazapteridines (DMDPs) against MAC and to assess their activities against MAC dihydrofolate reductase recombinant enzyme (rDHFR). Seventy-seven DMDP derivatives were evaluated initially for in vitro activity against one to three strains of MAC (NJ168, NJ211, and/or NJ3404). MICs were determined with 10-fold dilutions of drug and a colorimetric (Alamar Blue) microdilution broth assay. MAC rDHFR 50% inhibitory concentrations versus those of human rDHFR were also determined. Substitutions at position 5 of the pteridine moiety included -CH(3), -CH(2)CH(3), and -CH(2)OCH(3) groups. Additionally, different substituted and unsubstituted aryl groups were linked at position 6 through a two-atom bridge of either -CH(2)NH, -CH(2)N(CH(3)), -CH(2)CH(2), or -CH(2)S. All but 4 of the 77 derivatives were active against MAC NJ168 at concentrations of < or =13 microg/ml. Depending on the MAC strain used, 81 to 87% had MICs of < or =1.3 microg/ml. Twenty-one derivatives were >100-fold more active against MAC rDHFR than against human rDHFR. In general, selectivity was dependent on the composition of the two-atom bridge at position 6 and the attached aryl group with substitutions at the 2' and 5' positions on the phenyl ring. Using this assessment, a rational synthetic approach was implemented that resulted in a DMDP derivative that had significant intracellular activity against a MAC-infected Mono Mac 6 monocytic cell line. These results demonstrate that it is possible to synthesize pteridine derivatives that have selective activity against MAC.

Tobacco budworm dihydrofolate reductase is a promising target for insecticide discovery

Structural differences in dihydrofolate reductases from different species have been exploited to develop specific inhibitory molecules, such as chemotherapeutic agents, antibiotics or antihelminthics, that show species specificity or selectivity. As dihydrofolate reductase (DHFR) is a crucial enzyme for the synthesis of purines, pyrimidines and some amino acids, and also because developing insects show a remarkably rapid rate of cell division, DHFR is a potentially promising target for the discovery of novel insecticides. We have thus isolated and characterized the enzyme from a serious agricultural pest, Heliothis (Helicoverpa) virescens, the tobacco budworm. Sequencing tryptic peptides of the 35 000-fold purified DHFR allowed the subsequent isolation of a partial cDNA, with the full Dhfr gene sequence obtained from a genomic library. The H. virescens Dhfr spans 4 kb, with three introns, and encodes 185 amino acids. The enzyme shows an overall similarity of approximately 68% with DHFR from other metazoans, which has facilitated the molecular modeling of the protein. DHFRs from insects appear to have strikingly reduced sensitivity to inhibition by methotrexate, compared with the vertebrate enzymes, and this reduction was also reflected in the total binding energy seen after modeling experiments. Four residues that may be characteristic of insect DHFR, as well as a unique cysteine in the H. virescens DHFR active site, offer insight into the nature of inhibitor selectivity and provide suitable target sites for insecticide discovery.

Structure-based design of selective inhibitors of dihydrofolate reductase: synthesis and antiparasitic activity of 2, 4-diaminopteridine analogues with a bridged diarylamine side chain

As part of a larger search for potent as well as selective inhibitors of dihydrofolate reductase (DHFR) enzymes from opportunistic pathogens found in patients with AIDS and other immune disorders, N-[(2,4-diaminopteridin-6-yl)methyl]dibenz[b,f]azepine (4a) and the corresponding dihydrodibenz[b,f]azepine, dihydroacridine, phenoxazine, phenothiazine, carbazole, and diphenylamine analogues were synthesized from 2, 4-diamino-6-(bromomethyl)pteridine in 50-75% yield by reaction with the sodium salts of the amines in dry tetrahydrofuran at room temperature. The products were tested for the ability to inhibit DHFR from Pneumocystis carinii (pcDHFR), Toxoplasma gondii (tgDHFR), Mycobacterium avium (maDHFR), and rat liver (rlDHFR). The member of the series with the best combination of potency and species selectivity was 4a, with IC(50) values against the four enzymes of 0. 21, 0.043, 0.012, and 4.4 microM, respectively. The dihydroacridine, phenothiazine, and carbazole analogues were also potent, but nonselective. Of the compounds tested, 4a was the only one to successfully combine the potency of trimetrexate with the selectivity of trimethoprim. Molecular docking simulations using published 3D structural coordinates for the crystalline ternary complexes of pcDHFR and hDHFR suggested a possible structural interpretation for the binding selectivity of 4a and the lack of selectivity of the other compounds. According to this model, 4a is selective because of a unique propensity of the seven-membered ring in the dibenz[b,f]azepine moiety to adopt a puckered orientation that allows it to fit more comfortably into the active site of the P. carinii enzyme than into the active site of the human enzyme. Compound 4a was also evaluated for the ability to be taken up into, and retard the growth of, P. carinii and T. gondii in culture. The IC(50) of 4a against P. carinii trophozoites after 7 days of continuous drug treatment was 1.9 microM as compared with previously observed IC(50) values of >340 microM for trimethoprim and 0.27 microM for trimetrexate. In an assay involving [(3)H]uracil incorporation into the nuclear DNA of T. gondii tachyzoites as the surrogate endpoint for growth, the IC(50) of 4a after 5 h of drug exposure was 0.077 microM. The favorable combination of potency and enzyme selectivity shown by 4a suggests that this novel structure may be an interesting lead for structure-activity optimization.

Assay of dihydrofolate reductase activity by monitoring tetrahydrofolate using high-performance liquid chromatography with electrochemical detection

We developed a method to determine dihydrofolate reductase (DHFR) activity at pH 7.4 (37 degrees C) by monitoring its product, tetrahydrofolate (H(4)folate), using HPLC with electrochemical detection. After the assay mixture was deproteinized by 0.5 M perchloric acid, the H(4)folate concentration was measured. Using sodium ascorbate at 20 mM, H(4)folate was stable in our assay system. The enzyme activity was also stable. The detection limit of this method was less than 1 nM of H(4)folate in the enzyme assay system, which was 1/100 lower than those for the NADPH-spectrophotometric assay, which is commonly used for analysis of DHFR activity. This value of 1 nM allowed us to control the conversion from dihydrofolate (H(2)folate) to H(4)folate less than 10% of initial substrate concentrations during assay, when we used a concentration around K(m) values reported for DHFR from various sources. The rate of reduction showed a linearity at concentrations around the K(m). The reduction rate must be evaluated exactly around the K(m), in order to obtain an accurate profile of Michaelis-Menten kinetics. This assay method has a sensitivity high enough to determine the reduction rate at H(2)folate concentrations around K(m). In addition, the assay procedure is very simple. Therefore, our method may be useful for studying DHFR.

High-affinity inhibitors of dihydrofolate reductase: antimicrobial and anticancer activities of 7,8-dialkyl-1,3-diaminopyrrolo[3,2-f]quinazolines with small molecular size

A series of 7,8-dialkylpyrrolo[3,2-f]quinazolines were prepared as inhibitors of dihydrofolate reductase (DHFR). On the basis of an apparent inverse relationship between compound size and antifungal activity, the compounds were designed to be relatively small and compact. Inhibitor design was aided by GRID analysis of the three-dimensional structure of Candida albicans DHFR, which suggested that relatively small, branched alkyl groups at the 7- and 8-positions of the pyrroloquinazoline ring system would provide optimal interactions with a hydrophobic region of the protein. The compounds were potent inhibitors of fungal and human DHFR, with K(i) values as low as 7.1 and 0.1 pM, respectively, and were highly active against C. albicans and an array of tumor cell lines. In contrast to known lipophilic inhibitors of DHFR such as trimetrexate and piritrexim, members of this series of pyrroloquinazolines were not susceptible to P-glycoprotein-mediated multidrug resistance and also showed significant distribution into lung and brain tissue. The compounds were active in lung and brain tumor models and displayed in vivo activity against Pneumocystis carinii and C. albicans.

Synthesis and biological activity of folic acid and methotrexate analogues containing L-threo-(2S,4S)-4-fluoroglutamic acid and DL-3,3-difluoroglutamic acid

The stereospecific syntheses of L-threo-gamma-fluoromethotrexate (1t) and L-threo-gamma-fluorofolic acid (3t) are reported. Compounds 1t and 3t have no substrate activity with folylpoly-gamma-glutamate synthetase isolated from CCRF-CEM human leukemia cells, and compound 1t inhibits human dihydrofolate reductase at similar levels as methotrexate. The synthesis of DL-3,3-difluoroglutamic acid (6) and its incorporation into DL-beta,beta-difluorofolic acid (4) are also reported. Compound 4 acts as a better substrate for human CCRF-CEM folylpoly-gamma-glutamate synthetase than folic acid (V/K = ca. 7-fold greater). Thus, replacement of the glutamate moiety of methotrexate and folic acid with 4-fluoroglutamic acid and 3,3-difluoroglutamic acid results in folates and antifolates with altered polyglutamylation activity.

Activation of chicken liver dihydrofolate reductase in concentrated urea solutions

The activation and inactivation of dihydrofolate reductase from chicken liver during denaturation in a wide concentration range of urea are compared with changes in intrinsic fluorescence. At 2 M urea the enzyme is activated 3.6-fold and is stable up to 12 h in the activated form. At 4 M urea, the enzyme activity increases about 5-fold initially but the activated enzyme loses activity rapidly to a level well below that of the native enzyme. The activated enzyme is stabilized in presence of either DHF or NADPH. The Kd and Km of the enzyme for the substrates at various urea concentrations were determined and compared. In the presence of 3 M urea, the values of Kd for DHF and NADPH increase 4-fold and 10-fold, respectively, whereas the corresponding Km values increase 25-fold and 3-fold. A large increase in Vmax is mainly responsible for the activation. The inactivation and unfolding in urea are both biphasic processes. For the fast phase, the rate constant of inactivation is 10-fold greater than that of unfolding in 4 M urea. The effect of (NH4)2SO4 on the activation and unfolding of the enzyme was also studied. The results suggest that the active site of the enzyme is more easily perturbed by denaturants; and the activated enzyme appears to have a more open and flexible conformation at the active site, which is favorable for the full expression of the catalytic power of the enzyme. A scheme for the sequential activation and inactivation of DHFR accompanying its unfolding by increasing concentrations of urea is proposed.

A fully active variant of dihydrofolate reductase with a circularly permuted sequence

The amino acid sequence of mouse dihydrofolate reductase was permuted circularly at the level of the gene. By transposing the 3'-terminal half of the coding sequence to its 5' terminus, the naturally adjacent amino and carboxyl termini of the native protein were fused, and one of the flexible peptide loops at the protein surface was cleaved. The steady-state kinetic constants, the dissociation constants of folate analogues, and the degree of activation by both mercurials and salt as well as the resistance toward digestion by trypsin were almost indistinguishable from those of a recombinant wild-type protein. Judged by these criteria, the circularly permuted variant has the same active site and overall structure as the wild-type enzyme. The only significant difference was the lower stability toward guanidinium chloride and the lower solubility of the circularly permuted variant. This behavior may be due to moving a mononucleotide binding fold from the interior of the sequence to the carboxyl terminus. Thus, dihydrofolate reductase requires neither the natural termini nor the cleaved loop for stability, for the conformational changes that accompany catalysis as well as the binding of inhibitors, and for the folding process.

Conversion of arginine to lysine at position 70 of human dihydrofolate reductase: generation of a methotrexate-insensitive mutant enzyme

Arginine-70 of human dihydrofolate reductase (hDHFR) is a highly conserved residue which X-ray crystallographic data have shown to interact with the alpha-carboxylate of the terminal L-glutamate moiety of either folic acid or methotrexate (MTX). The rationale for this study was to introduce a conservative amino acid residue change at position 70 (Arg----Lys) which might function as a titratable group and, thus, reveal possible quantitative changes in ligand binding and kinetic parameters as a function of pH. Such a mutant enzyme (R70K) has been constructed and expressed by using site-directed mutagenesis techniques. This substitution has a dramatic effect on the binding of MTX, which displays a 22,600-fold increase in the dissociation constant (KD) at pH 7.5 compared to that of the reported wild-type enzyme value. At this pH, the KD value for dihydrofolate (FAH2) for the R70K enzyme shows only a 7-fold increase over that for the wild-type hDHFR. The pH profiles of the Michaelis and dissociation constants for FAH2 and KD values for MTX for the mutant enzyme all show a 7-8-fold increase from pH 7.5 to 8.5 as compared to its wild-type counterpart. The binding of NADPH or the nonclassical inhibitor trimetrexate (TMQ) to either the wild-type or the mutant enzyme does not show such pH-dependent characteristics. Thus, since FAH2 and MTX interact with the guanidinium side chain of arginine-70 in the wild-type hDHFR, the replacement of this residue with a lysine in the R70K mutant appears to have resulted in the introduction of a titratable group with a perturbed pKa value of ca. 8.3.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of tightly bound substrates in pure preparations of dihydrofolate reductase: implications for studies on enzymes

Investigations have been made to determine the identity and binding characteristics of the pterins that are bound tightly to dihydrofolate reductases which are isolated from vertebrate sources by a well established procedure. This procedure involves the binding of enzyme to a Sepharose-methotrexate column, elution with dihydrofolate and removal of free dihydrofolate by dialysis or by size-exclusion chromatography. Addition to such preparations of NADPH results in oxidation of the nucleotide and from the progress curves so obtained, it is possible to identify the bound pterin and to calculate the stoichiometry of binding. The data indicated that stoichiometric amounts of dihydrofolate or folate plus dihydrofolate were bound to the dihydrofolate reductases. It can also be concluded that binding occurs at the active sites of the enzymes. Enzyme preparations from which bound pterin had been removed by isoelectric focussing reacted with folate or dihydrofolate to form 1:1 enzyme-pterin complexes from which the pterin could not be removed by dialysis or by size-exclusion chromatography. From the magnitude of the dissociation constants for the enzyme-pterin complexes and the concentration of enzyme present in fractions after the step involving affinity chromatography on Sepharose-methotrexate, it could be concluded that the presence of bound folate and/or dihydrofolate in pure preparations of dihydrofolate reductase is simply a consequence of an association-dissociation reaction. Preparations of the enzyme from bacterial sources were also found to contain bound pterins. The findings have implications with respect to kinetic and thermodynamic studies on dihydrofolate reductase and other enzymes which are isolated by affinity chromatography techniques.

Role of lysine-54 in determining cofactor specificity and binding in human dihydrofolate reductase

Lysine-54 of human dihydrofolate reductase (hDHFR) appears to be involved in the interaction with the 2'-phosphate of NADPH and is conserved as a basic residue in other species. Studies have suggested that in Lactobacillus casei dihydrofolate reductase Arg-43, the homologous residue at this position, plays an important role in the binding of NADPH and in the differentiation of Km values for NADPH and NADH. A Lys-54 to Gln-54 mutant (K54Q) of hDHFR has been constructed by oligodeoxynucleotide-directed mutagenesis in order to study the role of Lys-54 in differentiating Km and Kcat values for NADPH and NADH as well as in other functions of hDHFR. The purpose of this paper is to delineate in quantitative terms the magnitude of the effect of the Lys-54 to Gln-54 replacement on the various kinetic parameters of hDHFR. Such quantitative effects cannot be predicted solely on the basis of X-ray structures. The Km for NADPH for the K54Q mutant enzyme is 58-fold higher, while the Km for NADH for K54Q is only 3.9-fold higher than that of the wild type, indicating that the substitution of Lys-54 with Gln-54 decreases the apparent affinity of the enzyme for NADPH dramatically, but has a lesser effect on the apparent affinity for NADH.(ABSTRACT TRUNCATED AT 250 WORDS)

The purification of dihydrofolate reductase from Drosophila melanogaster

Dihydrofolate reductase (DHFR) has been purified over 30,000-fold from Drosophila adults with a yield of 35%, using a combination of low pH extraction, (NH4)2SO4 precipitation, Sephadex gel filtration, Affi-Gel blue affinity chromatography, ion exchange and gel filtration FPLC. The Drosophila enzyme is a soluble, 17-22 kDa monomeric protein displaying the two pH optima characteristic of eukaryotic DHFRs. The sequence of the first 23 amino acids from the amino-terminal end of the protein shows that Drosophila DHFR is more homologous to the mosquito and vertebrate DHFRs than to the prokaryotic enzymes. However, the percent similarity between the two insect enzymes is not as close as expected when compared to the virtually identical initial sequence conservation of mammalian DHFRs.

Anti-idiotypic antibodies elicited by pterin recognize active site epitopes in dihydrofolate reductases and dihydropteridine reductase

Monoclonal antibodies (mAbs) against antipterin immunoglobulin and dihydropteridine reductase (DHPR) and also polyclonal antibodies against human dihydrofolate reductase (DHFR) were obtained. The anti-idiotypic mAbs and anti-DHPR mAbs bind specifically to human DHFR, Escherichia coli DHFR, soybean seedling DHFR, and human DHPR in solid-phase immunoassays. Further, the mAbs bind to the native but not to the denatured forms of DHFRs. The monoclonal antibodies also inhibit the enzymatic activity of human DHFR but not that of human DHPR. Competitive solid-phase immunoassays show stoichiometric inhibition by methotrexate and partial inhibition by NADPH of mAb binding to human DHFR. Cyanogen bromide fragments derived from human DHFR (residues 15-52 and 53-111), containing several active site residues, bind partially to some of the monoclonal antibodies. Accordingly, polyclonal antibodies to peptide 53-111 of human DHFR cross-react to some extent with human DHPR. Data from competitive immunoassays in which the binding of the various mAbs was tested singly and in combination with other mAbs suggest that these antibodies bind to a common region on human DHFR. The results also indicate that the mAbs display some heterogeneity with respect to specific epitopes. These data suggest that despite the absence of significant amino acid sequence homologies among the various DHFRs and DHPR, they have a fundamentally similar topography at the site of binding of the pterin moiety that is recognized by the anti-idiotypic mAbs generated by pterin. In the relatively simple structure of the pterin ring system there are different substituent groups at positions C4 and C6 in methotrexate, 7,8-dihydrofolate, and 7,8-dihydrobiopterin, suggesting that these antibodies are specific for regions on various proteins that interact with the remainder of the pterin moiety. These mAbs and similar mAbs specified by substituent groups on pterin may thus be used as specific probes or inhibitors of various folate-dependent enzymes and transport proteins. They should also provide insights into some of the general features of antibody recognition of protein antigens.

Tricyclic 2,4-diaminopyrimidines with broad antifolate activity and the ability to inhibit Pneumocystis carinii growth in cultured human lung fibroblasts in the presence of leucovorin

A selected number of 1,3-diaminobenzo[f]quinazolines and 1,3-diamino-5,6-dihydrobenzo[f]quinazolines, which may be viewed as tricyclic analogues of the lipid-soluble antifolates pyrimethamine (PM), metoprine (DDMP), and etoprine (DDEP), were tested as inhibitors of purified dihydrofolate reductase (DHFR) from WI-L2 lymphoblasts, and as inhibitors of the growth of Streptococcus faecium ATCC 8043 and L1210 murine leukemia cells in culture. In addition, these tricyclic compounds were tested for antimalarial activity against Plasmodium berghei in mice, and for the ability to inhibit the growth of Pneumocystis carinii trophozoites in WI-38 human lung fibroblast cultures in the presence of leucovorin (LV). The most potent analogues were those with chlorine substitution in the ring distal to the 2,4-diaminopyrimidine moiety. Fully aromatic compounds tended to be more active than those in which the 5,6-bond was reduced, suggesting that planarity favors binding to the DHFR active site and may be favorable for cellular uptake. Several of the 2,4-diaminopyrimidine analogues showed greater potency than PM, DDMP or DDEP, and were more nearly comparable to the bicyclic 2,4-diaminopyrimidine antifolates trimetrexate (TMQ) or piritrexim (BW301U), which are known to be selectively toxic to P. carinii in the presence of LV. Two of the tricyclic compounds, 1,3-diamino-8-chlorobenzo[f]quinazoline and 1,3-diamino-9-chlorobenzo[f]quinazoline, proved to have activity similar to TMQ and BW301U in this system.

Effects of conversion of phenylalanine-31 to leucine on the function of human dihydrofolate reductase

Oligonucleotide-directed, site-specific mutagenesis was used to convert phenylalanine-31 of human recombinant dihydrofolate reductase (DHFR) to leucine. This substitution was of interest in view of earlier chemical modification studies (Kumar et al., 1981) and structural studies based on X-ray crystallographic data (Matthews et al., 1985a,b) which had implicated the corresponding residue in chicken liver DHFR, Tyr-31, in the binding of dihydrofolate. Furthermore, this particular substitution allowed testing of the significance of protein sequence differences between mammalian and bacterial reductases at this position with regard to the species selectivity of trimethoprim. Both wild-type (WT) and mutant (F31L) enzymes were expressed and purified by using a heterologous expression system previously described (Prendergast et al., 1988). Values of the inhibition constants (Ki values) for trimethoprim were 1.00 and 1.08 microM for WT and F31L, respectively. Thus, the presence of phenylalanine at position 31 in human dihydrofolate reductase does not contribute to the species selectivity of trimethoprim. The Km values for nicotinamide adenine dinucleotide phosphate (reduced) (NADPH) and dihydrofolate were elevated 10.8-fold and 9.4-fold, respectively, for the mutant enzyme, whereas the Vmax increased only 1.8-fold. Equilibrium dissociation constants (KD values) were obtained for the binding of NADPH and dihydrofolate in binary complexes with each enzyme. The KD for NADPH is similar in both WT and F31L, whereas the KD for dihydrofolate is 43-fold lower in F31L. Values for dihydrofolate association rate constants (kon) with enzyme and enzyme-NADPH complexes were measured by stopped-flow techniques.(ABSTRACT TRUNCATED AT 250 WORDS)

A novel fluorometric assay for quantitative analysis of dihydrofolate reductase activity in biological samples

In an effort to study the level of dihydrofolate reductase (DHFR), the main molecular target of antifolate drugs, in healthy and malignant tissues of human origin, a new and convenient fluorometric enzymatic assay has been developed. The technique measures the overall decrease in fluorescence emission at 454 nm (lambda ex = 342 nm) due to the contributions from coenzyme oxidation and substrate reduction. This technique was developed by using an enzyme purified from beef liver. All criteria of quality were checked: sensitivity, reproducibility and specificity made it suitable for low activity measurements. It was successfully applied to human tissue crude extracts.

Purification and characterization of dihydrofolate reductase from soybean seedlings

Dihydrofolate reductase (DHFR; EC 1.5.1.3) was purified to homogeneity from soybean seedlings by affinity chromatography on methotrexate-aminohexyl Sepharose, gel filtration on Ultrogel AcA-54, and Blue Sepharose chromatography. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the enzyme gave a single protein band corresponding to a molecular weight of 22,000. The enzyme is not a 140,000 Da heteropolymer as reported by others. Amino acid sequence-specific antibodies to intact human DHFR and also antibodies to CNBr-generated fragments of human DHFR bound to the plant enzyme on Western blots and cross-reacted significantly in immunoassays, indicating the presence of sequence homology between the two enzymes. The plant and human enzymes migrated similarly on nondenaturing polyacrylamide electrophoretic gels as monitored by activity staining with a tetrazolium dye. The specific activity of the plant enzyme was 15 units/mg protein, with a pH optimum of 7.4. Km values of the enzyme for dihydrofolate and NADPH were 17 and 30 microM, respectively. Unlike other eukaryotic enzymes, the plant enzyme showed no activation with organic mercurials and was inhibited by urea and KCl. The affinity of the enzyme for folate was relatively low (I50 = 130 microM) while methotrexate bound very tightly (KD less than 10(-10) M). Binding of pyrimethamine to the plant enzyme was weaker, while trimethoprim binding was stronger than to vertebrate DHFR. Trimetrexate, a very potent inhibitor of the human and bacterial enzymes showed weak binding to the plant enzyme. However, certain 2,4-diaminoquinazoline derivatives were very potent inhibitors of the plant DHFR. Thus, the plant DHFR, while showing similarity to the vertebrate and bacterial enzymes in terms of molecular weight and immunological cross-reactivity, can be distinguished from them by its kinetic properties and interaction with organic mercurials, urea, KCl and several antifolates.

Inhibition of dihydrofolate reductase, methotrexate transport, and growth of methotrexate-sensitive and -resistant L1210 leukemia cells in vitro by 5-substituted 2,4-diaminoquinazolines

A series of eighteen 2,4-diaminoquinazoline analogues of folic, isofolic, pteroic and isopteroic acids having various substituents at position 5 was studied. Each compound was evaluated as an inhibitor of L1210 dihydrofolate reductase, methotrexate influx into L1210 leukemia cells, and growth of methotrexate-sensitive and -resistant L1210 cells in vitro. Bridge reversal at positions 9 and 10 reduced the effectiveness of the classical analogues only with regard to the inhibition of the drug-sensitive cells as compared to methotrexate (MTX). Absence of the glutamate moiety adversely affected the potency of the compounds, particularly when coupled with reversal of the 9,10-bridge. However, the presence of -Cl at position 5 restored significantly the potency of these compounds. The pteroate and isopteroate analogue ethyl esters were generally more effective inhibitors of cell growth than their non-esterified counterparts. Regarding the effects of substituents at position 5, the data suggest that -Cl greater than -CH3 greater than -H for inhibition of methotrexate transport and growth of methotrexate-sensitive L1210 cells. The 5-Cl pteroate analogue and its corresponding ethyl ester were highly effective as growth inhibitors of methotrexate-resistant, transport-defective, L1210 cells in vitro.

Comparative activity of rat liver dihydrofolate reductase with 7,8-dihydrofolate and other 7,8-dihydropteridines

The various interactions of rat liver dihydrofolate reductase with two unconjugated 7,8-dihydropteridines, 7,8-dihydrobiopterin and 6-methyl-7,8-dihydropteridine, have been compared with those of 7,8-dihydrofolate and folate. Of particular interest was the reactivity demonstrated by 7,8-dihydrobiopterin because of the potential physiological significance of this reaction both in the regeneration of tetrahydrobiopterin, a cofactor for various biological hydroxylations, and as a step in the biosynthesis of this compound from GTP. Kinetic experiments gave Km values of 0.17, 6.42, and 10.2 microM for 7,8-dihydrofolate, 7,8-dihydrobiopterin, and 6-methyl-7,8-dihydropteridine, respectively, with Vmax = 6.22, 2.39, and 1.54 mumol min-1 mg-1. With folate the enzyme showed high affinity (Km = 0.88 microM) but low Vmax (0.20 mumol min-1 mg-1). The natural cofactor was NADPH and a Km of approximately 0.7 microM was measured with each substrate. The enzyme was activated by both p-hydroxymercuribenzoate and urea when assayed with 7,8-dihydrofolate but was inhibited when 7,8-dihydrobiopterin was the substrate. The pH optimum for dihydrofolate reduction was 4 with enhancement at pH greater than or equal to 5.5 in the presence of 1 M NaCl. Peak activity with 7,8-dihydrobiopterin occurred at pH 4.8; this was shifted to pH 5.3 but was not enhanced by 1 M NaCl. Inhibition with methotrexate was similar whether the enzyme was assayed with either the conjugated or unconjugated 7,8-dihydro derivatives. The rat liver enzyme, highly unstable after purification, was stabilized in the presence of the nonionic detergent, Tween-20 (0.1%); however, the comparative properties toward the conjugated and unconjugated substrates were not altered by this treatment.

Polymorphism of dihydrofolate reductase from a methotrexate-resistant subline of L1210 cells

Dihydrofolate reductase, purified to homogeneity (as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis), from a subline of L1210 murine leukemia cells resistant to 10(-6) M methotrexate, was resolved into two principal forms (1 and 2) by polyacrylamide gel electrophoresis at pH 8.3 or isoelectric focusing. In the latter procedure, these forms had pI values of 7.4 and 8.2, respectively; both stained for protein and catalytic activity. Form 1 appears to be a single component, comprising ca. 10% of the total protein and at least 20% of the total catalytic activity. It is also more sensitive to inhibition by MTX, more heat-stable, and less susceptible to activation than form 2. Multiple components of 2 were observed by narrowing the pH range in isoelectric focusing, and further resolution was achieved by urea denaturation. Substrate and inhibitor complexes of 1 and 2, differentiated by polyacrylamide gel electrophoresis or isoelectric focusing, provided information about the ability of the enzyme to undergo conformational changes. Interconversion of 1 with one of the components of 2 may also involve conformational isomerism. These conclusions are consistent with the well-known ability of eukaryotic dihydrofolate reductases to exhibit increased catalytic activity (attributed to transformations to more open conformations) when treated with salts, chaotropes, or cysteine-modifying agents. Treatment of the L1210/R6 enzyme preparation with one of these activating agents, 5,5'-dithiobis(2-nitrobenzoic acid), derivatized both 1 and 2 (changing their pI values to 7.3 and 6.9, respectively) and altered the enzyme such that stoichiometric inhibition for MTX was observed.

Lack of dihydrofolate reductase in human tumor and leukemia cells in vivo

Dihydrofolate reductase (DHFR), the main target for methotrexate and other antifolate compounds was found to be present in 100-200 times higher concentration in human cell lines grown in vitro than in human tumors or cells obtained in situ. The DHFR content of human cell lines in vitro however were equivalent to rodent tumor lines also measured in vitro. The enzyme was quantitated by [3H]methotrexate binding, [3H]dihydrofolate reduction to [3H]tetrahydrofolate, and immunoprecipitation with a monospecific anti-serum to DHFR. Additional studies revealed only a liver sample to contain significant amounts of an inhibitor of DHFR activity. It is postulated either that low levels of DHFR in fresh human tissue reflect low cell turnover or conversely that high levels in vitro and in animal tissues reflect high levels of enzyme due to selection because of high levels of folic acid in culture medium and prepared feeds.

Biochemical pharmacology of the lipophilic antifolate, trimetrexate

Trimetrexate is a novel lipophilic folate antagonist that causes growth inhibition, inhibition of nucleic acid biosynthesis, and cytotoxicity at nanomolar concentrations in tissue cultures. The potency of trimetrexate cytotoxicity against most cell lines is greater than that of methotrexate. Trimetrexate has antitumor activity in vivo in several murine leukemia and solid tumor systems, including tumors in which methotrexate is inactive. Antitumor activity was seen following oral, intravenous, or intraperitoneal administration. Trimetrexate causes a pronounced and early depression in incorporation of deoxyuridine into DNA. In tumor cell lines resistant to methotrexate because of a drug transport defect, trimetrexate retains activity. In many such cases the methotrexate-resistant tumors show collateral sensitivity to trimetrexate. In methotrexate-resistant cells with impaired drug transport, trimetrexate sensitivity was even more pronounced when cells were grown in folate-free medium supplemented with physiological levels of tetrahydrofolate cofactor. In the human tumor stem cell colony assay, trimetrexate, at concentrations achievable in vivo, gave activity against many human tumors, including samples that were unresponsive to methotrexate. Trimetrexate crosses the blood-brain barrier, and at very high doses may cause neurotoxicity. At conventional doses the primary toxic effects in mice are gastrointestinal. This toxicity is reversible at therapeutic doses. Unlike earlier lipophilic antifolates, trimetrexate has rapid plasma clearance (t1/2 in mice of 45 minutes). Trimetrexate is a tight-binding competitive inhibitor of dihydrofolate reductase. The Ki,slope for inhibition of the human enzyme was 4 X 10(-11) M. A dose-dependent decrease in cellular purine ribonucleotide pools is given by trimetrexate. Pyrimidine ribonucleotide pools tend to increase in treated cells. Trimetrexate caused a marked depression of cellular pools of dTTP and dGTP, and a lesser depression in dATP. Cytotoxicity of trimetrexate in vitro was prevented by leucovorin. Leucovorin also protected mice from trimetrexate toxicity. Thymidine protected cells from lethal effects of low concentrations of trimetrexate, but not from high concentrations. The combination of thymidine and hypoxanthine completely protected cells from low and high concentrations of trimetrexate. A new, stable and highly water-soluble formulation of trimetrexate has been developed. Because of the interesting biochemical and pharmacological properties of trimetrexate, and its experimental antitumor activity, clinical trials are planned.