Inhibition of folate biosynthesis and function as a basis for chemotherapy

Folate depletion induces erythroid differentiation through perturbation of de novo purine synthesis

Folate, an essential vitamin, is a one-carbon acceptor and donor in key metabolic reactions. Erythroid cells harbor a unique sensitivity to folate deprivation, as revealed by the primary pathological manifestation of nutritional folate deprivation: megaloblastic anemia. To study this metabolic sensitivity, we applied mild folate depletion to human and mouse erythroid cell lines and primary murine erythroid progenitors. We show that folate depletion induces early blockade of purine synthesis and accumulation of the purine synthesis intermediate and signaling molecule, 5'-phosphoribosyl-5-aminoimidazole-4-carboxamide (AICAR), followed by enhanced heme metabolism, hemoglobin synthesis, and erythroid differentiation. This is phenocopied by inhibition of folate metabolism using the inhibitor SHIN1, and by AICAR supplementation. Mechanistically, the metabolically driven differentiation is independent of mechanistic target of rapamycin complex 1 (mTORC1) and adenosine 5'-monophosphate-activated protein kinase (AMPK) and is instead mediated by protein kinase C. Our findings suggest that folate deprivation-induced premature differentiation of erythroid progenitor cells is a molecular etiology to folate deficiency-induced anemia.

Regulatory mechanisms of one-carbon metabolism enzymes

One-carbon metabolism is a central metabolic pathway critical for the biosynthesis of several amino acids, methyl group donors, and nucleotides. The pathway mostly relies on the transfer of a carbon unit from the amino acid serine, through the cofactor folate (in its several forms), and to the ultimate carbon acceptors that include nucleotides and methyl groups used for methylation of proteins, RNA, and DNA. Nucleotides are required for DNA replication, DNA repair, gene expression, and protein translation, through ribosomal RNA. Therefore, the one-carbon metabolism pathway is essential for cell growth and function in all cells, but is specifically important for rapidly proliferating cells. The regulation of one-carbon metabolism is a critical aspect of the normal and pathological function of the pathway, such as in cancer, where hijacking these regulatory mechanisms feeds an increased need for nucleotides. One-carbon metabolism is regulated at several levels: via gene expression, posttranslational modification, subcellular compartmentalization, allosteric inhibition, and feedback regulation. In this review, we aim to inform the readers of relevant one-carbon metabolism regulation mechanisms and to bring forward the need to further study this aspect of one-carbon metabolism. The review aims to integrate two major aspects of cancer metabolism-signaling downstream of nutrient sensing and one-carbon metabolism, because while each of these is critical for the proliferation of cancerous cells, their integration is critical for comprehensive understating of cellular metabolism in transformed cells and can lead to clinically relevant insights.

Using biologically synthesized TiO2 nanoparticles as potential remedy against multiple drug resistant Staphylococcus aureus of bovine mastitis

Presently, there is considerable emphasis on biological synthesis of nanoparticles containing bioactive reducing compounds with an aim to mitigate the harmful effects of pollutants. The approach under study is simple and ideal for the production of durable antimicrobial nanomaterials by novel single-step green synthesis of TiO2 metal oxide nanostructures using ginger and garlic crude aqueous extracts with bactericidal and catalytic activity. A variety of experimental techniques were used to characterize the synthesized nanomaterials. As demonstrated using x-ray diffraction and ultra-violet visible spectroscopy, the produced nanoparticles exhibited high absorption at 318 nm with size varying between 23.38 nm for ginger and 58.64 nm for garlic in biologically-reduced TiO2. At increasing concentrations (500, 1000 µg/50 µl), nanoparticles reduced with garlic exhibited enhanced bactericidal efficacy against multiple drug-resistant S. aureus and effectively decomposed toxic methylene blue (MB) dye. In conclusion, biologically-reduced TiO2 nanoparticles may prove an effective tool in the fight against microbial illnesses and drug resistance.

Kinetic Barrier to Enzyme Inhibition Is Manipulated by Dynamical Local Interactions in E. coli DHFR

Dihydrofolate reductase (DHFR) is an important drug target and a highly studied model protein for understanding enzyme dynamics. DHFR's crucial role in folate synthesis renders it an ideal candidate to understand protein function and protein evolution mechanisms. In this study, to understand how a newly proposed DHFR inhibitor, 4'-deoxy methyl trimethoprim (4'-DTMP), alters evolutionary trajectories, we studied interactions that lead to its superior performance over that of trimethoprim (TMP). To elucidate the inhibition mechanism of 4'-DTMP, we first confirmed, both computationally and experimentally, that the relative binding free energy cost for the mutation of TMP and 4'-DTMP is the same, pointing the origin of the characteristic differences to be kinetic rather than thermodynamic. We then employed an interaction-based analysis by focusing first on the active site and then on the whole enzyme. We confirmed that the polar modification in 4'-DTMP induces additional local interactions with the enzyme, particularly, the M20 loop. These changes are propagated to the whole enzyme as shifts in the hydrogen bond networks. To shed light on the allosteric interactions, we support our analysis with network-based community analysis and show that segmentation of the loop domain of inhibitor-bound DHFR must be avoided by a successful inhibitor.

In-Vitro Catalytic and Antibacterial Potential of Green Synthesized CuO Nanoparticles against Prevalent Multiple Drug Resistant Bovine Mastitogen Staphylococcus aureus

Nanoparticles prepared from bio-reduction agents are of keen interest to researchers around the globe due to their ability to mitigate the harmful effects of chemicals. In this regard, the present study aims to synthesize copper oxide nanoparticles (CuO NPs) by utilizing root extracts of ginger and garlic as reducing agents, followed by the characterization and evaluation of their antimicrobial properties against multiple drug resistant (MDR) S. aureus. In this study, UV-vis spectroscopy revealed a reduced degree of absorption with an increase in the extract amount present in CuO. The maximum absorbance for doped NPs was recorded around 250 nm accompanying redshift. X-ray diffraction analysis revealed the monoclinic crystal phase of the particles. The fabricated NPs exhibited spherical shapes with dense agglomeration when examined with FE-SEM and TEM. The crystallite size measured by using XRD was found to be within a range of 23.38-46.64 nm for ginger-doped CuO and 26-56 nm for garlic-doped CuO. Green synthesized NPs of ginger demonstrated higher bactericidal tendencies against MDR S. aureus. At minimum and maximum concentrations of ginger-doped CuO NPs, substantial inhibition areas for MDR S. aureus were (2.05-3.80 mm) and (3.15-5.65 mm), and they were measured as (1.1-3.55 mm) and (1.25-4.45 mm) for garlic-doped NPs. Conventionally available CuO and crude aqueous extract (CAE) of ginger and garlic roots reduced MB in 12, 21, and 38 min, respectively, in comparison with an efficient (100%) reduction of dye in 1 min and 15 s for ginger and garlic doped CuO NPs.

Metabolic response to radiation therapy in cancer

Tumor metabolism has emerged as a hallmark of cancer and is involved in carcinogenesis and tumor growth. Reprogramming of tumor metabolism is necessary for cancer cells to sustain high proliferation rates and enhanced demands for nutrients. Recent studies suggest that metabolic plasticity in cancer cells can decrease the efficacy of anticancer therapies by enhancing antioxidant defenses and DNA repair mechanisms. Studying radiation-induced metabolic changes will lead to a better understanding of radiation response mechanisms as well as the identification of new therapeutic targets, but there are few robust studies characterizing the metabolic changes induced by radiation therapy in cancer. In this review, we will highlight studies that provide information on the metabolic changes induced by radiation and oxidative stress in cancer cells and the associated underlying mechanisms.

Bisubstrate inhibitors of 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase: Transition state analogs for high affinity binding

6-Hydroxymethyl-7,8-dihydropterin pyrophosphokinase (HPPK) is a key enzyme in the folate biosynthesis pathway. It catalyzes pyrophosphoryl transfer from ATP to 6-hydroxymethyl-7,8-dihydropterin (HP). HPPK is essential for microorganisms but absent in mammals; therefore, it is an attractive target for developing novel antimicrobial agents. Previously, based on our studies of the structure and mechanism of HPPK, we created first-generation bisubstrate inhibitors by linking 6-hydroxymethylpterin to adenosine through phosphate groups, and developed second-generation inhibitors by replacing the phosphate bridge with a linkage that contains a piperidine moiety. Here, we report third-generation inhibitors designed based on the piperidine-containing inhibitor, mimicking the transition state. We synthesized two such inhibitors, characterized their protein-binding and enzyme inhibition properties, and determined their crystal structures in complex with HPPK, advancing the development of such bisubstrate analog inhibitors.

Methotrexate elicits pro-respiratory and anti-growth effects by promoting AMPK signaling

One-carbon metabolism fuels the high demand of cancer cells for nucleotides and other building blocks needed for increased proliferation. Although inhibitors of this pathway are widely used to treat many cancers, their global impact on anabolic and catabolic processes remains unclear. Using a combination of real-time bioenergetics assays and metabolomics approaches, we investigated the global effects of methotrexate on cellular metabolism. We show that methotrexate treatment increases the intracellular concentration of the metabolite AICAR, resulting in AMPK activation. Methotrexate-induced AMPK activation leads to decreased one-carbon metabolism gene expression and cellular proliferation as well as increased global bioenergetic capacity. The anti-proliferative and pro-respiratory effects of methotrexate are AMPK-dependent, as cells with reduced AMPK activity are less affected by methotrexate treatment. Conversely, the combination of methotrexate with the AMPK activator, phenformin, potentiates its anti-proliferative activity in cancer cells. These data highlight a reciprocal effect of methotrexate on anabolic and catabolic processes and implicate AMPK activation as a metabolic determinant of methotrexate response.

Preventing BRCA1/ZBRK1 repressor complex binding to the GOT2 promoter results in accelerated aspartate biosynthesis and promotion of cell proliferation

Breast cancer susceptibility gene 1 (BRCA1) has been implicated in modulating metabolism via transcriptional regulation. However, direct metabolic targets of BRCA1 and the underlying regulatory mechanisms are still unknown. Here, we identified several metabolic genes, including the gene which encodes glutamate‐oxaloacetate transaminase 2 (GOT2), a key enzyme for aspartate biosynthesis, which are repressed by BRCA1. We report that BRCA1 forms a co‐repressor complex with ZBRK1 that coordinately represses GOT2 expression via a ZBRK1 recognition element in the promoter of GOT2. Impairment of this complex results in upregulation of GOT2, which in turn increases aspartate and alpha ketoglutarate production, leading to rapid cell proliferation of breast cancer cells. Importantly, we found that GOT2 can serve as an independent prognostic factor for overall survival and disease‐free survival of patients with breast cancer, especially triple‐negative breast cancer. Interestingly, we also demonstrated that GOT2 overexpression sensitized breast cancer cells to methotrexate, suggesting a promising precision therapeutic strategy for breast cancer treatment. In summary, our findings reveal that BRCA1 modulates aspartate biosynthesis through transcriptional repression of GOT2, and provides a biological basis for treatment choices in breast cancer.

Treatment of Toxoplasmosis and Neosporosis in Farm Ruminants: State of Knowledge and Future Trends

Toxoplasmosis and neosporosis are closely related protozoan diseases that lead to important economic impacts in farm ruminants. Toxoplasma gondii infection mainly causes reproductive failure in small ruminants and is a widespread zoonosis, whereas Neospora caninum infection is one of the most important causes of abortion in cattle worldwide. Vaccination has been considered the most economic measure for controlling these diseases. However, despite vaccine development efforts, only a live-attenuated T. gondii vaccine has been licensed for veterinary use, and no promising vaccines against ne-osporosis have been developed; therefore, vaccine development remains a key goal. Additionally, drug therapy could be a valuable strategy for disease control in farm ruminants, as several drugs that limit T. gondii and N. caninum proliferation and dissemination have been evaluated. This approach may also be relevant to performing an initial drug screening for potential human therapy for zoonotic parasites. Treat-ments can be applied against infections in adult ruminants to minimize the outcomes of a primo-infection or the reactivation of a chronic infection during gestation or in newborn ruminants to avoid infection chronification. In this review, the current status of drug development against toxoplasmosis and neosporo-sis in farm ruminants is presented, and in an effort to promote additional treatment options, prospective drugs that have shown efficacy in vitro and in laboratory animal models of toxoplasmosis and neosporosis are examined

Canonical Wnt is inhibited by targeting one-carbon metabolism through methotrexate or methionine deprivation

Metabolism, one of the most conserved features across all domains of life, is interwoven with cellular signaling networks and the posttranslational modification state of proteins. Methionine is an essential amino acid in one-carbon metabolism that serves as the substrate for S-adenosylmethionine, the universal methyl donor for cellular methylation. Here we report that the use of methionine-depleted medium inhibits Wnt-induced endolysosomal formation and canonical Wnt/β-catenin signaling. Use of methotrexate, a folate cycle inhibitor used in cancer chemotherapy, prevented PRMT1 and GSK3 vesicular sequestration, decreased Wnt-driven endolysosomal activity, and reduced canonical Wnt β-catenin luciferase signaling. In sum, these results imply a role for one-carbon metabolism in the regulation of Wnt signaling and endolysosomal biology.

The nutrient-sensing metabolite S-adenosylmethionine (SAM) controls one-carbon metabolism by donating methyl groups to biochemical building blocks, DNA, RNA, and protein. Our recent work uncovered a requirement for cytoplasmic arginine methylation during Wnt signaling through the activity of protein arginine methyltransferase 1 (PRMT1), which transfers one-carbon groups from SAM to many protein substrates. Here, we report that treatments that decrease levels of the universal methyl donor SAM were potent inhibitors of Wnt signaling and of Wnt-induced digestion of extracellular proteins in endolysosomes. Thus, arginine methylation provides the canonical Wnt pathway with metabolic sensing properties through SAM. The rapid accumulation of Wnt-induced endolysosomes within 30 minutes was inhibited by the depletion of methionine, an essential amino acid that serves as the direct substrate for SAM production. We also found that methionine is required for GSK3 sequestration into multivesicular bodies through microautophagy, an essential step in Wnt signaling activity. Methionine starvation greatly reduced Wnt-induced endolysosomal degradation of extracellular serum proteins. Similar results were observed by addition of nicotinamide (vitamin B3), which serves as a methyl group sink. Methotrexate, a pillar in the treatment of cancer since 1948, decreases SAM levels. We show here that methotrexate blocked Wnt-induced endocytic lysosomal activity and reduced canonical Wnt signaling. Importantly, the addition of SAM during methionine depletion or methotrexate treatment was sufficient to rescue endolysosomal function and Wnt signaling. Inhibiting the Wnt signaling pathway by decreasing one-carbon metabolism provides a platform for designing interventions in Wnt-driven disease.

Histidine catabolism is a major determinant of methotrexate sensitivity

The chemotherapeutic drug methotrexate inhibits the enzyme DHFR (dihydrofolate reductase)¹, which generates tetrahydrofolate (THF), an essential cofactor in nucleotide synthesis². Depletion of THF causes cell death by suppressing DNA and RNA production³. While methotrexate is widely used as an anti-cancer agent and the subject of over a thousand ongoing clinical trials⁴, its high toxicity often leads to the premature termination of its use, diminishing its potential efficacy⁵. To identify genes that modulate the response of cancer cells to methotrexate, we performed a CRISPR/Cas9-based screen^6,7. This screen yielded FTCD, which encodes an enzyme (formimidoyltransferase cyclodeaminase) needed for the catabolism of the amino acid histidine⁸, a process not previously linked to methotrexate sensitivity. In cultured cancer cells, depletion of multiple genes in the histidine catabolism pathway dramatically decreased sensitivity to methotrexate. Mechanistically, histidine catabolism drains the cellular pool of THF, which is particularly detrimental to methotrexate-treated cells. Moreover, expression of the rate-limiting enzyme in histidine catabolism is associated with methotrexate sensitivity in cancer cell lines and with survival rate in patients. In vivo dietary supplementation of histidine increased flux through the histidine degradation pathway and enhanced the sensitivity of leukemia xenografts to methotrexate. Thus, the histidine degradation pathway significantly influences the sensitivity of cancer cells to methotrexate and may be exploited to improve methotrexate efficacy through a simple dietary intervention.

Directed evolution of multiple genomic loci allows the prediction of antibiotic resistance

Antibiotic development is frequently plagued by the rapid emergence of drug resistance. However, assessing the risk of resistance development in the preclinical stage is difficult. Standard laboratory evolution approaches explore only a small fraction of the sequence space and fail to identify exceedingly rare resistance mutations and combinations thereof. Therefore, new rapid and exhaustive methods are needed to accurately assess the potential of resistance evolution and uncover the underlying mutational mechanisms. Here, we introduce directed evolution with random genomic mutations (DIvERGE), a method that allows an up to million-fold increase in mutation rate along the full lengths of multiple predefined loci in a range of bacterial species. In a single day, DIvERGE generated specific mutation combinations, yielding clinically significant resistance against trimethoprim and ciprofloxacin. Many of these mutations have remained previously undetected or provide resistance in a species-specific manner. These results indicate pathogen-specific resistance mechanisms and the necessity of future narrow-spectrum antibacterial treatments. In contrast to prior claims, we detected the rapid emergence of resistance against gepotidacin, a novel antibiotic currently in clinical trials. Based on these properties, DIvERGE could be applicable to identify less resistance-prone antibiotics at an early stage of drug development. Finally, we discuss potential future applications of DIvERGE in synthetic and evolutionary biology.

Orphan toxin OrtT (YdcX) of Escherichia coli reduces growth during the stringent response

Toxin/antitoxin (TA) systems are nearly universal in prokaryotes; toxins are paired with antitoxins which inactivate them until the toxins are utilized. Here we explore whether toxins may function alone; i.e., whether a toxin which lacks a corresponding antitoxin (orphan toxin) is physiologically relevant. By focusing on a homologous protein of the membrane-damaging toxin GhoT of the Escherichia coli GhoT/GhoS type V TA system, we found that YdcX (renamed OrtT for orphan toxin related to tetrahydrofolate) is toxic but is not part of TA pair. OrtT is not inactivated by neighboring YdcY (which is demonstrated to be a protein), nor is it inactivated by antitoxin GhoS. Also, OrtT is not inactivated by small RNA upstream or downstream of ortT. Moreover, screening a genomic library did not identify an antitoxin partner for OrtT. OrtT is a protein and its toxicity stems from membrane damage as evidenced by transmission electron microscopy and cell lysis. Furthermore, OrtT reduces cell growth and metabolism in the presence of both antimicrobials trimethoprim and sulfamethoxazole; these antimicrobials induce the stringent response by inhibiting tetrahydrofolate synthesis. Therefore, we demonstrate that OrtT acts as an independent toxin to reduce growth during stress related to amino acid and DNA synthesis.

One-carbon metabolic pathway rewiring in Escherichia coli reveals an evolutionary advantage of 10-formyltetrahydrofolate synthetase (Fhs) in survival under hypoxia

In cells, N(10)-formyltetrahydrofolate (N(10)-fTHF) is required for formylation of eubacterial/organellar initiator tRNA and purine nucleotide biosynthesis. Biosynthesis of N(10)-fTHF is catalyzed by 5,10-methylene-tetrahydrofolate dehydrogenase/cyclohydrolase (FolD) and/or 10-formyltetrahydrofolate synthetase (Fhs). All eubacteria possess FolD, but some possess both FolD and Fhs. However, the reasons for possessing Fhs in addition to FolD have remained unclear. We used Escherichia coli, which naturally lacks fhs, as our model. We show that in E. coli, the essential function of folD could be replaced by Clostridium perfringens fhs when it was provided on a medium-copy-number plasmid or integrated as a single-copy gene in the chromosome. The fhs-supported folD deletion (ΔfolD) strains grow well in a complex medium. However, these strains require purines and glycine as supplements for growth in M9 minimal medium. The in vivo levels of N(10)-fTHF in the ΔfolD strain (supported by plasmid-borne fhs) were limiting despite the high capacity of the available Fhs to synthesize N(10)-fTHF in vitro. Auxotrophy for purines could be alleviated by supplementing formate to the medium, and that for glycine was alleviated by engineering THF import into the cells. The ΔfolD strain (harboring fhs on the chromosome) showed a high NADP(+)-to-NADPH ratio and hypersensitivity to trimethoprim. The presence of fhs in E. coli was disadvantageous for its aerobic growth. However, under hypoxia, E. coli strains harboring fhs outcompeted those lacking it. The computational analysis revealed a predominant natural occurrence of fhs in anaerobic and facultative anaerobic bacteria.

Crystallographic and molecular dynamics simulation analysis of Escherichia coli dihydroneopterin aldolase

Background

Dihydroneopterin aldolase (DHNA) catalyzes the conversion of 7,8-dihydroneopterin to 6-hydroxymethyl-7,8-dihydropterin and also the epimerization of DHNP to 7,8-dihydromonapterin. Previously, we determined the crystal structure of Staphylococcus aureus DHNA (SaDHNA) in complex with the substrate analogue neopterin (NP). We also showed that Escherichia coli DHNA (EcDHNA) and SaDHNA have significantly different binding and catalytic properties by biochemical analysis. On the basis of these structural and functional data, we proposed a catalytic mechanism involving two proton wires.

Results

To understand the structural basis for the biochemical differences and further investigate the catalytic mechanism of DHNA, we have determined the structure of EcDHNA complexed with NP at 1.07-Å resolution [PDB:2O90], built an atomic model of EcDHNA complexed with the substrate DHNP, and performed molecular dynamics (MD) simulation analysis of the substrate complex. EcDHNA has the same fold as SaDHNA and also forms an octamer that consists of two tetramers, but the packing of one tetramer with the other is significantly different between the two enzymes. Furthermore, the structures reveal significant differences in the vicinity of the active site, particularly in the loop that connects strands β3 and β4, mainly due to the substitution of nearby residues. The building of an atomic model of the complex of EcDHNA and the substrate DHNP and the MD simulation of the complex show that some of the hydrogen bonds between the substrate and the enzyme are persistent, whereas others are transient. The substrate binding model and MD simulation provide the molecular basis for the biochemical behaviors of the enzyme, including noncooperative substrate binding, indiscrimination of a pair of epimers as the substrates, proton wire switching during catalysis, and formation of epimerization product.

Conclusions

The EcDHNA and SaDHNA structures, each in complex with NP, reveal the basis for the biochemical differences between EcDHNA and SaDHNA. The atomic substrate binding model and MD simulation offer insights into substrate binding and catalysis by DHNA. The EcDHNA structure also affords an opportunity to develop antimicrobials specific for Gram-negative bacteria, as DHNAs from Gram-negative bacteria are highly homologous and E. coli is a representative of this class of bacteria.

Structural enzymology and inhibition of the bi-functional folate pathway enzyme HPPK-DHPS from the biowarfare agent Francisella tularensis

Two valid targets for antibiotic development, 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase (HPPK) and dihydropteroate synthase (DHPS), catalyze consecutive reactions in folate biosynthesis. In Francisella tularensis (Ft), these two activities are contained in a single protein, FtHPPK-DHPS. Although Pemble et al. (PLoS One 5, e14165) determined the structure of FtHPPK-DHPS, they were unable to measure the kinetic parameters of the enzyme. In this study, we elucidated the binding and inhibitory activities of two HPPK inhibitors (HP-18 and HP-26) against FtHPPK-DHPS, determined the structure of FtHPPK-DHPS in complex with HP-26, and measured the kinetic parameters for the dual enzymatic activities of FtHPPK-DHPS. The biochemical analyses showed that HP-18 and HP-26 have significant isozyme selectivity, and that FtHPPK-DHPS is unique in that the catalytic efficiency of its DHPS activity is only 1/260,000 of that of Escherichia coli DHPS. Sequence and structural analyses suggest that HP-26 is an excellent lead for developing therapeutic agents for tularemia, and that the very low DHPS activity is due, at least in part, to the lack of a key residue that interacts with the substrate p-aminobenzoic acid (pABA). A BLAST search of the genomes of ten F. tularensis strains indicated that the bacterium contains a single FtHPPK-DHPS. The marginal DHPS activity and the single copy existence of FtHPPK-DHPS in F. tularensis make this bacterium more vulnerable to DHPS inhibitors. Current sulfa drugs are ineffective against tularemia; new inhibitors targeting the unique pABA-binding pocket may be effective and less subject to resistance because any mutations introducing resistance may make the marginal DHPS activity unable to support the growth of F. tularensis.

DATABASE

The coordinates and structure factors have been deposited in the Protein Data Bank under accession code 4PZV.

Glucarpidase to combat toxic levels of methotrexate in patients

In January 2012, glucarpidase (Voraxaze^®) received approval from the US Food and Drug Administration for intravenous treatment of toxic plasma methotrexate concentrations due to impaired renal clearance. Methotrexate, an antifolate agent, has been used for over 60 years in the treatment of various cancers. High-dose methotrexate has been particularly useful in the treatment of leukemias and lymphomas. However, even with aggressive hydration and urine alkalinization, such regimens can lead to acute renal dysfunction, as indicated by decreases in urine production and concomitant increases in blood urea nitrogen and serum creatinine levels. Because methotrexate is largely excreted by the kidneys, this can greatly potentiate tissue damage. Toxic levels of blood methotrexate can be rapidly and effectively decreased by intravenous administration of glucarpidase. Glucarpidase is a recombinant form of carboxypeptidase G2, a bacterial enzyme that rapidly cleaves methotrexate to form the amino acid glutamate and 2,4-diamino-N¹⁰-methylpteroic acid. Catabolites of methotrexate are much less toxic than the parent compound, and are primarily excreted by hepatic mechanisms. Glucarpidase has been available on a compassionate basis since the 1990s, and a variety of case reports and larger clinical trials have demonstrated the safety and efficacy of this drug in patients ranging in age from infants to the elderly and in a variety of races and ethnic groups. Glucarpidase should not be administered within 2 hours of leucovorin, because this agent is a reduced folate which competes with methotrexate for the enzyme and glucarpidase inactivates leucovorin. Side effects of glucarpidase are rare and relatively mild, and include paraesthesia, flushing, nausea, vomiting, pruritus, and headache. Glucarpidase has seen limited use in intrathecal treatment of methotrexate toxicity for which it is also effective. Future applications of this enzyme in chemotherapy continue to be an active area of research.

Bisubstrate analog inhibitors of 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase: new lead exhibits a distinct binding mode

6-Hydroxymethyl-7,8-dihydropterin pyrophosphokinase (HPPK), a key enzyme in the folate biosynthesis pathway catalyzing the pyrophosphoryl transfer from ATP to 6-hydroxymethyl-7,8-dihydropterin, is an attractive target for developing novel antimicrobial agents. Previously, we studied the mechanism of HPPK action, synthesized bisubstrate analog inhibitors by linking 6-hydroxymethylpterin to adenosine through phosphate groups, and developed a new generation of bisubstrate inhibitors by replacing the phosphate bridge with a piperidine-containing linkage. To further improve linker properties, we have synthesized a new compound, characterized its protein binding/inhibiting properties, and determined its structure in complex with HPPK. Surprisingly, this inhibitor exhibits a new binding mode in that the adenine base is flipped when compared to previously reported structures. Furthermore, the side chain of amino acid residue E77 is involved in protein-inhibitor interaction, forming hydrogen bonds with both 2' and 3' hydroxyl groups of the ribose moiety. Residue E77 is conserved among HPPK sequences, but interacts only indirectly with the bound MgATP via water molecules. Never observed before, the E77-ribose interaction is compatible only with the new inhibitor-binding mode. Therefore, this compound represents a new direction for further development.

Monitoring antimalarial drug resistance: Applying lessons learned from the past in a fast-moving present

The need for robust surveillance of antimalarial drugs is more urgent than it has ever been. In the western region of Cambodia, artemisinin resistance has emerged in Plasmodium falciparum and threatens to undermine the efficacy of highly effective artemisinin combination therapies. Although some manfestations of artemisinin tolerance are unique to this class of drug, many of its properties mirror previous experience in understanding and tracking resistance to other antimalarials. In this review we outline the spectrum of approaches that were developed to understand the evolution and spread of antifolate resistance, highlighting the importance of integrating information from different methodologies towards a better understanding of the underlying biologic processes. We consider how to apply our experience in investigating and attempting to contain antifolate resistance to inform our prospective assessment of novel antimalarial resistance patterns and their subsequent spread.

Bisubstrate analogue inhibitors of 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase: New design with improved properties

6-Hydroxymethyl-7,8-dihydropterin pyrophosphokinase (HPPK), a key enzyme in the folate biosynthetic pathway, catalyzes the pyrophosphoryl transfer from ATP to 6-hydroxymethyl-7,8-dihydropterin. The enzyme is essential for microorganisms, is absent from humans, and is not the target for any existing antibiotics. Therefore, HPPK is an attractive target for developing novel antimicrobial agents. Previously, we characterized the reaction trajectory of HPPK-catalyzed pyrophosphoryl transfer and synthesized a series of bisubstrate analog inhibitors of the enzyme by linking 6-hydroxymethylpterin to adenosine through 2, 3, or 4 phosphate groups. Here, we report a new generation of bisubstrate analog inhibitors. To improve protein binding and linker properties of such inhibitors, we have replaced the pterin moiety with 7,7-dimethyl-7,8-dihydropterin and the phosphate bridge with a piperidine linked thioether. We have synthesized the new inhibitors, measured their K(d) and IC(50) values, determined their crystal structures in complex with HPPK, and established their structure-activity relationship. 6-Carboxylic acid ethyl ester-7,7-dimethyl-7,8-dihydropterin, a novel intermediate that we developed recently for easy derivatization at position 6 of 7,7-dimethyl-7,8-dihydropterin, offers a much high yield for the synthesis of bisubstrate analogs than that of previously established procedure.

Role of protein conformational dynamics in the catalysis by 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase

Enzymatic catalysis has conflicting structural requirements of the enzyme. In order for the enzyme to form a Michaelis complex, the enzyme must be in an open conformation so that the substrate can get into its active center. On the other hand, in order to maximize the stabilization of the transition state of the enzymatic reaction, the enzyme must be in a closed conformation to maximize its interactions with the transition state. The conflicting structural requirements can be resolved by a flexible active center that can sample both open and closed conformational states. For a bisubstrate enzyme, the Michaelis complex consists of two substrates in addition to the enzyme. The enzyme must remain flexible upon the binding of the first substrate so that the second substrate can get into the active center. The active center is fully assembled and stabilized only when both substrates bind to the enzyme. However, the side-chain positions of the catalytic residues in the Michaelis complex are still not optimally aligned for the stabilization of the transition state, which lasts only approximately 10(-13) s. The instantaneous and optimal alignment of catalytic groups for the transition state stabilization requires a dynamic enzyme, not an enzyme which undergoes a large scale of movements but an enzyme which permits at least a small scale of adjustment of catalytic group positions. This review will summarize the structure, catalytic mechanism, and dynamic properties of 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase and examine the role of protein conformational dynamics in the catalysis of a bisubstrate enzymatic reaction.

Chemical genomics in Escherichia coli identifies an inhibitor of bacterial lipoprotein targeting

One of the most significant hurdles to developing new chemical probes of biological systems and new drugs to treat disease is that of understanding the mechanism of action of small molecules discovered with cell-based small-molecule screening. Here we have assembled an ordered, high-expression clone set of all of the essential genes from Escherichia coli and used it to systematically screen for suppressors of growth inhibitory compounds. Using this chemical genomic approach, we demonstrate that the targets of well-known antibiotics can be identified as high copy suppressors of chemical lethality. This approach led to the discovery of MAC13243, a molecule that belongs to a new chemical class and that has a unique mechanism and promising activity against multidrug-resistant Pseudomonas aeruginosa. We show that MAC13243 inhibits the function of the LolA protein and represents a new chemical probe of lipoprotein targeting in bacteria with promise as an antibacterial lead with Gram-negative selectivity.

In vitro activities of three new dihydrofolate reductase inhibitors against clinical isolates of gram-positive bacteria

BAL0030543, BAL0030544, and BAL0030545 are dihydrophthalazine inhibitors with in vitro potency against gram-positive pathogens. The MIC(50)s for methicillin (meticillin)-sensitive Staphylococcus aureus, methicillin-resistant Staphylococcus aureus, hetero-vancomycin-resistant Staphylococcus aureus, and vancomycin-resistant Staphylococcus aureus (VISA) range from 0.015 to 0.25 microg/ml (MIC(90)s < or = 0.5 microg/ml). MIC(50)s for beta-hemolytic streptococci range from 0.03 to 0.06 microg/ml, MIC(50)s for Streptococcus pneumoniae range from 0.06 to 0.12 microg/ml, MIC(50)s for Listeria monocytogenes range from 0.015 to 0.06 microg/ml, and MIC(50)s for Streptococcus mitis are < or = 0.015 microg/ml. These three dihydrophthalazine antifolates have improved potency compared to that of trimethoprim and activity against gram-positive pathogens resistant to other drug classes. (This work was presented in part at the 48th Interscience Conference on Antimicrobial Agents and Chemotherapy, Washington, DC, 2008.).

R2(2)(8) motifs in Aminopyrimidine sulfonate/carboxylate interactions: crystal structures of pyrimethaminium benzenesulfonate monohydrate (2:2:1) and 2-amino-4,6-dimethylpyrimidinium sulfosalicylate dihydrate (4:2:2)

BACKGROUND

Pyrimethamine [2,4-diamino-5-(p-chlorophenyl)-6-ethylpyrimidine] is an antifolate drug used in anti-malarial chemotherapy. Pyrimidine and aminopyrimidine derivatives are biologically important compounds owing to their natural occurrence as components of nucleic acids.

RESULTS

In the crystal structures of two organic salts, namely pyrimethaminium benzenesulfonate monohydrate 1 and 2-amino-4, 6-dimethylpyrimidinium 3-carboxy-4-hydroxy benzenesulfonate dihydrate 2, pyrimethamine (PMN) and 2-amino-4,6-dimethylpyrimidine (AMPY) are protonated at one of the nitrogens in the pyrimidine rings. In both the PMN and AMPY sulfonate complexes, the protonated pyrimidine rings are hydrogen bonded to the sulfonate groups, forming a hydrogen-bonded bimolecular ring motif with graph-set notation R2(2)(8). The sulfonate group mimics the carboxylate anion's mode of association, which is more commonly seen when binding with 2-aminopyrimidines. In compound 1, the PMN moieties are centrosymmetrically paired through a complementary DADA array of hydrogen bonds. In compound 2, two types of bimolecular cyclic hydrogen bonded R2(2)(8) motifs (one involving the carboxylate group and the other involving sulfonate group) coexist. Furthermore, this compound is stabilized by intra and intermolecular O-H...O hydrogen bonds.

CONCLUSION

The crystal structures of pyrimethaminium benzenesulfonate monohydrate and 2-amino-4,6-dimethylpyrimidinium sulfosalicylate dihydrate have been investigated in detail. In compound 1, the R2(2)(8) motif involving the sulfonate group is present. The role the sulfonic acid group plays in mimicking the carboxylate anions is thus evident. In compound 2, two types of bimolecular cyclic hydrogen bonded R2(2)(8) motifs (one involving the carboxylate group and the other involving sulfonate group) coexist. In both the compounds base pairing also occurs. Thus homo and hetero synthons are present.

Safety and toxicity of sulfadoxine/pyrimethamine: implications for malaria prevention in pregnancy using intermittent preventive treatment

Plasmodium falciparum infection during pregnancy is strongly associated with maternal anaemia and low birth weight, contributing to substantial morbidity and mortality in sub-Saharan Africa. Intermittent preventive treatment in pregnancy with sulfadoxine/pyrimethamine (IPTp-SP) has been one of the most effective approaches to reduce the burden of malaria during pregnancy in Africa. IPTp-SP is based on administering >or=2 treatment doses of sulfadoxine/pyrimethamine to pregnant women at predefined intervals after quickening (around 18-20 weeks). Randomised, controlled trials have demonstrated decreased rates of maternal anaemia and low birth weight with this approach. The WHO currently recommends IPTp-SP in malaria-endemic areas of sub-Saharan Africa. However, implementation has been suboptimal in part because of concerns of potential drug toxicities. This review evaluates the toxicity data of sulfadoxine/pyrimethamine, including severe cutaneous adverse reactions, teratogenicity and alterations in bilirubin metabolism. Weekly sulfadoxine/pyrimethamine prophylaxis is associated with rare but potentially fatal cutaneous reactions. Fortunately, sulfadoxine/pyrimethamine use in IPTp programmes in Africa, with 2-4 treatment doses over 6 months, has been well tolerated in multiple IPTp trials. However, sulfadoxine/pyrimethamine should not be administered concurrently with cotrimoxazole given their redundant mechanisms of action and synergistic worsening of adverse drug reactions. Therefore, HIV-infected pregnant women in malaria endemic areas who are already receiving cotrimoxazole prophylaxis should not also receive IPTp-SP. Although folate antagonist use in the first trimester is associated with neural tube defects, large case-control studies have demonstrated that sulfadoxine/pyrimethamine administered as IPTp (exclusively in the second and third trimesters and after organogenesis) does not result in an increased risk of teratogenesis. Folic acid supplementation is recommended for all pregnant women to reduce the rate of congenital anomalies but high doses of folic acid (5 mg/day) may interfere with the antimalarial efficacy of sulfadoxine/pyrimethamine. However, the recommended standard dose of folic acid supplementation (0.4 mg/day) does not affect antimalarial efficacy and may provide the optimal balance to prevent neural tube defects and maintain the effectiveness of IPTp-SP. No clinical association between sulfadoxine/pyrimethamine use and kernicterus has been reported despite the extensive use of sulfadoxine/pyrimethamine and related compounds to treat maternal malaria and congenital toxoplasmosis in near-term pregnant women and newborns. Although few drugs in pregnancy can be considered completely safe, sulfadoxine/pyrimethamine - when delivered as IPTp - has a favourable safety profile. Improved pharmacovigilance programmes throughout Africa are now needed to confirm its safety as access to IPTp-SP increases. Given the documented benefits of IPTp-SP in malaria endemic areas of Africa, access to this treatment for pregnant women should continue to expand.

2,4-diaminopteridine-based compounds as precursors for de novo synthesis of antifolates: a novel class of antimalarials

We have tested the hypothesis that 2,4-diamino-6-hydroxymethyl-pteridine (DAP), 2,4-diaminopteroic acid (DAPA), and 2,4 diamino-N10-methyl-pteroic acid (DAMPA) could be converted into aminopterin (from DAP and DAPA) and methotrexate (from DAMPA), both of which are potent inhibitors of dihydrofolate reductase, a proven drug target for Plasmodium falciparum. DAP, DAPA, and DAMPA inhibited parasite growth in the micromolar range; DAMPA was the most active, with 50% inhibitory concentrations in vitro of 446 ng/ml against the antifolate-sensitive strain and 812 ng/ml against the highly resistant strain under physiological folate conditions. DAMPA potentiates the activity of the sulfone dapsone, an inhibitor of dihydropteroate synthase, but not that of chlorcycloguanil, a known inhibitor of dihydrofolate reductase (DHFR). Experiments with a Saccharomyces cerevisiae strain dependent upon the P. falciparum DHFR enzyme showed that DHFR is a target of DAMPA in that system. We hypothesize that DAMPA is converted to methotrexate by the parasite dihydrofolate synthase, which explains the synergy of DAMPA with dapsone but not with chlorcycloguanil. This de novo synthesis will not occur in the host, since it lacks the complete folate pathway. If this hypothesis holds true, the de novo synthesis of the toxic compounds could be used as a framework for the search for novel potent antimalarial antifolates.

Structure, dynamics, and catalytic function of dihydrofolate reductase

Molecular motions are widely regarded as contributing factors in many aspects of protein function. The enzyme dihydrofolate reductase (DHFR), and particularly that from Escherichia coli, has become an important system for investigating the linkage between protein dynamics and catalytic function, both because of the location and timescales of the motions observed and because of the availability of a large amount of structural and mechanistic data that provides a detailed context within which the motions can be interpreted. Changes in protein dynamics in response to ligand binding, conformational change, and mutagenesis have been probed using numerous experimental and theoretical approaches, including X-ray crystallography, fluorescence, nuclear magnetic resonance (NMR), molecular dynamics simulations, and hybrid quantum/classical dynamics methods. These studies provide a detailed map of changes in conformation and dynamics throughout the catalytic cycle of DHFR and give new insights into the role of protein motions in the catalytic activity of this enzyme.

Pyrimethamine and WR99210 exert opposing selection on dihydrofolate reductase from Plasmodium vivax

Plasmodium vivax is a major public health problem in Asia and South and Central America where it is most prevalent. Until very recently, the parasite has been effectively treated with chloroquine, but resistance to this drug has now been reported in several areas. Affordable alternative treatments for vivax malaria are urgently needed. Pyrimethamine-sulfadoxine is an inhibitor of dihydrofolate reductase (DHFR) that has been widely used to treat chloroquine-resistant Plasmodium falciparum malaria. DHFR inhibitors have not been considered for treatment of vivax malaria, because initial trials showed poor efficacy against P. vivax. P. vivax cannot be grown in culture; the reason for its resistance to DHFR inhibitors is unknown. We show that, like P. falciparum, point mutations in the dhfr gene can cause resistance to pyrimethamine in P. vivax. WR99210 is a novel inhibitor of DHFR, effective even against the most pyrimethamine-resistant P. falciparum strains. We have found that it is also an extremely effective inhibitor of the P. vivax DHFR, and mutations that confer high-level resistance to pyrimethamine render the P. vivax enzyme exquisitely sensitive to WR99210. These data suggest that pyrimethamine and WR99210 would exert opposing selective forces on the P. vivax population. If used in combination, these two drugs could greatly slow the selection of parasites resistant to both drugs. If that is the case, this novel class of DHFR inhibitors could provide effective and affordable treatment for chloroquine- and pyrimethamine-resistant vivax and falciparum malaria for many years to come.

Efficacies of lipophilic inhibitors of dihydrofolate reductase against parasitic protozoa

Competitive inhibitors of dihydrofolate reductase (DHFR) are used in chemotherapy or prophylaxis of many microbial pathogens, including the eukaryotic parasites Plasmodium falciparum and Toxoplasma gondii. Unfortunately, point mutations in the DHFR gene can confer resistance to inhibitors specific to these pathogens. We have developed a rapid system for testing inhibitors of DHFRs from a variety of parasites. We replaced the DHFR gene from the budding yeast Saccharomyces cerevisiae with the DHFR-coding region from humans, P. falciparum, T. gondii, Pneumocystis carinii, and bovine or human-derived Cryptosporidium parvum. We studied 84 dicyclic and tricyclic 2,4-diaminopyrimidine derivatives in this heterologous system and identified those most effective against the DHFR enzymes from each of the pathogens. Among these compounds, six tetrahydroquinazolines were effective inhibitors of every strain tested, but they also inhibited the human DHFR and were not selective for the parasites. However, two quinazolines and four tetrahydroquinazolines were both potent and selective inhibitors of the P. falciparum DHFR. These compounds show promise for development as antimalarial drugs.

Dihydropteroate synthase of Mycobacterium leprae and dapsone resistance

Two Mycobacterium leprae genes, folP1 and folP2, encoding putative dihydropteroate synthases (DHPS), were studied for enzymatic activity and for the presence of mutations associated with dapsone resistance. Each gene was cloned and expressed in a folP knockout mutant of Escherichia coli (C600DeltafolP::Km(r)). Expression of M. leprae folP1 in C600DeltafolP::Km(r) conferred growth on a folate-deficient medium, and bacterial lysates exhibited DHPS activity. This recombinant displayed a 256-fold-greater sensitivity to dapsone (measured by the MIC) than wild-type E. coli C600, and 50-fold less dapsone was required to block (expressed as the 50% inhibitory concentration [IC(50)]) the DHPS activity of this recombinant. When the folP1 genes of several dapsone-resistant M. leprae clinical isolates were sequenced, two missense mutations were identified. One mutation occurred at codon 53, substituting an isoleucine for a threonine residue (T53I) in the DHPS-1, and a second mutation occurred in codon 55, substituting an arginine for a proline residue (P55R). Transformation of the C600DeltafolP::Km(r) knockout with plasmids carrying either the T53I or the P55R mutant allele did not substantially alter the DHPS activity compared to levels produced by recombinants containing wild-type M. leprae folP1. However, both mutations increased dapsone resistance, with P55R having the greatest affect on dapsone resistance by increasing the MIC 64-fold and the IC(50) 68-fold. These results prove that the folP1 of M. leprae encodes a functional DHPS and that mutations within this gene are associated with the development of dapsone resistance in clinical isolates of M. leprae. Transformants created with M. leprae folP2 did not confer growth on the C600DeltafolP::Km(r) knockout strain, and DNA sequences of folP2 from dapsone-susceptible and -resistant M. leprae strains were identical, indicating that this gene does not encode a functional DHPS and is not involved in dapsone resistance in M. leprae.

Efficacy of trimethoprim in murine experimental infection with a thymidine kinase-deficient mutant of Escherichia coli

The antimicrobial activity of trimethoprim is antagonized by thymidine in in vitro susceptibility tests. The purpose of this investigation was to determine whether this antagonism also occurred during experimental infection in mice, which have high serum thymidine concentrations. We derived a mutant strain of Escherichia coli, TT-48, incapable of utilizing exogenous thymidine from parent strain E. coli KC-14 and then investigated the in vitro and in vivo antimicrobial activities of trimethoprim, sulfamethoxazole, cefdinir, and ofloxacin against these strains. E. coli TT-48 lacked the activity of thymidine kinase, which catalyzes the conversion of thymidine to thymidylate, but its growth curve remained close to that of the parent strain. The MICs of all of the antimicrobial agents tested, except cefdinir, for the mutant strain were slightly inferior to those for the parent strain. The bactericidal effect of trimethoprim against the parent strain was antagonized by thymidine at concentrations of more than 1 microg/ml, while that against the mutant strain was not affected by thymidine even at the highest concentration (10 microg/ml). The therapeutic efficacy of trimethoprim in experimental murine infections was significantly higher when the mutant rather than the parent strain was used, whereas the therapeutic efficacy of cefdinir or ofloxacin, whose antimicrobial action is independent of folic acid synthesis, was the same with both strains. Unexpectedly, sulfamethoxazole also had similar efficacy against both strains. Thus, high thymidine concentrations antagonized the antimicrobial activity of trimethoprim in vitro and in vivo.

Genetic characterization of trimethoprim resistance in Haemophilus influenzae

We previously demonstrated that trimethoprim (Tmp) resistance in Haemophilus influenzae is mediated by chromosomally encoded dihydrofolate reductase (DHFR) with a modified primary structure and distinct kinetic properties. To gain insight into the relationship of the DHFR structure and the level of Tmp resistance that it confers on the host bacterium, we cloned and characterized the folH genes of one Tmp-susceptible and two Tmp-resistant H. influenzae strains. Differences were observed between Tmp-susceptible and Tmp-resistant isolates both in the promoter region and in the coding sequences. The effect of differences between H. influenzae folH genes on Tmp susceptibility was investigated in Escherichia coli. Various folH gene hybrids were constructed, and their influence on Tmp susceptibility was determined. Resistance in E. coli mediated by folH from H. influenzae strain R1047 was associated with alterations in the promoter and the central part of folH. In contrast, the E. coli Tmp resistance phenotype associated with the folH gene of H. influenzae R1042 was characterized by alterations in one or more of three amino acid residues at the C-terminal part of the protein. These data indicate that Tmp resistance is not only related to alterations in the promoter region of the folH gene and the Tmp binding domains at the N-terminal and central part of DHFR. Alterations in the C-terminal part may also cause Tmp resistance, probably as a result of a change in secondary structure and the subsequent loss of Tmp binding affinity.

Antimalarial activities of oligodeoxynucleotide phosphorothioates in chloroquine-resistant Plasmodium falciparum

Synthetic oligonucleotides and their chemical modifications have been shown to inhibit viral and cellular gene expression by sequence-specific antisense hybridization to target mRNAs. We now report that oligodeoxynucleotide phosphorothioates and their nuclease-resistant modifications are effective in micromolar and submicromolar concentrations against the growth of both chloroquine-resistant and chloroquine-sensitive strains of Plasmodium falciparum in vitro. Parasitized human erythrocytes were found to be accessible to radioactively labeled oligodeoxynucleotides, whereas the uninfected erythrocytes did not permit any cellular entry of the same compounds. The dihydrofolate reductase-thymidylate synthase gene of P. falciparum was demonstrated to be a good target for sequence-dependent inhibition of plasmodial growth by exogenously administered modified oligonucleotides. The antimalarial activities observed in vitro were identical for chloroquine-sensitive and chloroquine-resistant strains of P. falciparum. The antimalarial activity of oligodeoxynucleotide phosphorothioates is related to sequence complementarity to certain regions of the plasmodial genome as well as to non-sequence-defined activities.

Role of purine biosynthetic intermediates in response to folate stress in Escherichia coli

Folic acid plays a central role in anabolic metabolism by supplying single-carbon units at varied levels of oxidation for both nucleotide and amino acid biosyntheses. It has been proposed that 5-amino-4-imidazole carboxamide riboside 5'-triphosphate (ZTP), an intermediate in de novo purine biosynthesis, serves as a signal of cellular folate stress and mediates a physiologically beneficial response to folate stress in Salmonella typhimurium (B. R. Bochner, and B. N. Ames, Cell 29:929-937, 1982). We examined the physiological response of Escherichia coli to folate stress induced by the drugs psicofuranine, trimethoprim, and sodium sulfathiazole or by p-aminobenzoic acid (pABA) starvation. Analysis of nucleotide pools showed that psicofuranine or trimethoprim treatment of a prototrophic strain or growth of a pABA auxotroph on limiting pABA induced the production of the nucleotide ZTP, as previously observed in S. typhimurium by Bochner and Ames. Accumulation of ZTP and its precursor 5-amino-4-imidazole carboxamide riboside 5'-monophosphate (ZMP) did not correlate well with folate stress in E. coli, as measured by determination of the folate/protein ratios of extracts of treated cells. Treatment of cells with psicofuranine caused a marked accumulation of 5-amino-4-imidazole carboxamide ribonucleotides (Z-ribonucleotides) but a statistically insignificant drop in the folate/protein ratio of cell extracts. Sodium sulfathiazole treatment at a drug concentration that led to a threefold drop in the growth rate and in the folate/protein ratio of treated cells led to little accumulation of Z-ribonucleotides in E. coli A purF his+ strain which produces ZTP and ZMP when treated with trimethoprim was constructed. In this strain, histidine represses the synthesis of both ZMP and ZTP. Treatment of cells of this strain with trimethoprim resulted in a decrease in the folate/protein ratio of cell extracts, but a blockade of Z-ribonucleotide accumulation did not affect the extent of folate depletion seen in treated cells and had only a small effect on the resistance of this strain to growth inhibition by trimethoprim. The patterns of protein expression induced by treatment of this strain with trimethoprim or psicofuranine were examined by two-dimensional electrophoretic resolution of the total cellular proteins. No differences in protein expression were seen when the treatment were performed in media containing or lacking histidine. These studies failed to provide evidence in E. coli for a folate stress regulon controlled by ZTP.

Azithromycin, a macrolide antibiotic with potent activity against Toxoplasma gondii

Doses of 200 mg of azithromycin per kg (body weight) administered by the oral route daily for 10 days completely protected mice against death caused by intraperitoneal infection with Toxoplasma gondii. The same treatment regimen also protected 80% of mice infected intracerebrally, which suggests that azithromycin attains active concentrations in the inflamed central nervous system.

Potentiating effect of pyrimethamine and sulfadoxine against dihydrofolate reductase from pyrimethamine-sensitive and pyrimethamine-resistant Plasmodium chabaudi

Dihydrofolate reductase was partially purified from a pyrimethamine-sensitive Plasmodium chabaudi clone and a pyrimethamine-resistant clone derived from it and used in a study of the inhibitory effect of pyrimethamine and sulfadoxine, both alone and in combination. Kinetic analysis of the inhibitory effect of sulfadoxine against the enzyme from pyrimethamine-sensitive and -resistant parasites revealed that the drug inhibited the former enzyme competitively, with an inhibition constant (Kis) of 0.7 +/- 0.4 mM, but inhibited the latter enzyme noncompetitively, with Kis and Kii of 8.9 +/- 1.2 and 4.1 +/- 1.2 mM, respectively. Previous studies also showed competitive inhibition by pyrimethamine on the former enzyme and noncompetitive inhibition on the latter enzyme, with some 200-fold-lower affinity. Sulfadoxine and pyrimethamine exhibited a mutually potentiating effect on the enzyme activity, as revealed by the concave isoboles and the fractional inhibitions of less than unity. A potentiating effect was observed for the enzymes from both sources and was not dependent on the degree of the purification of the enzyme. Our results can be explained by assuming simultaneous binding of two inhibitors on the enzyme.

Purification and characterization of dihydrofolate reductase from Mycobacterium phlei

The dihydrofolate reductase from Mycobacterium phlei was purified and characterized; it has an Mr of 15 000 and a pI of 4.8. It is competitively inhibited by both methotrexate and trimethoprim, although the affinity is less than for other bacterial dihydrofolate reductases.

Erythema multiforme (Stevens-Johnson) precipitated by Fansidar

A case of Stevens-Johnson syndrome precipitated by the sulphonamide-containing anti-malarial drug, Fansidar, is described.

Competitive inhibition by diacylglycerol of specific phorbol ester binding

Emerging evidence suggests that the phorbol ester receptor in brain may be the same as the Ca2+-phospholipid-dependent protein kinase (protein kinase C). Since protein kinase C activity is stimulated by unsaturated diacylglycerol and the phorbol esters can substitute for diacylglycerol in this stimulation, we have examined the effect of diacylglycerols on phorbol ester binding. Assays were carried out with the mouse brain cytosolic phorbol ester aporeceptor, which requires phospholipids for activity. In the presence of phosphatidylserine at 0.96 mg/ml, diolein inhibited specific binding of [3H]phorbol 12,13-dibutyrate ( [3H]PBt2) in a dose-dependent fashion to less than 10% of control levels. The inhibition curves fit the curve expected for a competitive inhibitor and yielded a Ki of 3.6 +/- 0.8 micrograms/ml (n = 5) [0.38% (wt/wt) the concentration of phosphatidylserine]. Scatchard analysis confirmed the competitive nature of the inhibition. At constant phospholipid concentrations, the Ki determined for diolein was independent of the diolein concentrations over the range of 1.5-80 micrograms/ml, suggesting that the inhibition did not arise simply by perturbation of the phospholipid bilayers. The Ki of diolein was approximately proportional to the absolute phospholipid concentration. With phosphatidylserine at 4.8 micrograms/ml, for example, the Ki was 52 ng/ml (1.1% of phosphatidylserine). In addition to diolein, the short-chain saturated diacylglycerol derivatives dicaprylin and dicaproin also inhibited [3H]PBt2 binding, whereas the long-chain saturated derivatives dipalmitin and distearin were much less active. Our results suggest (i) that diacylglycerol may act as an endogenous ligand for the phorbol ester receptor and (ii) that variation in lipid composition provides a mechanism for modulating phorbol ester receptor affinity.

Trimethoprim activity in media selective for Campylobacter jejuni

The activity of trimethoprim (TMP) in two selective media used for isolation of Campylobacter jejuni was evaluated. The two selective media, Campy-BAP and Skirrow medium, contain TMP in addition to other antimicrobial agents. The minimal inhibitory concentrations of TMP in blood agar base (basal agar for Skirrow medium) or brucella agar (basal agar for Campy-BAP) for three sensitive control organisms were compared with those in Mueller-Hinton agar, which contains low levels of thymidine. TMP was inactive in both blood agar base and brucella agar, even when lysed horse blood or thymidine phosphorylase was added. TMP had activity when used in combination with the other antimicrobial agents normally included in Skirrow medium and Campy-BAP, probably indicating synergism between TMP and one or more of the other antimicrobial agents. Sheep blood could be substituted for lysed horse blood in Skirrow medium without compromising the activity of TMP.

Novel mechanism of resistance to folate analogues: ribonucleoside diphosphate reductase deficiency in bacteriophage T4

Some spontaneously occurring bacteriophage T4 mutants (far mutants) were able to form plaques in the presence of concentrations of folate analogues that completely inhibit plaque formation by wild-type phage T4. Some of these far mutants were shown to be ribonucleoside diphosphate (RDP) reductase (EC 1.17.4.1) deficient, and some independently isolated RDP reductase-deficient mutants (nrd mutants) were shown to be folate analogue resistant. The map positions of the RDP reductase-deficient far mutants were shown to be within the genes controlling the phage-induced RDP reductase activity.

Diffusion of trimethoprim and sulfamethoxazole from susceptibility disks into agar medium

The standard practice of using a single susceptibility disk for the antimicrobial combination Septra (trimethoprim/sulfamethoxazole) has been further justified by a direct measurement of the diffusion rates of each compound through agar medium. [(14)C]trimethoprim and [(35)S]sulfamethoxazole, singly and in combination, were applied to blank susceptibility disks which were incubated on 4-inch (10.16-cm) agar plates (Mueller-Hinton medium) at 37 C. The migration of each compound from the disk and diffusion through agar were measured with time by determining the radioactivity in concentric zones extending from the origin. The two compounds diffuse with similar rates, maintaining approximately a 1:20 concentration ratio which is approximately the ratio of trimethoprim to sulfamethoxazole observed in plasma during treatment. The diffusion rate is independent of the presence of the other compound; greater than 95% of the radioactivity is transferred from the disk to the agar in 24 h.

Isotope-dilution analysis of rate-limiting steps and pools affecting the incorporation of thymidine and deoxycytidine into cultured thymus cells

1. Rat thymus cells were incubated in homologous serum (10%) and medium 199. The effects of various quantities of thymidine or deoxycytidine (0-30mum) on the radioactive labelling of cells with the corresponding radioactive deoxynucleoside were examined. From plots of the reciprocal of the radioactivity incorporated against the total deoxynucleoside concentration (;isotope-dilution plots'), values were obtained for (a) the V(max.) of the rate-limiting step governing incorporation of the deoxynucleoside, and (b) the concentration of the pool of compounds competing with the radioactive deoxynucleoside at that rate-limiting step. From changes in these values under different experimental conditions inferences were drawn on the position and control of the rate-limiting step within intact cells. 2. Isotope-dilution plots for deoxycytidine were linear, whereas plots for thymidine were bimodal, indicating an abrupt increase in both the V(max.) and pool concentration at a critical thymidine concentration (approx. 5mum). The bimodality was removed by amethopterin. The V(max.) determined with deoxy[U-(14)C]cytidine was approximately equal to the sum of the V(max.) determined with deoxy[5-(3)H]cytidine and the V(max.) determined with [Me-(3)H]thymidine at thymidine concentrations above 5mum. 3. The thymidine competitor pool at thymidine concentrations above 5mum was approximately equal to the sum of the deoxycytidine competitor pool and the thymidine competitor pool at thymidine concentrations below 5mum. The pools were independent of cell concentration and dependent on serum concentration. 4. These results were explained on the following basis. Deoxycytidine in serum (16mum) is the major source of both cytosine and, by way of thymidylate synthetase, thymine, in the DNA of thymus cells. Serum deoxycytidine normally maintains a sufficient intracellular concentration of dTTP to inhibit partially the activity of thymidine kinase. When the dTTP concentration is lowered, either by decreasing the concentration of deoxycytidine or by inhibiting thymidylate synthetase, the activity of thymidine kinase increases. The activity of thymidine kinase may also be increased by concentrations of thymidine greater than 5mum, which overcome the inhibition of the enzyme by dTTP. At concentrations of thymidine below 5mum, thymidine kinase limits the rate of labelling with [Me-(3)H]thymidine and the radioactivity is diluted by a pool of unlabelled thymidine in serum (4mum). At thymidine concentrations greater than 5mum, the activity of DNA polymerase limits the rate of labelling and the radioactivity is diluted both by serum thymidine and, indirectly, by serum deoxycytidine.

The effect of trimethoprim on macromolecular synthesis in Escherichia coli

In trimethoprim-inhibited RC(str) strains of Escherichia coli, the expression of the RC control of stable RNA synthesis arose primarily from a decrease in the intracellular concentrations of glycine and methionine, and not from inhibition of the initiation of new protein chains. In non-supplemented cultures, experiments with rifampicin showed that the immediate response to the addition of trimethoprim was a rapid decrease in the rate of initiation of RNA chains. This was followed after a few minutes by a sufficiently large fall in the rate of endogenous synthesis of nucleotide bases to cause a decrease in the rate of RNA chain polymerization. Inhibition of RNA chain initiation was thus overridden by an accumulation of DNA-dependent RNA polymerases upon the cistrons. RC(rel) strains also accumulated polymerases upon the DNA in similar circumstances, but did not suffer the initial effects on chain initiation. If purines were supplied before adding trimethoprim, RC(str) strains polymerized RNA chains at normal rates, but initiation rates were permanently decreased. In either situation, an increased% of the RNA formed was mRNA. However, in RC(rel) strains supplemented with bases, trimethoprim did not affect either the rate of initiation of new chains or their rates of polymerization or the relative rates of synthesis of stable RNA and mRNA. Protein synthesis was also severely inhibited by trimethoprim. Though the addition of glycine and methionine to base-supplemented, trimethoprim-inhibited RC(str) strains did not apparently affect the decreased rate of protein synthesis, RNA accumulation resumed at its normal rate. Thus, the inhibition of protein chain initiation had no effect on the rate of RNA accumulation in either RC(str) or RC(rel) bacteria. The RC control does not express itself through inhibitions of protein synthesis at this level.

Cross-resistance and collateral susceptibility to antifolic antimalarial compounds

Series of strains of Streptococcus faecium ATCC 8043, Lactobacillus casei ATCC 7469, and Pediococcus cerevisiae ATCC 8081 with increasing resistance to the active antifolate antimalarial drugs chlorguanide triazine (CGT), pyrimethamine (PM), and trimethoprim (TMP) were isolated. These mutant strains, stable for at least 3 to 5 years, were examined for cross-resistance and collateral susceptibility to the above compounds and to methotrexate (MTX). Generally, they exhibited cross-resistance to all four compounds, but resistance of a strain to one compound did not predict accurately its resistance to another drug. Unexpectedly, L. casei resistant to CGT exhibited collateral susceptibility to MTX, TMP, and PM varying from 5- to 20-fold. P. cerevisiae developed resistance to CGT readily but maintained its susceptibility to PM and TMP after prolonged exposure to these compounds. Resistance to these antimalarial antifolates was accompanied by only low-grade cross-resistance to MTX, a representative antileukemic antifolate agent.

Effect of the folic acid analogue, trimethoprim, on growth, macromolecular synthesis, and incorporation of exogenous thymine in Escherichia coli

The effect of trimethoprim [2,4-diamino-5(2',4',5'trimethoxybenzyl)-pyrimidine] in the presence of thymine on Escherichia coli B temperature-sensitive and non-temperature-sensitive Thy(') strains and a phosphodeoxyribomutase-negative mutant was studied. The inhibitory effect of 5 mug of trimethoprim per ml on the growth of E. coli B was not overcome by thymine, thymidine, or thymidylate even in the presence of one-carbon metabolites and related metabolites. Deoxyribonucleic acid (DNA) and protein synthesis were more severely inhibited than ribonucleic acid (RNA) synthesis. The inhibition of DNA synthesis was partially reversed by addition of deoxyadenosine to increase the incorporation of exogenous thymine. By contrast, the inhibition of protein was not reversed even with one-carbon metabolites present, in keeping with the requirement for formylmethionyl-transfer RNA(F) for initiation. However, the inhibition of both DNA and protein synthesis in a phosphodeoxyribomutase-negative strain by 1 mug of trimethoprim per ml with thymine present was partially reversed by deoxyadenosine and one-carbon metabolites, and nearly normal growth occurred. 5-Fluorodeoxyuridine added at the time of addition of trimethoprim prevented the inhibition. Sulfadiazine in the presence of thymine inhibited both Thy(+) and Thy(-) strains whereas trimethoprim (with thymine) did not inhibit Thy(-) organisms. The effect of trimethoprim on the incorporation of labeled thymine into DNA was also studied. These experiments support the concept that trimethoprim in conjunction with the action of thymidylate synthetase inhibits the growth of Thy(+) cells because of a depletion of tetrahydrofolate. DNA synthesis is inhibited initially by a limitation of thymine nucleotide precursor, resulting from the indirect inhibition of thymidylate synthetase and the poor incorporation of exogenous thymine.