Anti-malarial drug targets: screening for inhibitors of 2C-methyl-D-erythritol 4-phosphate synthase (IspC protein) in Mediterranean plants.Abstract
The recently discovered non-mevalonate pathway of isoprenoid isoprenoid
Class of organic compounds made up of two or more structural units derived from isoprene. Isoprene is a five-carbon hydrocarbon with a branched-chain structure, two double bonds (see bonding), and the molecular formula C biosynthesis Biosynthesis
The synthesis of more complex molecules from simpler ones in cells by a series of reactions mediated by enzymes. The overall economy and survival of the cell is governed by the interplay between the energy gained from the breakdown of compounds serves as the unique source of terpenoids in numerous pathogenic eubacteria eubacteria
Term formerly used to describe and differentiate the true bacteria from the archaebacteria. Today, the true bacteria form the domain Bacteria, and the archaebacteria (also an obsolete term) form the domain Archaea. and in apicoplast-type protozoa, most notably Plasmodium plasmodium, name for a stage in the life cycle of a slime mold. Also, Plasmodium is the name given to the genus of the protozoan parasite that causes malaria. , but is absent in mammalian cells. It is therefore an attractive target for anti-infective chemotherapy. The first committed step of the non-mevalonate pathway is catalyzed by 2C-methyl-D-erythritol 4-phosphate synthase (IspC). Using photometric pho·tom·e·try
Measurement of the properties of light, especially luminous intensity.
photo·met and NMR NMR: see magnetic resonance. spectroscopic spec·tro·scope
An instrument for producing and observing spectra.
spectro·scop assays, we screened extracts of Mediterranean plants for inhibitors of the enzyme. Strongest inhibitory activity was found in leaf extracts of Cercis siliquastrum.
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Keywords: Terpene terpene /ter·pene/ (ter´pen) any hydrocarbon of the formula C10H16.
Any of various unsaturated hydrocarbons in essential oils and certain resins of plants and used in organic ; Deoxyxylulose inhibitors of 2C-methyl-D-erythritol 4-phosphate sythase (IspCprotein); Antibiotic; Malaria; Cercis siliquastrum
On a worldwide basis, infections by microorganisms, viruses and parasites are among the most important factors of morbidity and mortality Morbidity and Mortality can refer to:
(Hebrew: “priest”) Jewish priest descended from Zadok (a descendant of Aaron), priest at the First Temple of Jerusalem. The biblical priesthood was hereditary and male. , 2000; Falkow and Kennedy, 2001). In light of the rapid development of resistance against all anti-infective agents, there is an urgent need for the rapid development and deployment of novel drugs.
Any molecular target for anti-infective agents must be serving essential functions in the infectious agent. It is also desirable that the target should be essential in a wide variety of infectious organisms. On the other hand, it would be preferable if the target function were absent in the host in order to reduce the risks of toxicity. These requirements are all fulfilled by plasmodial plas·mo·di·al
Relating to a plasmodium or a species of the genus Plasmodium. enzymes of the fatty acid biosynthetic pathway and of the recently discovered non-mevalonate pathway of isoprenoid precursors (Ralph et al., 2004).
Classical work by Bloch, Cornforth and Lynen on mammalian and yeast cells in the 1950s culminated in the elucidation of the mevalonate pathway for the biosynthesis of the universal isoprenoid precursors, isopentenyl diphosphate di·phos·phate
An ester of phosphoric acid containing two phosphate groups. (IPP) (7) and dimethylallyl diphosphate (DMAPP DMAPP Dimetilalil Pirofosfato ) (8) via acetyl-CoA (11), 3-hydroxy-3-methylglutaryl-CoA (10) and 5-diphospho-mevalonate (9) (for review, see Qureshi and Porter, 1981; Bach, 1995; Bloch, 1992; Bochar et al., 1999) (Fig. 1). For several decades, this pathway was believed to be the universal source of IPP and DMAPP in all taxonomic kingdoms. However, it is now clear that the mevalonate pathway is the unique source for isoprenoids in animals and certain eubacteria, whereas many eubacteria including most human pathogens use exclusively the non-mevalonate pathway [for review, see Boucher and Doolittle, 2000; Eisenreich et al., 2004) (Fig. 1). Notable exceptions are Gram-positive cocci cocci /coc·ci/ (kok´si) plural of coccus.
[L.] plural of coccus. including Staphylococcus and Streptococcus spp. (Wilding et al., 2000). It is also important to note that apicoplast protozoa including Plasmodium depend exclusively on the non-mevalonate pathway (Gardner et al., 2002) which has already been shown to be a valid anti-malarial target (Jomaa et al., 1999; Missinou et al., 2002). Specifically, the isoprenoid side chains of essential quinones can only be made via the non-mevalonate pathway in all intraerythrocytic stages of Plasmodium falciparum (Cassera et al., 2004).
[FIGURE 1 OMITTED]
The non-mevalonate pathway starts with the condensation of D-glyceraldehyde 3-phosphate (2) and pyruvate pyruvate /py·ru·vate/ (pi´roo-vat) a salt, ester, or anion of pyruvic acid. Pyruvate is the end product of glycolysis and may be metabolized to lactate or to acetyl CoA.
n. (1) affording 1-deoxy-D-xylulose 5-phosphate (3) and carbon dioxide (Sprenger et al., 1997; Lois et al., 1998). A skeletal rearrangement of 3 affords the branched carbohydrate, 2C-methyl-D-erythrose 4-phosphate, which is then reduced under formation of 2C-methyl-D-erythritol 4-phosphate (4) (Takahashi et al., 1998). These consecutive reactions are both catalyzed by 2C-methyl-D-erythritol 4-phosphate synthase (IspC protein). In subsequent steps, the catalytic action of IspDEFGH enzymes finally affords a mixture of IPP and DMAPP (Fig. 1; for review, see Eisenreich et al., 2004).
In the 1980s, the antibiotic compound, fosmidomycin, was discovered in extracts of Streptomyces Streptomyces (strĕp'təmī`sēz), bacterial genus of the order Actinomycetales, members of which resemble fungi in their branching filamentous structure. Various species produce such antibiotics as streptomycin and various tetracyclines. lavendulae (Okuhara et al., 1980; Kuemmerle et al., 1985), but was subsequently abandoned. However, the compound attracted renewed interest, when it was shown that fosmidomycin acts as a potent inhibitor of IspC protein in the non-mevalonate biosynthetic pathway (Kuzuyama et al., 1998). A structure of the IspC protein complexed with fosmidomycin confirmed that the inhibitor acts as a substrate analogue that strongly binds to the active center of the enzyme (Steinbacher et al., 2003) (Fig. 2B). Indeed, the compound was able to cure malaria in laboratory studies with mice (Missinou et al., 2002) and in a clinical trial with 20 patients (Lell et al., 2003). Clinical studies using fosmidomycin in combination with the established anti-malarial antibiotic, clindamycin, showed that the mixture is highly efficient in the treatment of uncomplicated malaria with the exception of children < 3 years (Wiesner et al., 2003; Borrmann et al., 2006). In light of this recent result, improved inhibitors against IspC protein are desirable.
In this manuscript, we report on functional assays to test plant extracts for inhibitory activity against IspC protein. In a pilot screen using extracts from Mediterranean plants, we found strong inhibition of the IspC protein from Escherichia coli in leaf extracts of Cercis siliquastrum.
[FIGURE 2 OMITTED]
Materials and methods
The preparation of unlabeled 1-deoxy-D-xylulose 5-phosphate (3) and [3,4,5-[.sup.13.C.sub.3]]-3 has been described elsewhere (Hecht et al., 2001b; Illarionova et al., 2006). Recombinant 2C-methyl-D-erythritol 4-phosphate synthase (IspC protein) from E. coli was prepared according to published procedures (Hecht et al., 2001c). Fosmidomycin was obtained from Molecular Probes (Invitrogen).
Plants were collected at the Botanical Garden of Tel-Aviv University, in the Negev desert and in the Gaza Strip during 2003. When available in sufficient quantity, flower, fruit, inflorescence inflorescence
Cluster of flowers on one or a series of branches, which together make a large showy blossom. Categories depend on the arrangement of flowers on an elongated main axis (peduncle) or on sub-branches from the main axis, and on the timing and position of flowering. , seeds, leaves, and stems were obtained separately. Immediately after harvesting, the plant material was submerged in liquid nitrogen and brought to the laboratory for storage at -80[degrees]C until extracted.
Preparation of plant extracts
Aliquots (1 g) of frozen plant material were ground in a pre-chilled mortar containing liquid nitrogen. Two ml of 50% methanol/water (v/v) were added, and the slurry was mixed and kept on ice for 15 min. The mixture was centrifuged (11,000 rpm, 5 min, room temperature). The supernatant was stored at -80[degrees]C for analysis.
Photometric assay of IspC activity
Assay mixtures contained 100mM Tris hydrochloride, pH 8.0, 2mM EDTA EDTA: see chelating agents. , 16.7mM magnesium chloride, 2.5 mM NADPH NADPH the reduced form of NADP.
The reduced form of NADP.
reduced form of nicotinamide adenine dinucleotide phosphate (NADP) used in a number of reductive synthesis such as fatty , 2.4 [micro]g of bovine serum albumin, and 1.23 [micro]g of IspC protein in a volume of 120 [micro]l. Plant extract (0.02-20 [micro]l) was added, and the reaction was started by the addition of 60 [micro]l of a solution containing 100mM Tris hydrochloride, pH 8.0, and 8 mM 3. In photometric assays to estimate the IC-50 value of the positive reference compound, fosmidomycin, 0.03-20 [micro]l of 60 [micro]M fosmidomycin were added instead of plant extract. The mixtures were incubated in a thermostatted cuvette cuvette /cu·vette/ (ku-vet´) [Fr.] a glass container generally having well-defined characteristics (dimensions, optical properties), to contain solutions or suspensions for study.
n. at 24[degrees]C, and absorbance absorbance /ab·sor·bance/ (-sor´bans)
1. in analytical chemistry, a measure of the light that a solution does not transmit compared to a pure solution. Symbol .
2. at 340 nm was recorded.
NMR assay of IspC activity
Assay mixtures contained 75 mM Tris hydrochloride, pH 8.0, 10mM magnesium chloride, 1.2 mM EDTA, 6mM NADPH, 6mM [3,4,5-[.sup.13.C.sub.3]]-3, 7.2 [micro]g of bovine serum albumin, 3.7 [micro]g of IspC protein, and 20% [D.sub.2]O (v/v) in a total volume of 600 [micro]l. Plant extract (6 [micro]l) or 12 [micro]l of a solution of 50 [micro]M fosmidomycin (final concentration in the assay, 1 [micro]M) was added. The mixtures were incubated at 37[degrees]C for 45min, and the reaction was terminated by the addition of EDTA to a final concentration of 20 mM. Without any further work-up, [.sup.13.C] NMR spectra were recorded using a DRX 500 spectrometer from Bruker Instuments, Karlsruhe, Germany. The [.sup.13.C] NMR signals of [3,4,5-[.sup.13.C.sub.3]]-3 and [1,3,4-[.sup.13.C.sub.3]]-4 have been assigned earlier (Hecht et al., 2001b, c).
Results and discussion
The evolution of 2C-methyl-D-erythritol 4-phosphate synthase (IspC protein) has been relatively conservative. A sequence of the IspC protein from E. coli and the catalytic domain of the P. falciparum protein indicates 145 identical (36%) and 216 similar (54%) amino acid residues (Fig. 2A). Virtually all residues directly involved in the binding of the substrate and NADPH and in the coordination of the essential divalent divalent /di·va·lent/ (di-va´lent) bivalent; carrying a valence of two.
di·va metal ions (cf. Fig. 2B) (Hecht et al., 2001b) are identical (Fig. 2A). The protein of E. coli, which can be easily obtained by recombinant overexpression appears as a valid model for the screening of enzyme inhibitors that may serve as lead compounds for the development of novel anti-bacterial and anti-malarial agents.
The conversion of 1-deoxy-D-xylulose 5-phosphate (3) into 2C-methyl-D-erythritol 4-phosphate (4) by IspC protein is accompanied by the dehydrogenation of stoichiometric stoi·chi·om·e·try
1. Calculation of the quantities of reactants and products in a chemical reaction.
2. The quantitative relationship between reactants and products in a chemical reaction. amounts of NADPH, which can be monitored photometrically pho·tom·e·try
Measurement of the properties of light, especially luminous intensity.
photo·met . On this basis, the effects of potential inhibitory compounds or compound mixtures can be tested in a rapid screening process involving ordered compound libraries (Illarionova et al., 2006) or plant extracts (this study). To validate the assay, a solution of the known IspC inhihibitor, fosmidomycin, was used as a positive control. The robustness of the method was also controlled in assays using different concentrations of fosmidomycin in five replicates for each concentration. The statistical analysis showed an excellent R2 value of 0.9986 (Fig. 3A). Under the given experimental conditions (for details, see Methods), an IC-50 value of 370 nM can be estimated for fosmidomycin.
Plants growing under extreme conditions are considered to produce unique natural metabolites as a metabolic answer to stress factors (e.g., heat, dryness, and salinity). With this in mind, we initiated a study to search for inhibitors of the IspC protein in Mediterranean plants including many desert plants. Specifically, we collected about 200 Mediterranean plant species. All specimens were taxonomically assigned. Plant tissue was extracted as described under Methods in a standardized manner (for [.sup.1.H] NMR fingerprints, see Supplemental Material), and the extracts were screened photometrically for IspC activity. Plant extracts that showed significant enzyme inhibition were again assayed at progressively higher dilution.
[FIGURE 3 OMITTED]
The cutoff for significant inhibitory activity was arbitrarily set to less than 20% residual enzyme activity observed after 4-fold dilution of the plant extract (reflecting about 5 [micro]l of a methanolic extract using 1 g of plant material and 2 ml of solvent). This level of inhibition was observed with 12 extracts of the plants under study (Table 1). The highest level of inhibition was observed with a leaf extract from Cercis siliquas-trum. Specifically, the addition of 0.2 [micro]l of plant extract to an assay volume of 200 [micro]l reduced the reaction rate to less than 25% (Fig. 3B). An apparent IC-50 value that equals the IC-50 value of 370 nM for fosmidomycin was obtained with C. siliquastrum extract after 380-fold dilution.
To verify the photometric read-out, extracts of C. siliquastrum were re-assayed by NMR spectroscopy. In order to enhance the sensitivity and selectivity of that assay, the enzyme substrate was multiply labeled with [.sup.13.C] ([3,4,5-[.sup.13.C.sub.3]]-3). As a consequence of the specific [.sup.13]C-labeling, [.sup.13.C]-signals of C-3, C-4 and C-5 of the substrate and of C-1, C-3 and C-4 of the product, [1,3,4-[.sup.13.C.sub.3]]-4, were detected with high selectivity in the [.sup.13.C] NMR spectrum of the assay without any prior work-up. Moreover, the multiplet signatures resulting from [.sup.13.C] coupling of the multiply [.sup.13.C]-labeled substrate and product were highly characteristic and enabled the unequivocal signal assignment. Enzyme, co-substrate and EDTA present in the analyzed mixture all had natural [.sup.13.C] abundance (i.e., with 1.1% [.sup.13.C]), and the intensities of their [.sup.13.C] NMR signals were therefore approximately two orders of magnitude below those of the signals of the [.sup.13.C]-labeled compounds. With typical measuring times (approximately 45min), only signals from [.sup.13.C]-labelled positions were detected (cf. Fig. 4). All [.sup.13.C] NMR signals and [.sup.13.C][.sup.13.C] coupling constants of [3,4,5-[.sup.13.C.sub.3]]-3 and [1,3,4-[.sup.13.C.sub.3]]-4 have been reported earlier (Hecht et al., 2001a, b). The signals of [3,4,5-[.sup.13.C.sub.3]]-3 in an assay mixture without protein are shown in Fig. 4A. The signals detected in a standard assay without an inhibitory agent are shown in Fig. 4B. It is immediately obvious that the signals due to [3,4,5-[.sup.13.C.sub.3]]-3 disappeared and that three new signals due to [1,3,4-[.sup.13.C.sub.3]]-4 appeared since the substrate was completely converted into the product. The [.sup.13.C] NMR signals of the same assay mixture in the presence of 6 [micro]l of an extract from C. siliquastrum is shown in Fig. 4C. Signals due to [1,3,4-[.sup.13.C.sub.3]]-4 were not detected showing that the enzyme reaction is completely inhibited. Fig. 4D displays the NMR signals in the presence of the positive reference compound, fosmidomycin at a concentration of 1 [micro]M. Only minor signals due to [1,3,4-[.sup.13.C.sub.3]]-4 were observed demonstrating the validity of the approach.
[FIGURE 4 OMITTED]
The surprisingly high number of plant extracts that showed strong inhibition (12 out of approximately 200 extracts) suggests that Mediterranean plants may represent a rich source for natural inhibitors of IspC protein from E. coli and its orthologous proteins. The nature of the inhibitory principle(s) in the extract of C. siliquastrum and the other plants listed in Table 1 is still unknown. Notably, the plants shown to be active belong to different families and it is therefore difficult to identify a common principle under the aspects of phylogenetic relationships. However, on the basis of the limited phytochemical phy·to·chem·i·cal
A nonnutritive bioactive plant substance, such as a flavonoid or carotenoid, considered to have a beneficial effect on human health. analyses of C. siliquastrum (Salatino et al., 2000; Torck et al., 1971), A. andrachne (Aburjai et al., 1999; Sakar et al., 1991) and some other plants listed in Table 1 (Danne et al., 1994; Fecka et al., 2001; Fayad and Al-Showiman, 1990; Meselhy et al., 1994), flavonoids flavonoids,
n.pl common plant pigment compounds that act as antioxidants, enhance the effects of vitamin C, and strengthen connective tissue around capillaries. or galloyl triterpenes appear to be major components in the plants tested positive. In this context, it is interesting to note that flavonoids were repeatedly reported to act as inhibitors of plasmodial growth under in vitro as well as under in vivo conditions (van Baren et al., 2006; Beldjoudi et al., 2003; Andrade-Neto et al., 2004; Andayi et al., 2006; Weniger et al., 2006).
This work was supported by the Deutsche Forschungsgemeinschaft. We also thank Mrs. Ulrike Stier (Bad Soden) and the Hans Fischer Gesellschaft e. V. for generous sponsoring of this research work.
Appendix A. Supplementary materials
Supplementary data associated with this article can be found in the online version at doi:10.1016/j.phymed.2006.12.018.
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J. Kaiser (a,1), M. Yassin (b,1), S. Prakash (c,1), N. Safi (b), M. Agami (d), S. Lauw (a), E. Ostrozhenkova (a), A. Bacher (a), F. Rohdich (a), W. Eisenreich (a,*), J. Safi (b,**), A. Golan-Goldhirsh (c,**)
(a) Lehrstuhl fur Organische Chemie und Biochemie, Technische Universitat Munchen, Lichtenbergstr. 4, D-85747 Garching, Germany
(b) Environmental Protection and Research Institute, Gaza, Palestinian National Authority Noun 1. Palestinian National Authority - combines the Gaza Strip and the West Bank under a political unit with limited autonomy and a police force; created in 1993 by an agreement between Israel and the PLO
Palestine Authority, Palestine National Authority
(c) Albert Katz Department of Dryland Biotechnologies, Ben Gurion University of the Negev, Jacob Blaustein Institutes for Desert Research The Jacob Blaustein Instiutes for Desert Research is part of the Ben-Gurion University, and is located in the Midreshet Ben-Gurion campus in the centre of the Negev desert in Israel. , Sede Boqer Campus, Israel
(d) Institute for Cereal Crops Improvement, Faculty of Life Sciences, Tel Aviv University Tel Aviv University (TAU, אוניברסיטת תל־אביב, את"א) is Israel's largest on-site university. , Israel
Received 28 August 2006; accepted 21 November 2006
* Corresponding author. Tel.: +49 89 289 13336; fax: +49 89 289 13363.
** Also to be corresponded to.
E-mail address: firstname.lastname@example.org (W. Eisenreich).
(1) These authors contributed equally to this work.
Table 1. Plants with strong inhibitory activity against IspC protein from E. coli Relative IspC activity (%) after addition of certain volumes ([micro]l) of plant extract to the assay (a) Species/family 5 [micro]l 2 [micro]l 0.5 [micro]l 0.2 [micro]l Gymnocarpos 9 17 60 83 decandrum (Caryophyllaceae) Helianthemum 4 16 21 49 ventosum (Cistaceae) Helianthemum 8 11 45 58 vesicarium (Cistaceae) Zizphus 18 28 65 66 spina-christi (L.) Desf. (Rhamnaceae) Phoenix 3 10 72 81 dactylifera L. (Palmae) Geranium molle L. 14 20 17 58 (Geraniaceae) Erodium gruinum 16 13 47 63 (L.) L'Her. (Geraniaceae) Cistus 12 16 37 60 salviifolius L. (Cistaceae) Sarcopoterium 19 31 40 56 spinosum (L.) Spach (Rosaceae) Fumana thymifolia 15 34 80 89 (L.) Webb (Cistaceae) Arbutus 11 12 24 34 andrachne L. (Ericaceae) Cercis 4 8 10 26 siliquastrum L. (Caesalpiniaceae) (a) The accuracy of the photometric test was tested with the positive reference compound, fosmidomycin (see Fig. 3A). On this basis, the standard deviations can be estimated to be below 5%.