Orotate phosphoribosyltransferase and orotidine 5'-monophosphate decarboxylase exist as multienzyme complex in human malaria parasite Plasmodium falciparum

In Silico Approach Triterpene Glycoside of H. atra Targeting Orotidine 5-Monophosphate Decarboxylase Protein (PfOMPDC) in P. falciparum Infection Mechanism

This study accessed the potential antimalarial activity of triterpene glycoside of H. atra through targeting orotidine 5-monophosphate decarboxylase protein (PfOMPDC) in P. falciparum by molecular docking. Nine triterpene glycosides from H. atra extract modeled the structure by the Corina web server and interacted with PfOMPDC protein by using Hex 8.0.0. The docking results were visualized and analyzed by Discovery Studio version 21.1.1. 17-Hydroxyfuscocineroside B showed the lowest binding energy in PfOMPDC interaction, which was -1,098.13 kJ/mol. Holothurin A3, echinoside A, and fuscocineroside C showed low binding energy. Nine triterpene glycosides of H. atra performed interaction with PfOMPDC protein at the same region. Holothurin A1 posed interaction with PfOMPDC protein by 8 hydrogen bonds, 3 hydrophobic interactions, and 8 unfavorable bonds. Several residues were detected in the same active sites of other triterpene glycosides. Residue TYR111 was identified in all triterpene glycoside complexes, except holothurin A3 and calcigeroside B. In summary, the triterpene glycoside of H. atra is potentially a drug candidate for malaria therapeutic agents. In vitro and in vivo studies were required for further investigation.

Targeting Kinetoplastid and Apicomplexan Thymidylate Biosynthesis as an Antiprotozoal Strategy

Kinetoplastid and apicomplexan parasites comprise a group of protozoans responsible for human diseases, with a serious impact on human health and the socioeconomic growth of developing countries. Chemotherapy is the main option to control these pathogenic organisms and nucleotide metabolism is considered a promising area for the provision of antimicrobial therapeutic targets. Impairment of thymidylate (dTMP) biosynthesis severely diminishes the viability of parasitic protozoa and the absence of enzymatic activities specifically involved in the formation of dTMP (e.g. dUTPase, thymidylate synthase, dihydrofolate reductase or thymidine kinase) results in decreased deoxythymidine triphosphate (dTTP) levels and the so-called thymineless death. In this process, the ratio of deoxyuridine triphosphate (dUTP) versus dTTP in the cellular nucleotide pool has a crucial role. A high dUTP/dTTP ratio leads to uracil misincorporation into DNA, the activation of DNA repair pathways, DNA fragmentation and eventually cell death. The essential character of dTMP synthesis has stimulated interest in the identification and development of drugs that specifically block the biochemical steps involved in thymine nucleotide formation. Here, we review the available literature in relation to drug discovery studies targeting thymidylate biosynthesis in kinetoplastid (genera Trypanosoma and Leishmania) and apicomplexan (Plasmodium spp and Toxoplasma gondii) protozoans. The most relevant findings concerning novel inhibitory molecules with antiparasitic activity against these human pathogens are presented herein.

Pyrimidine metabolism in schistosomes: A comparison with other parasites and the search for potential chemotherapeutic targets

Schistosomes are responsible for the parasitic disease schistosomiasis, an acute and chronic parasitic ailment that affects >240 million people in 70 countries worldwide. It is the second most devastating parasitic disease after malaria. At least 200,000 deaths per year are associated with the disease. In the absence of the availability of vaccines, chemotherapy is the main stay for combating schistosomiasis. The antischistosomal arsenal is currently limited to a single drug, Praziquantel, which is quite effective with a single-day treatment and virtually no host-toxicity. Recently, however, the question of reduced activity of Praziquantel has been raised. Therefore, the search for alternative antischistosomal drugs merits the study of new approaches of chemotherapy. The rational design of a drug is usually based on biochemical and physiological differences between pathogens and host. Pyrimidine metabolism is an excellent target for such studies. Schistosomes, unlike most of the host tissues, require a very active pyrimidine metabolism for the synthesis of DNA and RNA. This is essential for the production of the enormous numbers of eggs deposited daily by the parasite to which the granulomas response precipitates the pathogenesis of schistosomiasis. Furthermore, there are sufficient differences between corresponding enzymes of pyrimidine metabolism from the host and the parasite that can be exploited to design specific inhibitors or "subversive substrates" for the parasitic enzymes. Specificities of pyrimidine transport also diverge significantly between parasites and their mammalian host. This review deals with studies on pyrimidine metabolism in schistosomes and highlights the unique characteristic of this metabolism that could constitute excellent potential targets for the design of safe and effective antischistosomal drugs. In addition, pyrimidine metabolism in schistosomes is compared with that in other parasites where studies on pyrimidine metabolism have been more elaborate, in the hope of providing leads on how to identify likely chemotherapeutic targets which have not been looked at in schistosomes.

Bifunctional activity of fused Plasmodium falciparum orotate phosphoribosyltransferase and orotidine 5'-monophosphate decarboxylase

Fusion of the last two enzymes in the pyrimidine biosynthetic pathway in the inversed order by having a COOH-terminal orotate phosphoribosyltransferase (OPRT) and an NH₂-terminal orotidine 5'-monophosphate decarboxylase (OMPDC), as OMPDC-OPRT, are described in many organisms. Here, we produced gene fusions of Plasmodium falciparum OMPDC-OPRT and expressed the bifunctional protein in Escherichia coli. The enzyme was purified to homogeneity using affinity and anion-exchange chromatography, exhibited enzymatic activities and functioned as a dimer. The activities, although unstable, were stabilized by its substrate and product during purification and long-term storage. Furthermore, the enzyme expressed a perfect catalytic efficiency (k_cat/K_m). The k_cat was selectively enhanced up to three orders of magnitude, while the K_m was not much affected and remained at low μM levels when compared to the monofunctional enzymes. The fusion of the two enzymes, creating a "super-enzyme" with perfect catalytic power and more flexibility, reflects cryptic relationship of enzymatic reactivities and metabolic functions on molecular evolution.

Insights into the pyrimidine biosynthetic pathway of human malaria parasite Plasmodium falciparum as chemotherapeutic target

Malaria is a major cause of morbidity and mortality in humans. Artemisinins remain as the first-line treatment for Plasmodium falciparum (P. falciparum) malaria although drug resistance has already emerged and spread in Southeast Asia. Thus, to fight this disease, there is an urgent need to develop new antimalarial drugs for malaria chemotherapy. Unlike human host cells, P. falciparum cannot salvage preformed pyrimidine bases or nucleosides from the extracellular environment and relies solely on nucleotides synthesized through the de novo biosynthetic pathway. This review presents significant progress on understanding the de novo pyrimidine pathway and the functional enzymes in the human parasite P. falciparum. Current knowledge in genomics and metabolomics are described, particularly focusing on the parasite purine and pyrimidine nucleotide metabolism. These include gene annotation, characterization and molecular mechanism of the enzymes that are different from the human host pathway. Recent elucidation of the three-dimensional crystal structures and the catalytic reactions of three enzymes: dihydroorotate dehydrogenase, orotate phosphoribosyltransferase, and orotidine 5'-monophosphate decarboxylase, as well as their inhibitors are reviewed in the context of their therapeutic potential against malaria.

Structure of Plasmodium falciparum orotate phosphoribosyltransferase with autologous inhibitory protein-protein interactions

The most severe form of malaria is caused by the obligate parasite Plasmodium falciparum. Orotate phosphoribosyltransferase (OPRTase) is the fifth enzyme in the de novo pyrimidine-synthesis pathway in the parasite, which lacks salvage pathways. Among all of the malaria de novo pyrimidine-biosynthesis enzymes, the structure of P. falciparum OPRTase (PfOPRTase) was the only one unavailable until now. PfOPRTase that could be crystallized was obtained after some low-complexity sequences were removed. Four catalytic dimers were seen in the asymmetic unit (a total of eight polypeptides). In addition to revealing unique amino acids in the PfOPRTase active sites, asymmetric dimers in the larger structure pointed to novel parasite-specific protein-protein interactions that occlude the catalytic active sites. The latter could potentially modulate PfOPRTase activity in parasites and possibly provide new insights for blocking PfOPRTase functions.

Malaria parasite carbonic anhydrase: inhibition of aromatic/heterocyclic sulfonamides and its therapeutic potential

Plasmodium falciparum (P. falciparum) is responsible for the majority of life-threatening cases of human malaria, causing 1.5-2.7 million annual deaths. The global emergence of drug-resistant malaria parasites necessitates identification and characterization of novel drug targets and their potential inhibitors. We identified the carbonic anhydrase (CA) genes in P. falciparum. The pfCA gene encodes anα-carbonic anhydrase, a Zn(2+)-metalloenzme, possessing catalytic properties distinct from that of the human host CA enzyme. The amino acid sequence of the pfCA enzyme is different from the analogous protozoan and human enzymes. A library of aromatic/heterocyclic sulfonamides possessing a large diversity of scaffolds were found to be very good inhibitors for the malarial enzyme at moderate-low micromolar and submicromolar inhibitions. The structure of the groups substituting the aromatic-ureido- or aromatic-azomethine fragment of the molecule and the length of the parent sulfonamide were critical parameters for the inhibitory properties of the sulfonamides. One derivative, that is, 4- (3, 4-dichlorophenylureido)thioureido-benzenesulfonamide (compound 10) was the most effective in vitro Plasmodium falciparum CA inhibitor, and was also the most effective antimalarial compound on the in vitro P. falciparum growth inhibition. The compound 10 was also effective in vivo antimalarial agent in mice infected with Plasmodium berghei, an animal model of drug testing for human malaria infection. It is therefore concluded that the sulphonamide inhibitors targeting the parasite CA may have potential for the development of novel therapies against human malaria.

A unique insertion of low complexity amino acid sequence underlies protein-protein interaction in human malaria parasite orotate phosphoribosyltransferase and orotidine 5'-monophosphate decarboxylase

OBJECTIVE

To investigate the multienzyme complex formation of human malaria parasite Plasmodium falciparum (P. falciparum) orotate phosphoribosyltransferase (OPRT) and orotidine 5'-monophosphate decarboxylase (OMPDC), the fifth and sixth enzyme of the de novo pyrimidine biosynthetic pathway. Previously, we have clearly established that the two enzymes in the malaria parasite exist physically as a heterotetrameric (OPRT)2(OMPDC)2 complex containing two subunits each of OPRT and OMPDC, and that the complex have catalytic kinetic advantages over the monofunctional enzyme.

METHODS

Both enzymes were cloned and expressed as recombinant proteins. The protein-protein interaction in the enzyme complex was identified using bifunctional chemical cross-linker, liquid chromatography-mass spectrometric analysis and homology modeling.

RESULTS

The unique insertions of low complexity region at the α 2 and α 5 helices of the parasite OMPDC, characterized by single amino acid repeat sequence which was not found in homologous proteins from other organisms, was located on the OPRT-OMPDC interface. The structural models for the protein-protein interaction of the heterotetrameric (OPRT)2(OMPDC)2 multienzyme complex were proposed.

CONCLUSIONS

Based on the proteomic data and structural modeling, it is surmised that the human malaria parasite low complexity region is responsible for the OPRT-OMPDC interaction. The structural complex of the parasite enzymes, thus, represents an efficient functional kinetic advantage, which in line with co-localization principles of evolutional origin, and allosteric control in protein-protein-interactions.

Transition-state inhibitors of purine salvage and other prospective enzyme targets in malaria

Malaria is a leading cause of human death within the tropics. The gradual generation of drug resistance imposes an urgent need for the development of new and selective antimalarial agents. Kinetic isotope effects coupled to computational chemistry have provided the relevant details on geometry and charge of enzymatic transition states to facilitate the design of transition-state analogs. These features have been reproduced into chemically stable mimics through synthetic chemistry, generating inhibitors with dissociation constants in the pico- to femto-molar range. Transition-state analogs are expected to contribute to the control of malaria.

Expression of functional Plasmodium falciparum enzymes using a wheat germ cell-free system

One decade after the sequencing of the Plasmodium falciparum genome, 95% of malaria proteins in the genome cannot be expressed in traditional cell-based expression systems, and the targets of the best new leads for antimalarial drug discovery are either not known or not available in functional form. For a disease that kills up to 1 million people per year, routine expression of recombinant malaria proteins in functional form is needed both for the discovery of new therapeutics and for identification of targets of new drugs. We tested the general utility of cell-free systems for expressing malaria enzymes. Thirteen test enzyme sequences were reverse amplified from total RNA, cloned into a plant-like expression vector, and subjected to cell-free expression in a wheat germ system. Protein electrophoresis and autoradiography confirmed the synthesis of products of expected molecular masses. In rare problematic cases, truncated products were avoided by using synthetic genes carrying wheat codons. Scaled-up production generated 39 to 354 μg of soluble protein per 10 mg of translation lysate. Compared to rare proteins where cell-based systems do produce functional proteins, the cell-free yields are comparable or better. All 13 test products were enzymatically active, without failure. This general path to produce functional malaria proteins should now allow the community to access new tools, such as biologically active protein arrays, and lead to the discovery of new chemical functions, structures, and inhibitors of previously inaccessible malaria gene products.

The in silico screening and X-ray structure analysis of the inhibitor complex of Plasmodium falciparum orotidine 5'-monophosphate decarboxylase

Orotidine 5'-monophosphate decarboxylase from Plasmodium falciparum (PfOMPDC) catalyses the final step in the de novo synthesis of uridine 5'-monophosphate (UMP) from orotidine 5'-monophosphate (OMP). A defective PfOMPDC enzyme is lethal to the parasite. Novel in silico screening methods were performed to select 14 inhibitors against PfOMPDC, with a high hit rate of 9%. X-ray structure analysis of PfOMPDC in complex with one of the inhibitors, 4-(2-hydroxy-4-methoxyphenyl)-4-oxobutanoic acid, was carried out to at 2.1 Å resolution. The crystal structure revealed that the inhibitor molecule occupied a part of the active site that overlaps with the phosphate-binding region in the OMP- or UMP-bound complexes. Space occupied by the pyrimidine and ribose rings of OMP or UMP was not occupied by this inhibitor. The carboxyl group of the inhibitor caused a dramatic movement of the L1 and L2 loops that play a role in the recognition of the substrate and product molecules. Combining part of the inhibitor molecule with moieties of the pyrimidine and ribose rings of OMP and UMP represents a suitable avenue for further development of anti-malarial drugs.

Crystallization and preliminary X-ray diffraction analysis of orotate phosphoribosyltransferase from the human malaria parasite Plasmodium falciparum

Orotate phosphoribosyltransferase (OPRT) catalyzes the Mg(2+)-dependent condensation of orotic acid (OA) with 5-α-D-phosphorylribose 1-diphosphate (PRPP) to yield diphosphate (PP(i)) and the nucleotide orotidine 5'-monophosphate. OPRT from Plasmodium falciparum produced in Escherichia coli was crystallized by the sitting-drop vapour-diffusion method in complex with OA and PRPP in the presence of Mg(2+). The crystal exhibited tetragonal symmetry, belonging to space group P4(1) or P4(3), with unit-cell parameters a = b = 49.15, c = 226.94 Å. X-ray diffraction data were collected to 2.5 Å resolution at 100 K using a synchrotron-radiation source.

Molecular, kinetic and thermodynamic characterization of Mycobacterium tuberculosis orotate phosphoribosyltransferase

Tuberculosis (TB) is a chronic infectious disease caused mainly by Mycobacterium tuberculosis. The worldwide emergence of drug-resistant strains, the increasing number of infected patients among immune compromised populations, and the large number of latent infected individuals that are reservoir to the disease have underscored the urgent need of new strategies to treat TB. The nucleotide metabolism pathways provide promising molecular targets for the development of novel drugs against active TB and may, hopefully, also be effective against latent forms of the pathogen. The orotate phosphoribosyltransferase (OPRT) enzyme of the de novo pyrimidine synthesis pathway catalyzes the reversible phosphoribosyl transfer from 5'-phospho-α-D-ribose 1'-diphosphate (PRPP) to orotic acid (OA), forming pyrophosphate and orotidine 5'-monophosphate (OMP). Here we describe cloning and characterization of pyrE-encoded protein of M. tuberculosis H37Rv strain as a homodimeric functional OPRT enzyme. The M. tuberculosis OPRT true kinetic constants for forward reaction and product inhibition results suggest a Mono-Iso Ordered Bi-Bi kinetic mechanism, which has not been previously described for this enzyme family. Absence of detection of half reaction and isothermal titration calorimetry (ITC) data support the proposed mechanism. ITC data also provided thermodynamic signatures of non-covalent interactions between substrate/product and M. tuberculosis OPRT. These data provide a solid foundation on which to base target-based rational design of anti-TB agents and should inform us how to better design inhibitors of M. tuberculosis OPRT.

Structural determinants for the inhibitory ligands of orotidine-5'-monophosphate decarboxylase

In recent years, orotidine-5'-monophosphate decarboxylase (ODCase) has gained renewed attention as a drug target. As a part of continuing efforts to design novel inhibitors of ODCase, we undertook a comprehensive study of potent, structurally diverse ligands of ODCase and analyzed their structural interactions in the active site of ODCase. These ligands comprise of pyrazole or pyrimidine nucleotides including the mononucleotide derivatives of pyrazofurin, barbiturate ribonucleoside, and 5-cyanouridine, as well as, in a computational approach, 1,4-dihydropyridine-based non-nucleoside inhibitors such as nifedipine and nimodipine. All these ligands bind in the active site of ODCase exhibiting distinct interactions paving the way to design novel inhibitors against this interesting enzyme. We propose an empirical model for the ligand structure for rational modifications in new drug design and potentially new lead structures.

Co-expression of human malaria parasite Plasmodium falciparum orotate phosphoribosyltransferase and orotidine 5’-monophosphate decarboxylase as enzyme complex in Escherichia coli: a novel strategy for drug development

Background: Human malaria parasite Plasmodium falciparum operates de novo pyrimidine biosynthetic pathway. The fifth and sixth enzymes of the pathway form a heterotetrameric complex, containing two molecules each of orotate phosphoribosyltransferase (OPRT) and orotidine 5’-monophosphate decarboxylase (OMPDC). Objective: Define the function of OPRT-OMPDC enzyme complex of P. falciparum by co-expressing the enzymes in Escherichia coli. Methods: The constructed plasmids containing either P. falciparum OPRT or OMPDC were cloned in E. coli by co-transformation. Both genes were co-expressed as OPRT-OMPDC enzyme complex and the complex was purified by chromatographic techniques, including N2+-NTA affinity, Hi Trap Q HP anion-exchange, uridine 5’- monophosphate affinity, and Superose 12 gel-filtration columns. Physical and kinetic properties of the enzyme complex were analyzed for its molecular mass. Results: Co-transformation of PfOPRT and PfOMPDC plasmids in E. coli were achieved with a clone containing DNA ratio of 1:2, respectively. Both plasmids remained stable and were functionally expressed in the E. coli cell for at least 20 weeks. The P. falciparum OPRT-OMPDC enzyme complex were co-expressed and the complex was co-eluted in all chromatographic columns during purification and physical analysis. The molecular mass of the complex was 130 kDa, whereas the PfOPRT and PfOMPDC component were 35.6 and 41.5 kDa, respectively. The enzymatic activities of the complex were competitively inhibited by their products of each enzyme component. Conclusion: P. falciparum OPRT and OMPDC in E. coli as an enzyme complex were co-transformed and functionally co-expressed. These have similar properties to the native enzyme purified directly from P. falciparum, and this character is different from that of the human host organism. The enzyme complex would be suitable as new target to research selective inhibitors as suitable drugs to better control this disease.

Kinetic benefits and thermal stability of orotate phosphoribosyltransferase and orotidine 5'-monophosphate decarboxylase enzyme complex in human malaria parasite Plasmodium falciparum

We have previously shown that orotate phosphoribosyltransferase (OPRT) and orotidine 5'-monophosphate decarboxylase (OMPDC) in human malaria parasite Plasmodium falciparum form an enzyme complex, containing two subunits each of OPRT and OMPDC. To enable further characterization, we expressed and purified P. falciparum OPRT-OMPDC enzyme complex in Escherichia coli. The OPRT and OMPDC activities of the enzyme complex co-eluted in the chromatographic columns used during purification. Kinetic parameters (K(m), k(cat) and k(cat)/K(m)) of the enzyme complex were 5- to 125-folds higher compared to the monofunctional enzyme. Interestingly, pyrophosphate was a potent inhibitor to the enzyme complex, but had a slightly inhibitory effect for the monofunctional enzyme. The enzyme complex resisted thermal inactivation at higher temperature than the monofunctional OPRT and OMPDC. The result suggests that the OPRT-OMPDC enzyme complex might have kinetic benefits and thermal stability significantly different from the monofunctional enzyme.

Functional analysis of pyrimidine biosynthesis enzymes using the anticancer drug 5-fluorouracil in Caenorhabditis elegans

Pyrimidine biosynthesis enzymes function in many cellular processes and are closely associated with pyrimidine antagonists used in cancer chemotherapy. These enzymes are well characterized from bacteria to mammals, but not in a simple metazoan. To study the pyrimidine biosynthesis pathway in Caenorhabditis elegans, we screened for mutants exhibiting resistance to the anticancer drug 5-fluorouracil (5-FU). In several strains, mutations were identified in ZK783.2, the worm homolog of human uridine phosphorylase (UP). UP is a member of the pyrimidine biosynthesis family of enzymes and is a key regulator of uridine homeostasis. C. elegans UP homologous protein (UPP-1) exhibited both uridine and thymidine phosphorylase activity in vitro. Knockdown of other pyrimidine biosynthesis enzyme homologs, such as uridine monophosphate kinase and uridine monophosphate synthetase, also resulted in 5-FU resistance. Uridine monophosphate kinase and uridine monophosphate synthetase proteins are redundant, and show different, tissue-specific expression patterns in C. elegans. Whereas pyrimidine biosynthesis pathways are highly conserved between worms and humans, no human thymidine phosphorylase homolog has been identified in C. elegans. UPP-1 functions as a key regulator of the pyrimidine salvage pathway in C. elegans, as mutation of upp-1 results in strong 5-FU resistance.

Structure and inhibition of orotidine 5'-monophosphate decarboxylase from Plasmodium falciparum

Orotidine 5'-monophosphate (OMP) decarboxylase from Plasmodium falciparum (PfODCase, EC 4.1.1.23) has been overexpressed, purified, subjected to kinetic and biochemical analysis, and crystallized. The native enzyme is a homodimer with a subunit molecular mass of 38 kDa. The saturation curve for OMP as a substrate conformed to Michaelis-Menten kinetics with K m = 350 +/- 60 nM and V max = 2.70 +/- 0.10 micromol/min/mg protein. Inhibition patterns for nucleoside 5'-monophosphate analogues were linear competitive with respect to OMP with a decreasing potency of inhibition of PfODCase in the order: pyrazofurin 5'-monophosphate ( K i = 3.6 +/- 0.7 nM) > xanthosine 5'-monophosphate (XMP, K i = 4.4 +/- 0.7 nM) > 6-azauridine 5'-monophosphate (AzaUMP, K i = 12 +/- 3 nM) > allopurinol-3-riboside 5'-monophosphate ( K i = 240 +/- 20 nM). XMP is an approximately 150-fold more potent inhibitor of PfODCase compared with the human enzyme. The structure of PfODCase was solved in the absence of ligand and displays a classic TIM-barrel fold characteristic of the enzyme. Both the phosphate-binding loop and the betaalpha5-loop have conformational flexibility, which may be associated with substrate capture and product release along the reaction pathway.

Targeting purine and pyrimidine metabolism in human apicomplexan parasites

Synthesis de novo, acquisition by salvage and interconversion of purines and pyrimidines represent the fundamental requirements for their eventual assembly into nucleic acids as nucleotides and the deployment of their derivatives in other biochemical pathways. A small number of drugs targeted to nucleotide metabolism, by virtue of their effect on folate biosynthesis and recycling, have been successfully used against apicomplexan parasites such as Plasmodium and Toxoplasma for many years, although resistance is now a major problem in the prevention and treatment of malaria. Many targets not involving folate metabolism have also been explored at the experimental level. However, the unravelling of the genome sequences of these eukaryotic unicellular organisms, together with increasingly sophisticated molecular analyses, opens up possibilities of introducing new drugs that could interfere with these processes. This review examines the status of established drugs of this type and the potential for further exploiting the vulnerability of apicomplexan human pathogens to inhibition of this key area of metabolism.

Orotate phosphoribosyl transferase mRNA expression in oral squamous cell carcinoma and its relationship with the dihydropyrimidine dehydrogenase expression and the clinical effect of 5-fluorouracil

In this study, we investigated whether orotate phosphoribosyl transferase (OPRT) correlates with the clinicopathological features and effect of 5-fluorouracil (5-FU) in human oral carcinoma. We examined the expression of OPRT mRNA by in situ hybridization in surgical specimens of oral squamous cell carcinoma. The expression of OPRT mRNA in oral carcinoma was observed in all specimens and such expression was higher than that seen in normal control tissue specimens. There was no correlation between the expression of OPRT mRNA and clinical factors, but the expression of OPRT mRNA was significantly associated with histological differentiation. The expression of OPRT mRNA showed correlation with effect of 5-FU for oral carcinoma in either in vivo or in vitro. These results suggest that the OPRT expressions may therefore be a prognostic factor of 5-FU efficacy in patients with oral squamous cell carcinoma.

Crystallization and preliminary crystallographic analysis of orotidine 5'-monophosphate decarboxylase from the human malaria parasite Plasmodium falciparum

Orotidine 5'-monophosphate (OMP) decarboxylase (OMPDC; EC 4.1.1.23) catalyzes the final step in the de novo synthesis of uridine 5'-monophosphate (UMP) and defects in the enzyme are lethal in the malaria parasite Plasmodium falciparum. Active recombinant P. falciparum OMPDC (PfOMPDC) was crystallized by the seeding method in a hanging drop using PEG 3000 as a precipitant. A complete set of diffraction data from a native crystal was collected to 2.7 A resolution at 100 K using synchrotron radiation at the Swiss Light Source. The crystal exhibits trigonal symmetry (space group R3), with hexagonal unit-cell parameters a = b = 201.81, c = 44.03 A. With a dimer in the asymmetric unit, the solvent content is 46% (V(M) = 2.3 A3 Da(-1)).