Orotate phosphoribosyltransferase and orotidine-5'-phosphate decarboxylase in two parasitic kinetoplastid flagellates

Fresh insights into the pyrimidine metabolism in the trypanosomatids

The trypanosomatid parasites continue their killing spree resulting in significant annual mortality due to the lack of effective treatments and the prominence of these diseases in poorer countries. These dimorphic parasites thrive unchecked in the host system, outsmarting the immune mechanisms. An understanding of biology of these parasitic forms will help in the management and elimination of these fatal diseases. Investigation of various metabolic pathways in these parasites has shed light in the understanding of the unique biology of the trypansomatids. An understanding of these pathways have helped in tracing the soft targets in the metabolic pathways, which could be used as effective drug targets which would further impact the therupeutic implications. Pyrimidine pathway is a vital metabolic pathway which yields in the formation of pyrimidines, which are then integrated in nucleic acids (DNA and RNA) in sugars (UDP sugars) and lipids (CDP lipids). A wealth of data and information has been generated in the past decades by in-depth analyses of pyrimidine pathway in the trypanosomatid parasites, which can aid in the identification of anomalies between the parasitic and host counterpart which could be further harnessed to develop therapeutic interventions for the treatment of parasitic diseases. This review presents an updated and comprehensive detailing of the pyrimidine metabolism in the trypansomatids, their uniqueness and their distinctions, and its possible outcomes that would aid in the eradication of these parasitic diseases.

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.

Enzymes of pyrimidine biosynthesis in Crithidia luciliae

Evidence has been obtained for the presence of enzymes of both the de novo and salvage pyrimidine pathways in the protozoan parasite, Crithidia luciliae. Carbamyl phosphate synthetase-II activity could not be unequivocally demonstrated in crude extracts. However, a distinct peak of activity with a molecular weight of approximately 500 000 was observed following chromatography on Sepharose CL-6B. The enzyme preferentially utilised glutamine with respect to ammonia. It was inhibited by UTP and 5-phosphoribosyl-1-diphosphate had a small activating effect. Carbamyl phosphate synthesis by a 'phosphorolytic' citrullinase could not be demonstrated. The ensuing three de novo enzymes could also be separated on Sepharose CL-6B. Approximate molecular weights were estimated: aspartate transcarbamylase (150,000); dihydroorotase (90,000) and dihydroorotate dehydrogenase (70,000). As reported previously, orotate phosphoribosyltransferase and orotidylate decarboxylase were particulate, being associated with the glucosome. Activities of the salvage enzymes, uracil phosphoribosyltransferase, uridine phosphorylase and uridine nucleosidase were observed. All enzymes were cytoplasmic. No uridine kinase activity was detected.

Purine and pyrimidine metabolism in the Trypanosomatidae

The pathways leading to purine and pyrimidine nucleotide production in members of the family Trypanosomatidae are discussed with special emphasis on data relating to pathogenic species published from 1974 to 1983 inclusive. Trypanosomes and leishmania in general lack a de novo purine biosynthetic pathway, but have a multiplicity of possible routes for purine salvage. In contrast, pyrimidine nucleotides can be produced by either de novo or salvage pathways. The properties of these pathways in trypanosomatids are compared and contrasted with those of their hosts.

Orotate phosphoribosyltransferase and orotidylate decarboxylase from Crithidia luciliae: subcellular location of the enzymes and a study of substrate channeling

Orotate phosphoribosyltransferase (OPRTase) and orotidylate decarboxylase (ODCase) have been found to be particulate in the kinetoplastid protozoan, Crithidia luciliae. Sucrose density centrifugation indicated that these two enzymes are associated with the glycosome, a microbody which appears to be unique to the Kinetoplastida and which contains many of the glycolytic enzymes. The particulate location of OPRTase and ODCase was considered to be favorable for channeling of orotidine-5'-monophosphate (OMP), the product of the first enzyme and substrate for the second. The degree of channeling was determined by double radioactively labeled experiments designed to determine the relative efficiency of endogenous and exogenous OMP as substrates of ODCase. The efficiency of channeling was high, with an approximate 50-fold preference for endogenous OMP. By comparison, the degree of channeling for the yeast enzymes, which are soluble and unassociated, was less than 2-fold. The OPRTase-ODCase enzyme complex was solubilized using Triton X-100 in the presence of dimethyl sulfoxide, glycerol, and phosphoribosyldiphosphate. The percentage recovery of the overall enzyme activity was approximately 20%. The degree of channeling was reduced by approximately 10-fold for the solubilized complex. The Km for OMP changed from 7.5 (+/- 1.8) to 1.6 (+/- 0.3) microM in the ODCase reaction. There was no alteration in the Km for orotate in the OPRTase reaction.

Studies on the glycosomal orotate phosphoribosyl transferase of Trypanosoma cruzi

The orotate phosphoribosyltransferase of the epimastigote form of Trypanosoma cruzi was studied in its particulate state in preparations derived from glycosomes. Maximum activity was observed at pH 9. There was little activity in the absence of Mg2+; optimum [Mg2+] was related to [5'-phosphoribosyl-alpha-1-pyrophosphate]; Mn2+ could partially substitute for it. Kinetic analyses ruled out a substituted mechanism and suggested instead that the mechanism may be sequential. The apparent Km orotate was 2 microM; that for 5'-phosphoribosyl-alpha-1-pyrophosphate was 8 microM. The enzyme could not use uracil as substrate and was apparently not regulated by naturally-occurring nucleotides. It was, however, sensitive to inhibition by a wide range of pyrimidine analogues, the most active of which was 5-fluoroorotate. These inhibitors were as effective against the enzyme activity of intact glycosomes as broken preparations. This observation, when considered with an apparent lack of latency, suggests that the enzyme is located on the outside of the glycosome. The product of its reaction, orotidine 5'-phosphate, did not exchange readily with exogenous orotidine 5'-phosphate, suggesting that it is channeled directly to orotidine 5'-phosphate decarboxylase, the next enzyme in the pathway.

UMP synthesis in the kinetoplastida

All six enzymes of pyrimidine biosynthesis de novo have been detected in homogenates of the culture promastigote form of Leishmania mexicana amazonensis, the blood trypomastigote form of Trypanosoma brucei and the culture epimastigote, blood trypomastigote and intracellular amastigote forms of Trypanosoma cruzi. Dihydroorotate dehydrogenase is mitochondrial in mammals, but the isofunctional enzyme, dihydroorotate oxidase was found to be cytoplasmic, whereas orotate phosphoribosyltransferase and orotidine-5'-phosphate decarboxylase, which are cytoplasmic in mammals, were found to be particulate. Analysis by isopycnic sedimentation in sucrose showed that both particulate enzymes co-sedimented with glycosomal-(microbody-)marker enzymes such as hexokinase. Electron microscopy indicated that fractions containing these activities consisted essentially only of microbodies. It is concluded therefore that these enzymes are associated with glycosomes. Kinetic studies with intact glycosomal preparations suggested that there was no membrane barrier between 5-phosphoribose 1-pyrophosphate (P-Rib-PP) and orotate phosphoribosyltransferase, indicating either that the active site of this enzyme is probably on the outside of the glycosome or that the glycosome may have an efficient transport site for P-Rib-PP. Not all the UMP salvage enzymes assayed were detected. No uridine kinase activity was found in any of the species investigated, suggesting that uridine salvage might be routed via a uridine phosphorylase and uracil phosphoribosyltransferase. In agreement with this suggestion, these latter activities were detected in all organisms tested except the intracellular amastigote form of T. cruzi, where uracil phosphoribosyltransferase appeared absent. All the UMP salvage enzymes investigated occurred in cytoplamic fractions.

A comparative study of Leishmania mexicana amastigotes and promastigotes. Enzyme activities and subcellular locations

Leishmania mexicana mexicana amastigotes have been shown to contain greater activities than promastigotes of the enzymes that catalyse the beta-oxidation of fatty acids, but lower activities of several glycolytic enzymes, with the activity of pyruvate kinase being especially low. The results suggest the beta-oxidation of fatty acids is relatively more important to Leishmania amastigotes than promastigotes, whereas the reverse is true for glycolysis. Succinic dehydrogenase and peptidase activities were much higher in promastigotes than amastigotes. The activities of glucose-6-phosphatase, fructose-1,6-bisphosphatase, acid phosphatase and glucose-6-phosphate dehydrogenase varied less, although in each case the activity was significantly lower in the mammalian stage. A method for lysing and fractionating L. m. mexicana promastigotes has been developed. Using this procedure it has been established that many of the glycolytic and functionally related enzymes are located in cell organelles, that hexokinase is intimately connected with the particulate part of the parasite, and that the microsomal fraction of L. m. mexicana is very different in composition from the microsomes of mammalian liver cells.

Purine and pyrimidine salvage pathways in Leishmania donovani

Leishmania donovani, grown in culture, salvaged radiolabeled purine bases which were distributed into adenine and guanine ribonucleotides and into the RNA of these cells. De novo synthesis of purines in L. donovani does not occur [J. J. Marr, R. L. Berens and D. J. Nelson, Biochim. biophys. Acta 544, 360 (1978)]. [8-14C]Adenine was rapidly deaminated to hypoxanthine via the action of an adenine aminohydrolase (EC 3.5.4.2). [8-14C]Guanine was also rapidly deaminated by guanase (EC 3.5.4.3) to form zanthine in these cells. Therefore, the formation of nucleotides of hypoxanthine and xanthine are the first committed steps of purine salvage in L. donovani. While purines are efficiently conserved by this parasite, the salvage of pyrimidines is not so dramatic. [2-14C]Orotic acid was converted to OMP and then incorporated into the pyrimidine nucleotides and into RNA, indicating the existence of the later steps of de novo pyrimidine synthesis. [6-14C]Thymidine was salvaged by L. donovani, being incorporated into the thymine deoxyribonucleotides and into DNA. The major pathway of thymidine metabolism in this parasite, however, was cleavage of the deoxyriboside linkage to form thymine, probably via the action of a thymidine phosphorylase (EC 2.4.2.4).

A quick, simple method for purifying Leishmania mexicana amastigotes in large numbers

A rapid method for the bulk isolation of purified Leishmania mexicana mexicana amastigotes from parasite-induced lesions in experimentally infected mice is described. The procedure includes purification steps based on differences in net cell charge, lysis susceptibility and buoyant density between parasite and host cells. Yields of up to 2 x 10(10) untransformed amastigotes with minimal contamination with host cells and cell debris can be obtained. At least 90% of the purified amastigotes are viable as judged by light and electron microscopy, the staining of their lysosomes with acridine orange, their ability to transform to promastigotes and their infectivity to macrophages in vivo and in vitro.