Purine and pyrimidine metabolism in the Trypanosomatidae

Isolation and characterization of the gene encoding the surface membrane 3'-nucleotidase/nuclease of Leishmania donovani

Leishmania donovani and related trypanosomatid protozoa possess an externally oriented surface membrane enzyme capable of hydrolyzing both 3'-nucleotides and nucleic acids. By virtue of these activities, this 3'-nucleotidase/nuclease (3'-NT/Nu), previously shown to be analogous to fungal and plant class-I single-strand-specific nucleases, is thought to play a critical role in the salvage of purines, essential for the survival of these organisms. The 43-kDa 3'-NT/Nu was purified from L. donovani promastigotes and trypsin treated. Four of the released tryptic peptide fragments yielded amino-acid sequence information (Pept-1 to Pept-4) which provided the basis for the preparation of oligonucleotide primers used for PCR amplification of an approx. 300-bp DNA fragment. This fragment was cloned, sequenced and used to probe a genomic L. donovani cosmid library. Nucleotide sequence analysis of a 4.5-kb SmaI fragment, isolated from a cosmid clone, revealed an open reading frame (ORF) of 1434 nt encoding a 477-amino-acid protein. Pept-1 to Pept-4 were mapped onto the ORF-deduced protein sequence. Peptides corresponding to Pept-1 to Pept-4 were synthesized and used to immunize rabbits. The resulting anti-peptide antibodies recognized the 43-kDa protein on Western blots and immunoprecipitated the native 3'-nucleotidase activity from L. donovani membrane extracts. Further, the ORF-deduced protein shared significant sequence identity with the S1 and P1 fungal nucleases of Aspergillus oryzae and Penicillium citrinum, respectively. Cumulatively, these results demonstrated that the ORF corresponded to a gene for the L. donovani 3'-nucleotidase/nuclease. In Northern blots a nucleotide probe specific for the 3'-NT/Nu gene hybridized to a single 2.5-kb messenger RNA. Results of Southern blot analyses were consistent with the 3'-NT/Nu being encoded by a single copy gene. These data constitute the first report of the gene for this unique trypanosomatid surface membrane enzyme.

Purification and characterization of a soluble nucleoside diphosphate kinase in Trypanosoma cruzi

A soluble nucleoside diphosphate kinase (NDP kinase) was purified and characterized in epimastigote forms of Trypanosoma cruzi. The enzyme was purified by affinity chromatography on Blue-agarose and Q-Sepharose columns and by FPLC on a Superose 12 column. A membrane-associated NDP kinase was identified which accounts for 30% of total enzymatic activity. Western blot analysis of the soluble NDP kinase revealed a 16.5-kDa monomer recognized by polyclonal antibodies to NDP kinase from Dictyostelium discoideum, Candida albicans or human. Most of the T. cruzi NDP kinase is found in the cell as a hexamer composed of 16.5-kDa monomers. The Km values of the enzyme for ATP, GDP and dTDP were 0.2 +/- 0.008 mM, 0.125 +/- 0.012 mM and 0.4 +/- 0.009 mM, respectively. The parasite enzyme was stable, remained active at 65 degrees C and was found to tolerate up to 2.5 M urea. The 16.5-kDa subunit was phosphorylated with [gamma-32P]ATP or thiophosphorylated with [35S]GTP gamma S. The incubation of the 32P-labelled phosphoenzyme with unlabelled nucleoside 5'-diphosphates resulted in the formation of 32P-labelled nucleoside 5'-triphosphates without strict base specificity, indicating that the reaction mechanism of the T. cruzi enzyme is the same as reported for other NDP kinases. When the phosphoenzyme was incubated with a mixture of nucleoside 5'-diphosphates, GTP was preferentially formed.

Adenosine analogues as antimetabolites against Plasmodium falciparum malaria

Analogues of purine nucleosides and deoxynucleosides were tested for toxicity against the intraerythrocytic parasite Plasmodium falciparum in vitro culture. Sangivamycin (7-deaza-7-amido-adenosine) (IC37 of 0.3 microM), tubercidin (7-deaza-adenosine) (IC37 of 0.7 microM), 6-methylamino-deoxyadenosine (IC37 of 10 microM), 8-aza-2-amino-deoxy-adenosine (IC37 of 11 microM) and 2-chloro-adenosine (IC37 of 11 microM) were found to be the most toxic towards the parasite. Structure-activity analysis suggested that alteration of the purine ring at the 7 or 8 position significantly increased the toxicity of the compound against P. falciparum. Analysis by HPLC of parasite lysates which had been subjected to the cytotoxic compounds confirmed that alterations in the flux of the purine salvage pathways of the parasite had occurred. Comparison of the toxicity of these compounds against P. falciparum with the toxicity against a similar intraerythrocytic parasite, Babesia bovis, or human melanoma cell lines indicated a differential toxicity, in that many of the compounds toxic towards P. falciparum were relatively non-toxic towards human melanoma cell lines or B. bovis and vice versa. The mechanism of toxicity of the deoxyadenosine and adenosine analogues, whose normal metabolism involves transport, metabolism and incorporation into nucleic acids appears to vary significantly between P. falciparum, B. bovis and mammalian cells.

Nucleoside transport in Crithidia luciliae

Nucleoside transport was evaluated in the trypanosomatid Crithidia luciliae by a rapid sampling technique. C. luciliae was shown to possess two independent nucleoside transporters, one which transported adenosine, deoxyadenosine, tubercidin, sangivamycin and the pyrimidine nucleoside thymidine, while the second was specific for guanosine, inosine and deoxyguanosine. The rapid influx occurred by a process of facilitated transport. The apparent Km values for adenosine and guanosine were 9.34 +/- 1.30 and 10.6 +/- 2.60 microM, respectively. The pyrimidine nucleoside thymidine was transported at a rate approximately 50% lower than the purine nucleosides, whilst uridine, deoxyuridine and deoxycytidine were not transported. The optical isomer, L-adenosine entered the organism by simple diffusion rather than by facilitated transport. In contrast to mammalian cells, neither of the nucleoside transporters in C. luciliae were inhibited by nitrobenzylthioinosine, dilazep, or dipyridamole, potent inhibitors of nucleoside transport in mammalian cells, whilst p-chloromercuribenzoate sulphonate inhibited both nucleoside transporters in C. luciliae.

Expression of a channel-like pathway for adenosine transport in Leishmania donovani promastigotes

L. donovani promastigotes (MHOM/ET/67/HA3) transport adenosine by a route that is sensitive to inhibition by a nonspecific channel blocker, propranolol. At the logarithmic and stationary phases of growth, the transport of 1 microM-3H-adenosine was significantly inhibited (40-50%) by 100 microM-propranolol. In contrast, a strain of Leishmania donovani promastigotes clonally selected to grow in defined medium was only slightly (approximately 10%) inhibited at the logarithmic but not at the stationary phase. These results suggest that the differences in expression of adenosine in the parasites previously reported, may be related to uptake by a channel-like pathway in the promastigotes.

Pathways of pyrimidine nucleotide biosynthesis in gravid Angiostrongylus cantonensis

The incorporation of radioactive precursors into pyrimidine nucleotides via de novo and salvage pathways was measured in gravid Angiostrongylus cantonensis by HPLC and thin-layer chromatography. 14C-labelled orotate, uridine, uracil and deoxyuridine were traced to UMP, UDP, UTP, UDP-glucose, dTMP, CMP, CDP and CTP. 3H-labelled cytidine was also incorporated into both uracil and cytosine nucleotides in a ratio of 2:1. Cytosine was a major end-product for all the precursors. Cytosine nucleotides were probably formed from UTP by the action of CTP synthetase whose activity in crude cell-free extract was 31.5 +/- 4.9 pmol min-1 (mg protein)-1. It was dependent on glutamine, ATP and GTP and was inhibited by CTP. The total amount of pyrimidine nucleotides formed from uridine was 3 times of that from uracil. The presence of uracil in the metabolism of uridine indicates that UMP is formed by uracil phosphoribosyltransferase as well as by uridine kinase. UMP is a key intermediate for cytidylate and thymidylate biosynthesis in the gravid worms.

Salvage and interconversion of purines in developing Artemia

Incorporation of the radiolabelled purine bases adenine, guanine and hypoxanthine into acid soluble fraction, RNA and DNA nucleotides during the early larval development of Artemia sp. was studied. Adenine was the best precursor and guanine the poorest. The adenine phosphoribosyltransferase (APRT) activity was considerably higher than that of hypoxanthine-guanine phosphoribosyltransferase (HGPRT) and these activities did not significantly change throughout larval development. The pattern of purine interconversion was dependent on naupliar age. Conversion of [14C]adenine and [14C]hypoxanthine into guanine nucleotides increased with time of development. However, the conversion of [14C]guanine into [14C]adenine nucleotides was very low.

Arsenical-resistant trypanosomes lack an unusual adenosine transporter

The melaminophenyl arsenical melarsoprol is still used to treat African sleeping sickness, a disease caused by parasitic protozoa of the Trypanosoma brucei subgroup. Based on the observation that melamine antagonizes the trypanocidal activity of this class of drugs, we investigated whether other physiological compounds could compete for the same receptor. Here we report that the in vitro trypanolytic effect of melarsen oxide can be specifically abrogated by adenine, adenosine and dipyridamole, all of which compete for uptake by an adenosine transporter. Melarsen-sensitive trypanosomes have two high-affinity adenosine transport systems: a P1 type, which also transports inosine; and a P2 type, which also transports adenine and the melaminophenyl arsenicals. Melarsen-resistant trypanosomes lack P2 adenosine transport, suggesting that resistance to these arsenicals is due to loss of uptake.

Molecular biology studies of tubercidin resistance in Trypanosoma cruzi

Trypanosomatids are incapable of de novo purine synthesis; purines are obtained through the scavenging of exogenous nucleosides. To advance our understanding of purine utilization, we mutagenized a Trypanosoma cruzi stock and selected for resistance to high levels of tubercidin (7-deazaadenosine, TUB), a purine analog. The TUB-resistant stocks were > 100 times more resistant to TUB than was the parental stock. TUB and uridine transport in the TUB-resistant stocks decreased by 50%-90%, whereas thymidine and adenosine transport were unaffected. These data imply that TUB-resistant stocks have defects in the pathways involved in the transport of TUB and uridine but not in the thymidine and adenosine transport pathways. Karyotype analyses using specific probes showed that the deletion of a 950-kb chromosome-size DNA occurred in both of the TUB-resistant stocks. These data suggest that genes involved in nucleoside transport are located in this DNA region. This study will facilitate the identification and characterization of the specific genes involved in nucleoside transport and aid in the elucidation and development of new chemotherapeutics for Chagas' disease.

Purification and properties of 3'-nucleotidase of Leishmania donovani

A surface membrane 3'-nucleotidase from Leishmania donovani promastigotes has been purified to SDS/PAGE homogeneity. The enzyme has apparent subunit molecular mass of 38 kDa, pI 5.8 and a broad pH optimum, 5.5-7.5. EDTA partially inhibited the enzyme activity, which was fully restored by Co2+; Mg2+, Ca2+ or Mn2+ had no effect on the activity. ZnCl2 or dithiothreitol at 1 mM was inhibitory at pH 7.5, but was without effect at pH 5.5, whereas at both pH values 5 mM of either compound inhibited the enzyme. The substrate-specificity of the purified enzyme is restricted to ribonucleoside 3'-phosphates. 3'-AMP and 3'-IMP are the best substrates, whereas ADP, ATP, 2'-deoxyadenosine 3'-phosphate and 5'-AMP are competitive inhibitors of the enzyme. The enzyme showed low latency in intact-cell preparations. The kinetic properties and the surface membrane localization of the enzyme suggest its implication in the formation of nucleosides from 3'-nucleotides of the parasite's host.

Precursors of pyrimidine nucleotide biosynthesis for gravid Angiostrongylus cantonensis (Nematoda: Metastrongyloidea)

Gravid Angiostrongylus cantonensis can utilize radiolabelled bicarbonate, orotate, uracil, uridine and cytidine but not cytosine, thymine and thymidine for the synthesis of RNA and DNA. In cell-free extracts of the worm, a phosphoribosyltransferase was shown to convert orotate to OMP and uracil to UMP. A similar reaction was not observed with cytosine and thymine. Uridine was readily phosphorylated by a kinase but a similar reaction for thymidine and deoxyuridine was not found. Cytidine could be phosphorylated by a kinase or be deaminated by a deaminase to uridine. No deaminase for cytosine was detected. There was also no phosphotransferase activity for pyrimidine nucleosides in the cytosolic or membrane fractions. Pyrimidine nucleosides were, in general, converted to the bases by a phosphorylase reaction but only uracil and thymine could form nucleosides in the reverse reaction. The activity of thymidylate synthetase was also measured. These results indicate that the nematode synthesizes pyrimidine nucleotides by de novo synthesis and by utilization of uridine and uracil and that cytosine and thymine nucleotides are formed mainly through UMP. The thymidylate synthetase reaction appears to be vital for the growth of the parasite.

Uptake of the antitrypanosomal drug 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine (MDL 73811) by the purine transport system of Trypanosoma brucei brucei

An irreversible inhibitor of S-adenosyl-L-methionine decarboxylase (AdoMetDC), 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine (MDL 73811), was found to cure Trypanosoma brucei brucei and multidrug-resistant T. b. rhodesiense infections in mice [Bitonti, Byers, Bush, Casara, Bacchi, Clarkson, McCann & Sjoerdsma (1990) Antimicrob. Agents Chemother. 34, 1485-1490]. Doses of this drug which resulted in a rapid clearance of parasites from T. b. brucei-infected rats resulted in plasma levels of 50-60 microM-MDL 73811 and an intratrypanosomal MDL 73811 concentration of 1.9 mM within 10 min of administration [Byers, Bush, McCann & Bitonti (1991) Biochem. J. 274, 527-533[. Based on this finding we speculated that MDL 73811, which is an adenosine analogue, is a substrate for the trypanosome active purine transport system. We now report evidence that supports this hypothesis. MDL 73811 uptake by T. b. brucei in vitro was time- and temperature-dependent and was saturable over a time course in which MDL 73811 metabolism was undetectable, suggesting that MDL 73811 uptake is a transport-mediated phenomenon. Inhibition of MDL 73811 uptake by purine nucleosides is consistent with the drug being a substrate for the trypanosome purine transport system. The accumulation of MDL 73811 by cultured L1210 mouse leukaemia cells was significantly less than by trypanosomes exposed to the same pharmacologically relevant concentrations of MDL 73811. Given that the half-life of MDL 73811 in the plasma of rats and mice is approx. 10 min, it seems likely that the existence of a highly active parasite transport system for MDL 73811 is crucial for the sensitivity of trypanosomes towards MDL 73811 in vivo, and that the absence of active transport of MDL 73811 by the host's cells may play a role in the selectivity of this drug.

The toxicity of adenosine analogues against Babesia bovis in vitro

The toxicities of 20 analogues of deoxyadenosine or adenosine were tested in vitro against the intraerythrocytic parasite Babesia bovis. IC37 values (the concentration of compound required to reduce cell survival to 37%) were determined for each compound. Tubercidin (7-deaza-adenosine), 2-bromo-adenosine, 8-bromo-3-ribosyl adenine and 6-phenylamino-deoxyadenosine were shown to be the most toxic towards B. bovis. Comparison of the toxicity results for these compounds in B. bovis with those in human melanoma cell lines indicated a differential toxicity, in that many of the compounds were toxic towards B. bovis but were relatively non-toxic towards human melanoma cell lines and vice versa. These results suggest that the mechanism of toxicity of the deoxyadenosine and adenosine analogues, whose normal metabolism involves transport, metabolism and incorporation into nucleic acids, may vary significantly between B. bovis and mammalian cells, allowing such drugs to be considered for parasite chemotherapy.

Leishmania donovani: characteristics of adenosine and inosine transporters in promastigotes of two different strains

The nucleoside transport characteristics of two strains of Leishmania donovani promastigotes were studied. Strain S1, growing in fully defined medium, and strain S2 (MHOM/ET/67/HA3) both transported adenosine and inosine, but only strain S1 transported uridine and thymidine. Competition studies in the presence of 100 microM of unlabeled adenosine, inosine, guanosine, 2'-deoxyadenosine, tubercidin, formycin B, 3'-deoxyinosine as well as uridine, thymidine and cytidine, with either 1 microM [3H]adenosine or [3H]inosine as permeant, were carried out. The inhibition profile with [3H]inosine as permeant was essentially identical in S1 and S2 promastigotes, indicating that the same inosine transporter was present in both strains. However, with [3H] adenosine as permeant, significant differences were noted between the two strains. Thus, only adenosine, 2'-deoxyadenosine, tubercidin, uridine, and thymidine were strongly inhibitory in S1 promastigotes, while essentially all nucleosides tested were effective in S2 promastigotes. This indicates that adenosine transport in S2 promastigotes seems to involve a transporter differing from that described for S1 promastigotes.

Antiprotozoal activity of 3'-deoxyinosine. Inverse correlation to cleavage of the glycosidic bond

Two nucleosides related to the known antiprotozoal agent 1-(beta-D-ribofuranosyl)-1,5-dihydro-4H-pyrazolo-[3,4-d]pyrimidine-4-one (allopurinol riboside, 1) were prepared and evaluated against Leishmania donovani, Trypanosoma cruzi, and Trypanosoma gambiense. 3'-Deoxyinosine (2) exhibited potent antiprotozoal activity against the three protozoal pathogens with minimal toxicity for host cells. It was found to be especially effective against the Columbia strain of T. cruzi reported to be resistant to 1. The antiprotozoal activity of 2 appeared to be inversely related to the rate of cleavage of the glycosidic bond, as shown by metabolic profiles of 2 in the various pathogenic hemoflagellates and host cells. Combining the key structural elements of 1 and 2 led to the synthesis of 1-(3-deoxy-beta-D-erythro-pentofuranosyl)-1,5-dihydro-4H-pyrazolo[3,4-d] pyrimidin-4-one (3'-deoxy-allopurinol riboside, 3). which was found to be inactive as an antiprotozoal agent.

Antitrypanosomal effects of polyamine biosynthesis inhibitors correlate with increases in Trypanosoma brucei brucei S-adenosyl-L-methionine

We reported recently that administration of ([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine (MDL 73811), an enzyme-activated irreversible inhibitor of S-adenosyl-L-methionine decarboxylase (AdoMetDC; EC 4.1.1.50), a key enzyme in the synthesis of spermidine, cures African trypanosome infections in mice. The precise mechanism of action of MDL 73811 was not clear because a rapid disappearance of trypanosomes from the bloodstream of treated rats occurred before significant depletion of spermidine. Administration of MDL 73811 to Trypanosoma brucei brucei-infected rats resulted in a 70% decrease in parasitaemia within 1 h and a complete disappearance of parasites by 5 h. The reduction in parasitaemia was accompanied by complete inhibition of AdoMetDC activity by 10 min after injection of MDL 73811; inhibition was sustained for at least 4 h. Polyamine levels in trypanosomes were unaffected during the first 1 h in which the marked decrease in parasitaemia was observed, but parasite AdoMet levels increased 20-fold within this time. In contrast, exposure of cultured mammalian cells to MDL 73811 resulted in only a 1.5-2-fold increase in AdoMet levels over a 6 h time course. Experiments with inhibitors of ornithine decarboxylase (ODC) also suggested that the increased AdoMet levels might be an important factor for antitrypanosomal efficacy. Trypanosomes taken from rats treated for 36 h with eflornithine, an inhibitor of ODC, were depleted of putrescine and had markedly decreased spermidine levels. These organisms also had less than 10% of control AdoMetDC activity, and had elevated decarboxy AdoMet (greater than 4000-fold) and AdoMet (up to 50-fold) levels. The methyl ester of alpha-monofluromethyl-3,4-dehydro-ornithine (delta-MFMO-CH3), which cures murine T. b. brucei infections, and the ethyl ester analogue of this compound (delta-MFMO-C2H5), which does not cure this infection, become ODC inhibitors upon hydrolysis and thus were tested for their effects on trypanosomal polyamines, AdoMet and decarboxy AdoMet levels. Although both esters of delta-MFMO depleted trypanosomal polyamines, AdoMet and decarboxy AdoMet levels were elevated in T. b. brucei from infected mice treated with delta-MFMO-CH3 but not in parasites from mice treated with the delta-MFMO-C2H5. These data suggest that inhibition of AdoMetDC, either directly with MDL 73811 or indirectly with inhibitors of ODC, apparently leads to a trypanosome-specific elevation of AdoMet. It is possible that major changes in AdoMet, rather than changes in polyamines, may be responsible for the antitrypanosomal effects of these drugs.

Identification and characterization of variants of Crithidia luciliae with altered morphological and surface properties

Variants of a cloned laboratory stock of the trypanosomatid parasite Crithidia luciliae have been distinguished from "parental type" organisms. These variants accumulated spontaneously over time as the protozoan was maintained by continuous passage in a chemically defined medium. Cloned lines of these variants have been isolated by plating on nutrient agar and partially characterized on the basis of their growth characteristics in culture, their colony and cellular morphology as well as their surface protein expression. One cloned line consisted of motile, flagellated forms which, unlike "parental type" organisms, did not adhere to the surface of culture flasks. Another cloned line was composed of non-adherent, nonmotile, amastigote-like forms which were further distinguished from "parental type" cells by virtue of their constitutive expression, in nutrient-replete medium, of high levels of a surface membrane associated 3'-nucleotidase/nuclease (3'-N'ase) activity. Both the motile, flagellated and amastigote-like variants, like the "parental type" organisms, exhibited elevated levels of the 3'-N'ase activity upon exposure to purine starvation conditions. The variants described are of potential importance in elucidating the mechanism of induction of the highly regulated 3'-N'ase activity as well as for understanding the cytoskeletal systems and the surface properties of these protozoa.

Future prospects for the chemotherapy of human trypanosomiasis. 1. Novel approaches to the chemotherapy of trypanosomiasis

Chemotherapy of trypanosomiasis and leishmaniasis is far from satisfactory and the discovery of new drugs has been slow. Emphasis on the empirical approach to drug development is now shifting towards a more rational, directed approach, which employs the modern tools of science. This is illustrated with respect to the newly discovered drug target, trypanothione, a unique metabolite which is critical for survival of Trypanosoma and Leishmania and absent from the host.

Leishmania donovani: regulated changes in the level of expression of the surface 3'-nucleotidase/nuclease

Leishmania donovani promastigotes have previously been shown to possess a surface membrane bound 3'-nucleotidase/nuclease (3'-N'ase) capable of hydrolyzing both nucleic acids and 3'-ribonucleotides. The specific activity of the 3'-N'ase was increased following transfer of the parasites to fresh, nutrient-replete media or to media lacking purines and/or inorganic phosphate (Pi). In nutrient-replete media, the enzyme activity was transiently elevated during the lag and early logarithmic phases of the growth curve; enzyme activity fell as the cells continued into late log and stationary phases. Purine- and Pi-starved cells exhibited significantly greater levels of 3'-N'ase activity than nutrient-replete cells. These levels remained elevated as long as the organisms were maintained in the deficient media. Nutrient-replete and purine-starved 125I surface-labeled parasites displayed differences in electrophoretic patterns. Upon purine starvation, incorporation of radiolabel was increased in proteins which migrated with apparent molecular weights of 70, 43, and 40 kDa. Comigration, in both one- and two-dimensional systems, of 3'-N'ase activity with the radiolabeled 43-kDa band demonstrated that this band was the catalytically active protein. Peptide mapping of the 70-, 43-, and 40-kDa proteins failed to demonstrate similarities in peptide sequence consistent with either a degradation or a precursor/product relationship. Treatment of the 43- and 40-kDa peptides with N-Glycanase indicated that they were differentially glycosylated. The cumulative results of these studies indicated that L. donovani can respond to altered culture conditions by the differential expression of surface proteins. In particular, the differential expression of the protein responsible for 3'-N'ase activity is consistent with the role of this enzyme in purine acquisition.

Crithidia luciliae: starvation for purines and/or phosphate leads to the enhanced surface expression of a protein responsible for 3'-nucleotidase/nuclease activity

It has been shown previously that starvation of the trypanosomatid protozoan Crithidia luciliae for purines and/or inorganic phosphate results in increased levels of a surface membrane-associated 3'-nucleotidase/nuclease (3'-N'ase) activity which hydrolyzes both 3'-ribonucleotides and nucleic acids, thereby permitting the organisms to transport these essential nutrients across their cell membranes. A polypeptide with the requisite catalytic properties has been identified by an in situ gel activity assay following sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). In current studies, differential synthesis of the protein responsible for the 3'-N'ase activity was not demonstrable by comparisons of SDS-PAGE patterns of nutrient-replete or purine-starved parasites metabolically labeled with either [35S]methionine, [3H]leucine, or [3H]tyrosine. However, surface labeling of nutrient-replete and purine-starved cells revealed the enhanced expression of an 125I surface-labeled 43-kDa protein which comigrated with the 3'-N'ase activity in one- and two-dimensional electrophoretic systems. The amount of this surface-labeled peptide correlated with the level of 3'-N'ase activity as measured by test tube assay. Refeeding adenosine to purine-starved cells led to the loss of both the enzyme activity and the surface iodinatable 43-kDa band as a result of renewed cell division. Starvation of these organisms for phosphate also led to the enhanced expression of the 43-kDa radioiodinatable band. The results indicated that the 3'-N'ase protein, itself, is differentially expressed at the cell surface under conditions which lead to increased enzyme activity.

Immune and nonimmune regulation of the population of Trypanosoma musculi in infected host mice

A clear understanding of the population dynamics of trypanosome infections is lacking. In the case of murine Trypanosoma musculi infections, there are no answers to questions concerning (i) the nature of the prolonged plateau phase during which the number of parasites present in the host remains nearly constant (is it a static or dynamic steady state?); (ii) the origin of new parasites, if the plateau is a dynamic steady state, given the relatively early disappearance of generative forms from the bloodstream; and (iii) the role, if any, of a putative ablastin (reproduction-restricting antibody) in regulating the population dynamics of T. musculi infections. We describe here the results of studies of the number and distribution of mature and reproductive forms (RF) in the blood and peritoneal space of both immunocompetent and cyclophosphamide-treated mice throughout the course of infection. While the RF disappeared from the blood within a few days after parasite inoculation, a high fraction (20 to 30%) of the parasites in the peritoneal space were RF throughout the course of infection and, thus, represented a source of new parasites. If an ablastin is responsible for inhibiting RF in the blood, it appeared to have no effect on RF in the peritoneal space. The results of this investigation support the conclusion that the control of the dynamics of T. musculi infections is largely nonimmunological (until cure of the infection) and probably is exercised by the supply of nutrients and reproduction-inhibiting (nonimmunological) and maturation-promoting factors that affect the generative fraction of the population.

Effects of pyrimidine salvage inhibitors on uracil incorporation of Toxoplasma gondii

Metabolic inhibitors which act in the process of pyrimidine salvage influenced on the uracil incorporation into nucleic acids of Toxoplasma. Inhibitors of dihydrofolate reductase, pyrimethamine and methotrexate, and inhibitors of thymidylate synthase, fluoro-uridine, fluoro-dUMP and fluoro-uracil, diminished isotopic uracil uptake in dose-dependent manners. Azauridine which suppresses de novo pyrimidine biosynthesis did not affect the salvage even in a relatively high dose. These results suggested that the activation of uracil salvage should be closely related with the function of TMP biosynthetic enzymes. The pattern of thymidine uptake had no differences between control HL-60 cells and Toxoplasma infected cells, which did not reflect the specific proliferation of Toxoplasma. It can be exploited to characterize the effects of various compounds related with the proliferation of Toxoplasma, especially its DNA synthesis.

Purification and characterization of uracil phosphoribosyltransferase from Crithidia luciliae

1. Uracil phosphoribosyltransferase (UPRTase) was purified 370-fold from the protozoan parasite, Crithidia luciliae. 2. The enzyme was a dimer of mol. wt 80 000 and was highly specific for uracil. 3. GTP, which is an activator of UPRTase from E. coli had a slight inhibitory effect on the parasite enzyme. 4. The C. luciliae UPRTase demonstrated a broad specificity for activating divalent metal ions.

The surface membrane 3′-nucleotidase/nuclease of trypanosomatid protozoa

The plasma membrane of protozoan parasites has been the subject of intense research interest because it serves as the interface between the parasite and its host] This structure functions in the acquisition of nutrients, in the response to environmental signals, in the secretion and excretion of metabolites, in the recognition of host surfaces and in the avoidance of host-destructive factors including the components of the immune response. Recent technological advances, such as the use of monoclonal antibodies and recombinant DNA methodologies, have led to the identification of specific plasma membrane components and their coding; sequences, often in the absence of information regarding the physiological role of the component. In this article Michael Gottlieb considers the functional activity of o plasma membrane enzyme which has been identified in a number of protozoa belonging to the family Trypanosomatidae, and considers its adaptive significance and possible evolution in the different genera of this family.

Purine base transport in wild-type and mycophenolic acid-resistant Tritrichomonas foetus

The purine base transport systems of wild-type and mycophenolic acid-resistant (MPAR) Tritrichomonas foetus have been characterized. Wild-type T. foetus has two carriers, one for hypoxanthine (Km = 0.7 +/- 0.3 mM, Vm = 80 +/- 20 pmol microliters-1min-1) and guanine (Km = 0.09 +/- 0.02 mM, Vm = 17 +/- 3 pmol microliters-1min-1), and a second for xanthine (Km = 0.6 +/- 0.2 mM, Vm = 25 +/- 5 pmol microliters-1min-1). Adenine transport was not saturable (k = 0.16 +/- 0.01 min-1) and therefore appears to enter the parasite by passive diffusion through the membrane. T. foetus MPAR has lost the hypoxanthine/guanine transporter. Xanthine and adenine transport are similar in wild-type and MPAR T. foetus. No purine nucleoside transporter could be identified.

The enzymes of the classical pentose phosphate pathway display differential activities in procyclic and bloodstream forms of Trypanosoma brucei

The specific activities of each of the enzymes of the classical pentose phosphate pathway have been determined in both cultured procyclic and bloodstream forms of Trypanosoma brucei. Both forms contained glucose-6-phosphate dehydrogenase (EC 1.1.1.49), 6-phosphogluconolactonase (EC 3.1.1.31), 6-phosphogluconate dehydrogenase (EC 1.1.1.44), ribose-5-phosphate isomerase (EC 5.3.1.6) and transaldolase (EC 2.2.1.2). However, ribulose-5-phosphate 3'-epimerase (EC 5.1.3.1) and transketolase (EC 2.2.1.1) activities were detectable only in procyclic forms. These results clearly demonstrate that both forms of T. brucei can metabolize glucose via the oxidative segment of the classical pentose phosphate pathway in order to produce D-ribose-5-phosphate for the synthesis of nucleic acids and reduced NADP for other synthetic reactions. However, only procyclic forms are capable of using the non-oxidative segment of the classical pentose phosphate pathway to cycle carbon between pentose and hexose phosphates in order to produce D-glyceraldehyde 3-phosphate as a net product of the pathway. Both forms lack the key gluconeogenic enzyme, fructose-bisphosphatase (EC 3.1.3.11). Consequently, neither form should be able to engage in gluconeogenesis nor should procyclic forms be able to return any of the glyceraldehyde 3-phosphate produced in the pentose phosphate pathway to glucose 6-phosphate. This last specific metabolic arrangement and the restriction of all but the terminal steps of glycolysis to the glycosome may be the observations required to explain the presence of distinct cytosolic and glycosomal isoenzymes of glyceraldehyde-3-phosphate dehydrogenase and phosphoglycerate kinase. These same observations also may provide the basis for explaining the presence of cytosolic hexokinase and phosphoglucose isomerase without the presence of any cytosolic phosphofructokinase activity. The key enzymes of the Entner-Doudoroff pathway, 6-phosphogluconate dehydratase (EC 4.2.1.12) and 2-keto-3-deoxy-6-phosphogluconate aldolase (EC 4.1.2.14) were not detected in either procyclic or bloodstream forms of T. brucei.

Pyrimidine scavenging by Mycobacterium leprae

Mycobacterium leprae incorporated exogenously supplied pyrimidines as bases or nucleosides, but not as a nucleotide, into its nucleic acids. Notably, thymine was incorporated approximately 5 times more rapidly than thymidine by both suspensions of, or intracellular M. leprae. Thymine incorporation was significantly inhibited by clofazamine and dapsone at near-pharmacological levels. Therefore, incorporation of thymine is preferable as an activity for assessing viability of M. leprae. Nucleosides were converted to nucleotides through kinases, bases through phosphoribosyltransferases. Alternatively, thymine and uracil could first be converted to nucleosides. Cytosine and uracil bases were interconvertible, and uracil alone could supply all the pyrimidine requirements of M. leprae, though conversion to the thymine base was extremely slow. Overall, pyrimidine scavenging occurs at a slower rate than, and appears not to be so important as purine scavenging in M. leprae.

Pyrimidine biosynthesis de novo in M. leprae

Mycobacterium leprae can synthesise pyrimidines de novo. Although pyrimidine synthesis could not be detected in intact bacteria, extracts contained all four enzymes unique to the de novo pathway which are detectable in mycobacteria by the methods used. Inhibition of aspartate transcarbamylase by UTP and ATP suggested that lack of pyrimidine synthetic activity in whole M. leprae could be a result of strong feedback inhibition.

Novel biochemical pathways in parasitic protozoa

Throughout evolution, enzymes and their metabolites have been highly conserved. Parasites are no exception to this and differ most markedly by the absence of metabolic pathways that are present in the mammalian host. In general, parasites are metabolically lazy and rely on the metabolism of the host both for a supply of prefabricated components such as purines, fatty acids, sterols and amino acids and for the removal of end-products. Nonetheless, parasites are metabolically highly sophisticated in that (1) they retain the genetic capacity to induce many pathways, when needed, and (2) they have developed complex mechanisms for their survival in the host. Certain unique features of the metabolism of trypanosomes, leishmania, malaria and anaerobic protozoa will be discussed. This will include (1) glycolysis and electron transport with reference to the unique organelles: the glycosome and the hydrogenosome, (2) purine salvage, pyrimidine biosynthesis and folic acid metabolism and (3) polyamine and thiol metabolism with special reference to the role of the unique metabolite of trypanosomes and leishmanias, trypanothione.

Effects of anaerobiosis on adenine nucleotide levels and the release of ATP by Leishmania major promastigotes

1. Leishmania major promastigotes showed a large decrease in ATP and increases in ADP and AMP contents after 4 min of anaerobiosis. 2. When ADP was added to intact promastigotes, it was completely metabolized, apparently by its conversion to adenosine extracellularly followed by adenosine uptake, further metabolism intracellularly, and release of hypoxanthine. Under anaerobic conditions, adenosine uptake was strongly inhibited and ADP degradation was stopped at adenosine. 3. Under both aerobic and anaerobic conditions, ATP was released into the medium. ATP release was specific, since ADP and AMP were not detectable extracellularly even when their external degradation was inhibited with molybdate.

Nucleoside uptake in Trypanosoma cruzi: analysis of a mutant resistant to tubercidin

Nucleoside salvage pathways are vital to the parasitic protozoan Trypanosoma cruzi, and have become important targets in the development of new chemotherapeutic agents against this organism. We produced a mutant T. cruzi clone with a defect in the uptake of the adenosine analogue tubercidin which allowed us to hypothesize that there are at least two distinct nucleoside transport pathways in this parasite. The mutant shows a marked defect in the uptake of tubercidin and thymidine, whereas the uptake of adenosine and inosine are normal. Inhibition and metabolic studies suggest that the defect is related to transport and that there are two transport processes relatively specific for purines and pyrimidines, respectively, although tubercidin is transported via the latter. This is similar to the reported dual nucleoside transport pathways in Leishmania donovani and may be a common system in the Trypanosomatidae. These transport processes are markedly different from those which have been described for mammalian cells and may play an important role in the design of strategies for the chemotherapy of human infection with these pathogenic parasites.

Crithidia luciliae: factors affecting the expression of 3'-nucleotidase/nuclease activity

Crithidia luciliae, a trypanosomatid protozoan readily grown in axenic cultures, was shown to possess low levels of a surface membrane-bound ectoenzyme capable of hydrolyzing both 3'-ribonucleotides and nucleic acids. The specific activities of this 3'-nucleotidase/nuclease, with both mononucleotide and nucleic acid substrates, were greatly enhanced when the protozoa were deprived of purines, an essential nutrient. The catalytic activities were exhibited by a polypeptide which migrated in sodium dodecyl sulfate-polyacrylamide gel electrophoresis with an Mr of 47,000. Starvation of these cells for inorganic phosphate (Pi), in media with or without purines, also led to an increase in the specific activity of the ectoenzyme compared to that of Pi- and purine-replete cells. In contrast, the level of enzyme activity was not increased when the protozoa were starved, under purine-replete conditions, for either arginine or hemin, two other essential nutrients. Cells starved simultaneously for either of the latter two nutrients and for purines also did not show increased levels of the 3'-nucleotidase/nuclease. The activation of the enzyme was also prevented by sodium arsenite, cycloheximide, actinomycin D, and tunicamycin indicating that the activation presumably required metabolic energy as well as new transcription, translation, and protein modification. The results demonstrate that the control of 3'-nucleotidase/nuclease expression is a regulated, adaptive response to growth-limiting levels of essential nutrients.

The availability of purines influences both the number of parasites and the splenocyte levels of purine-metabolizing enzymes in trypanosome-infected mice

Growth on Trypanosoma musculi in the murine host was limited by the availability of host purines. A portion of the spleen cells of infected mice (many of them granulocytes) displayed high levels of adenosine deaminase (ADA) and purine nucleoside phosphorylase, probably as a compensatory response to extracellular purine deficiency. Injections of adenosine or 2-deoxycoformycin stimulated significant increases in the growth of parasites. 2-Deoxycoformycin treatment also diminished parasite-induced splenomegaly. Treatment of mice with polyethylene glycol-modified ADA, a slowly catabolized form of ADA, had no effect on the course of T. musculi infection, indicating that the parasites can utilize purines other than adenosine. The apparent competition between parasites and host cells for available purines suggests that depletion of extracellular purines should be considered as an approach to treating extracellular trypanosome infections.

Inactivation of Crithidia fasciculata carbamoyl phosphate synthetase II by the antitumor drug acivicin

In Crithidia fasciculata, carbamoyl phosphate synthetase II, which catalyses the first step of de novo pyrimidine biosynthesis, was separated from aspartate carbamoyltransferase by ammonium sulfate fractionation. The antitumor drug acivicin competitively inhibited the synthetase II activity with respect to L-glutamine, yielding an apparent Ki of 2 microM. In the absence of L-glutamine, acivicin resulted in a selective, time-dependent inactivation of L-glutamine-dependent activity of the enzyme, with an inactivation constant (Kinact) of 100 microM and a minimum inactivation half-time (T) of 0.2 min. L-Glutamine protected the enzyme from inactivation. These results are consistent with a postulate that acivicin is an active site-directed affinity analogue of L-glutamine, achieving irreversible inactivation. The inactivated enzyme retained ammonia-dependent activity. Acivicin stimulated the ammonia-dependent activity by increasing the Vmax value of the enzyme; apparent Km values for ammonia and MgATP were not affected. Differential action of acivicin on the Crithidia and mammalian synthetase II is discussed.

Kinetic properties of carbamoyl-phosphate synthetase II (glutamine-hydrolyzing) in the parasitic protozoan Crithidia fasciculata and separation of the enzyme from aspartate carbamoyltransferase

A high specific activity of carbamoyl-phosphate synthetase II (glutamine-hydrolyzing; EC 6.3.5.5) was demonstrated in extract of the cultured Crithidia fasciculata. The enzyme was separated from aspartate carbamoyltransferase by ammonium sulfate fractionation. Apparent Km for the synthetase for L-glutamine, NH4+, MgATP or bicarbonate was 0.27, 26, 1.7 or 1.7 mM at 2.0% dimethyl sulfoxide plus 0.3% glycerol. 8.6% dimethyl sulfoxide plus 1.4% glycerol decreased Km for L-glutamine to 0.10 mM, while Km for MgATP was unaffected. The higher solvent concentrations made Vmax markedly reduced, yielding the inhibition of the activity. These properties are unique to the Crithidia synthetase, compared with the mammalian enzyme.

Renaturation of Leishmania donovani 3'-nucleotidase following sodium dodecyl sulfate-polyacrylamide gel electrophoresis

1. Renaturation of a 3'-nucleotidase from the surface membrane of Leishmania donovani promastigotes was achieved following polyacrylamide gel electrophoresis (PAGE) in the presence of sodium dodecyl sulfate (SDS). 2. Enzyme activity was detected in situ in gels, following SDS removal, by incubating the gels in reaction mixtures containing 3'-AMP or 3'-UMP as substrate followed by staining for the inorganic phosphate (Pi) reaction product with malachite green-molybic acid solution. 3. Conditions for the removal of SDS by diffusion and for the renaturation of enzyme activity are described including evidence for the detergent requirement, which is best satisfied by 3[(3-cholamidopropyl)-dimethylammonio]2-hydroxy-1-propane sulfonate (CHAPSO). 4. Results indicate that the 3'-nucleotidase migrates under these conditions as a polypeptide with an Mr of 43,000.

Activity of inosine analogs against Pneumocystis carinii in culture

Three analogs of inosine, formycin B, allopurinol ribonucleoside, and 9-deazainosine, were tested for their ability to suppress proliferation of Pneumocystis carinii in culture with WI-38 cells. The organism was inhibited by 9-deazainosine at 10 micrograms/ml, and there was some inhibition at 1 microgram/ml. Formycin B was effective only at 40 micrograms/ml. Allopurinol ribonucleoside had little effect.

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.

Progress in molecular parasitology

Substantial progress has been made in the last ten years in understanding the structural and functional organization of parasitic protozoa and helminths and the complex physiological relationships that exist between these organisms and their hosts. By employing the new powerful techniques of biochemistry, molecular biology and immunology the genomic organization in parasites, the molecular basis of parasite's variation in surface antigens and the biosynthesis, processing, transport and membrane anchoring of these and other surface proteins were extensively investigated. Significant advances have also been made in our knowledge of the specific and often peculiar strategies of intermediary metabolism, cell compartmentation, the role of oxygen for parasites and the mechanisms of antiparasitic drug action. Further major fields of interest are currently the complex processes which enables parasites to evade the host's immune defense system and other mechanisms which have resulted in the specific adaptations which enabled parasites to survive within their host environments. Various approaches in molecular and biochemical parasitology and in immunoparasitology have been proven to be of high potential for serodiagnosis, immunoprophylaxis and drug design.