Parasite-induced permeation of nucleosides in Plasmodium falciparum malaria.

A mechanism which mediates the transport of the nonphysiological nucleoside, L-adenosine, was demonstrated in Plasmodium falciparum infected erythrocytes and naturally released merozoites. L-Adenosine was not a substrate for influx in freed intraerythrocytic parasites or in normal human erythrocytes nor was L-adenosine transported in a variety of cell types including other parasitic protozoa such as Crithidia luciliae, Trichomonas vaginalis, Giardia intestinalis, or the mammalian cells, Buffalo Green Monkey and HeLa cells. L-Adenosine transport in P. falciparum infected cells was nonsaturable, with a rate of 0.13 +/- 0.01 pmol/microliter cell water per s per microM L-adenosine, yet the transport was inhibited by furosemide, phloridzin and piperine with IC50 values between 1-13 microM, distinguishing the transport pathway from simple diffusion. The channel-like permeation was selective as disaccharides were not permeable to parasitised cells. In addition, an unusual metabolic property of parasitic adenosine deaminase was found in that L-adenosine was metabolised to L-inosine by both P. falciparum infected erythrocytes and merozoites, an activity which was inhibited by 50 nM deoxycoformycin. No other cell type examined displayed this enzymic activity. The results further substantiate that nucleoside transport in P. falciparum infected cells was significantly altered compared to uninfected erythrocytes and that L-adenosine transport and metabolism was a biochemical property of Plasmodium infected cells and merozoites and not found in normal erythrocytes nor any of the other cell types investigated.

Induction of nucleoside transport sites into the host cell membrane of Babesia bovis infected erythrocytes.

Normal bovine erythrocytes have negligible ability to transport adenosine and related nucleosides across their cell membrane. However, infection with the intraerythrocytic parasite Babesia bovis was found to induce a nucleoside permeation site into the host cell membrane. Transport experiments over periods of up to 30 s determined that the transport rate of 1 microM adenosine into the infected cell was 1.72 +/- 1.2 pmol incorporated (microliter cell water)-1s-1, a rate three times higher than for normal human erythrocytes. Incorporation studies over 6 h with labelled adenosine indicated that the purine moiety was incorporated into parasite nucleic acids. The mammalian nucleoside transport inhibitors, nitrobenzylthioinosine (NBMPR), nitrobenzylthioguanosine (NBTGR), dilazep and dipyridamole inhibited the induced nucleoside transport mechanism in the Babesia-infected erythrocytes, though at higher concentrations than those required to inhibit normal human erythrocyte transport. An ID50 value for NBMPR of 0.36 microM was determined. Phloretin and 5'-p-fluorosulphonyl benzoyl adenosine-HCl (5FSBA) were also shown to be inhibitory, with ID50 values of 0.11 and 0.18 microM, respectively, whilst phlorizin and verapamil at 1 microM had no effect. Binding studies with [3H]NBMPR indicated that high-affinity NBMPR binding sites could not be detected in either normal or B. bovis infected bovine erythrocytes. The results indicate that the induced nucleoside permeation site(s) in B. bovis infected erythrocytes has characteristics different from either human erythrocytes or erythrocytes infected with the malarial parasites Plasmodium falciparum or Plasmodium yoelii.

In vitro inhibition of Plasmodium falciparum by pyrazofurin, an inhibitor of pyrimidine biosynthesis de novo.

The effect of pyrazofurin, an inhibitor of UMP synthesis, on Plasmodium falciparum growth in vitro has been studied. ID50 values (concentration of compound causing 50% inhibition of [3H]hypoxanthine incorporation) for the FCQ-27, FCI-1 and K-1 (chloroquine-resistant) isolates were 10 +/- 8.7, 6.4 +/- 5.3 and 6.3 +/- 0.5 microM, respectively. Comparative ID50 values for chloroquine were 13.5 +/- 4.2, 22.8 +/- 7.6 and 343 +/- 114 microM, respectively. Over the 48-h intraerythrocytic cycle of tightly synchronized parasites, pyrazofurin both reduced the parasitemia and retarded the maturation of trophozoites and schizonts. Addition of uracil or uridine to the in vitro culture did not decrease the anti-parasitic activity of pyrazofurin. Chloroquine reduced the parasitemia, but did not retard development of the remaining viable parasites. Pyrazofurin (20 microM) caused a 50% inhibition of parasite orotate phosphoribosyltransferase (E.C. 2.4.2.10) and, in the presence of adenosine kinase and ATP, a 73% inhibition of orotidine-5'-phosphate decarboxylase (E.C. 4.1.1.23).

Stage-specific alteration of nucleoside membrane permeability and nitrobenzylthioinosine insensitivity in Plasmodium falciparum infected erythrocytes.

In human erythrocytes, the intracellular presence of malarial parasites (Plasmodium falciparum) markedly changed the permeation characteristics of the nucleosides, adenosine and tubercidin, an adenosine analogue. We report parasite-induced changes in the kinetics of cellular uptake of the nucleosides and in the appearance in infected cells of a nucleoside permeation route of low sensitivity to the classical inhibitor of erythrocytic nucleoside transport, nitrobenzylthioinosine (NBMPR). These changes and a diminution in NBMPR effectiveness during parasite maturation to the trophozoite or schizont stage, suggest the presence in the infected cells of an altered or new nucleoside permeation mechanism of low sensitivity to NBMPR. The incorporation of adenosine into polynucleotides was also of low sensitivity to 10 microM NBMPR. Binding studies of [3H]NBMPR with both normal erythrocytes and those harbouring parasites at each morphological stage indicated that fewer high affinity NBMPR binding sites were present on cells containing mature parasites than on the uninfected cells. The apparent low sensitivity to NBMPR of nucleoside permeation in erythrocytes containing P. falciparum forms may enable therapeutic measures with cytotoxic nucleosides to be directed with selectivity toward parasite-containing cells.

Effect of diamide on nucleoside and glucose transport in Plasmodium falciparum and Babesia bovis infected erythrocytes.

Normal human erythrocytes, preincubated with the oxidizing agent diamide, did not demonstrate any increased permeability, but showed a significant decrease in their ability to transport the nucleoside adenosine. Diamide appeared to have little effect on glucose permeation in uninfected and Plasmodium falciparum infected cells. The inhibition of adenosine transport in human erythrocytes by diamide pretreatment appeared to be unrelated to the inhibition by the established nucleoside transport inhibitor, nitrobenzylthioinosine (NBMPR). An ID50 for diamide of 0.3 mM was determined for 1 microM adenosine transport in human erythrocytes after preincubation for 45 min at 37 degrees C. However, preincubation of diamide (20 mM, 60 min at 37 degrees C) with Babesia bovis-infected bovine erythrocytes resulted in complete inhibition of the capacity of the parasitised cell to transport adenosine and partial inhibition of glucose permeation. By contrast, diamide was shown to have little or no effect on the new or induced nucleoside permeation site in P. falciparum (trophozoite) infected erythrocytes nor on the glucose transporter in these cells. The results further indicate the differences between the normal human erythrocyte nucleoside and glucose transporters and those new or altered transporters in the membrane of P. falciparum or B. bovis-infected red blood cells.

Enzymes of the de novo pyrimidine biosynthetic pathway in Toxoplasma gondii.

All six enzymes of the de novo biosynthetic pathway leading to the biosynthesis of UMP have been characterized in Toxoplasma gondii. The first three enzymes of the pathway, carbamyl phosphate synthetase-II (CPS-II), aspartate transcarbamylase (ATCase) and dihydroorotase (DHOase) could be consistently separated by sucrose gradient centrifugation. Their molecular weights were estimated to be approximately 540 000, 140 000 and 70 000, respectively. The last two enzymes, orotate phosphoribosyltransferase (OPRTase) and orotidylate decarboxylase (ODCase), cosedimented at the same position, corresponding also to a molecular weight of approximately 70 000. The fourth enzyme, dihydroorotate dehydrogenase (DHO-DHase), was associated with the particulate fraction. Apparent Km values for the respective enzymes were: CPS-II, MgATP2- (19.7 1.2 mM), L-glutamine (12.0 +/- 1.7 microM), ammonia (15.5 +/- 2.7 mM); ATCase, carbamyl phosphate (26.2 +/- 3.5 microM), L-aspartate (17.6 +/- 8.5 mM); DHOase (reverse direction) dihydroorotate (1.6 +/- 0.08 microM); ODCase, orotidine 5'-monophosphate (0.41 +/- 0.04 microM). MgUTP2- was found to act as an inhibitor of CPS-II, with an apparent Ki of 0.41 mM. However, 5-phospho-alpha-D-ribosyl-1-diphosphate, dimethyl sulphoxide and glycerol had no effect on the Km value for MgATP2-. The effect of some inhibitors, including pyrimidine and purine nucleotides and analogs and respiratory chain inhibitors, was also determined for the enzymes of the pathway.

Antimalarial action of nitrobenzylthioinosine in combination with purine nucleoside antimetabolites.

The infection of human erythrocytes by two strains of the human malarial parasite, Plasmodium falciparum (FCQ-27 or the multi-drug-resistant strain K-1), markedly changed the transport characteristics of the nucleosides, adenosine and tubercidin, compared to uninfected erythrocytes. A component of the transport of these nucleosides was insensitive to the classical mammalian nucleoside transport inhibitor nitrobenzylthioinosine (NBMPR). In vitro studies with tubercidin demonstrated ID50 values of 0.43 and 0.51 microM for FCQ-27 and K-1, respectively. In addition, the nucleoside transport inhibitors NBMPR, nitrobenzylthioguanosine (NBTGR), dilazep and dipyridamole also independently exhibited antimalarial activity in vitro. The combination of tubercidin and NBMPR or NBTGR in vitro demonstrated synergistic activity, whilst tubercidin together with dilazep or dipyridamole showed subadditive activity. Analysis by HPLC indicated that NBMPR could permeate the infected cell membrane and provided evidence for the catabolism of NBMPR in vitro, with subsequent alteration of the purine pool in the infected erythrocyte. These observations further indicated the possibility of the utilization of cytotoxic nucleosides against P. falciparum infection in conjunction with a nucleoside transport inhibitor to protect the host tissue.

Expression of substrate-specific transporters encoded by Plasmodium falciparum in Xenopus laevis oocytes.

When the malarial parasite Plasmodium falciparum multiplies in erythrocytes it dramatically increases uptake of essential metabolic precursors (nucleosides, nucleobases and glucose) and export of lactic acid by undefined mechanisms. The first evidence is provided here, by a detailed study in Xenopus laevis oocytes, that several specific nutrient transporters are the product of P. falciparum genes. We report the expression of nucleoside, nucleobase, hexose and monocarboxylate transport systems in Xenopus oocytes when injected with mRNA isolated from asexual stages of developing P. falciparum parasites. Their properties are distinct from transport events occurring at the infected erythrocyte membrane or the electrophysiologically identified channel localised to the parasitophorous vacuolar membrane. These novel transporters are substrate-specific and stereoselective, and represent a key regulatory step in the acquisition and export of metabolites by intraerythrocytic P. falciparum.

Inhibition of uridine phosphorylase from Giardia lamblia by pyrimidine analogs.

Fifty-six pyrimidine analogs were tested as possible inhibitors of uridine phosphorylase from Giardia lamblia. Values of Ki were determined for eight of these which demonstrated an inhibition greater than 60% under the standard conditions of uridine at 1 mM (approximately 1.5 times the Km) and inhibitor at 1 mM. All were competitive with respect to uridine. The most effective inhibitors were uracil analogs substituted at the C-5 position with electron withdrawing groups (nitro groups or halogens). The inhibitory effect at the 5-position appeared to be further enhanced by substitution at the C-6 position with electron releasing groups. The order of effectiveness as inhibitors was 6-methyl-5-nitrouracil greater than 6-amino-5-nitrouracil greater than 5-benzylacyclouridine greater than 5-nitrouracil greater than 5-fluorouracil greater than 5-bromouracil greater than 6-benzyl-2-thiouracil greater than 1,3-dimethyluracil with Ki values of 10, 12, 44, 56, 119, 230, 190 and greater than 1000 microM, respectively. The compounds were also effective inhibitors of the thymidine phosphorylase activity of the enzyme. The effect of the more potent compounds on G. lamblia in in vitro culture are currently under investigation.

Increased glucose permeability in Babesia bovis-infected erythrocytes.

Glucose influx into bovine erythrocytes was found to be significantly increased upon infection with the parasite, Babesia bovis. The influx of glucose into the infected cells over 4 min was not saturable at high concentrations of glucose (240 mM), nor was it affected by established inhibitors of mammalian glucose transport, such as cytochalasin B and phloretin (0.1-100 microM). Glucose uptake into the parasitized cells was, however, inhibited by phloridzin (phloretin-2-beta-glucoside) at concentrations over the range of 10-500 microM. Further inhibition of glucose uptake by adenosine (2.5-15 mM) was found to occur in B. bovis-infected bovine erythrocytes, suggesting an interaction of adenosine with the new or altered component of glucose transport in the parasitized cells.

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.

Stimulated transport of adenosine, guanosine and hypoxanthine in Crithidia luciliae: metabolic machinery in which the parasite has a distinct advantage over the host.

Nutritional insufficiency is a common environmental extreme to which parasitic protozoa are routinely exposed. In this study of purine salvage mechanisms we illustrate some successful adaptations of the parasite Crithidia luciliae to its environment, particularly in the case of purine stress. In purine-depleted conditions, the insect trypanosome C. luciliae has the ability to increase the rates of transport of adenosine, guanosine and hypoxanthine and the activity of the exoenzyme 3'nucleotidase (3'NTase) during the growth cycle. The dramatic increase in these activities appears after a 72-h period in culture. The increased activity of the purine transporters and 3'NTase could be suppressed by addition to the medium of a purine supplement such as adenosine or hypoxanthine (100 microM). Under conditions where the concentration of purines in the medium could be closely regulated, C. luciliae grown in purine-replete medium (> or = 75 microM purine) exhibited low rates of purine transport and activity of 3'NTase. In comparison, parasites transferred to medium with a low purine source (< or = 7.5 microM adenosine) had levels of adenosine, guanosine and hypoxanthine transport elevated 25-40-fold. The results link the simultaneous increase in activity of the nucleoside and base transporters, 3'NTase activity and a general increase in the purine salvage of C. luciliae to the concentration of purines available at any time to the parasite.

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.

Reduced transport of adenosine in erythrocytes from patients with beta-thalassaemia.

The transport of adenosine into blood from beta-thalassaemia subjects was measured to provide a background to the relationship between resistance of malaria infection and beta-thalassaemia. Adenosine transport was significantly reduced in the abnormal cells in the blood samples. As adenosine is one of the major purines salvaged by P. falciparum malaria, we suggest that the resistance to malaria in beta-thalassaemia subjects may be due to a nutrient deficiency in the abnormal red 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.

Routine screening for potential babesicides using cultures of Babesia bovis.

A procedure for screening of potential anti-malarial agents, based on the incorporation of [3H]hypoxanthine by the parasite, was adapted for the testing of anti-metabolites against Babesia bovis (Lismore and Samford isolates) cultured in vitro. A close correlation was found between [3H]hypoxanthine incorporation in a standard assay and percentage of parasitized cells as determined by microscopic examination. The concentrations of compounds causing 50% inhibition of [3H]hypoxanthine incorporation (ID50 values) for the established babesicides, Imidocarb and Amicarbalide, were determined to be 3 ng ml-1 (8.6 nM) and 5-10 ng ml-1 (17-34 nM), respectively. A variety of other anti-metabolites were tested in the system. ID50 values for some of the more effective compounds were tubercidin (75 nM), tetracycline (25 microM), menoctone (100 nM) and TN 108, a di-Mannich base derived from 4-(7'-trifluoromethyl-quinolin-4'-ylamino)phenol (0.13 microM). No significant differences between results with the two isolates of B. bovis were observed.

Amplifications of phleomycin and bleomycin-induced antibiotic activity in Escherichia coli by aromatic cationic compounds.

A wide range of aromatic compounds has been shown to amplify phleomycin-induced cell killing in Escherichia coli. They include acridines, acridinium chlorides, dihydroanthracenes, anthracenes, dianthracenes, phenanthridinium salts, phenazinium chlorides, phenoxazones, triphenyl methane dyes, benzoquinolizinium chloride, diphenylmethane derivatives, stilbene and diphenyl derivatives. Low concentrations of these amplifiers also amplified the DNA breakage and degradation effects of phleomycin. The minimum structural specification for activity as an amplifying agent is suggested. A representative sample of compounds effective as amplifiers of phleomycin also amplified the antibiotic effects of bleomycins B4 and B6. The amplifiers described are known to vary in their ability to penetrate and accumulate in different organisms or tissues. This suggests the possibility of developing a series of antibiotic regimes using these amplifiers (or the large number of derivative compounds also likely to be active) where the therapeutic index is determined by the properties of the amplifier chosen rather than of the phleomycin or the bleomycin.

Pyrimidine de novo synthesis during the life cycle of the intraerythrocytic stage of Plasmodium falciparum.

The 6 enzymes involved in de novo synthesis of pyrimidines were measured in Plasmodium falciparum isolated by saponin lysis from RBC's nonsynchronized and synchronized in vitro cultures. The total activities were found to be dependent on the stage of the P. falciparum cycle. In parasites isolated from synchronized cultures, the highest activities for all enzymes were found at about 27 hr after synchronization in the late trophozoite stage, or just before schizont formation. Merozoites and ring forms contained little de novo activity. The first enzyme of the pathway, carbamyl phosphate synthetase (CPS-II) preferentially utilized glutamine. Ammonia was a poor substrate. CPS-II was unstable in the absence of the cryoprotectants, dimethylsulfoxide and glycerol. The apparent Km for MgATP--was 3.8 +/- 0.7 mM and the enzyme in all morphological forms of P. falciparum (ring, mature trophozoites and schizonts) was inhibited by UTP. The activity of the fourth enzyme of the pathway, dihydroorotate dehydrogenase, appeared to be linked to the cell's respiratory chain; inhibitors of mammalian electron transport such as cyanide, amytal, antimycin A, thenoyltrifluoroacetone and ubiquinone analogs also inhibited the P. falciparum enzyme. The demonstration of the variation of activity of the pyrimidine enzymes correlates with the increased synthesis of nucleic acids in the late trophozoite stage. These observations provide a basis for the testing of the effectiveness of pyrimidine analogs as potential antimetabolites against various forms of the parasite.

Laboratory and field comparisons of adenosine influx in Plasmodium falciparum and Plasmodium vivax infected erythrocytes with genetic abnormalities from patients in Myanmar.

Influx of the purine nucleoside, adenosine, was assessed in erythrocytes from both normal subjects and from subjects with a range of genetically determined erythrocyte disorders from Myanmar. The latter included alpha-thalassemia major (Myanmar variant), beta-thalassemia major (Myanmar variant), beta-thalassemia trait, HbEE and HbAE erythrocytes and two variants of glucose-6-phosphate dehydrogenase (G6PDH) deficiency. Significant reductions (p < 0.01) of adenosine influx were observed in erythrocytes from individuals with alpha- and beta-thalassemia major and severe G6PDH deficiency. Abnormal erythrocytes infected with the malarial parasites, Plasmodium falciparum or Plasmodium vivax, demonstrated a reduction in adenosine transport which correlated with the proportion of abnormal erythrocytes present in the samples obtained. The effect of nitrobenzylthioinosine (NBMPR) on adenosine influx was explored in normal and abnormal erythrocytes. In all these cases, NBMPR completely inhibited the transport of adenosine. However, transport of adenosine into P. falciparum and P. vivax-infected normal erythrocytes and abnormal cells was only inhibited 50-60% by NBMPR. The combination of tubercidin and NBMPR completely blocked adenosine transport into both normal and abnormal erythrocytes infected with either P. falciparum or P. vivax.