Transport of aspartic acid, arginine, and tyrosine by the opportunistic protist Pneumocystis carinii

In order to improve culture media and to discover potential drug targets, uptake of an acidic, a basic, and an aromatic amino acid were investigated. Current culture systems, axenic or co-cultivation with mammalian cells, do not provide either the quantity or quality of cells needed for biochemical studies of this organism. Insight into nutrient acquisition can be expected to lead to improved culture media and improved culture growth. Aspartic acid uptake was directly related to substrate concentration, Q(10) was 1.10 at pH 7.4. Hence the organism acquired this acidic amino acid by simple diffusion. Uptake of the basic amino acid arginine and the aromatic amino acid tyrosine exhibited saturation kinetics consistent with carrier-mediated mechanisms. Kinetic parameters indicated two carriers (K(m)=22.8+/-2.5 microM and K(m)=3.6+/-0.3 mM) for arginine and a single carrier for tyrosine (K(m)=284+/-23 microM). The effects of other L-amino acids showed that the tyrosine carrier was distinct from the arginine carriers. Tyrosine and arginine transport were independent of sodium and potassium ions, and did not appear to require energy from ATP or a proton motive force. Thus facilitated diffusion was identified as the mechanism of uptake. After 30 min of incubation, these amino acids were incorporated into total lipids and the sedimentable material following lipid extraction; more than 90% was in the cellular soluble fraction.

Putrescine and spermidine transport in Leishmania

The transport of putrescine and spermidine into Leishmnania donovani promastigotes and Leishmania mexicana promastigotes and amastigotes has been characterised. Polyamine transport was shown to be saturable and temperature-sensitive for both developmental stages of Leishmania. Transport was pH-dependent with pH optima of 7.4 and 5.5 for promastigotes and amastigotes, respectively. The uptake process was independent of extracellular Na+, but inhibited by protonophores and H+-ATPase inhibitors. Kinetic analyses of polyamine transport showed that Km and Vmax differed between promastigotes of the two species and between promastigotes and amastigotes of L. mexicana. Inhibition data suggest that putrescine and spermidine use different transporters. The aromatic diamidine pentamidine, the drug of choice for treatment of antimonial-resistant cases of leishmaniasis, inhibited both putrescine and spermidine transport non-competitively.

Modification of kinetoplast DNA minicircle composition in pentamidine-resistant Leishmania

Pentamidine, an antiprotozoal drug, was shown to have various cellular and molecular targets depending on the organism. In Leishmania, ultrastructural modifications of kinetoplast and mitochondria have been observed but no data is available on cellular and molecular events involved in development of pentamidine-resistance. The absence of modification of minicircle DNA in pentamidine treated L. donovani and L. amazonensis promastigotes suggested that topoisomerase II activity is not a target. This result was confirmed by quantitation of the enzyme by immunodetection. Southern blot experiments indicated that the kDNA network was altered in resistant clones. Molecular cloning and sequence analysis of kDNA minicircles showed transkinetoplastidy hitherto reported only for arsenite- and tunicamycin-resistant Leishmania. Comparison of wild-type and resistant sequences showed only 32-51% homology. The AT-rich regions, known as binding sites, of the drug occurred less frequently in the resistant clones and their locations were different. These minicircle sequence modifications leading to decreased binding sites for the drug might contribute to pentamidine-resistance in Leishmania.

Alterations in membrane fluidity, lipid metabolism, mitochondrial activity, and lipophosphoglycan expression in pentamidine-resistant Leishmania

Cytoplasmic and mitochondrial membranes were studied in wild-type promastigotes of Leishmania donovani and L. amazonensis treated with pentamidine and in the parasites resistant to this drug. Analyses by flow cytometry showed membrane fluidification in resistant cells and a modification of the lipidic metabolism in pentamidine-treated cells and in resistant clones as compared with wild-type ones. Pentamidine decreased the amounts of membrane polar lipids, i.e., phospholipids, and neutral lipidic droplets in the cytoplasm. Perturbation of lipid constituents did not induce modifications of membrane acid phosphatase and protease activities. Lipophosphoglycan, the major cell-surface glycoconjugate, was no longer detected by Western blotting in resistant cells. The use of rhodamine 123 showed a decrease in transmembrane mitochondrial potential in wild-type cells treated with pentamidine and in resistant parasites. In conclusion, cytoplasmic and mitochondrial membranes and lipidic metabolism are altered in pentamidine-resistant parasites.

Effects of pentamidine on polyamine level and biosynthesis in wild-type, pentamidine-treated, and pentamidine-resistant Leishmania

Polyamine biosynthesis was studied in wild-type promastigotes of Leishmania donovani and Leishmania amazonensis treated with pentamidine and in the parasites resistant to this drug. Treatment of wild-type clones with low pentamidine concentrations for 24 hr provoked a strong decrease in arginine, ornithine, and putrescine pools, while the level of intracellular spermidine remained unchanged. In these cells, the activity of the enzyme ornithine decarboxylase was found to be decreased. Compared to wild-type cells, resistant clones had a lower level of putrescine, higher pools of arginine and ornithine, and a similar spermidine content. Analysis by Western blot and DFMO-binding showed reduced amount of ornithine decarboxylase. Furthermore, in the resistant cells, the kinetic parameters of the enzyme spermidine synthase were markedly changed, showing increased affinity to putrescine and decreased affinity to pentamidine. Thus, it seems that polyamine biosynthesis pathway is a target of pentamidine in Leishmania and is altered in resistant clones.

Altered transport properties of pentamidine-resistant Leishmania donovani and L. amazonensis promastigotes

Pentamidine-resistant clones of Leishmania donovani and L. amazonensis promastigotes were developed by increase of the drug pressure in the culture medium and characterized. The resistant clones could grow in 40 and 20 microM pentamidine as determined for L. donovani and L. amazonensis, respectively, with 50% inhibitory concentrations (IC50 values) being 140 and 60 microM, which were 18 and 75 times higher than those recorded for the parental clones, respectively. Biochemical analysis of the clones showed that the acquired pentamidine resistance was specific (no cross-resistance to unrelated drugs and no reversibility with verapamil) and stable in vitro and in vivo. Pentamidine resistance is related to decreased drug uptake and highly increased efflux in both clones of Leishmania spp., accompanied by an alteration in polyamine carriers. Furthermore, a modification of the uptake of pyrimidine nucleosides and several amino acids by these resistant clones indicates alterations in the surface membrane.

Pentamidine uptake in Leishmania donovani and Leishmania amazonensis promastigotes and axenic amastigotes

A transport system for pentamidine in Leishmania donovani and Leishmania amazonensis promastigotes and axenic amastigotes has been identified and characterized. Pentamidine is not metabolized by these parasites. Its uptake process is saturable, carrier-mediated and energy-dependent. This drug does not inhibit purine or pyrimidine uptake, whereas it inhibits uptake of several amino acids non-competitively and that of putrescine and spermidine competitively. The results suggest that pentamidine shares polyamine-carrier systems in these parasites.

Sinefungin shares AdoMet-uptake system to enter Leishmania donovani promastigotes

The involvement of a carrier for sinefungin (SF) uptake in Leishmania donovani promastigotes is indicated by saturation kinetics, competition studies and SF accumulation against a 270-fold concentration gradient across the cell membrane. Whether SF uptake occurs via nucleoside- or AdoMet-carrier systems was investigated by competition experiments and comparison of the uptake of various molecules in wild-type and SF-resistant cells. Results show that SF did not inhibit purine or pyrimidine uptake whereas it competitively inhibited AdoMet uptake. Furthermore, the uptake of nucleosides in SF-resistant cells is similar to that in wild-type cells, whereas uptake of SF and AdoMet is lower.