Arginase Is Essential for Survival of Leishmania donovani Promastigotes but Not Intracellular Amastigotes.

Studies of Leishmania donovani have shown that both ornithine decarboxylase and spermidine synthase, two enzymes of the polyamine biosynthetic pathway, are critical for promastigote proliferation and required for maximum infection in mice. However, the importance of arginase (ARG), the first enzyme of the polyamine pathway in Leishmania, has not been analyzed in L. donovani To test ARG function in intact parasites, we generated Δarg null mutants in L. donovani and evaluated their ability to proliferate in vitro and trigger infections in mice. The Δarg knockout was incapable of growth in the absence of polyamine supplementation, but the auxotrophic phenotype could be bypassed by addition of either millimolar concentrations of ornithine or micromolar concentrations of putrescine or by complementation with either glycosomal or cytosolic versions of ARG. Spermidine supplementation of the medium did not circumvent the polyamine auxotrophy of the Δarg line. Although ARG was found to be essential for ornithine and polyamine synthesis, ornithine decarboxylase appeared to be the rate-limiting enzyme for polyamine production. Mouse infectivity studies revealed that the Δarg lesion reduced parasite burdens in livers by an order of magnitude but had little impact on the numbers of parasites recovered from spleens. Thus, ARG is essential for proliferation of promastigotes but not intracellular amastigotes. Coupled with previous studies, these data support a model in which L. donovani amastigotes readily salvage ornithine and have some access to host spermidine pools, while host putrescine appears to be unavailable for salvage by the parasite.

GMP reductase and genetic uncoupling of adenylate and guanylate metabolism in Leishmania donovani parasites.

Purine acquisition is an essential nutritional process for Leishmania. Although purine salvage into adenylate nucleotides has been investigated in detail, little attention has been focused on the guanylate branch of the purine pathway. To characterize guanylate nucleotide metabolism in Leishmania and create a cell culture model in which the pathways for adenylate and guanylate nucleotide synthesis can be genetically uncoupled for functional studies in intact cells, we created and characterized null mutants of L. donovani that were deficient in either GMP reductase alone (Δgmpr) or in both GMP reductase and its paralog IMP dehydrogenase (Δgmpr/Δimpdh). Whereas wild type parasites were capable of utilizing virtually any purine nucleobase/nucleoside, the Δgmpr and Δgmpr/Δimpdh null lines exhibited highly restricted growth phenotypes. The Δgmpr single mutant could not grow in xanthine, guanine, or their corresponding nucleosides, while no purine on its own could support the growth of Δgmpr/Δimpdh cells. Permissive growth conditions for the Δgmpr/Δimpdh necessitated both xanthine, guanine, or the corresponding nucleosides, and additionally, a second purine that could serve as a source for adenylate nucleotide synthesis. Interestingly, GMPR, like its paralog IMPDH, is compartmentalized to the leishmanial glycosome, a process mediated by its COOH-terminal peroxisomal targeting signal. The restricted growth phenotypes displayed by the L. donovani Δgmpr and Δgmpr/Δimpdh null mutants confirms the importance of GMPR in the purine interconversion processes of this parasite.

A role for adenine nucleotides in the sensing mechanism to purine starvation in Leishmania donovani.

Purine salvage by Leishmania is an obligatory nutritional process that impacts both cell viability and growth. Previously, we have demonstrated that the removal of purines in culture provokes significant metabolic changes that enable Leishmania to survive prolonged periods of purine starvation. In order to understand how Leishmania sense and respond to changes in their purine environment, we have exploited several purine pathway mutants, some in which adenine and guanine nucleotide metabolism is uncoupled. While wild type parasites grow in any one of a variety of naturally occurring purines, the proliferation of these purine pathway mutants requires specific types or combinations of exogenous purines. By culturing purine pathway mutants in high levels of extracellular purines that are either permissive or non-permissive for growth and monitoring for previously defined markers of the adaptive response to purine starvation, we determined that adaptation arises from a surveillance of intracellular purine nucleotide pools rather than from a direct sensing of the extracellular purine content of the environment. Specifically, our data suggest that perturbation of intracellular adenine-containing nucleotide pools provides a crucial signal for inducing the metabolic changes necessary for the long-term survival of Leishmania in a purine-scarce environment.

Adenine and adenosine salvage in Leishmania donovani.

6-aminopurine metabolism in Leishmania is unique among trypanosomatid pathogens since this genus expresses two distinct routes for adenine salvage: adenine phosphoribosyltransferase (APRT) and adenine deaminase (AAH). To evaluate the relative contributions of APRT and AAH, adenine salvage was evaluated in Δaprt, Δaah, and Δaprt/Δaah null mutants of L. donovani. The data confirm that AAH plays the dominant role in adenine metabolism in L. donovani, although either enzyme alone is sufficient for salvage. Adenosine salvage was also evaluated in a cohort of null mutants. Adenosine is also primarily converted to hypoxanthine, either intracellularly or extracellularly, but can also be phosphorylated to the nucleotide level by adenosine kinase when the predominant pathways are genetically or pharmacologically blocked. These data provide genetic verification for the relative contributions of 6-aminopurine metabolizing pathways in L. donovani and demonstrate that all of the pathways can function under appropriate conditions of genetic or pharmacologic perturbation.

Adenylosuccinate synthetase and adenylosuccinate lyase deficiencies trigger growth and infectivity deficits in Leishmania donovani.

Leishmania are auxotrophic for purines, and consequently purine acquisition from the host is a requisite nutritional function for the parasite. Both adenylosuccinate synthetase (ADSS) and adenylosuccinate lyase (ASL) have been identified as vital components of purine salvage in Leishmania donovani, and therefore Δadss and Δasl null mutants were constructed to test this hypothesis. Unlike wild type L. donovani, Δadss and Δasl parasites in culture exhibited a profoundly restricted growth phenotype in which the only permissive growth conditions were a 6-aminopurine source in the presence of 2'-deoxycoformycin, an inhibitor of adenine aminohydrolase activity. Although both knock-outs showed a diminished capacity to infect murine peritoneal macrophages, only the Δasl null mutant was profoundly incapacitated in its ability to infect mice. The enormous discrepancy in parasite loads observed in livers and spleens from mice infected with either Δadss or Δasl parasites can be explained by selective accumulation of adenylosuccinate in the Δasl knock-out and consequent starvation for guanylate nucleotides. Genetic complementation of a Δasl lesion in Escherichia coli implied that the L. donovani ASL could also recognize 5-aminoimidazole-(N-succinylocarboxamide) ribotide as a substrate, and purified recombinant ASL displayed an apparent Km of ∼24 µm for adenylosuccinate. Unlike many components of the purine salvage pathway of L. donovani, both ASL and ADSS are cytosolic enzymes. Overall, these data underscore the paramount importance of ASL to purine salvage by both life cycle stages of L. donovani and authenticate ASL as a potential drug target in Leishmania.

Purine salvage in Leishmania: complex or simple by design?

Purine nucleotides function in a variety of vital cellular and metabolic processes including energy production, cell signaling, synthesis of vitamin-derived cofactors and nucleic acids, and as determinants of cell fate. Unlike their mammalian and insect hosts, Leishmania cannot synthesize the purine ring de novo and are absolutely dependent upon them to meet their purine requirements. The obligatory nature of purine salvage in these parasites, therefore, offers an attractive paradigm for drug targeting and, consequently, the delineation of the pathway has been under scientific investigation for over 30 years. Here, we review recent developments that reveal how purines flux in Leishmania and offer a potential 'Achilles' heel' for future validation.

Genetic dissection of pyrimidine biosynthesis and salvage in Leishmania donovani.

Protozoan parasites of the Leishmania genus express the metabolic machinery to synthesize pyrimidine nucleotides via both de novo and salvage pathways. To evaluate the relative contributions of pyrimidine biosynthesis and salvage to pyrimidine homeostasis in both life cycle stages of Leishmania donovani, individual mutant lines deficient in either carbamoyl phosphate synthetase (CPS), the first enzyme in pyrimidine biosynthesis, uracil phosphoribosyltransferase (UPRT), a salvage enzyme, or both CPS and UPRT were constructed. The Δcps lesion conferred pyrimidine auxotrophy and a growth requirement for medium supplementation with one of a plethora of pyrimidine nucleosides or nucleobases, although only dihydroorotate or orotate could circumvent the pyrimidine auxotrophy of the Δcps/Δuprt double knockout. The Δuprt null mutant was prototrophic for pyrimidines but could not salvage uracil or any pyrimidine nucleoside. The capability of the Δcps parasites to infect mice was somewhat diminished but still robust, indicating active pyrimidine salvage by the amastigote form of the parasite, but the Δcps/Δuprt mutant was completely attenuated with no persistent parasites detected after a 4-week infection. Complementation of the Δcps/Δuprt clone with either CPS or UPRT restored infectivity. These data establish that an intact pyrimidine biosynthesis pathway is essential for the growth of the promastigote form of L. donovani in culture, that all uracil and pyrimidine nucleoside salvage in the parasite is mediated by UPRT, and that both the biosynthetic and salvage pathways contribute to a robust infection of the mammalian host by the amastigote. These findings impact potential therapeutic design and vaccine strategies for visceral leishmaniasis.

Adenine aminohydrolase from Leishmania donovani: unique enzyme in parasite purine metabolism.

Adenine aminohydrolase (AAH) is an enzyme that is not present in mammalian cells and is found exclusively in Leishmania among the protozoan parasites that infect humans. AAH plays a paramount role in purine metabolism in this genus by steering 6-aminopurines into 6-oxypurines. Leishmania donovani AAH is 38 and 23% identical to Saccharomyces cerevisiae AAH and human adenosine deaminase enzymes, respectively, catalyzes adenine deamination to hypoxanthine with an apparent K(m) of 15.4 µM, and does not recognize adenosine as a substrate. Western blot analysis established that AAH is expressed in both life cycle stages of L. donovani, whereas subcellular fractionation and immunofluorescence studies confirmed that AAH is localized to the parasite cytosol. Deletion of the AAH locus in intact parasites established that AAH is not an essential gene and that Δaah cells are capable of salvaging the same range of purine nucleobases and nucleosides as wild type L. donovani. The Δaah null mutant was able to infect murine macrophages in vitro and in mice, although the parasite loads in both model systems were modestly reduced compared with wild type infections. The Δaah lesion was also introduced into a conditionally lethal Δhgprt/Δxprt mutant in which viability was dependent on pharmacologic ablation of AAH by 2'-deoxycoformycin. The Δaah/Δhgprt/Δxprt triple knock-out no longer required 2'-deoxycoformycin for growth and was avirulent in mice with no persistence after a 4-week infection. These genetic studies underscore the paramount importance of AAH to purine salvage by L. donovani.

IMP dehydrogenase deficiency in Leishmania donovani causes a restrictive growth phenotype in promastigotes but is not essential for infection in mice.

Leishmania cannot synthesize purines de novo and therefore must scavenge purines from its host for survival and growth. Biochemical and genomic analyses have indicated that Leishmania species express three potential routes for the synthesis of guanylate nucleotides: (1) a two-step pathway that converts IMP to GMP; (2) a three-step pathway that starts with the deamination of guanine to xanthine, followed by phosphoribosylation to XMP and then conversion to GMP; or (3) direct guanine phosphoribosylation by HGPRT. To determine the role of the first of these pathways to guanylate nucleotide synthesis, an L. donovani line deficient in IMP dehydrogenase (IMPDH), the first step in the IMP to GMP pathway, was constructed by targeted gene replacement. The Δimpdh lesion triggered a highly restrictive growth phenotype in promastigotes in culture but did not impact parasitemias in mice. The dispensability of IMPDH in vivo is the first definitive demonstration that intracellular L. donovani amastigotes have access to a sufficient pool of guanine, xanthine, or guanylate precursors from the host.

The Leishmania donovani UMP synthase is essential for promastigote viability and has an unusual tetrameric structure that exhibits substrate-controlled oligomerization.

The final two steps of de novo uridine 5'-monophosphate (UMP) biosynthesis are catalyzed by orotate phosphoribosyltransferase (OPRT) and orotidine 5'-monophosphate decarboxylase (OMPDC). In most prokaryotes and simple eukaryotes these two enzymes are encoded by separate genes, whereas in mammals they are expressed as a bifunctional gene product called UMP synthase (UMPS), with OPRT at the N terminus and OMPDC at the C terminus. Leishmania and some closely related organisms also express a bifunctional enzyme for these two steps, but the domain order is reversed relative to mammalian UMPS. In this work we demonstrate that L. donovani UMPS (LdUMPS) is an essential enzyme in promastigotes and that it is sequestered in the parasite glycosome. We also present the crystal structure of the LdUMPS in complex with its product, UMP. This structure reveals an unusual tetramer with two head to head and two tail to tail interactions, resulting in two dimeric OMPDC and two dimeric OPRT functional domains. In addition, we provide structural and biochemical evidence that oligomerization of LdUMPS is controlled by product binding at the OPRT active site. We propose a model for the assembly of the catalytically relevant LdUMPS tetramer and discuss the implications for the structure of mammalian UMPS.

Characterization of amplicons that suppress the conditional lethal growth phenotype of a Leishmania donovani mutant lacking normal purine salvage mechanisms.

A conditionally lethal mutant of Leishmania donovani that lacks both hypoxanthine-guanine phosphoribosyltransferase and xanthine phosphoribosyltransferase exhibits a strikingly restricted growth phenotype, can only survive as the promastigote under pharmacological constraints, and is profoundly compromised in its ability to infect macrophages and mice. Interestingly, the conditionally lethal growth phenotype displayed by these mutant parasites can be suppressed in vitro by selection of strains that have markedly amplified the adenine phosphoribosyltransferase gene on extrachromosomal elements that are unique to these suppressor strains. Employing pulsed field gel electrophoresis, we have now determined that the amplicons in two of these suppressor lines are linear molecules by: (1) their pulse time-dependent mobility; (2) the failure of γ-irradiation to generate new discrete bands; (3) their susceptibility to λ exonuclease digestion; and (4) the presence of telomeric sequences. Pulsed field gel electrophoresis also shows these amplicons to be approximately 200-275kb in size. However, quantitative polymerase chain reaction and Southern blot analyses demonstrated that the amplification units are ∼40kb in length, implying that the formation of these amplicons involved additional chromosomal rearrangements or oligomerization.

Amplification of adenine phosphoribosyltransferase suppresses the conditionally lethal growth and virulence phenotype of Leishmania donovani mutants lacking both hypoxanthine-guanine and xanthine phosphoribosyltransferases.

Leishmania donovani cannot synthesize purines de novo and obligatorily scavenge purines from the host. Previously, we described a conditional lethal Deltahgprt/Deltaxprt mutant of L. donovani (Boitz, J. M., and Ullman, B. (2006) J. Biol. Chem. 281, 16084-16089) that establishes that L. donovani salvages purines primarily through hypoxanthine-guanine phosphoribosyltransferase (HGPRT) and xanthine phosphoribosyltransferase (XPRT). Unlike wild type L. donovani, the Deltahgprt/Deltaxprt knock-out cannot grow on 6-oxypurines and displays an absolute requirement for adenine or adenosine and 2'-deoxycoformycin, an inhibitor of parasite adenine aminohydrolase activity. Here, we demonstrate that the ability of Deltahgprt/Deltaxprt parasites to infect mice was profoundly compromised. Surprisingly, mutant parasites that survived the initial passage through mice partially regained their virulence properties, exhibiting a >10-fold increase in parasite burden in a subsequent mouse infection. To dissect the mechanism by which Deltahgprt/Deltaxprt parasites persisted in vivo, suppressor strains that had regained their capacity to grow under restrictive conditions were cloned from cultured Deltahgprt/Deltaxprt parasites. The ability of these suppressor clones to grow in and metabolize 6-oxypurines could be ascribed to a marked amplification and overexpression of the adenine phosphoribosyltransferase (APRT) gene. Moreover, transfection of Deltahgprt/Deltaxprt cells with an APRT episome recapitulated the suppressor phenotype in vitro and enabled growth on 6-oxypurines. Biochemical studies further showed that hypoxanthine, unexpectedly, was an inefficient substrate for APRT, evidence that could account for the ability of the suppressors to metabolize hypoxanthine. Subsequent analysis implied that APRT amplification was also a potential contributory mechanism by which Deltahgprt/Deltaxprt parasites displayed persistence and increased virulence in mice.

Leishmania donovani ornithine decarboxylase is indispensable for parasite survival in the mammalian host.

Mutations within the polyamine biosynthetic pathway of Leishmania donovani, the etiological agent of visceral leishmaniasis, confer polyamine auxotrophy to the insect vector or promastigote form of the parasite. However, whether the infectious or amastigote form of the parasite requires an intact polyamine pathway has remained an open question. To address this issue, conditionally lethal Deltaodc mutants lacking ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine biosynthesis, were created by double targeted gene replacement within a virulent strain of L. donovani. ODC-deficient promastigotes and axenic amastigotes were auxotrophic for polyamines and capable of robust growth only when exogenous putrescine was supplied in the culture medium, confirming that polyamine biosynthesis is an essential nutritional pathway for L. donovani promastigotes. To assess whether the Deltaodc lesion also affected the ability of amastigotes to sustain a robust infection, macrophage and mouse infectivity experiments were performed. Parasite loads in murine macrophages infected with each of two independent Deltaodc knockout lines were decreased approximately 80% compared to their wild-type counterpart. Furthermore, alpha-difluoromethylornithine, a suicide inhibitor of ODC, inhibited growth of wild-type L. donovani amastigotes and effectively cured macrophages of parasites, thereby preventing host cell destruction. Strikingly, however, parasitemias of both Deltaodc null mutants were reduced by 6 and 3 orders of magnitude, respectively, in livers and spleens of BALB/c mice. The compromised infectivity phenotypes of the Deltaodc knockouts in both macrophages and mice were rescued by episomal complementation of the genetic lesion. These genetic and pharmacological studies strongly implicate ODC as an essential cellular determinant that is necessary for the viability and growth of both L. donovani promastigotes and amastigotes and intimate that pharmacological inhibition of ODC is a promising therapeutic paradigm for the treatment of visceral and perhaps other forms of leishmaniasis.

Purine and pyrimidine metabolism in Leishmania.

Purines and pyrimidines are indispensable to all life, performing many vital functions for cells: ATP serves as the universal currency of cellular energy, cAMP and cGMP are key second messenger molecules, purine and pyrimidine nucleotides are precursors for activated forms of both carbohydrates and lipids, nucleotide derivatives of vitamins are essential cofactors in metabolic processes, and nucleoside triphosphates are the immediate precursors for DNA and RNA synthesis. Unlike their mammalian and insect hosts, Leishmania lack the metabolic machinery to make purine nucleotides de novo and must rely on their host for preformed purines. The obligatory nature of purine salvage offers, therefore, a plethora of potential targets for drug targeting, and the pathway has consequently been the focus of considerable scientific investigation. In contrast, Leishmania are prototrophic for pyrimidines and also express a small complement of pyrimidine salvage enzymes. Because the pyrimidine nucleotide biosynthetic pathways of Leishmania and humans are similar, pyrimidine metabolism in Leishmania has generally been considered less amenable to therapeutic manipulation than the purine salvage pathway. However, evidence garnered from a variety of parasitic protozoa suggests that the selective inhibition of pyrimidine biosynthetic enzymes offers a rational therapeutic paradigm. In this chapter, we present an overview of the purine and pyrimidine pathways in Leishmania, make comparisons to the equivalent pathways in their mammalian host, and explore how these pathways might be amenable to selective therapeutic targeting.

Kinetic characterization of inosine monophosphate dehydrogenase of Leishmania donovani.

Trypanosomatid protozoan pathogens are purine auxotrophs that are highly dependent on the enzyme inosine monophosphate dehydrogenase (IMPDH) for the synthesis of guanylate nucleotides. Enzymatic characterization of the Leishmania donovani IMPDH (LdIMPDH) overexpressed in E. coli revealed that this enzyme was highly specific for the substrates IMP and NAD(+) with K(m)(app) values of 33 and 390 microM, respectively. In contrast to other IMPDHs, LdIMPDH exhibits no substrate inhibition in high concentrations of NAD(+). Kinetic studies revealed that XMP and GMP were inhibitors with K(i) values of approximately 26 and 210 microM, respectively, suggesting that these nucleotides may regulate LdIMPDH activity. Mycophenolic acid was also a potent inhibitor of L. donovani IMPDH with a K(i) value of approximately 25 nM. Confocal immunofluorescence microscopy and subcellular fractionation localized LdIMPDH to the glycosome. Protein-protein interaction assays revealed that LdIMPDH associated tightly with glycosomal protein sorting receptor LdPEX5.

Functional characterization of nucleoside transporter gene replacements in Leishmania donovani.

Leishmania donovani express two nucleoside transporters of non-overlapping ligand selectivity. To evaluate the physiological role of nucleoside transporters in L. donovani, homozygous null mutants of the genes encoding the LdNT1 adenosine-pyrimidine nucleoside transporter and the LdNT2 inosine-guanosine transporter were created singly and in combination by single targeted gene replacement followed by selection for loss-of-heterozygosity. The mutant alleles were verified by Southern blotting, and the effects of gene replacement on transport phenotype were evaluated by rapid sampling transport measurements and by drug resistance profiles. The Deltaldnt1, Deltaldnt2, and Deltaldnt1/Deltaldnt2 mutants were all capable of proliferation in defined culture medium supplemented with any of a spectrum of purine nucleobases or nucleosides, except that a Deltaldnt2 lesion conferred an inability to efficiently salvage exogenous xanthosine, a newly discovered ligand of LdNT2. Each of the three knockout strains was viable as promastigotes and axenic amastigotes and capable of maintaining an infection in J774 and bone marrow-derived murine macrophages. These genetic studies demonstrate: (1) that L. donovani promastigotes, axenic amastigotes, and tissue amastigotes are viable in the absence of nucleoside transport; (2) that nucleoside transporters are not essential for sustaining an infection in mammalian host cells; (3) that the phagolysosome of macrophages is likely to contain purines that are not LdNT1 or LdNT2 ligands, i.e., nucleobases. Furthermore, the Deltaldnt1, Deltaldnt2, and Deltaldnt1/Deltaldnt2 knockouts offer a unique genetically defined null background for the biochemical and genetic characterization of nucleoside transporter genes and cDNAs from phylogenetically diverse species and of genetically manipulated LdNT1 and LdNT2 constructs.

Leishmania donovani singly deficient in HGPRT, APRT or XPRT are viable in vitro and within mammalian macrophages.

Leishmania species express three phosphoribosyltransferase enzymes, hypoxanthine-guanine phosphoribosyltransferase (HGPRT), adenine phosphoribosyltransferase (APRT), and xanthine phosphoribosyltransferase (XPRT), which enable this genus to acquire purine nutrients from their hosts. To test whether any of these enzymes is essential for viability, transformation into amastigotes, and infectivity and proliferation within mammalian macrophages, Deltahgprt, Deltaaprt, and Deltaxprt null mutants were created by targeted gene replacement within a virulent background of Leishmania donovani. Each of the three knockout strains was viable as promastigotes and axenic amastigotes and capable of maintaining an infection in bone marrow-derived murine macrophages. These data support the hypothesis that none of the three phosphoribosyltransferases is essential for purine salvage or viability by itself and that purine salvage occurs through multiple anabolic routes in both parasite life cycle stages. In addition these studies revealed the presence of an adenine aminohydrolase enzyme in L. donovani axenic amastigotes, an activity previously thought to be restricted to promastigotes.

A conditional mutant deficient in hypoxanthine-guanine phosphoribosyltransferase and xanthine phosphoribosyltransferase validates the purine salvage pathway of Leishmania donovani.

Leishmania donovani cannot synthesize purines de novo and express a multiplicity of enzymes that enable them to salvage purines from their hosts. Previous efforts to generate an L. donovani strain deficient in both hypoxanthine-guanine phosphoribosyl-transferase (HGPRT) and xanthine phosphoribosyltransferase (XPRT) using gene replacement approaches were not successful, lending indirect support to the hypothesis that either HGPRT or XPRT is crucial for purine salvage by the parasite. We now report the genetic confirmation of this hypothesis through the construction of a conditional delta hgprt/delta xprt mutant strain that exhibits an absolute requirement for 2'-deoxycoformycin, an inhibitor of the leishmanial adenine aminohydrolase enzyme, and either adenine or adenosine as a source of purine. Unlike wild type parasites, the delta hgprt/delta xprt strain cannot proliferate indefinitely without 2'-deoxycoformycin or with hypoxanthine, guanine, xanthine, guanosine, inosine, or xanthosine as the sole purine nutrient. The delta hgprt/delta xprt mutant infects murine bone marrow-derived macrophages <5% as effectively as wild type parasites and cannot sustain an infection. These data establish genetically that either HGPRT or XPRT is absolutely essential for purine acquisition, parasite viability, and parasite infectivity of mouse macrophages, that all exogenous purines are funneled to hypoxanthine and/or xanthine by L. donovani, and that the purine sources within the macrophage to which the parasites have access are HGPRT or XPRT substrates.

Subcellular localization of adenine and xanthine phosphoribosyltransferases in Leishmania donovani.

The subcellular location of a protein is a critical factor in its physiological function and an important consideration in therapeutic paradigms that target the protein. Because Leishmania donovani cannot synthesize purine nucleotides de novo, they rely predominantly upon therapeutically germane phosphoribosyltransferase (PRT) enzymes, hypoxanthine-guanine PRT (HGPRT), adenine PRT (APRT), and xanthine PRT (XPRT), for purine acquisition from the host. Previous studies have shown that the L. donovani HGPRT is localized to the glycosome, a fuel-metabolizing microbody that is unique to kinetoplastid parasites [J. Biol. Chem. 273 (1998) 1534]. The sequences of the other two PRTs indicate that XPRT, but not APRT, possesses a COOH-terminal tripeptide that mediates protein targeting to the glycosome. To determine definitively the intracellular milieu of APRT and XPRT, polyclonal antibodies were raised to each recombinant protein. APRT and XPRT were then shown by immunofluorescence to be localized to the cytosol and glycosome, respectively. The glycosomal milieu for XPRT was also verified by immunoelectron microscopy. Amputation of the glycosomal targeting signal from XPRT resulted in protein mislocalization to the cytosol, but the cytosolic xprt was still functional with respect to purine salvage. These studies establish that APRT is cytosolic and XPRT, like the homologous HGPRT, is glycosomal and demonstrate that a mutant xprt protein that mislocalizes to the cytosol is still functional and supports parasite viability.