Human Hypoxanthine-guanine Phosphoribosyltransferase Research Articles

Metabolic and neurobehavioral disturbances induced by purine recycling deficiency in Drosophila.

Adenine phosphoribosyltransferase (APRT) and hypoxanthine-guanine phosphoribosyltransferase (HGPRT) are two structurally related enzymes involved in purine recycling in humans. Inherited mutations that suppress HGPRT activity are associated with Lesch-Nyhan disease (LND), a rare X-linked metabolic and neurological disorder in children, characterized by hyperuricemia, dystonia, and compulsive self-injury. To date, no treatment is available for these neurological defects and no animal model recapitulates all symptoms of LND patients. Here, we studied LND-related mechanisms in the fruit fly. By combining enzymatic assays and phylogenetic analysis, we confirm that no HGPRT activity is expressed in Drosophila melanogaster, making the APRT homolog (Aprt) the only purine-recycling enzyme in this organism. Whereas APRT deficiency does not trigger neurological defects in humans, we observed that Drosophila Aprt mutants show both metabolic and neurobehavioral disturbances, including increased uric acid levels, locomotor impairments, sleep alterations, seizure-like behavior, reduced lifespan, and reduction of adenosine signaling and content. Locomotor defects could be rescued by Aprt re-expression in neurons and reproduced by knocking down Aprt selectively in the protocerebral anterior medial (PAM) dopaminergic neurons, the mushroom bodies, or glia subsets. Ingestion of allopurinol rescued uric acid levels in Aprt-deficient mutants but not neurological defects, as is the case in LND patients, while feeding adenosine or N6-methyladenosine (m6A) during development fully rescued the epileptic behavior. Intriguingly, pan-neuronal expression of an LND-associated mutant form of human HGPRT (I42T), but not the wild-type enzyme, resulted in early locomotor defects and seizure in flies, similar to Aprt deficiency. Overall, our results suggest that Drosophila could be used in different ways to better understand LND and seek a cure for this dramatic disease.

Metabolic and neurobehavioral disturbances induced by purine recycling deficiency in Drosophila

Adenine phosphoribosyltransferase (APRT) and hypoxanthine-guanine phosphoribosyltransferase (HGPRT) are two structurally related enzymes involved in purine recycling in humans. Inherited mutations that suppress HGPRT activity are associated with Lesch–Nyhan disease (LND), a rare X-linked metabolic and neurological disorder in children, characterized by hyperuricemia, dystonia, and compulsive self-injury. To date, no treatment is available for these neurological defects and no animal model recapitulates all symptoms of LND patients. Here, we studied LND-related mechanisms in the fruit fly. By combining enzymatic assays and phylogenetic analysis, we confirm that no HGPRT activity is expressed in Drosophila melanogaster, making the APRT homolog (Aprt) the only purine-recycling enzyme in this organism. Whereas APRT deficiency does not trigger neurological defects in humans, we observed that Drosophila Aprt mutants show both metabolic and neurobehavioral disturbances, including increased uric acid levels, locomotor impairments, sleep alterations, seizure-like behavior, reduced lifespan, and reduction of adenosine signaling and content. Locomotor defects could be rescued by Aprt re-expression in neurons and reproduced by knocking down Aprt selectively in the protocerebral anterior medial (PAM) dopaminergic neurons, the mushroom bodies, or glia subsets. Ingestion of allopurinol rescued uric acid levels in Aprt-deficient mutants but not neurological defects, as is the case in LND patients, while feeding adenosine or N6-methyladenosine (m6A) during development fully rescued the epileptic behavior. Intriguingly, pan-neuronal expression of an LND-associated mutant form of human HGPRT (I42T), but not the wild-type enzyme, resulted in early locomotor defects and seizure in flies, similar to Aprt deficiency. Overall, our results suggest that Drosophila could be used in different ways to better understand LND and seek a cure for this dramatic disease.

Gas‐phase experimental and computational studies of human hypoxanthine‐guanine phosphoribosyltransferase substrates: Intrinsic properties and biological implications

AbstractThe gas‐phase acidity and proton affinity of nucleobases that are substrates for the enzyme human hypoxanthine‐guanine phosphoribosyltransferase (HGPRT) have been examined using both theoretical and experimental methods. These thermochemical values have not heretofore been measured and provide experimental data to benchmark the computational results. HGPRT is important for human health and is also a key target for antiparasitic chemotherapy. We use our gas‐phase results to lend insight into the HGPRT mechanism and also propose kinetic isotope studies that could potentially differentiate between possible mechanisms.

Abstract 1840: Isolation and evaluation of human nanobodies against hypoxanthine guanine phosphoribosyltransferase (HPRT) and their potential use for the detection and targeting of lung, breast and colon cancer cells

Abstract Since the clinical relevance of hypoxanthine guanine phosphoribosyltransferase (HPRT) as both a cancer biomarker and a potential tumor target is increasing, we isolated human nanobodies (Nbs) against HPRT and tested their potential to detect and target cytosolic and membrane associated HPRT in lung, breast and colon cancer cells. HPRT is a purine salvage pathway enzyme that catalyzes the conversion of hypoxanthine to inosine monophosphate and guanine to guanosine monophosphate. Until recently, HPRT was considered as a housekeeping gene and its clinical relevance was mainly limited to congenital central nervous disorders such as Lesch-Nyhan disease. However, recent gene expression analysis across all cancers in The Cancer Genome Atlas (TCGA) database and analysis of HPRT expression within cancer patients' tissues has revealed that the expression of HPRT is elevated in the majority of malignant tissues in comparison to normal tissues. Moreover, multiple studies have recently shown that HPRT may be associated with the cell membrane in lung, colon cancer and lymphoma cells, and not in normal cells. Thus, HPRT is a potential immunotherapeutic target for cancer. Using phage display technology we isolated 384 nanobody clones from the Christ human single domain antibody (dAb) library against human HPRT. Ten of the most sensitive Nbs were sequenced and evaluated to determine their sensitivity using dose response curves. Four parameter logistic regression analysis of the top 10 HPRT nanobodies showed R squared values ranging from 0.9388-0.9989 and a sigmoidal shape denoting a concentration-dependent antigen-ligand relationship. The anti-HPRT Nbs were able to detect minimum HPRT amounts ranging from 10-20 ng/ml. Validation of the anti-HPRT Nbs was conducted with a siRNA HPRT knockdown in A549 cells. A549 siRNA HPRT knockdown cell lysate showed at least a 5-fold reduction in the overall signal in phage ELISA compared to A549 wild type cell lysate (p<0.001) indicating the specificity of the Nbs to HPRT. In addition, flow cytometry analysis showed that the anti-HPRT Nbs were able to detect membrane associated HPRT on A549 (20%), NCI-H460 (23%), HT-29 (41%) , SW620 (46%) and MDA-MB-231 (32%) cancer cell lines. Anti-HPRT Nbs did not show significant binding to normal mononuclear cells compared to cancer cells (p<0.002). Anti-HPRT Nbs were successfully expressed and purified using the pET-scFv expression system. Human anti-HPRT Nbs may be used for the detection of both cytosolic and membrane associate HPRT. Future directions of this research will explore the uses of anti-HPRT-Nb-based therapies and their use in other applications such as chimeric antigen receptors (CARs) for cell adoptive therapies. Citation Format: Edwin J. Velazquez, Tyler B. Humpherys, Toni O. Mortimer, David M. Bellini, Kathryn R. Smith, Rachel M. Morris, Abigail Goodman, Kim L. O'Neill. Isolation and evaluation of human nanobodies against hypoxanthine guanine phosphoribosyltransferase (HPRT) and their potential use for the detection and targeting of lung, breast and colon cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1840.

Biosensor-based monitoring of the central metabolic pathway metabolites

In vivo biosensors have a wide range of applications, from the detection of metabolites to the regulation of metabolic networks, providing a versatile tool for cell biology and metabolic engineering. However, compared with the vast array of small molecules present in nature, the existing range of biosensors is far from sufficient. Here we describe the use of human hypoxanthine guanine phosphoribosyltransferase (HGPRT) as a ligand binding domain (LBD) protein, that acts by ligand-dependent stabilization, to build a biosensor for detection of the pentose phosphate pathway metabolite 5-phospho-α-D-ribose 1-diphosphate (PRPP). Using this protein as a template, we computationally redesigned a new pocket de novo according to the pose of the ligand, creating a binding mode exclusive to recognize another pentose phosphate metabolite, D-erythrose 4-phosphate (E4P), and glycerate-3-phosphate (3PG), from the glycolysis pathway. Furthermore, E4P biosensor was developed by fluorescence-activated cell sorting (FACS) and application of it enabled successful screening for the highest phenylalanine-producing strain reported to date. This work provides a strategy for computational design and development of biosensors for a broad range of molecules.

Evaluating Iso-Mukaadial Acetate and Ursolic Acid Acetate as Plasmodium falciparum Hypoxanthine-Guanine-Xanthine Phosphoribosyltransferase Inhibitors.

To date, Plasmodium falciparum is one of the most lethal strains of the malaria parasite. P. falciparum lacks the required enzymes to create its own purines via the de novo pathway, thereby making Plasmodium falciparum hypoxanthine-guanine-xanthine phosphoribosyltransferase (PfHGXPT) a crucial enzyme in the malaria life cycle. Recently, studies have described iso-mukaadial acetate and ursolic acid acetate as promising antimalarials. However, the mode of action is still unknown, thus, the current study sought to investigate the selective inhibitory and binding actions of iso-mukaadial acetate and ursolic acid acetate against recombinant PfHGXPT using in-silico and experimental approaches. Recombinant PfHGXPT protein was expressed using E. coli BL21 cells and homogeneously purified by affinity chromatography. Experimentally, iso-mukaadial acetate and ursolic acid acetate, respectively, demonstrated direct inhibitory activity towards PfHGXPT in a dose-dependent manner. The binding affinity of iso-mukaadial acetate and ursolic acid acetate on the PfHGXPT dissociation constant (KD), where it was found that 0.0833 µM and 2.8396 µM, respectively, are indicative of strong binding. The mode of action for the observed antimalarial activity was further established by a molecular docking study. The molecular docking and dynamics simulations show specific interactions and high affinity within the binding pocket of Plasmodium falciparum and human hypoxanthine-guanine phosphoribosyl transferases. The predicted in silico absorption, distribution, metabolism and excretion/toxicity (ADME/T) properties predicted that the iso-mukaadial acetate ligand may follow the criteria for orally active drugs. The theoretical calculation derived from ADME, molecular docking and dynamics provide in-depth information into the structural basis, specific bonding and non-bonding interactions governing the inhibition of malarial. Taken together, these findings provide a basis for the recommendation of iso-mukaadial acetate and ursolic acid acetate as high-affinity ligands and drug candidates against PfHGXPT.

In vitro and in vivo characterization of the multiple isoforms of Schistosoma mansoni hypoxanthine-guanine phosphoribosyltransferases

Schistosoma mansoni, the parasite responsible for schistosomiasis, lacks the “de novo” purine biosynthetic pathway and depends entirely on the purine salvage pathway for the supply of purines. Numerous reports of praziquantel resistance have been described, as well as stimulated efforts to develop new drugs against schistosomiasis. Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) is a key enzyme of the purine salvage pathway. Here, we describe a crystallographic structure of the S. mansoni HPGRT-1 (SmHGPRT), complexed with IMP at a resolution of 2.8 Ǻ. Four substitutions were identified in the region of the active site between SmHGPRT-1 and human HGPRT. We also present data from RNA-Seq and WISH, suggesting that some isoforms of HGPRT might be involved in the process related to sexual maturation and reproduction in worms; furthermore, its enzymatic assays show that the isoform SmHGPRT-3 does not present the same catalytic efficiency as other isoforms. Finally, although other studies have previously suggested this enzyme as a potential antischistosomal chemotherapy target, the kinetics parameters reveal the impossibility to use SmHGPRT as an efficient chemotherapeutic target.

Pyrrolidine nucleoside bisphosphonates as antituberculosis agents targeting hypoxanthine-guanine phosphoribosyltransferase

Therapeutic treatment of tuberculosis (TB) is becoming increasingly problematic due to the emergence of drug resistant Mycobacterium tuberculosis (Mt). Thus, new targets for anti-TB drug discovery need to be identified to combat and eradicate this disease. One such target is hypoxanthine-guanine phosphoribosyltransferase (HGPRT) which synthesises the 6-oxopurine nucleoside monophosphates essential for DNA/RNA production. [3R,4R]-4-Hypoxanthin-9-yl-3-((S)-2-hydroxy-2-phosphonoethyl)oxy-1-N-(phosphonopropionyl)pyrrolidine and [3R,4R]-4-guanin-9-yl-3-((S)-2-hydroxy-2-phosphonoethyl)oxy-1-N-(phosphonopropionyl)pyrrolidine (compound 6) are the most potent inhibitors of MtHGPRT yet discovered having Ki values of 60 nM. The crystal structure of the MtHGPRT.6 complex was obtained and compared with that of human HGPRT in complex with the same inhibitor. These structures provide explanations for the 60-fold difference in the inhibition constants between these two enzymes and a foundation for the design of next generation inhibitors. In addition, crystal structures of MtHGPRT in complex with two pyrrolidine nucleoside phosphosphonate inhibitors plus pyrophosphate provide insights into the final stage of the catalytic reaction. As the first step in ascertaining if such compounds have the potential to be developed as anti-TB therapeutics, the tetra-(ethyl L-phenylalanine) tetraamide prodrug of 6 was tested in cell based assays. This compound arrested the growth of virulent Mt not only in its replicating phase (IC50 of 14 μΜ) but also in its latent phase (IC50 of 29 μΜ). Furthermore, it arrested the growth of Mt in infected macrophages (MIC50 of 85 μΜ) and has a low cytotoxicity in mammalian cells (CC50 of 132 ± 20 μM). These inhibitors are therefore viewed as forerunners of new anti-TB chemotherapeutics.

Prioritization of Deleterious Variations in the Human Hypoxanthine-Guanine Phosphoribosyltransferase Gene

Prioritization of Deleterious Variations in the Human Hypoxanthine-Guanine Phosphoribosyltransferase Gene

Synthesis and Evaluation of Asymmetric Acyclic Nucleoside Bisphosphonates as Inhibitors of Plasmodium falciparum and Human Hypoxanthine-Guanine-(Xanthine) Phosphoribosyltransferase.

Acyclic nucleoside bisphosphonates (ANbPs) have previously been shown to be good inhibitors of human hypoxanthine-guanine phosphoribosyltransferase (HGPRT) and Plasmodium falciparum (Pf) hypoxanthine-guanine-xanthine phosphoribosyltransferase (PfHGXPRT). On the basis of this scaffold, a new series of ANbPs was synthesized. One of these new ANbPs, [3-(guanine-9-yl)-2-((2-phosphonoethoxy)methyl)propoxy]methylphosphonic acid, exhibited Ki values of 6 and 70 nM for human HGPRT and Pf HGXPRT, respectively. These low Ki values were achieved by inserting an extra carbon atom in the linker connecting the N9 atom of guanine to one of the phosphonate groups. The crystal structure of this ANbP in complex with human HGPRT was determined at 2.0 Å resolution and shows that it fills three key pockets in the active site. The most potent phosphoramidate prodrugs of these compounds have IC50 values in the low micromolar range in Pf lines and low toxicity in human A549 cells, demonstrating that these ANbPs are excellent antimalarial drug leads.

Crystal structures of Apo and GMP bound hypoxanthine-guanine phosphoribosyltransferase from Legionella pneumophila and the implications in gouty arthritis.

Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) (EC 2.4.2.8) reversibly catalyzes the transfer of the 5-phophoribosyl group from 5-phosphoribosyl-alpha-1-pyrophosphate (PRPP) to hypoxanthine or guanine to form inosine monophosphate (IMP) or guanosine monophosphate (GMP) in the purine salvage pathway. To investigate the catalytic mechanism of this enzyme in the intracellular pathogen Legionella pneumophila, we determined the crystal structures of the L. pneumophila HGPRT (LpHGPRT) both in its apo-form and in complex with GMP. The structures reveal that LpHGPRT comprises a core domain and a hood domain which are packed together to create a cavity for GMP-binding and the enzymatic catalysis. The binding of GMP induces conformational changes of the stable loop II. This new binding site is closely related to the Gout arthritis-linked human HGPRT mutation site (Ser103Arg). Finally, these structures of LpHGPRT provide insights into the catalytic mechanism of HGPRT.

Differential Distortion of Purine Substrates by Human and Plasmodium falciparum Hypoxanthine-Guanine Phosphoribosyltransferase to Catalyse the Formation of Mononucleotides.

Plasmodium falciparum (Pf) hypoxanthine-guanine phosphoribosyltransferase (HGPRT) is a potential therapeutic target. Compared to structurally homologous human enzymes, it has expanded substrate specificity. In this study, 9-deazapurines are used as in situ probes of the active sites of human and Pf HGPRTs. Through the use of these probes it is found that non-covalent interactions stabilise the pre-transition state of the HGPRT-catalysed reaction. Vibrational spectra reveal that the bound substrates are extensively distorted, the carbonyl bond of nucleobase moiety is weakened and the substrate is destabilised along the reaction coordinate. Raman shifts of the human and Pf enzymes are used to quantify the differing degrees of hydrogen bonding in the homologues. A decreased Raman cross-section in enzyme-bound 9-deazaguanine (9DAG) shows that the phenylalanine residue (Phe186 in human and Phe197 in Pf) of HGPRT stacks with the nucleobase. Differential loss of the Raman cross-section suggests that the active site is more compact in human HGPRT as compared to the Pf enzyme, and is more so in the phosphoribosyl pyrophosphate (PRPP) complex 9DAG-PRPP-HGPRT than in 9-deazahypoxanthine (9DAH)-PRPP-HGPRT.

Deciphering Differential Distortion of Purine Substrates by Human and Plasmodium falciparum HGPRT

Enzymes of the salvage pathway are essential to understanding the biochemistry of parasitic protozoa. These parasites lack de novo pathway for nucleotide synthesis and depend solely on purines salvage from their host. Hypoxanthine guanine phosphoribosyltransferase (HGPRT) from Plasmodium falciparum has long been regarded as a potential therapeutic target. Although structurally homologous to human enzyme, PfHGPRT differs from human (h) HGPRT in its expanded substrate specificity and activity towards common substrates. We used substrate analogues, 9‐deazapurines, in combination with UV resonance Raman spectroscopy to capture non‐covalent interactions in enzyme‐substrate complexes. Raman spectral band positions are sensitive to sub‐angstrom changes in bond length. We observed perturbations in Raman spectra that provide quantitative information about hydrogen bonding and stacking‐interactions with the enzyme active‐site. The bound ligands are extensively distorted towards the transition‐state of phosphoribosyltransferase reaction. Differential loss in Raman crosssection of the ligands bound to two enzymes suggests that active‐site is more compact in hHGPRT than in PfHGPRT. The rigidity of active‐site in hHGPRT is attributed to tighter stacking interaction between phenylalanine residue of the active‐site and the nucleobase.Research was supported by IISER, India and Council for Scientific and Industrial Research, India.

HIV-1 TAT-mediated protein transduction of human HPRT into deficient cells

Lesch–Nyhan disease (LND) is a severe and incurable X-linked genetic syndrome caused by the deficiency of hypoxanthine–guanine phosphoribosyltransferase (HPRT), resulting in severe alterations of central nervous system, hyperuricemia and subsequent impaired renal functions. Therapeutic options consist in supportive care and treatments of complications, but the disease remains largely untreatable. Enzyme replacement of the malfunctioning cytosolic protein might represent a possible therapeutic approach for the LND treatment. Protein transduction domains, such as the TAT peptide derived from HIV TAT protein, have been used to transduce macromolecules into cells in vitro and in vivo. The present study was aimed to the generation of TAT peptide fused to human HPRT for cell transduction in enzyme deficient cells. Here we document the construction, expression and delivery of a functional HPRT enzyme into deficient cells by TAT transduction domain and by liposome mediated protein transfer. With this approach we demonstrate the correction of the enzymatic defect in HPRT deficient cells.Our data show for the first time the feasibility of the enzyme replacement therapy for the treatment of LND.

Role of Human Hypoxanthine Guanine Phosphoribosyltransferase in Activation of the Antiviral Agent T-705 (Favipiravir)

6-Fluoro-3-hydroxy-2-pyrazinecarboxamide (T-705) is a novel antiviral compound with broad activity against influenza virus and diverse RNA viruses. Its active metabolite, T-705-ribose-5'-triphosphate (T-705-RTP), is recognized by influenza virus RNA polymerase as a substrate competing with GTP, giving inhibition of viral RNA synthesis and lethal virus mutagenesis. Which enzymes perform the activation of T-705 is unknown. We here demonstrate that human hypoxanthine guanine phosphoribosyltransferase (HGPRT) converts T-705 into its ribose-5'-monophosphate (RMP) prior to formation of T-705-RTP. The anti-influenza virus activity of T-705 and T-1105 (3-hydroxy-2-pyrazinecarboxamide; the analog lacking the 6-fluoro atom) was lost in HGPRT-deficient Madin-Darby canine kidney cells. This HGPRT dependency was confirmed in human embryonic kidney 293T cells undergoing HGPRT-specific gene knockdown followed by influenza virus ribonucleoprotein reconstitution. Knockdown for adenine phosphoribosyltransferase (APRT) or nicotinamide phosphoribosyltransferase did not change the antiviral activity of T-705 and T-1105. Enzymatic assays showed that T-705 and T-1105 are poor substrates for human HGPRT having Km(app) values of 6.4 and 4.1 mM, respectively. Formation of the RMP metabolites by APRT was negligible, and so was the formation of the ribosylated metabolites by human purine nucleoside phosphorylase. Phosphoribosylation and antiviral activity of the 2-pyrazinecarboxamide derivatives was shown to require the presence of the 3-hydroxyl but not the 6-fluoro substituent. The crystal structure of T-705-RMP in complex with human HGPRT showed how this compound binds in the active site. Since conversion of T-705 by HGPRT appears to be inefficient, T-705-RMP prodrugs may be designed to increase the antiviral potency of this new antiviral agent.

Identification of Novel Mutations in the Human HPRT Gene

Inherited mutation of the purine salvage enzyme, hypoxanthine guanine phosphoribosyltransferase (HPRT) gives rise to Lesch–Nyhan syndrome (LNS) or Lesch–Nyhan variants (LNVs). We report three novel independent mutations in the coding region of HPRT gene: exon 3: c.141delA, p.D47fs53X; exon 5: c.400G>A, p.E134K; exon 7: c.499A>G, p.R167G from three LNS affected male patients.

Novel Mutations in the Human HPRT Gene

Inherited mutation of a purine salvage enzyme, hypoxanthine guanine phosphoribosyltransferase (HPRT), gives rise to Lesch-Nyhan Syndrome (LNS) or HPRT-related gout. Here, we report five novel independent mutations in the coding region of the HPRT gene from five unrelated male patients manifesting different clinical phenotypes associated with LNS: exon 2: c.133A > G, p.45R > G; c.35A > C, p.12D > A; c.88delG; exon 7: c.530A > T, p.177D > V; and c.318 + 1G > C: IVS3 + 1G > C splice site mutation.

Hypoxanthine Guanine Phosphoribosyltransferase Distorts the Purine Ring of Nucleotide Substrates and Perturbs the pKa of Bound Xanthosine Monophosphate

Enzymatic efficiency and structural discrimination of substrates from nonsubstrate analogues are attributed to the precise assembly of binding pockets. Many enzymes have the additional remarkable ability to recognize several substrates. These apparently paradoxical attributes are ascribed to the structural plasticity of proteins. A partially defined active site acquires complementarity upon encountering the substrate and completing the assembly. Human hypoxanthine guanine phosphoribosyltransferase (hHGPRT) catalyzes the phosphoribosylation of guanine and hypoxanthine, while the Plasmodium falciparum HGPRT (PfHGPRT) acts on xanthine as well. Reasons for the observed differences in substrate specificities of the two proteins are not clear. We used ultraviolet resonance Raman spectroscopy to study the complexes of HGPRT with products (IMP, GMP, and XMP), in both organisms, in resonance with the purine nucleobase electronic absorption. This led to selective enhancement of vibrations of the purine ring over those of the sugar-phosphate backbone and protein. Spectra of bound nucleotides show that HGPRT distorts the structure of the nucleotides. The distorted structure resembles that of the deprotonated nucleotide. We find that the two proteins assemble similar active sites for their common substrates. While hHGPRT does not bind XMP, PfHGPRT perturbs the pK(a) of bound XMP. The results were compared with the mutant form of hHGPRT that catalyzed xanthine but failed to perturb the pK(a) of XMP.

Structural and functional studies of the human phosphoribosyltransferase domain containing protein 1

Human hypoxanthine-guanine phosphoribosyltransferase (HPRT) (EC 2.4.2.8) catalyzes the conversion of hypoxanthine and guanine to their respective nucleoside monophosphates. Human HPRT deficiency as a result of genetic mutations is linked to both Lesch-Nyhan disease and gout. In the present study, we have characterized phosphoribosyltransferase domain containing protein 1 (PRTFDC1), a human HPRT homolog of unknown function. The PRTFDC1 structure has been determined at 1.7 Å resolution with bound GMP. The overall structure and GMP binding mode are very similar to that observed for HPRT. Using a thermal-melt assay, a nucleotide metabolome library was screened against PRTFDC1 and revealed that hypoxanthine and guanine specifically interacted with the enzyme. It was subsequently confirmed that PRTFDC1 could convert these two bases into their corresponding nucleoside monophosphate. However, the catalytic efficiency (k(cat)/K(m)) of PRTFDC1 towards hypoxanthine and guanine was only 0.26% and 0.09%, respectively, of that of HPRT. This low activity could be explained by the fact that PRTFDC1 has a Gly in the position of the proposed catalytic Asp of HPRT. In PRTFDC1, a water molecule at the position of the aspartic acid side chain position in HPRT might be responsible for the low activity observed by acting as a weak base. The data obtained in the present study indicate that PRTFDC1 does not have a direct catalytic role in the nucleotide salvage pathway.

Erythrocytic Adenosine Monophosphate as an Alternative Purine Source in Plasmodium falciparum

Plasmodium falciparum is a purine auxotroph, salvaging purines from erythrocytes for synthesis of RNA and DNA. Hypoxanthine is the key precursor for purine metabolism in Plasmodium. Inhibition of hypoxanthine-forming reactions in both erythrocytes and parasites is lethal to cultured P. falciparum. We observed that high concentrations of adenosine can rescue cultured parasites from purine nucleoside phosphorylase and adenosine deaminase blockade but not when erythrocyte adenosine kinase is also inhibited. P. falciparum lacks adenosine kinase but can salvage AMP synthesized in the erythrocyte cytoplasm to provide purines when both human and Plasmodium purine nucleoside phosphorylases and adenosine deaminases are inhibited. Transport studies in Xenopus laevis oocytes expressing the P. falciparum nucleoside transporter PfNT1 established that this transporter does not transport AMP. These metabolic patterns establish the existence of a novel nucleoside monophosphate transport pathway in P. falciparum.