Molecular cloning and characterization of expressed human ecto-nucleoside triphosphate diphosphohydrolase 8 (E-NTPDase 8) and its soluble extracellular domain

Nucleoside Triphosphate Diphosphohydrolase 1 Exhibits Enzymatic Activity toward Tenofovir Diphosphate

Tenofovir (TFV; prescribed as TFV disoproxil fumarate and TFV alafenamide prodrugs) is currently used for HIV prevention and treatment. TFV must be phosphorylated twice into TFV-diphosphate (TFV-DP) to become pharmacologically active. Previously, we reported heterogeneity in TFV-DP distribution in colorectal tissue (a putative site of HIV infection) sections collected from research participants receiving a TFV-containing enema. This observed heterogeneity is likely multifactorial. Of note, TFV-DP is structurally similar to ATP. It is known that nucleotidases such as nucleoside triphosphate diphosphohydrolases (NTPDases) dephosphorylate ATP. Thus, it was hypothesized that NTPDase-mediated dephosphorylation plays a role in regulating TFV-DP levels in colorectal tissue. To test this hypothesis, recombinant NTPDase proteins (NTPDase 1, 3, 4, 5, 6, and 8) were incubated, individually, with TFV-DP to determine their abilities to dephosphorylate TFV-DP in vitro. Following incubations, TFV-DP dephosphorylation was determined using both malachite green phosphate assays and ultrahigh-performance liquid chromatography tandem mass spectrometry. From these, NTPDase 1 exhibited the highest activity toward TFV-DP. Further, enzyme kinetic analysis revealed Michaelis-Menten kinetics for NTPDase 1-mediated TFV-DP dephosphorylation. Next, immunoblot analyses were conducted to confirm the expression of NTPDase 1 protein in human colorectal tissue. Liquid chromatography coupled to mass spectrometry proteomics analysis was used to measure the relative abundance of NTPDases in human colorectal tissue among healthy adult individuals (n = 4). These analyses confirmed the high abundance of NTPDase 1 in human colorectal tissue. Taken together, results suggest that NTPDase 1 may contribute to the regulation of TFV-DP levels. The above data provide important insights into the dephosphorylation of TFV-DP. SIGNIFICANCE STATEMENT: Nucleoside triphosphate diphosphohydrolases (NTPDases) that are involved in enzymatic ATP dephosphorylation may contribute to tenofovir-diphosphate (TFV-DP) dephosphorylation, leading to its inactivation. In this study, the NTPDases responsible for TFV-DP dephosphorylation in vitro and their expression in human colorectal tissue were investigated. Through this work, it was demonstrated that NTPDase 1 has the highest activity toward TFV-DP dephosphorylation, and it was abundant in human colorectal tissue. Importantly, these studies will increase our understanding of TFV-DP disposition.

Fluorescence polarization immunoassays for monitoring nucleoside triphosphate diphosphohydrolase (NTPDase) activity

Novel and very sensitive fluorescence polarization immunoassays (FPIA) for the screening of NTPDases have been successfully established and validated.

Ecto-nucleotidases and Ecto-phosphatases from Leishmania and Trypanosoma parasites

Ecto-enzymes can be defined as membrane-bound proteins that have their active site facing the extracellular millieu. In trypanosomatids, the physiological roles of these enzymes remain to be completed elucidated; however, many important events have already been related to them, such as the survival of parasites during their complex life cycle and the successful establishment of host infection. This chapter focuses on two remarkable classes of ecto-enzymes: ecto-nucleotidases and ecto-phosphatases, summarizing their occurrence and possible physiological roles in Leishmania and Trypanosoma genera. Ecto-nucleotidases are characterized by their ability to hydrolyze extracellular nucleotides, playing an important role in purinergic signaling. By the action of these ecto-enzymes, parasites are capable of modulating the host immune system, which leads to a successful parasite infection. Furthermore, ecto-nucleotidases are also involved in the purine salvage pathway, acting in the generation of nucleosides that are able to cross plasma membrane via specialized transporters. Another important ecto-enzyme present in a vast number of pathogenic organisms is the ecto-phosphatase. These enzymes are able to hydrolyze extracellular phosphorylated substrates, releasing free inorganic phosphate that can be internalized by the cell, crossing the plasma membrane through a Pi-transporter. Ecto-phosphatases are also involved in the invasion and survival of parasite in the host cells. Several alternative functions have been suggested for these enzymes in parasites, such as participation in their proliferation, differentiation, nutrition and protection. In this context, the present chapter provides an overview of recent discoveries related to the occurrence of ecto-nucleotidase and ecto-phosphatase activities in Leishmania and Trypanosoma parasites.

Mutagenesis of apyrase conserved region 1 alters the nucleotide substrate specificity

Two apyrases having different substrate specificity, MP67 and MpAPY2, are present in Mimosa pudica. The substrate specificity of MP67 is quite high against ADP, and is distinct from any other apyrase. This might be attributed to the nucleotide binding motif (DXG) in apyrase conserved region 1. We performed a single amino acid substitution at position X in the motif. The ratio of the velocity of ATP/ADP hydrolysis was higher (approximately 1) for the S63A-MP67 mutant than for wild type-MP67 (0.19). Binding affinity for ADP of A75S-MpAPY2 mutant was increased to a level higher than that of the wild type MpAPY2. Thus, the residue at position X in the DXG motif plays an important role in determining nucleotide preference.

Cellular function and molecular structure of ecto-nucleotidases

Ecto-nucleotidases play a pivotal role in purinergic signal transmission. They hydrolyze extracellular nucleotides and thus can control their availability at purinergic P2 receptors. They generate extracellular nucleosides for cellular reuptake and salvage via nucleoside transporters of the plasma membrane. The extracellular adenosine formed acts as an agonist of purinergic P1 receptors. They also can produce and hydrolyze extracellular inorganic pyrophosphate that is of major relevance in the control of bone mineralization. This review discusses and compares four major groups of ecto-nucleotidases: the ecto-nucleoside triphosphate diphosphohydrolases, ecto-5'-nucleotidase, ecto-nucleotide pyrophosphatase/phosphodiesterases, and alkaline phosphatases. Only recently and based on crystal structures, detailed information regarding the spatial structures and catalytic mechanisms has become available for members of these four ecto-nucleotidase families. This permits detailed predictions of their catalytic mechanisms and a comparison between the individual enzyme groups. The review focuses on the principal biochemical, cell biological, catalytic, and structural properties of the enzymes and provides brief reference to tissue distribution, and physiological and pathophysiological functions.

The GDA1_CD39 superfamily: NTPDases with diverse functions

The first comprehensive review of the ubiquitous "ecto-ATPases" by Plesner was published in 1995. A year later, a lymphoid cell activation antigen, CD39, that had been cloned previously, was shown to be an ecto-ATPase. A family of proteins, related to CD39 and a yeast GDPase, all containing the canonical apyrase conserved regions in their polypeptides, soon started to expand. They are now recognized as members of the GDA1_CD39 protein family. Because proteins in this family hydrolyze nucleoside triphosphates and diphosphates, a unifying nomenclature, nucleoside triphosphate diphopshohydrolases (NTPDases), was established in 2000. Membrane-bound NTPDases are either located on the cell surface or membranes of intracellular organelles. Soluble NTPDases exist in the cytosol and may be secreted. In the last 15 years, molecular cloning and functional expression have facilitated biochemical characterization of NTPDases of many organisms, culminating in the recent structural determination of the ecto-domain of a mammalian cell surface NTPDase and a bacterial NTPDase. The first goal of this review is to summarize the biochemical, mutagenesis, and structural studies of the NTPDases. Because of their ability in hydrolyzing extracellular nucleotides, the mammalian cell surface NTPDases (the ecto-NTPDases) which regulate purinergic signaling have received the most attention. Less appreciated are the functions of intracellular NTPDases and NTPDases of other organisms, e.g., bacteria, parasites, Drosophila, plants, etc. The second goal of this review is to summarize recent findings which demonstrate the involvement of the NTPDases in multiple and diverse physiological processes: pathogen-host interaction, plant growth, eukaryote cell protein and lipid glycosylation, eye development, and oncogenesis.

Crystal structure of a Legionella pneumophila ecto -triphosphate diphosphohydrolase, a structural and functional homolog of the eukaryotic NTPDases

Many pathogenic bacteria have sophisticated mechanisms to interfere with the mammalian immune response. These include the disruption of host extracellular ATP levels that, in humans, is tightly regulated by the nucleoside triphosphate diphosphohydrolase family (NTPDases). NTPDases are found almost exclusively in eukaryotes, the notable exception being their presence in some pathogenic prokaryotes. To address the function of bacterial NTPDases, we describe the structures of an NTPDase from the pathogen Legionella pneumophila (Lpg1905/Lp1NTPDase) in its apo state and in complex with the ATP analog AMPPNP and the subtype-specific NTPDase inhibitor ARL 67156. Lp1NTPDase is structurally and catalytically related to eukaryotic NTPDases and the structure provides a basis for NTPDase-specific inhibition. Furthermore, we demonstrate that the activity of Lp1NTPDase correlates directly with intracellular replication of Legionella within macrophages. Collectively, these findings provide insight into the mechanism of this enzyme and highlight its role in host-pathogen interactions.

Proline residues link the active site to transmembrane domain movements in human nucleoside triphosphate diphosphohydrolase 3 (NTPDase3)

The active sites of the membrane-bound nucleoside triphosphate diphosphohydrolases (NTPDases) regulate and are regulated by coordinated and spatially distant movements of their transmembrane helices, modulating enzyme activity, and substrate specificity. Using site-directed mutagenesis, the roles of the conserved proline residues (N-terminal: P52 and P53; C-terminal: P472, P476, P481, P484, and P485) of human NTPDase3, located in the "linker regions" that connect the N- and C-terminal transmembrane helices with the extracellular active site, were examined. Single cysteine substitutions were strategically placed in the transmembrane domain (N-terminal helix: V42C; C-terminal helix: G489C) to serve as cross-linking "sensors" of helical interactions. These "sensor" background mutant proteins (V42C and G489C NTPDase3) are enzymatically active and are cross-linked by copper phenanthroline less efficiently in the presence of adenosine triphosphate (ATP). Proline to alanine substitutions at P53, P481, P484, and P485 in the V42C background, as well as P53, P481, and P484 in the G489C background, exhibited decreased nucleotidase activities. More importantly, alanine substitutions at P53 and P481 in the V42C background and P481 in the G489C background no longer exhibited the ATP-induced decrease in transmembrane cross-linking efficiency. Interestingly, the P485A mutation abolished oxidative cross-linking at G489C both in the presence and absence of ATP. Taken together, these results suggest a role for proline residues 53 and 481 in the linker regions of human NTPDase3 for coupling nucleotide binding at the enzyme active site to movements and/or rearrangements of the transmembrane helices necessary for optimal nucleotide hydrolysis.

The stability of chicken nucleoside triphosphate diphosphohydrolase 8 requires both of its transmembrane domains

Chicken nucleoside triphosphate diphosphohydrolase 8 (NTPDase8) is a cell surface ectonucleotidase with a large extracellular domain (ECD) containing the active site and is anchored to the membrane by two transmembrane domains (TMDs) at the N- and C-termini. Unlike other cell surface NTPDases that have been characterized, the chicken NTPDase8 is not susceptible to substrate inactivation or agents that cause membrane perturbation. To determine if the stability of the enzyme is inherent in its ECD, the cDNA construct of the soluble chicken NTPDase8 was expressed and the protein purified. The ATPase activity of the purified soluble chicken NTPDase8 was less than 15% of that of the purified full-length enzyme. Strikingly, in contrast to the membrane-bound enzyme, the activity of the soluble chicken NTPDase8 decreased with time in a temperature-dependent manner as a result of inactivation by ATP, ADP, and P(i). Truncated mutants in which the ECD is anchored by a single TMD at either the N- or the C-terminus by the native chicken NTPDase TMDs or a TMD from a different NTPDase, human NTPDase2, also displayed a nonlinear time course of ATP hydrolysis. While removal of the N- or C-terminal TMD affected protein expression differently, the truncated mutants were generally similar to the soluble chicken NTPDase8 with respect to ATP, ADP, and P(i) inactivation. Other biochemical characteristics, e.g., ATPase/ADPase ratios, inhibition by azide, and affinity for ATP, were also altered when one or both of the TMDs were removed from the chicken NTPDase8. These results indicate that (1) both TMDs of the chicken NTPDase8 are required to maintain stability of the enzyme and maximal catalytic activity and (2) the conformations of the ectodomain in the soluble enzyme and the truncated mutants differ from that of the full-length chicken NTPDase8.

Possible effects of microbial ecto-nucleoside triphosphate diphosphohydrolases on host-pathogen interactions

In humans, purinergic signaling plays an important role in the modulation of immune responses through specific receptors that recognize nucleoside tri- and diphosphates as signaling molecules. Ecto-nucleoside triphosphate diphosphohydrolases (ecto-NTPDases) have important roles in the regulation of purinergic signaling by controlling levels of extracellular nucleotides. This process is key to pathophysiological protective responses such as hemostasis and inflammation. Ecto-NTPDases are found in all higher eukaryotes, and recently it has become apparent that a number of important parasitic pathogens of humans express surface-located NTPDases that have been linked to virulence. For those parasites that are purine auxotrophs, these enzymes may play an important role in purine scavenging, although they may also influence the host response to infection. Although ecto-NTPDases are rare in bacteria, expression of a secreted NTPDase in Legionella pneumophila was recently described. This ecto-enzyme enhances intracellular growth of the bacterium and potentially affects virulence. This discovery represents an important advance in the understanding of the contribution of other microbial NTPDases to host-pathogen interactions. Here we review other progress made to date in the characterization of ecto-NTPDases from microbial pathogens, how they differ from mammalian enzymes, and their association with organism viability and virulence. In addition, we postulate how ecto-NTPDases may contribute to the host-pathogen interaction by reviewing the effect of selected microbial pathogens on purinergic signaling. Finally, we raise the possibility of targeting ecto-NTPDases in the development of novel anti-infective agents based on potential structural and clear enzymatic differences from the mammalian ecto-NTPDases.

The single NTPase gene of Drosophila melanogaster encodes an intracellular nucleoside triphosphate diphosphohydrolase 6 (NTPDase6)

I report here the cloning and characterization of a nucleoside triphosphate diphosphohydrolase 6 (NTPDase6) encoded by the single Dmel/NTPase gene of Drosophila melanogaster. S2 cells stably transfected with the Drosophila NTPDase6 cDNA displayed strong UDPase activity only after addition of NP-40, indicating the intracellular location of the enzyme. The enzyme hydrolyzed UDP, GDP, and IDP equally well whereas other NDP and NTP were poor substrates. It was not or only partially inhibited by several modulators of the cell surface NTPDases, but was strongly inhibited upon oxidative cross-linking by copper phenanthroline. The decrease of activity correlated with dimer formation. Mutagenesis studies indicated that dimer formation required C42 in the transmembrane domain and C447 in the exoplasmic domain. Fluorescence microscopy revealed that the protein was located primarily in the ER. The substrate specificity and cellular localization of the Drosophila NTPDase6 suggest that it participates in Drosophila glycoprotein processing.

Structure-activity relationships of anthraquinone derivatives derived from bromaminic acid as inhibitors of ectonucleoside triphosphate diphosphohydrolases (E-NTPDases)

Reactive blue 2 (RB-2) had been characterized as a relatively potent ectonucleoside triphosphate diphosphohydrolase (E-NTPDase) inhibitor with some selectivity for NTPDase3. In search for the pharmacophore and to analyze structure-activity relationships we synthesized a series of truncated derivatives and analogs of RB-2, including 1-amino-2-sulfo-4-ar(alk)ylaminoanthraquinones, 1-amino-2-methyl-4-arylaminoanthraquinones, 1-amino-4-bromoanthraquinone 2-sulfonic acid esters and sulfonamides, and bis-(1-amino-4-bromoanthraquinone) sulfonamides, and investigated them in preparations of rat NTPDase1, 2, and 3 using a capillary electrophoresis assay. Several 1-amino-2-sulfo-4-ar(alk)ylaminoanthraquinone derivatives inhibited E-NTPDases in a concentration-dependent manner. The 2-sulfonate group was found to be required for inhibitory activity, since 2-methyl-substituted derivatives were inactive. 1-Amino-2-sulfo-4-p-chloroanilinoanthraquinone (18) was identified as a nonselective competitive blocker of NTPDases1, 2, and 3 (K_i 16–18 μM), while 1-amino-2-sulfo-4-(2-naphthylamino)anthraquinone (21) was a potent inhibitor with preference for NTPDase1 (K_i 0.328 μM) and NTPDase3 (K_i 2.22 μM). Its isomer, 1-amino-2-sulfo-4-(1-naphthylamino)anthraquinone (20), was a potent and selective inhibitor of rat NTPDase3 (K_i 1.5 μM).

Structural insight into signal conversion and inactivation by NTPDase2 in purinergic signaling

Cell surface-located nucleoside triphosphate diphosphohydrolases (NTPDase1, -2, -3, and -8) are oligomeric integral membrane proteins responsible for signal conversion and inactivation in extracellular nucleotide-mediated "purinergic" signaling. They catalyze the sequential hydrolysis of the signaling molecule ATP via ADP to AMP. Here we present the structure of the extracellular domain of Rattus norvegicus NTPDase2 in an active state at resolutions between 1.7 A and 2.1 A in four different forms: (i) apo form, (ii) ternary complex with the nonhydrolyzable ATP analog AMPPNP and cofactor Ca(2+), (iii) quaternary complex with Ca(2+) and bound products AMP and phosphate, and (iv) binary product complex with AMP only. Analysis of the ATP (analog) binding mode explains the importance of several residues for activity and allows suggestion of a catalytic mechanism. The carboxylate group of E165 serves as a catalytic base and activates a water molecule, which is well positioned for nucleophilic attack on the terminal phosphate. Based on analysis of the two product complex structures in which AMP adopts different conformations, a substrate binding mode for ADP hydrolysis is proposed. This allows for an understanding of how the same hydrolytic site can be engaged in ATP and ADP but not AMP hydrolysis.

Enzymatic properties of an ecto-nucleoside triphosphate diphosphohydrolase from Legionella pneumophila: substrate specificity and requirement for virulence

Legionella pneumophila is the predominant cause of Legionnaires disease, a severe and potentially fatal form of pneumonia. Recently, we identified an ecto-nucleoside triphosphate diphosphohydrolase (NTPDase) from L. pneumophila, termed Lpg1905, which enhances intracellular replication of L. pneumophila in eukaryotic cells. Lpg1905 is the first prokaryotic member of the CD39/NTPDase1 family of enzymes, which are characterized by the presence of five apyrase conserved regions and the ability to hydrolyze nucleoside tri- and diphosphates. Here we examined the substrate specificity of Lpg1905 and showed that apart from ATP and ADP, the enzyme catalyzed the hydrolysis of GTP and GDP but had limited activity against CTP, CDP, UTP, and UDP. Based on amino acid residues conserved in the apyrase conserved regions of eukaryotic NTPDases, we generated five site-directed mutants, Lpg1905E159A, R122A, N168A, Q193A, and W384A. Although the mutations E159A, R122A, Q193A, and W384A abrogated activity completely, N168A resulted in decreased activity caused by reduced affinity for nucleotides. When introduced into the lpg1905 mutant strain of L. pneumophila, only N168A partially restored the ability of L. pneumophila to replicate in THP-1 macrophages. Following intratracheal inoculation of A/J mice, none of the Lpg1905 mutants was able to restore virulence to an lpg1905 mutant during lung infection, thereby demonstrating the importance of NTPDase activity to L. pneumophila infection. Overall, the kinetic studies undertaken here demonstrated important differences to mammalian NTPDases and different sensitivities to NTPDase inhibitors that may reflect underlying structural variations.

IL-6 downregulates transcription of NTPDase2 via specific promoter elements

Bile ductular proliferation is markedly upregulated in biliary fibrosis and cirrhosis. However, the mechanisms regulating this upregulation in bile ductular proliferation have not been defined. Recently, we demonstrated that expression of the ectonucleotidase nucleoside triphosphate diphosphohydrolase-2 (NTPDase2/Entpd2) by portal fibroblasts (PF) is a critical regulator of bile ductular proliferation. Since interleukin 6 (IL-6) is markedly upregulated in biliary cirrhosis, our aims were to determine the role and mechanism of IL-6 in the regulation of NTPDase2 by PF. We found that IL-6 downregulated NTPDase2 protein expression in a concentration-dependent and time-dependent fashion but did not alter PF alpha-smooth muscle actin expression. IL-6 markedly downregulated NTPDase2 mRNA expression. Expression of the IL-6 receptor gp130 but not the IL-6 receptor gp80 was detected in PF. Two transcription start sites were identified in rat Entpd2 by the method of RNA ligase-mediated rapid amplification of 5' cDNA ends. The minimal promoter construct, but not shorter constructs, was downregulated by IL-6. Three putative IL-6 response elements were identified in silico and mutated. Mutation of all three response elements, but not fewer elements, completely abolished the IL-6 response. Thus IL-6 transcriptionally downregulates NTPDase2 expression by PF via actions at specific promoter elements independently of myofibroblastic differentiation. This effect may represent a novel signaling pathway by which bile ductular proliferation is dysregulated in biliary cirrhosis and thus provides a potential therapeutic approach for the regulation of bile ductular growth.

Transmembrane domain interactions affect the stability of the extracellular domain of the human NTPDase 2

Human NTPDase2 and chicken NTPDase8 are cell surface nucleotidases that contain two transmembrane domains (TMD) and five apyrase conserved regions (ACRs). ACR1 is located near the N-terminal TMD whereas ACR5 is located near the C-terminal TMD. The human NTPDase2 activity is decreased by low concentration of NP-40 and at temperatures higher than 37 degrees C, and undergoes substrate inactivation, whereas the chicken NTPDase8 activity is not. When freed from membrane anchorage, the soluble human NTPDase2 is no longer inactivated by detergents, high temperature, and substrate. These characteristics are retained in the hu-ck ACR1,5 chimera in which the extracellular domain is anchored to the membrane by the two TMDs of the chicken NTPDase8. The hu-ck ACR1,5 chimera is the first chimeric NTPDase reported that shows a resistance to membrane perturbation and substrate inactivation. Our results indicate that the strengths of interaction of the respective TMD pairs of the human NTPDase2 and chicken NTPDase8 determine their different responses to membrane perturbation and substrate.

Cloning, purification, and identification of the liver canalicular ecto-ATPase as NTPDase8

Extracellular nucleotides regulate critical liver functions via the activation of specific transmembrane receptors. The hepatic levels of extracellular nucleotides, and therefore the related downstream signaling cascades, are modulated by cell-surface enzymes called ectonucleotidases, including nucleoside triphosphate diphosphohydrolase-1 (NTPDase1/CD39), NTPDase2/CD39L1, and ecto-5'-nucleotidase/CD73. The goal of this study was to determine the molecular identity of the canalicular ecto-ATPase/ATPDase that we hypothesized to correspond to the recently cloned NTPDase8. Human and rat NTPDase8 cDNAs were cloned, and the genes were located on chromosome loci 9q34 and 3p13, respectively. The recombinant proteins, expressed in COS-7 and HEK293T cells, were biochemically characterized. NTPDase8 was also purified from rat liver by Triton X-100 solubilization, followed by DEAE, Affigel Blue, and concanavalin A chromatographies. Importantly, NTPDase8 was responsible for the major ectonucleotidase activity in liver. The ion requirement, apparent K(m) values, nucleotide hydrolysis profile, and preference as well as the resistance to azide were similar for recombinant NTPDase8s and both purified rat NTPDase8 and porcine canalicular ecto-ATPase/ATPDase. The partial NH(2)-terminal amino acid sequences of all NTPDase8s share high identity with the purified liver canalicular ecto-ATPase/ATPDase. Histochemical analysis showed high ectonucleotidase activities in bile canaliculi and large blood vessels of rat liver, in agreement with the immunolocalization of NTPDase1, 2, and 8 with antibodies developed for this study. No NTPDase3 expression could be detected in liver. In conclusion, NTPDase8 is the canalicular ecto-ATPase/ATPDase and is responsible for the main hepatic NTPDase activity. The canalicular localization of this enzyme suggests its involvement in the regulation of bile secretion and/or nucleoside salvage.

Characterization of an alternative splice variant of human nucleoside triphosphate diphosphohydrolase 3 (NTPDase3): a possible modulator of nucleotidase activity and purinergic signaling

Nucleoside triphosphate diphosphohydrolase 3 (NTPDase3) is a cell surface, membrane-bound enzyme that hydrolyzes extracellular nucleotides, thereby modulating purinergic signaling. An alternatively spliced variant of NTPDase3 was obtained and analyzed. This alternatively spliced variant, termed "NTPDase3beta", is produced through the use of an alternative terminal exon (exon 11) in place of the terminal exon (exon 12) in the full-length NTPDase3, now termed "NTPDase3alpha". This results in an expressed protein lacking the C-terminal cytoplasmic sequence, the C-terminal transmembrane helix, and apyrase conserved region 5. The cDNA encoding this truncated splice variant was detected in a human lung library by PCR. Like the full-length NTPDase3alpha, the alternatively spliced NTPDase3beta was expressed in COS cells after transfection, but only the full-length NTPDase3alpha is enzymatically active and properly trafficked to the plasma membrane. However, when the truncated NTPDase3beta was co-transfected with full-length NTPDase3alpha, there was a significant reduction in the amount of NTPDase3alpha that was properly processed and trafficked to the plasma membrane as active enzyme, indicating that the truncated form interferes with normal biosynthetic processing of the full-length enzyme. This suggests a role for the NTPDase3beta variant in the regulation of NTPDase3 nucleotidase activity, and therefore the control of purinergic signaling, in those cells and tissues expressing both NTPDase3alpha and NTPDase3beta.

Polyoxometalates--a new class of potent ecto-nucleoside triphosphate diphosphohydrolase (NTPDase) inhibitors

Polyoxotungstates were identified as potent inhibitors of NTPDases1, 2, and 3. The most potent compound was K(6)H(2)[TiW(11)CoO(40)], exhibiting K(i) values of 0.140 microM (NTPDase1), 0.910 microM (NTPDase2), and 0.563 microM (NTPDase3). One of the compounds, (NH(4))(18)[NaSb(9)W(21)O(86)], was selective for NTPDases2 and 3 versus NTPDase1. NTPDase inhibition might contribute to the described biological effects of polyoxometalates, including their anti-cancer activity.