Ectonucleotidases: Some recent developments and a note on nomenclature

Leishmania amazonensis: Biological and biochemical characterization of ecto-nucleoside triphosphate diphosphohydrolase activities

The presence of Leishmania amazonensis ecto-nucleoside triphosphate triphosphohydrolase activities was demonstrated using antibodies against different NTPDase members by Western blotting, flow cytometry, and immunoelectron microscopy analysis. Living promastigote cells sequentially hydrolyzed the ATP molecule generating ADP, AMP, and adenosine, indicating that this surface enzyme may play a role in the salvage of purines from the extracellular medium. The L. amazonensis ecto-NTPDase activities were insensitive to Triton X-100, but they were enhanced by divalent cations, such as Mg(2+). In addition, the ecto-NTPDase activities decreased with time for 96 h when promastigotes were grown in vitro. On the other hand, these activities increased considerably when measured in living amastigote forms. Furthermore, the treatment with adenosine, a mediator of several relevant biological phenomena, induced a decrease in the reactivity with anti-CD39 antibody, raised against mammalian E-NTPDase, probably because of down regulation in the L. amazonensis ecto-NTPDase expression. Also, adenosine and anti-NTPDase antibodies induced a significant diminishing in the interaction between promastigotes of L. amazonensis and mouse peritoneal macrophages.

Ecto-nucleotide pyrophosphatase/phosphodiesterase as part of a multiple system for nucleotide hydrolysis by platelets from rats: kinetic characterization and biochemical properties

In this study, we describe an ecto-nucleotide pyrophosphatase/phosphodiesterase (E-NPP) activity in rat platelets. Using p-nitrophenyl 5'-thymidine monophosphate (p-Nph-5'-TMP) as a substrate for E-NPP, we demonstrate an enzyme activity that shares the major biochemical properties described for E-NPPs: alkaline pH dependence, divalent cation dependence and blockade of activity by metal ion chelator. K(m) and V(max) values for p-Nph-5'-TMP hydrolysis were found to be 106 +/- 18 microM and 3.44 +/- 0.18 nmol p-nitrophenol/min/mg (mean +/- SD, n = 5). We hypothesize that an E-NPP is co-localized with an ecto-nucleoside triphosphate diphosphohydrolase and an ecto-5'-nucleotidase on the platelet surface, as part of a multiple system for nucleotide hydrolysis, since they can act under distinct physiological conditions and can be differently regulated. Thus, 0.25 mM suramin inhibited p-Nph-5'-TMP, ATP and ADP hydrolysis, while 0.5 mM AMP decreased only p-Nph-5'-TMP hydrolysis. Besides, 5.0, 10 and 20 mM sodium azide just inhibited ATP and ADP hydrolysis. Angiotensin II (5.0 and 10 nM) affected only ADP hydrolysis. Gadolinium chloride (0.2 and 0.5 mM) strongly inhibited the ATP and ADP hydrolysis. The E-NPP described here represents a novel insight into the control of platelet purinergic signaling.

Enzymes that hydrolyze adenine nucleotides in chronic renal failure: relationship between hemostatic defects and renal failure severity

The activities of the enzymes NTPDase (E.C.3.6.1.5, apyrase, ATP diphosphohydrolase, ecto-CD 39) and 5'-nucleotidase (E.C.3.1.3.5, CD 73) were analyzed in platelets from patients with chronic renal failure (CRF), both undergoing hemodialysis treatment (HD) and not undergoing hemodialysis (ND), as well as from a control group. The results showed an increase in platelet NTPDase activity in CRF patients on HD treatment (52.88%) with ATP as substrate (P<0.0001). ADP hydrolysis was decreased (33.68% and 39.75%) in HD and ND patients, respectively. In addition, 5'-nucleotidase activity was elevated in the HD (160%) and ND (81.49%) groups when compared to the control (P<0.0001). Significant correlation was found among ATP, ADP and AMP hydrolysis and plasma creatinine and urea levels (P<0.0001). Patients were compared statistically according the time of hemodialysis treatment. We found enhanced NTPDase and 5'-nucleotidase activities between 49 and 72 months on HD patients. Our result suggests the existence of alterations in nucleotide hydrolysis in platelets of CRF patients. Possibly, this altered nucleotide hydrolysis could contribute to hemostasis abnormalities found in CRF.

P2 receptors and cancer

Purinergic signalling has been implicated in many biological processes, and ATP and other extracellular nucleotides might have therapeutic potential in the treatment of cancer by signalling through P2 receptors. Different P2 receptor subtypes have been identified in a variety of different cancer types, in both primary samples of human cancer tissue and cell lines. Recent evidence suggests that different receptor subtypes mediate different pathophysiological functions such as proliferation, differentiation and apoptosis. In vivo studies of the use of ATP suggest that it can decrease the rate of cancer growth, and the first clinical trials have been undertaken. Thus, agents acting at P2 receptors might provide novel therapeutic tools in the treatment of cancer. In this article, background information about purinergic signalling and purinoceptor subtypes will be provided and then the proposed role of ATP in different cancers will be discussed in detail, including a discussion of in vivo studies and animal models, clinical trials and the specific P2 receptor subtypes involved.

Characterization of an ecto-ATPase activity in Fonsecaea pedrosoi

In this work, we characterized an ecto-ATPase activity in intact mycelial forms of Fonsecaea pedrosoi, the primary causative agent of chromoblastomycosis. In the presence of 1 mM EDTA, fungal cells hydrolyzed adenosine-5'-triphosphate (ATP) at a rate of 84.6 +/- 11.3 nmol Pi h(-1) mg(-1) mycelial dry weight. The ecto-ATPase activity was increased at about five times (498.3 +/- 27.6 nmol Pi h(-1) mg(-1)) in the presence of 5 mM MgCl2, with values of Vmax and apparent Km for Mg-ATP(2-) corresponding to 541.9 +/- 48.6 nmol Pi h(-1) mg(-1) cellular dry weight and 1.9 +/- 0.2 mM, respectively. The Mg2+-stimulated ecto-ATPase activity was insensitive to inhibitors of intracellular ATPases such as vanadate (P-ATPases), bafilomycin A1(V-ATPases), and oligomycin (F-ATPases). Inhibitors of acid phosphatases (molybdate, vanadate, and fluoride) or alkaline phosphatases (levamizole) had no effect on the ecto-ATPase activity. The surface of the Mg2+ -stimulated ATPase in F. pedrosoi was confirmed by assays in which 4,4'-diisothiocyanostylbene-2,2'-disulfonic acid (DIDS), a membrane impermeant inhibitor, and suramin, an inhibitor of ecto-ATPase and antagonist of P2 purinoreceptors. Based on the differential expression of ecto-ATPases in the different morphological stages of F. pedrosoi, the putative role of this enzyme in fungal biology is discussed.

Pathophysiology and therapeutic potential of purinergic signaling

The concept of a purinergic signaling system, using purine nucleotides and nucleosides as extracellular messengers, was first proposed over 30 years ago. After a brief introduction and update of purinoceptor subtypes, this article focuses on the diverse pathophysiological roles of purines and pyrimidines as signaling molecules. These molecules mediate short-term (acute) signaling functions in neurotransmission, mechanosensory transduction, secretion and vasodilatation, and long-term (chronic) signaling functions in cell proliferation, differentiation, and death involved in development and regeneration. Plasticity of purinoceptor expression in pathological conditions is frequently observed, including an increase in the purinergic component of autonomic cotransmission. Recent advances in therapies using purinergic-related drugs in a wide range of pathological conditions will be addressed with speculation on future developments in the field.

Historical review: ATP as a neurotransmitter

Purinergic signalling is now recognized to be involved in a wide range of activities of the nervous system, including neuroprotection, central control of autonomic functions, neural-glial interactions, control of vessel tone and angiogenesis, pain and mechanosensory transduction and the physiology of the special senses. In this article, I give a personal retrospective of the discovery of purinergic neurotransmission in the early 1970s, the struggle for its acceptance for approximately 20 years, the expansion into purinergic cotransmission and its eventual acceptance when receptor subtypes for ATP were cloned and characterized and when purinergic synaptic transmission between neurons in the brain and peripheral ganglia was described in the early 1990s. I also discuss the current status of the field, including recent interest in the pathophysiology of purinergic signalling and its therapeutic potential.

In vitro effect of homocysteine on nucleotide hydrolysis by blood serum from adult rats

During the past few years, elevated blood levels of homocysteine (Hcy) have been linked to increased risk of premature coronary artery disease, stroke and thromboembolism. These processes can be also related to the ratio adenine nucleotide/adenosine, since extracellularly these nucleotides are associated with modulation of processes such as platelet aggregation, vasodilatation and coronary flow. Furthermore, there are some studies that suggest a relationship between Hcy and plasma adenosine concentrations. The sequential hydrolysis of ATP to adenosine by soluble nucleotidases constitutes one of the systems for rapid inactivation of circulating adenine nucleotides. Thus, the main objective of this study was to evaluate if Hcy can participate in the modulation of the extracellular adenine nucleotide hydrolysis by rat blood serum. Our results showed that Hcy, at final concentrations of 5.0 mM, inhibits in vitro ATP, ADP and AMP hydrolysis by 26, 21 and 16%, respectively. Also Hcy, at final concentrations of 8.0mM, inhibited the in vitro hydrolysis of ATP, ADP and AMP by 46, 44 and 44%, respectively. Kinetic analysis showed that the inhibitions of the three adenine nucleotide hydrolyses in the presence of Hcy, by serum of adult rats, is of the uncompetitive type. The IC50 calculated from the results obtained were 6.52+/-1.75 mM (n = 4), 5.18 +/- 0.64 mM (n = 3) and 5.16 +/- 1.22 mM (n = 3) for ATP, ADP and AMP hydrolysis, respectively.

Nociceptive response and adenine nucleotide hydrolysis in synaptosomes isolated from spinal cord of hypothyroid rats

Purinergic system exerts a significant influence on the modulation of pain pathways at the spinal site. Adenosine has antinociceptive properties in experimental and clinical situations, while ATP exerts pronociceptive actions in different pain models. In this study we investigated the hydrolysis of ATP to adenosine in synaptosomes from spinal cord in parallel with the nociceptive response of rats at different ages after hypothyroidism induction. Hypothyroidism elicited a significant increase in AMP hydrolysis to adenosine in synaptosomes from spinal cord of rats subjected to neonatal hypothyroidism and in 420-day-old rats submitted to thyroidectomy. Accordingly, these rats presented an analgesic response as a consequence of hypothyroidism. In contrast, the ATP hydrolysis was decreased in the spinal cord of 60-day-old hypothyroid rats in parallel with a significant increase in nociceptive response. These results indicate the involvement of adenine nucleotides in the control of the hypothyroidism-induced nociceptive response during development.

Ecto-hydrolysis of adenine nucleotides in rat blood platelets are altered by ovariectomy

There is evidence that estrogen is associated with a reduction on cardiovascular disease risk through inhibition of platelet aggregation and action on vascular function. The process of haemostasis can also be affected by adenine nucleotides and adenosine. Consequently, regulation of enzymes that hydrolyze these nucleotides in the bloodstream is essential in the modulation of the processes of platelet aggregation, vasodilatation and coronary flow. Ecto-ATP diphosphohydrolase and ecto-5'-nucleotidase from platelets are enzymes related to nucleotide hydrolysis. In the present study, we examined the effect of ovariectomy (OVX) and estradiol replacement therapy (ER) on the activity of the enzymes that degrade adenine nucleotides in platelets of female rats. The OVX group significantly decrease the hydrolysis of ATP, ADP and AMP by 42, 52 and 29.3%, respectively, when compared to a control group. ER did not reverse the inhibition of nucleotide hydrolysis observed in OVX rats. Our findings indicate that hormonal deprivation affects ATP, ADP and AMP hydrolysis by platelets and consequently the level of these nucleotides and adenosine in the circulation. Since, ADP is the most important platelet agonist and recruiting agent present in the microenvironment of the thrombus, our findings was contribute to a better comprehension of the cardiovascular complications described in alterations of sexual hormonal status.

Nucleoside triphosphate diphosphohydrolase-2 (NTPDase2/CD39L1) is the dominant ectonucleotidase expressed by rat astrocytes

Inflammatory and degenerative pathophysiological processes within the CNS are important causes of human disease. Astrocytes appear to modulate these reactions and are a major source of inflammatory mediators, e.g. extracellular adenine nucleotides, in nervous tissues. Actions following extracellular nucleotides binding to type 2 purinergic receptors are regulated by ectonucleotidases, including members of the CD39/ecto-nucleoside triphosphate diphosphohydrolase family. The ectonucleotidases of astrocytes expressed by rat brain rapidly convert extracellular ATP to ADP, ultimately to AMP. RT-PCR, immunocytochemistry as well as Western blotting analysis demonstrated expression of multiple ecto-nucleoside triphosphate diphosphohydrolase family members at both the mRNA and protein level. By quantitative real-time PCR, we identified Entpd2 (CD39L1) as the dominant Entpd gene expressed by rat hippocampal, cortical and cerebellar astrocytes. These data in combination with the elevated ecto-ATPase activity observed in these brain regions, suggest that NTPDase2, an ecto-enzyme that preferentially hydrolyzes ATP, is the major ecto-nucleoside triphosphate diphosphohydrolase expressed by rat astrocytes. NTPDase2 may modulate inflammatory reactions within the CNS and could represent a useful therapeutic target in human disease.

A capillary electrophoresis method for the characterization of ecto-nucleoside triphosphate diphosphohydrolases (NTPDases) and the analysis of inhibitors by in-capillary enzymatic microreaction

A capillary electrophoresis (CE) method for the characterization of recombinant NTPDases 1, 2, and 3, and for assaying NTPDase inhibitors has been developed performing the enzymatic reaction within the capillary. After hydrodynamic injection of plugs of substrate solution with or without inhibitor in reaction buffer, followed by a suspension of an enzyme-containing membrane preparation, and subsequent injection of another plug of substrate solution with or without inhibitor, the reaction took place close to the capillary inlet. After 5 min, the electrophoretic separation of the reaction products was initiated by applying a constant current of -60 muA. The method employing a polyacrylamide-coated capillary and reverse polarity mode provided baseline resolution of substrates and products within a short separation time of less than 7 min. A 50 mM phosphate buffer (pH 6.5) was used for the separations and the products were detected by their UV absorbance at 210 nm. The Michaelis-Menten constants (K (m)) for the recombinant rat NTPDases 1, 2, and 3 obtained with this method were consistent with previously reported data. The inhibition studies revealed pronounced differences in the potency of reactive blue 2, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), suramin, and N (6)-diethyl-beta,gamma-dibromomethylene-ATP (ARL67156) towards the NTPDase isoforms. Notably, ARL67156 does not inhibit all NTPDases, having only a minor inhibitory effect on NTPDase2. Dipyridamole is not an inhibitor of the NTPDase isoforms investigated. The new method is fast and accurate, it requires only tiny amounts of material (nanoliter scale), no sample pretreatment and can be fully automated; thus it is clearly superior to the current standard methods.

HIV infection is associated with increased NTPDase activity that correlates with CD39-positive lymphocytes

Infection with the human immunodeficiency virus (HIV) results in alterations in immune cells such as an increase or decrease of cytokine secretion and immunodeficiency. HIV causes a state of chronic cellular activation that can induce apoptosis in lymphocyte T-helpers, making the patient susceptive to opportunistic infections. The biochemical mechanisms involved in this immune response to HIV have been researched. Here, we have shown for the first time that ATP and ADP hydrolysis are essential for the immune response to HIV. Our results clearly indicate an increase of NTPDase-1 (EC 3.6.1.5) activity in lymphocytes of HIV-positive patients, confirmed by an enhanced CD39 expression on its surface. These results suggest that NTPDase-1 may be important to keep an adequate balance between the generation and consumption of ATP and to preserve cellular integrity and immune response to the HIV infection.

YND1 interacts with CDC55 and is a novel mediator of E4orf4-induced toxicity

Adenovirus E4orf4 (early region 4 open reading frame 4) protein induces protein phosphatase 2A-dependent non-classical apoptosis in mammalian cells and irreversible growth arrest in Saccharomyces cerevisiae. Oncogenic transformation sensitizes cells to E4orf4-induced cell death. To uncover additional components of the E4orf4 network required for induction of its unique mode of apoptosis, we used yeast genetics to select gene deletions conferring resistance to E4orf4. Deletion of YND1, encoding a yeast Golgi apyrase, conferred partial resistance to E4orf4. However, Ynd1p apyrase activity was not required for E4orf4-induced toxicity. Ynd1p and Cdc55p, the yeast protein phosphatase 2A-B subunit, contributed additively to E4orf4-induced toxicity. Furthermore, concomitant overexpression of one and deletion of the other was detrimental to yeast growth, demonstrating a functional interaction between the two proteins. YND1 and CDC55 also interacted genetically with CDC20 and CDH1/HCT1, encoding activating subunits of the anaphase-promoting complex/cyclosome. In addition to their functional interaction, Ynd1p and Cdc55p interacted physically, and this interaction was disrupted by E4orf4, which remained associated with both proteins. The results suggested that Ynd1p and Cdc55p share a common downstream target whose balanced modulation by the two E4orf4 partners is crucial to viability. Disruption of this balance by E4orf4 may lead to cell death. NTPDase-4/Lalp70/UDPase, the closest mammalian homologue of Ynd1p, associated with E4orf4 in mammalian cells, suggesting that the results in yeast are relevant to the mammalian system.

Biochemical brain markers and purinergic parameters in rat CSF after seizure induced by pentylenetetrazol

Cellular and molecular mechanisms involved in the generation of seizures and the magnitude of neural cells injury are not fully understood. We evaluated astrocyte and/or neuronal injury in rats in the pentylenetetrazol model of acute seizures by measuring S100B and NSE levels in cerebrospinal fluid. Additionally, we determined ADP and GDP hydrolysis by soluble nucleoside triphosphate diphosphohydrolase in the cerebrospinal fluid, and the concentration of nucleosides adenosine, inosine and guanosine as putative markers of brain injury. After pentylenetetrazol-induced seizures: (i) S100B values increased from 10 to 30 min, returning to control levels at 24 h; NSE levels presented a biphasic increase: an increase at 10 to 30 min returning to control levels, and again at 240 min followed by a decline at 24 h; (ii) nucleotidase activities increased from 10 min, returning to control levels at 240 min; (iii) guanosine and inosine levels increased exclusively after 30 min. In summary, this study showed biochemical changes in the cerebrospinal fluid occurring after seizures induced by pentylenetetrazol. Such events may have a modulating effect upon seizure expression, particularly nucleoside triphosphate diphosphohydrolase activities and nucleoside concentrations, but are nevertheless followed by neural death as evidenced by the increase in NSE and S100B levels.

Heterogeneity in extracellular nucleotide hydrolysis among clinical isolates of Trichomonas vaginalis

Trichomonas vaginalis is a parasitic protozoan that causes trichomonosis, a sexually-transmitted disease, with serious sequelae to women and men. As the host-parasite relationship is complex, it is important to investigate biochemical aspects of the parasite that contribute to our understanding of trichomonal biology and pathogenesis. Nucleoside triphosphate diphosphohydrolase 1 (NTPDase 1), which hydrolyses extracellular ATP and ADP, and ecto-5'-nucleotidase, which hyrolyses AMP, have been characterized in laboratory isolates of T. vaginalis. Here we show that the extracellular ATP: ADP hydrolysis ratio varies among fresh clinical isolates, which presented higher ATPase and ADPase activities than long-term-grown isolates. Growth of parasites in iron-replete and iron-depleted medium resulted in different, albeit minor, patterns in extracellular ATP and ADP hydrolysis among isolates. Importantly, some isolates had low or absent ecto-5'-nucleotidase activity, regardless of environmental conditions tested. For isolates with ecto-5'-nucleotidase activity, high- and low-iron trichomonads had increased and decreased levels of activity, respectively, compared to organisms grown in normal TYM-serum medium. This suggests a regulation in expression of either the enzyme amounts and/or activity under the control of iron. Finally, we found no correlation between the presence or absence of dsRNA virus infection among trichomonad isolates and NTPDase and ecto-5'-nucleotidase activities.

Carbofuran and malathion inhibit nucleotide hydrolysis in zebrafish (Danio rerio) brain membranes

Carbofuran and malathion are broad spectrum pesticides widely used in agricultural practice throughout the world. Toxicity of these pesticides has been correlated with their inhibitory effects on acetylcholinesterase activity. Nucleotides are extracellular signaling molecules, which trigger multiple biological effects. Studies have demonstrated the co-transmission of acetylcholine and ATP at the nerve endings. The control of neurotransmitter ATP levels is promoted by enzymes named ectonucleotidases, which include nucleoside triphosphate diphosphohydrolase (NTPDase) family and ecto-5'-nucleotidase. Since acetylcholine and ATP are co-released at the synapse and the acetylcholinesterase inhibition is an important target for pesticide action, here we verified the effect of exposure in vitro and in vivo to carbofuran and malathion on ectonucleotidase activities from brain membranes of zebrafish. To verify if carbofuran and malathion have a direct inhibitory effect on NTPDase and 5'-nucleotidase activities in brain membranes of zebrafish, we have tested in vitro concentrations of pesticides varying from 0.25 to 5 mM. Carbofuran, in vitro, inhibited ATP and ADP hydrolysis in an uncompetitive manner, but no effect was observed on AMP hydrolysis. Malathion decreased ATP and ADP hydrolysis in competitive and an uncompetitive manner, respectively, but not altered AMP hydrolysis. After exposure to carbofuran (50 and 500 microg/L) during 7 days, ADP hydrolysis was significantly decreased in both concentrations tested (by 19 and 24.5%, respectively). Malathion, at 500 microg/L, was able to inhibit ADP and AMP hydrolysis (by 28 and 58.5%, respectively). This study has shown that ectonucleotidases from brain membranes of zebrafish can be a potential target for pesticide neurotoxicity.

Cloning and characterization of the ecto-nucleotidase NTPDase3 from rat brain: Predicted secondary structure and relation to other members of the E-NTPDase family and actin

The protein family of ecto-nucleoside triphosphate diphosphohydrolases (E-NTPDase family) contains multiple members that hydrolyze nucleoside 5'-triphosphates and nucleoside 5'-diphosphates with varying preference for the individual type of nucleotide. We report the cloning and functional expression of rat NTPDase3. The rat brain-derived cDNA has an open reading frame of 1590 bp encoding 529 amino acid residues, a calculated molecular mass of 59.1 kDa and predicted N- and C-terminal hydrophobic sequences. It shares 94.3% and 81.7% amino acid identity with the mouse and human NTPDase3, respectively, and is more closely related to cell surface-located than to the intracellularly located members of the enzyme family. The NTPDase3 gene is allocated to chromosome 8q32 and organized into 11 exons. Rat NTPDase3 expressed in CHO cells hydrolyzed both nucleoside triphosphates and nucleoside diphosphates with hydrolysis ratios of ATP:ADP of 5:1 and UTP:UDP of 8:1. After addition of ATP, ADP is formed as an intermediate product that is further hydrolyzed to AMP. The enzyme is preferentially activated by Ca(2+) over Mg(2+) and reveals an alkaline pH optimum. Immunocytochemistry confirmed expression of heterologously expressed NTPDase3 to the surface of CHO cells. PC12 cells express endogenous surface-located NTPDase3. An immunoblot analysis detects NTPDase3 in all rat brain regions investigated. An alignment of the secondary structure domains of actin conserved within the actin/HSP70/sugar kinase superfamily to those of all members of the NTPDase family reveals apparent similarity. It infers that NTPDases share the two-domain structure with members of this enzyme superfamily.

C-terminal splicing of NTPDase2 provides distinctive catalytic properties, cellular distribution and enzyme regulation

The present study provides functional characterization of alternative splicing of the NTPDase2 (ecto-nucleoside triphosphate diphosphohydrolase-2) involved in the regulation of extracellular nucleotide concentrations in a range of organ systems. A novel NTPDase2beta isoform produced by alternative splicing of the rat NTPDase2 gene provides an extended intracellular C-terminus and distinguishes itself from NTPDase2alpha isoform in gaining several intracellular protein kinase CK2 (casein kinase 2) phosphorylation sites and losing the intracellular protein kinase C motif. The plasmids containing NTPDase2alpha or NTPDase2beta cDNA were used to stably transfect Chinese-hamster ovary-S cells. Imaging studies showed that NTPDase2alpha was predominantly membrane-bound, whereas NTPDase2beta had combined cell surface and intracellular localization. alpha and beta isoforms showed variations in divalent cation dependence and substrate specificity for nucleoside-5'-triphosphates and nucleoside-5'-diphosphates. NTPDase2beta exhibited reduced ATPase activity and no apparent ADPase activity. NTPDase2 isoforms demonstrated similar sensitivity to inhibitors such as suramin and pyridoxal phosphate-6-azophenyl-2',4'-disulphonic acid, and differential regulation by protein kinases. NTPDase2beta was up-regulated by intracellular protein kinase CK2 phosphorylation, whereas NTPDase2alpha activity was down-regulated by protein kinase C phosphorylation. The results demonstrate that alternative coding of the intracellular C-terminal domain contributes distinctive phenotypic variation with respect to extracellular nucleotide specificity, hydrolysis kinetics, protein kinase-dependent intracellular regulation and protein trafficking. These findings advance the molecular physiology of this enzyme system by characterizing the contribution of the C-terminal domain to many of the enzyme's signature properties.

L-NAME administration prevents the inhibition of nucleotide hydrolysis by rat blood serum subjected to hyperargininemia

The main objective of the present study was to evaluate the in vivo and in vitro effect of Arg on serum nucleotide hydrolysis. The action of Nomega-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthase, on the effects produced by Arg was also examined. Sixty-day-old rats were treated with a single or a triple (with an interval of 1 h between each injection) intraperitoneal injection of saline (group I), Arg (0.8 g/kg) (group II), L-NAME (2.0 mg/kg or 20 mg/kg) (group III) or Arg (0.8 g/kg) plus L-NAME (2.0 mg/kg or 20 mg/kg) (group IV) and were killed 1 h later. The present results show that a triple Arg administration decreased ATP, ADP and AMP hydrolysis. Simultaneous injection of L-NAME (20 mg/kg) prevented such effects. Arg in vitro did not alter nucleotide hydrolysis. It is suggested that in vivo Arg administration reduces nucleotide hydrolysis in rat serum, probably through nitric oxide or/and peroxynitrite formation.

Ecto-5'-nucleotidase activity in brain membranes of zebrafish (Danio rerio)

Adenosine, a well-known neuromodulator, may be formed intracellularly in the CNS from degradation of AMP and then exit via bi-directional nucleoside transporters, or extracellularly by the metabolism of released nucleotides. This study reports the enzymatic properties of an ecto-5'-nucleotidase activity in brain membranes of zebrafish (Danio rerio). This enzyme was cation-dependent, with a maximal rate for AMP hydrolysis in a pH range of 7.0-7.5 in the presence of Mg(2+). The enzyme presented a maximal activity for AMP hydrolysis at 37 degrees C. The apparent K(m) and V(max) values for Mg(2+)-AMP were 135.3+/-16 microM and 29+/-4.2 nmol Pi.min(-1).mg(-1) protein, respectively. The enzyme was able to hydrolyze both purine and pyrimidine monophosphate nucleotides, such as UMP, GMP and CMP. Levamisole and tetramisole (1 mM), specific inhibitors of alkaline phosphatases, did not alter the enzymatic activity. However, a significant inhibition of AMP hydrolysis (42%) was observed in the presence of 100 microM alpha,beta-methylene-ADP, a known inhibitor of ecto-5'-nucleotidase. Since 5'-nucleotidase represents the major enzyme responsible for the formation of extracellular adenosine, the enzymatic characterization is important to understand its role in purinergic systems and the involvement of adenosine in the regulation of neurotransmitter release.

Comparative hydrolysis of P2 receptor agonists by NTPDases 1, 2, 3 and 8

Nucleoside triphosphate diphosphohydrolases 1, 2, 3 and 8 (NTPDases 1, 2, 3 and 8) are the dominant ectonucleotidases and thereby expected to play important roles in nucleotide signaling. Distinct biochemical characteristics of individual NTPDases should allow them to regulate P2 receptor activation differentially. Therefore, the biochemical and kinetic properties of these enzymes were compared. NTPDases 1, 2, 3 and 8 efficiently hydrolyzed ATP and UTP with K (m) values in the micromolar range, indicating that they should terminate the effects exerted by these nucleotide agonists at P2X(1-7) and P2Y(2,4,11) receptors. Since NTPDase1 does not allow accumulation of ADP, it should terminate the activation of P2Y(1,12,13) receptors far more efficiently than the other NTPDases. In contrast, NTPDases 2, 3 and 8 are expected to promote the activation of ADP specific receptors, because in the presence of ATP they produce a sustained (NTPDase2) or transient (NTPDases 3 and 8) accumulation of ADP. Interestingly, all plasma membrane NTPDases dephosphorylate UTP with a significant accumulation of UDP, favoring P2Y(6) receptor activation. NTPDases differ in divalent cation and pH dependence, although all are active in the pH range of 7.0-8.5. Various NTPDases may also distinctly affect formation of extracellular adenosine and therefore adenosine receptor-mediated responses, since they generate different amounts of the substrate (AMP) and inhibitor (ADP) of ecto-5'-nucleotidase, the rate limiting enzyme in the production of adenosine. Taken together, these data indicate that plasma membrane NTPDases hydrolyze nucleotides in a distinctive manner and may therefore differentially regulate P2 and adenosine receptor signaling.

Hypo-and hyperthyroidism affect the ATP, ADP and AMP hydrolysis in rat hippocampal and cortical slices

The presence of severe neurological symptoms in thyroid diseases has highlighted the importance of thyroid hormones in the normal functioning of the mature brain. Since, ATP is an important excitatory neurotransmitter and adenosine acts as a neuromodulatory structure inhibiting neurotransmitters release in the central nervous system (CNS), the ectonucleotidase cascade that hydrolyzes ATP to adenosine, is also involved in the control of brain functions. Thus, we investigated the influence of hyper-and hypothyroidism on the ATP, ADP and AMP hydrolysis in hippocampal and cortical slices from adult rats. Hyperthyroidism was induced by daily injections of l-thyroxine (T4) 25 microg/100 g body weight, for 14 days. Hypothyroidism was induced by thyroidectomy and methimazole (0.05%) added to their drinking water for 14 days. Hypothyroid rats were hormonally replaced by daily injections of T4 (5 microg/100 g body weight, i.p.) for 5 days. Hyperthyroidism significantly inhibited the ATP, ADP and AMP hydrolysis in hippocampal slices. In brain cortical slices, hyperthyroidism inhibited the AMP hydrolysis. In contrast, hypothyroidism increased the ATP, ADP and AMP hydrolysis in both hippocampal and cortical slices and these effects were reverted by T4 replacement. Furthermore, hypothyroidism increased the expression of NTPDase1 and 5'-nucleotidase, whereas hyperthyroidism decreased the expression of 5'-nucleotidase in hippocampus of adult rats. These findings demonstrate that thyroid disorders may influence the enzymes involved in the complete degradation of ATP to adenosine and possibly affects the responses mediated by adenine nucleotides in the CNS of adult rats.

ATP and ADP hydrolysis in the kidney and liver of fish, chickens and rats

We investigated NTPDase-like activity [ecto-nucleoside triphosphate diphosphohydrolases (E-NTPDases)] in liver and kidney membrane from silver catfish (Rhamdia quelen), chicken (Gallus gallus) and rat (Rattus norvegicus) under different conditions and in the presence of several inhibitors. The cation concentration required for maximal activity was 0.5, 1.5 and 2.0 mM for fish, chicken and rat liver, respectively (with ATP and ADP as substrates). The maximal activity in the kidney was observed at calcium concentrations of 0.5, 2.0, 1.5 mM (ATP) and 0.5, 1.5, 1.0 (ADP) for fish, chickens and rats, respectively. The results showed that the pH optimum for all animals and for the two tissues was close to 8.0. The temperature chosen was 25 degrees C for fish and 36 degrees C for chicken and rat preparations. Ouabain had no effect on the NTPDase-like activity of fish, chickens or rats. NTPDase activity was decreased in the presence of lanthanum in the chicken (ADP) and rat (ATP and ADP) liver. In the kidney, lanthanum inhibited fish ATP and rat ATP and ADP (0.2 mM) hydrolysis. N-ethylmaleimide (NEM) had an inhibitory effect on the kidney of all species at the concentration of 3.0 mM (ADP). Orthovanadate only inhibited fish membrane NTPDase; azide only inhibited the preparation at high concentrations (10 mM) and fluoride inhibited it at 10 mM (fish and chicken) and 5 mM (rat). Trifluoperazine (0.05-0.2 mM) and suramin (0.03-0.3 mM) inhibited NTPDase at all concentrations tested. These results suggest that NTPDase-like activity shows a different behavior among the vertebrate species and tissues studied. Additionally, we propose that NTPDase1 is the main enzyme present in this preparation.

Developmental profile of NTPDase activity in synaptic plasma membranes isolated from rat cerebral cortex

In the present study the developmental profile of ATP-hydrolyzing activity promoted by NTPDase 1, its kinetic properties and the enzyme protein abundance associated with synaptic plasma membrane from rat cerebral cortex were characterized. NTPDase 1 activity increased from birth to day 30; afterwards it decreased and remained unchanged from adulthood (90 days) to senescence (365 days). Kinetic analysis revealed that enzyme exhibited the highest specific activity at day 30 and highest apparent affinity for ATP at day 365; however, V(max)/K(m) values remained unchanged for each age studied. Immunoblot analysis demonstrated that relative abundance of NTPDase 1 is highest at day 15 during ontogeny. The discrepancy between maximum enzyme activity and maximum enzyme protein abundance indicates that NTPDase 1 may have an additional role during development.

Ovariectomy and estradiol replacement therapy alters the adenine nucleotide hydrolysis in rat blood serum

The low prevalence of coronary heart disease in premenopausal women and its increase after menopause are well established. Many studies have suggested that steroid hormones can inhibit platelet aggregation, reducing the cardiovascular risk. In addition, a number of studies have shown an effect of estrogen on vascular function. The process of haemostasis and thrombus formation can be also affected by adenine nucleotides and adenosine. Consequently, the regulation of enzymes that hydrolyze these nucleotides in the bloodstream is essential in the modulation of the processes of platelet aggregation, vasodilatation and coronary flow. Thus, in this study, we examined the effect of ovariectomy (OVX), estradiol replacement therapy and the in vitro administration of 17beta-estradiol, dehydroisoandrosterone 3-sulfate (DHEAS) and pregnenolone (PREG) on the activity of the enzymes that degrade adenosine triphosphate (ATP), adenosine diphosphate (ADP) and adenosine monophosphate (AMP) in the blood serum of female rats. OVX significantly increased the hydrolysis of ATP, ADP and AMP, whilst phosphodiesterase activity was unchanged. Estradiol replacement therapy significantly decreased the hydrolysis of the adenine nucleotides and of the substrate marker of phosphodiesterase. In vitro, the addition of steroid hormones did not have any effect on the nucleotide hydrolysis by rat serum. These results suggest the presence of a strong relation between these enzymes and the hormonal system. In addition, the alterations observed are important, because these enzymes control the nucleotides/nucleosides ratio in the circulation and thus the events related to haemostasis.

A large hinge bending domain rotation is necessary for the catalytic function of Escherichia coli 5'-nucleotidase

Two variants of Escherichia coli 5'-nucleotidase with disulfide bridges that were engineered to link the two domains of the protein were used to demonstrate that a large domain rotation is required for the catalytic mechanism of the enzyme. Kinetic analysis demonstrates that the variant trapped in the open form is almost inactive but can be activated up to 250-fold by reduction of the disulfide bridge. The second variant can adopt a closed but also a half-open conformation despite the presence of the cystine linkage. As a result of this flexibility, the mutant is still active in its oxidized state, although it shows a more pronounced substrate inhibition than the wild-type protein. A theoretical model is proposed that allows estimation of the flexibility of the proteins in the presence of the disulfide domain cross-link. Despite the unexpected residual flexibility of the trapped mutants, the enzymes could be used as conformational reporters in CD spectroscopy, revealing that the wild-type protein exists predominantly in an open conformation in solution. The kinetic, spectroscopic, and theoretical data are brought together to discuss the domain rotation in terms of the kinetic functioning of E. coli 5'-nucleotidase.

Nucleotidase activities in membrane preparations of nervous ganglia and digestive gland of the snail Helix aspersa

Nucleotide-metabolizing enzymes play an important role in the regulation of nucleotide levels. In the present report, we demonstrated an enzyme activity with different kinetic properties in membrane preparations of the nervous ganglia and digestive gland from Helix aspersa. ATPase and ADPase activities were dependent on Ca2+ and Mg2+ with pH optima approximately 7.2 and between 6.0 and 8.0 in digestive gland and nervous ganglia, respectively. The enzyme activities present in membrane preparations of these tissues preferentially hydrolyzed triphosphate nucleotides. In nervous ganglia, the enzyme was insensitive to the classical ATPases inhibitors. In contrast, in digestive gland, N-ethylmaleimide (NEM) produced 45% inhibition of Ca(2+)-ATP hydrolysis. Sodium azide, at 100 microM and 20 mM, inhibited Mg(2+)-ATP hydrolysis by 36% and 55% in digestive gland, respectively. The presence of nucleotide-metabolizing enzymes in these tissues may be important for the modulation of nucleotide and nucleoside levels, controlling their actions on specific purinoceptors in these species.

Acetylsalicylic acid inhibits ATP diphosphohydrolase activity by platelets from adult rats

BACKGROUND AND METHODS

The in vitro effect of the nonsteroidal anti-inflammatory drug, acetylsalicylic acid (ASA), on the extracellular adenine nucleotide hydrolysis by intact rat blood platelets was studied.

RESULTS

Our results demonstrate that aspirin, at final concentrations of 2.0 and 3.0 mM, inhibits ATP extracellular hydrolysis in vitro by approximately 17% and 21%, respectively. Aspirin, at a final concentration of 3.0 mM, also inhibited in vitro extracellular ADP hydrolysis by approximately 41%. The same concentrations of this drug, however, did not alter AMP hydrolysis by intact rat blood platelets under similar assay conditions. The kinetic analysis demonstrated that the inhibition of ADP and ATP hydrolysis by aspirin in rat platelets is of the uncompetitive type.

CONCLUSION

In this study, we demonstrated an inhibitory effect of ASA upon E-NTPDase 3 activity of platelets from adult rats and discussed the significance of our findings.

Aqueous extract of Ilex paraguariensis decreases nucleotide hydrolysis in rat blood serum

Mate is a xanthine-containing beverage, which is prepared as an infusion of the dried and ground leaves of Ilex paraguariensis St. Hil. (Aquifoliacea). Previous reports have shown that Ilex paraguariensis has the highest levels of caffeine and theobromine when compared to other Ilex species. Furthermore, mate is able to interfere in the circulatory system, acting as a diuretic and hypotensive agent. Many processes of vascular injury result in the release of adenine nucleotides, which exert a variety of effects. Nucleoside 5' tri- and diphosphates may be hydrolyzed by members of the ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase) family. The synchronic action of a NTPDase and a 5'-nucleotidase promotes the catabolism of ATP to adenosine, which is able to control the extracellular nucleotides/nucleosides ratio. The chronic ingestion of aqueous extract of Ilex paraguariensis by rats during 15 days significantly decreased ATP (55%), ADP (50%) and AMP (40%) hydrolysis in blood serum. These results suggest changes in the balance of purine levels induced by Ilex paraguariensis ingestion. Considering the potential effects of Ilex paraguariensis in the circulatory system, these results may be relevant since NTPDases are a novel drug target for the treatment of cardiovascular diseases.

Inhibitory effect of novel pyrimidines on ATP and ADP hydrolysis in synaptosomes from rat cerebral cortex

In this study, a new series of trihalomethyl-substituted pyrimidines and dihydropyrimidines were synthesized and tested as potential NTPDase inhibitors. For this purpose, synaptosomes from rat cerebral cortex were used as the enzyme source and ATP and ADP were used as the substrate. Among the new compounds, 4-methyl-2-methylsulfanyl-6-trichloromethylpyrimidine (2b) was found to be the most effective noncompetitive inhibitor, with an estimated K(i) value of 0.18 and 0.55 mM for ATP and ADP, respectively. Other pyrimidines inhibited NTPDase activity with the following rank order of inhibitory potency: 3,6-dimethyl-2-methylsulfanyl-4-trifluoromethyl-3,4-dihydro-pyrimidin-4-ol (3a) > 5-methyl-2-(4-methyl-6-trifluoromethyl-pyrimidin-2-yl)-3-trifluoromethyl-3,4-dihydro-2H-3-pyrazol-3-ol (6a) > 5-bromo-4,6-dimethoxy-4-trichloromethyl-1,2,3,4-tetrahydro-2-pyrimidin-2-one (9) for ATP and 6a > 9 > 3a for ADP. Our results demonstrate that a novel group of pyrimidines compounds can act as inhibitors of ATP and ADP hydrolysis in synaptosomes from rat cerebral cortex. These results can contribute for the understanding of the NTPDase activity and for further studies involving new compounds that can enlist as it inhibitors.

Comparative hydrolysis of extracellular adenine nucleotides and adenosine in synaptic membranes from porcine brain cortex, hippocampus, cerebellum and medulla oblongata

We have investigated the metabolism of extracellular adenine nucleotides and adenosine in porcine brain. The cortex synaptic plasma membranes hydrolyzed ATP to ADP, AMP and adenosine. We also observed a slow hydrolysis of adenosine with the concomitant accumulation of inosine. These results indicate that NTPDase1, NTPDase2, ecto-5'-nucleotidase, and adenosine deaminase are present in cortex synaptic membranes from porcine brain. We further showed that all these enzymes are also abundant in synaptic membranes from hippocampus, cerebellum, and medulla oblongata and compared their specific activities. Brain cortex and hippocampus exhibited higher activities of NTPDase1 and NTPDase2 than cerebellum and medulla oblongata. It was consistent with the high level of the expression of NTPDases in the two first structures. Adenosine deaminase activity was found in all brain structures analyzed; however, it was lower than the activity of ecto-nucleotidases. Taken together, our data suggest that investigated enzymes have a ubiquitous abundance in porcine brain, and observed differences in their activities in cortex, hippocampus, cerebellum, and medulla oblongata may correlate with the pattern of P2 receptor expression in these brain areas. In addition, low activity of adenosine deaminase may indicate that nonenzymatic mechanism(s) are responsible for the termination of P1 receptor signaling in porcine brain.

Effects of steroid hormones on synaptosomal ectonucleotidase activities from hippocampus and cortex of adult female rats

Over the last few years, the effects of steroid hormones on the brain have been intensively discussed. It has been demonstrated that ATP (acting as a neurotransmitter) is hydrolyzed to adenosine in the synaptic cleft by the conjugated action of ectonucleotidases, which include an enzyme of the E-NTPDase family (NTPDase3, apyrase, EC 3.6.1.5) and a 5'-nucleotidase (EC 3.1.3.5). The 5'-nucleotidase enzyme is able to hydrolyze AMP as well as other monophosphate nucleotides. The importance of this enzyme in the central nervous system is to participate in the adenosine formation, a nucleoside with neuroprotective properties and modulatory effects. However, several questions have been raised about the mechanisms of steroid hormones and the possible neuroprotective effects of estrogen. Thus, we examined the effects of gonadal steroid hormone deprivation, induced by ovary removal (OVX) and estradiol replacement therapy, on the ectonucleotidase activities in synaptosomes from hippocampus and cerebral cortex of adult rats. ATP and ADP hydrolysis in synaptosomes from cerebral cortex and hippocampus did not change as a function of OVX and results demonstrated an increase in AMP hydrolysis (82%) in the animals submitted to OVX in cerebral cortex, but not in hippocampus, when compared to control and sham-operated groups. Estradiol replacement therapy reversed this effect. RT-PCR analysis showed that the enhancement of enzyme activity in cerebral cortex could be explained by the higher expression of 5'-nucleotidase, following OVX. The hormones 17beta-estradiol (cyclodextrin-encapsulated 17beta-estradiol), DHEAS, and pregnenolone (1.0, 2.5, and 5.0 microM) did not alter the nucleotide hydrolysis, in vitro, in synaptosomes from cortex and hippocampus of female adult rats. Results presented, herein, should be considered relevant for hormone replacement therapy, since much controversy exists surrounding this area and the relationship between adenosine and sex steroids is still poorly understood.

Expression of NTPDase1 and NTPDase2 in murine kidney: relevance to regulation of P2 receptor signaling

The regulation of renal function by extracellular nucleotides encompasses alterations in glomerular hemodynamics, microvascular function, tubuloglomerular feedback, tubular transport, cell growth or apoptosis, and transport of water and solutes in the medullary collecting duct. Nearly all cells can release ATP or other nucleotides that are then rapidly hydrolyzed in the extracellular milieu. However, little information is available on the cellular expression of ectoenzymes that hydrolyze extracellular nucleotides within the kidney. Nucleoside triphosphate diphosphohydrolases (NTPDases) are plasma membrane-bound ectonucleotidases. NTPDase1 has identity with CD39, a B lymphocyte activation marker, and hydrolyzes extracellular ATP and ADP to AMP within the vasculature, whereas NTPDase2/CD39L(ike)1 preferentially converts ATP to ADP outside of blood vessels. Using immunohistochemical and in situ hybridization approaches, we localized the protein and mRNA of NTPDase1 and 2 in murine renal tissues. In the renal cortex, NTPDase1 is expressed by vascular smooth muscle cells and endothelium in interlobular arteries, afferent glomerular arterioles, and peritubular capillaries. In the inner medulla, NTPDase1 is expressed in ascending thin limbs of Henle's loop, ducts of Bellini, and in the pelvic wall. In contrast, NTPDase2 is expressed in Bowman's capsule, glomerular arterioles, adventitia of blood vessels, and pelvic wall. Thus the distribution patterns of NTPDases have parallels to the known distribution of P2 receptors within the kidney. NTPDases may modulate regulatory effects of ATP and degradation products within the vasculature and other sites and thereby potentially influence physiological as well as multiple pathological events in the kidney.

Regulation of P2Y1 receptor-mediated signaling by the ectonucleoside triphosphate diphosphohydrolase isozymes NTPDase1 and NTPDase2

Ectonucleoside triphosphate diphosphohydrolases (NTPDases) control the concentration of released extracellular nucleotides, but the precise physiological roles played by these isozymes in modulation of P2 receptor signaling remain unclear. Activation of the human P2Y(1) receptor was studied in the presence of NTPDase1 or NTPDase2 expressed either in the same cell as the receptor or in P2Y(1) receptor-expressing cells cocultured with NTPDaseexpressing cells. Coexpression of NTPDase1 with the P2Y(1) receptor resulted in increases in the EC(50) for 2'-methylthioadenosine 5'-diphosphate (2MeSADP; 12-fold), ADP (50-fold), and ATP (10-fold) for activation of phospholipase C. Similar effects were observed when the P2Y(1) receptor and NTPDase1 were expressed on different cells. These results are explained by the capacity of NTPDase1 to hydrolyze both nucleoside triphosphates and diphosphates. NTPDase2 preferentially hydrolyzes nucleoside triphosphates, and the presence of NTPDase2 under either coexpression or coculture conditions did not change the EC(50) of 2MeSADP, ADP, or adenosine 5'-O-(2-thiodiphosphate) for activation of the P2Y(1) receptor. However, the EC(50) for ATP was 15-fold lower in the presence of NTPDase2 than in cells expressing the P2Y(1) receptor alone. Whereas expression of NTPDase1 decreased basal activity of the P2Y(1) receptor, the presence of the NTPDase2 resulted in P2Y(1) receptor-dependent increases in basal activity. These results suggest that basal activity of the P2Y(1) receptor is maintained by paracrine or autocrine release of receptor agonists and that the biological and/or pharmacological response mediated by P2Y receptors in target tissues is highly dependent on the types of ectonucleotidases expressed in the vicinity of the receptor.

The ratio of ADP- to ATP-ectonucleotidase activity is reduced in patients with coronary artery disease

INTRODUCTION

CD39 (NTPDase1), an endothelial cell membrane glycoprotein, is the predominant ATP diphosphohydrolase (ATPDase) in vascular endothelium. It hydrolyses both triphosphonucleosides and diphosphonucleosides at comparable rates, thus terminating platelet aggregation and recruitment responses to ADP and other platelet agonists. This occurs even when nitric oxide (NO) formation and prostacyclin production are inhibited. Thus, CD39 represents the main control system for platelet reactivity. Reduced or deficient local ecto-nucleotidase activity may predispose to development of vascular disease. Based on data in animal models and in vitro, CD39 constitutes a new therapeutic modality for vascular disease with a novel and unique mode of action.

MATERIALS AND METHODS

Lymphocytes were isolated from 46 patients with angiographically proven coronary artery disease (CAD) as well as from matched healthy control subjects. Ectonucleotidase ADPase and ATPase activities (prototypical for the ATPDase activity of endothelial cells) were measured using established radio-TLC procedures.

RESULTS AND DISCUSSION

In the patients, a decreased ratio of ADPase to ATPase activities (from 1.26 to 1.04) was observed despite increases in both ADPase and ATPase activities. Coronary artery disease was the only independent predictor of a difference in the ADPase/ATPase activity ratio by multivariate linear regression analysis (P=0.0035). This altered ADPase/ATPase activity ratio in patients may represent a reduction in endogenous defense systems against platelet-driven thrombotic events. These data may identify a population of patients with excessive platelet reactivity in their circulation. Increased generation of prothrombotic ADP in these patients implies a potential benefit from therapeutic intervention with soluble forms of CD39.

Chronic treatment with lithium increases the ecto-nucleotidase activities in rat hippocampal synatosomes

Lithium is a mood-stabilizing treatment used in bipolar and other psychiatric disorders. The molecular mechanisms underlying lithium action remain poorly understood. Adenosine is a neuromodulator that possesses anticonvulsant and neuroprotective properties and the ecto-nucleotidase pathway is a metabolic source of the extracellular adenosine. Here we investigated the effect of lithium on the ecto-nucleotidase pathway in synaptosomes from hippocampus and cerebral cortex of adult rats. Male Wistar rats received standard rat chow with lithium chloride (2.5 mg/g of chow) and NaCl (17 mg/g of chow) during 4 weeks. The serum lithium levels were 1.18 +/- 0.05 mEq./L. ATP and AMP hydrolysis was significantly increased (20 and 35%, respectively) in hippocampal synaptosomes of rats chronically treated with lithium chloride. No significant differences were observed in the hydrolysis of the three nucleotides by cortical synaptosomes. In conclusion, the modulation of the ecto-nucleotidase pathway may be a new explanation for the potential neuroprotective lithium action in hippocampal lesions.

Ecto-ATPases of clinical and non-clinical isolates of Acanthamoeba

Acanthamoeba are opportunistic protozoan parasites that can cause fatal granulomatous amoebic encephalitis and eye keratitis, however the pathogenic mechanisms of Acanthamoeba remain unclear. In this study, we described the ability of live Acanthamoeba to hydrolyse extracellular ATP. Both clinical and non-clinical isolates belonging to genotypes, T1, T2, T3, T4 and T7 exhibited ecto-ATPase activities in vitro. Using non-denaturing polyacrylamide gel electrophoresis, ecto-ATPases were further characterized. All Acanthamoeba isolates tested, exhibited a single ecto-ATPase band (approximate molecular weight of 272 kDa). However, clinical isolates exhibited additional bands suggesting that ecto-ATPases may play a role in the pathogenesis of Acanthamoeba. This was supported using suramin (ecto-ATPase inhibitor), which inhibited Acanthamoeba-induced host cell cytotoxicity. Previously, we and others have shown that Acanthamoeba binds to host cells using their mannose-binding protein and binding can be blocked using exogenous alpha-mannose. In this study, we observed that alpha-mannose significantly increased ecto-ATPase activities of pathogenic Acanthamoeba belonging to T1, T2, T3 and T4 genotypes but had no effect on non-pathogenic Acanthamoeba (belonging to T7 genotype). Overall, we have shown, for the first time, that Acanthamoeba exhibit ecto-ATPase activities, which may play a role in the pathogenesis of Acanthamoeba as well as their potential role in the differentiation of pathogenic Acanthamoeba.

Characterization of NTPDase (NTPDase1; ecto-apyrase; ecto-diphosphohydrolase; CD39; EC 3.6.1.5) activity in human lymphocytes

Human lymphocytes contain NTPDase (NTPDase-1; ecto-apyrase; ecto-diphosphohydrolase; CD39; EC 3.6.1.5), a cation-dependent enzyme that hydrolyzes ATP and ADP and also other di- and triphosphate nucleosides, acting at an optimum pH of 8.0. A significant inhibition of ATP and ADP hydrolysis (P<0.05) was observed in the presence of 20 mM sodium azide. NTPDase inhibitors, 20 mM sodium fluoride, 0.2 mM trifluoperazine and 0.3 mM suramin, significantly decreased ATP and ADP hydrolysis (P<0.05) and ADP hydrolysis was only inhibited by 0.5 mM orthovanadate (P<0.05). ATP and ADP hydrolysis was not inhibited in the presence of 0.01 mM Ap5A (P1,P5-di(adenosine-5')pentaphosphate), 0.1 mM ouabain, 1 mM levamisole, 2 microg/mL oligomycin, 0.1 mM N-ethylmaleimide (NEM), or 5 mM sodium azide. With respect to kinetic behavior, apparent K(m) values of 77.6+/-10.2 and 106.8+/-21.0 microM, and V(max) values of 68.9+/-8.1 and 99.4+/-8.5 (mean+/-S.E., n=3) nmol Pi/min/mg protein were obtained for ATP and ADP, respectively. A Chevilard plot demonstrated that only one enzymatic site is responsible for the hydrolysis of ATP and ADP. The presence of CD39 was determined by flow cytometry, showing a low density of 2.72+/-0.24% (mean+/-S.E.; n=30) in human peripheral lymphocytes. The study of NTPDase activity in human lymphocytes may be important to determine the immune response status against infectious agents related to ATP and ADP hydrolysis.

Noise exposure induces up-regulation of ecto-nucleoside triphosphate diphosphohydrolases 1 and 2 in rat cochlea

Extracellular ATP acting via P2 receptors in the inner ear initiates a variety of signaling pathways that may be involved in noise-induced cochlear injury. Nucleoside triphosphate diphosphohydrolase (NTPDase)1/CD39 and NTPDase2/CD39L1 are key elements for regulation of extracellular nucleotide concentrations and P2 receptor signaling in the cochlea. This study characterized the effect of noise exposure on regulation of NTPDase1 and NTPDase2 expression in the cochlea using a combination of real-time RT-PCR, immunohistochemistry and functional studies. Adult Wistar rats were exposed to broad band noise at 90 dB and 110 dB sound pressure level (SPL) for 72 h. Exposure to 90 dB SPL induced a small and temporary change of auditory thresholds (temporary threshold shift), while exposure to 110 dB SPL induced a robust and permanent change of auditory thresholds (permanent threshold shift). NTPDase1 and NTPDase2 mRNA transcripts were upregulated in the cochlea exposed to 110 dB SPL, while mild noise (90 dB SPL) altered only NTPDase1 mRNA expression levels. Changes in NTPDases expression did not correlate with levels of circulating corticosterone, implying that the up-regulation of NTPDases expression was not stress-related. Semi-quantitative immunohistochemistry in the cochlea exposed to 110 dB SPL localized the increased NTPDase1 and NTPDase2 immunostaining in the stria vascularis and up-regulation of NTPDase2 in the intraganglionic spiral bundle. In contrast, NTPDase1 was down-regulated in the cell bodies of the spiral ganglion neurones. Distribution of NTPDases was not altered in the cochlea exposed to 90 dB SPL. Functional studies revealed increased ectonucleotidase activities in the cochlea after exposure to 110 dB SPL, consistent with up-regulation of NTPDases. The changes in NTPDases expression may reflect adaptive response of cochlear tissues to limit ATP signaling during noise exposure.

Cloning and characterization of mouse nucleoside triphosphate diphosphohydrolase-8

A novel mammalian plasma membrane bound nucleoside triphosphate diphosphohydrolase (NTPDase), named NTPDase8, has been cloned and characterized. Analysis of cDNA reveals an open reading frame of 1491 base pairs encoding a protein of 497 amino acid residues with an estimated molecular mass of 54650 Da and a predicted isoelectric point of 5.94. In a mouse, the genomic sequence is located on chromosome 2A3 and is comprised of 10 exons. The deduced amino acid sequence reveals eight putative N-glycosylation sites, two transmembrane domains, five apyrase-conserved regions, and 20-50% amino acid identity with other mammalian NTPDases. mRNA expression was detected in liver, jejunum, and kidney. Both intact cells and crude cell lysates from COS-7 cells expressing NTPDase8 hydrolyzed P2 receptor agonists, namely, ATP, ADP, UTP, and UDP, but did not hydrolyze AMP. There was an absolute requirement for divalent cations for the catalytic activity (Ca(2+) > Mg(2+)) with an optimal pH between 5.5 and 8.0 for ATP and 6.4 for ADP hydrolysis. Kinetic parameters derived from analysis of crude cell lysates showed that the enzyme had lower apparent K(m) values for adenine nucleotides and for triphosphonucleosides (K(m,app) of 13 microM for ATP, 41 microM for ADP, 47 microM for UTP, and 171 microM for UDP). Hydrolysis of triphosphonucleosides resulted in a transient accumulation of the corresponding diphosphonucleoside, as expected from the apparent K(m) values. Enzymatic properties of NTPDase8 differ from those of other NTPDases suggesting an alternative way to modulate nucleotide levels and consequently P2 receptor activation.

Cloning and characterization of mouse nucleoside triphosphate diphosphohydrolase-3

We have cloned and characterized the nucleoside triphosphate diphosphohydrolase-3 (NTPDase3) from mouse spleen. Analysis of cDNA shows an open reading frame of 1587 base pairs encoding a protein of 529 amino acids with a predicted molecular mass of 58953Da and an estimated isoelectric point of 5.78. The translated amino acid sequence shows the presence of two transmembrane domains, eight potential N-glycosylation sites and the five apyrase conserved regions. The genomic sequence is located on chromosome 9F4 and is comprised of 11 exons. Intact COS-7 cells transfected with an expression vector containing the coding sequence for mouse NTPDase3 hydrolyzed P2 receptor agonists (ATP, UTP, ADP and UDP) but not AMP. NTPDase3 required divalent cations (Ca2+ > Mg2+) for enzymatic activity. Interestingly, the enzyme had two optimum pHs for ATPase activity (pH 5.0 and 7.4) and one for ADPase activity (pH 8.0). Consequently, the ATP/ADP and UTP/UDP hydrolysis ratios were two to four folds higher at pH 5.0 than at pH 7.4, for both, intact cells and protein extracts. At pH 7.4 mouse NTPDase3 hydrolyzed ATP, UTP, ADP and UDP according to Michaelis-Menten kinetics with apparent K(m)s of 11, 10, 19 and 27 microM, respectively. In agreement with the K(m) values, the pattern of triphosphonucleoside hydrolysis showed a transient accumulation of the corresponding diphosphonucleoside and similar affinity for uracil and adenine nucleotides. NTPDase3 hydrolyzes nucleotides in a distinct manner than other plasma membrane bound NTPDases that may be relevant for the fine tuning of the concentration of P2 receptor agonists.

ire-1-dependent Transcriptional Up-regulation of a Lumenal Uridine Diphosphatase from Caenorhabditis elegans

Lumenal ecto-nucleoside tri- and di-phosphohydrolases (ENTPDases) of the secretory pathway of eukaryotes hydrolyze nucleoside diphosphates resulting from glycosyltransferase-mediated reactions, yielding nucleoside monophosphates. The latter are weaker inhibitors of glycosyltransferases than the former and are also antiporters for the transport of nucleotide sugars from the cytosol to the endoplasmic reticulum (ER) and Golgi apparatus (GA) lumen. Here we describe the presence of two cation-dependent nucleotide phosphohydrolase activities in membranes of Caenorhabditis elegans: one, UDA-1, is a UDP/GDPase encoded by the gene uda-1, whereas the other is an apyrase encoded by the gene ntp-1. UDA-1 shares significant amino acid sequence similarity to yeast GA Gda1p and mammalian UDP/GDPases and has a lumenal active site in vesicles displaying an intermediate density between those of the ER and GA when expressed in S. cerevisiae. NTP-1 expressed in COS-7 cells appeared to localize to the GA. The transcript of uda-1 but not those of two other C. elegans ENTPDase mRNAs (ntp-1 and mig-23) was induced up to 3.5-fold by high temperature, tunicamycin, and ethanol. The same effectors triggered the unfolded protein response as shown by the induction of expression of green fluorescent protein under the control of the BiP chaperone promoter and the UDP-glucose:glycoprotein glucosyltransferase. Up-regulation of uda-1 did not occur in ire-1-deficient mutants, demonstrating the role of this ER stress sensor in this event. We hypothesize that up-regulation of uda-1 favors hydrolysis of the glucosyltransferase inhibitory product UDP to UMP, and that the latter product then exits the lumen of the ER or pre-GA compartment in a coupled exchange with the entry of UDP-glucose, thereby further relieving ER stress by favoring protein re-glycosylation.

P2X7 receptors stimulate AKT phosphorylation in astrocytes

1. Emerging evidence indicates that nucleotide receptors are widely expressed in the nervous system. Here, we present evidence that P2Y and P2X receptors, particularly the P2X(7) subtype, are coupled to the phosphoinositide 3-kinase (PI3K)/Akt pathway in astrocytes. 2. P2Y and P2X receptor agonists ATP, uridine 5'-triphosphate (UTP) and 2',3'-O-(4-benzoyl)-benzoyl ATP (BzATP) stimulated Akt phosphorylation in primary cultures of rat cortical astrocytes. BzATP induced Akt phosphorylation in a concentration- and time-dependent manner, similar to the effect of BzATP on Akt phosphorylation in 1321N1 astrocytoma cells stably transfected with the rat P2X(7) receptor. Activation was maximal at 5 - 10 min and was sustained for 60 min; the EC(50) for BzATP was approximately 50 microM. In rat cortical astrocytes, the positive effect of BzATP on Akt phosphorylation was independent of glutamate release. 3. The effect of BzATP on Akt phosphorylation in rat cortical astrocytes was significantly reduced by the P2X(7) receptor antagonist Brilliant Blue G and the P2X receptor antagonist iso-pyridoxal-5'-phosphate-6-azophenyl-2',4'-disulfonic acid, but was unaffected by trinitrophenyl-ATP, oxidized ATP, suramin and reactive blue 2. 4. Results with specific inhibitors of signal transduction pathways suggest that extracellular and intracellular calcium, PI3K and a Src family kinase are involved in the BzATP-induced Akt phosphorylation pathway. 5. In conclusion, our data indicate that stimulation of astrocytic P2X(7) receptors, as well as other P2 receptors, leads to Akt activation. Thus, signaling by nucleotide receptors in astrocytes may be important in several cellular downstream effects related to the Akt pathway, such as cell cycle and apoptosis regulation, protein synthesis, differentiation and glucose metabolism.

Characterization of an ecto-5'-nucleotidase (EC 3.1.3.5) activity in intact cells of Trichomonas vaginalis

The enzymatic properties of an ecto-5'-nucleotidase were described in Trichomonas vaginalis. The enzyme hydrolyzes nucleoside monophosphates in optimum pH values of 7.5 and 6.5 for the 30236 strain and for the 30238 strain, respectively. Mg(2+) and Ca(2+) were activators of AMP hydrolysis in both strains. The apparent K(m) (Michaelis constant) values for Mg(2+)-AMP were 111.4+/-28.1 microM (mean+/-SD, n=3) for 30236 strain and 420.2+/-35.7 microM (mean+/-SD, n=3) for 30238 strain. The ecto-5'-nucleotidase activity was insensitive to levamisole and tetramisole, inhibitors of alkaline phosphatases, whereas alpha,beta-methylene-ADP inhibited the enzymatic activity of both strains. Our results showed that the AMP hydrolysis presents differences in some kinetic parameters between the two strains investigated. An analysis of the enzymatic chain involved in the ATP hydrolysis to adenosine will contribute to understanding the biochemical aspects of the parasite and the mechanisms related to host-parasite interactions.

Ecto-nucleotidase activities in spinal cord of rats changes as function of age

An increase in ADP hydrolysis was observed in spinal cord synaptosomal fractions of 2-month-old Wistar male rats, when compared to other ages (1, 4 and 6 months of age), while no change in ATPase activity was observed. Conversely, in female rats, whilst no change in ADPase activity was observed in the spinal cord synaptosomal fraction, ATPase activity diminished with age, in 1-6-month-old animals. 5'-Nucleotidase activity was higher in the 4-month-old male and female rats in relation to 1 and 2-month-old animals. In the female rats, this activity continued to increase at least until 6 months of age. In conclusion, adenine nucleotides hydrolysis in synaptosomes from rat spinal cord is influenced by age and by gender. Since both ATP and adenosine may act as neuromodulators in the spinal cord, influencing several processes such as nociception, the regulation of ATP-metabolizing enzymes in spinal cord is probably important for the normal function of this tissue at different ages.

Enzymatic and transcriptional regulation of human ecto-ATPase/E-NTPDase 2

We have characterized the regulation of expressed human ecto-ATPase (E-NTPDase 2), a cell surface integral membrane glycoprotein. Ecto-ATPase activity is inhibited by parameters that decrease membrane protein interaction, i.e., detergents and high temperatures. These inhibitory effects are overcome when membranes are pretreated with concanavalin A or chemical cross-linking agents that increase the amounts of ecto-ATPase oligomers. Cross-linking agents also abrogate substrate inactivation of the ecto-ATPase, a unique characteristic of the enzyme. These effects indicate that the magnitude of negative substrate regulation is dependent on quaternary structures of the protein, which likely involves interaction of transmembrane domains. The importance of transmembrane domains of ecto-ATPase in activity modulation is demonstrated further by the stimulatory effect of digitonin, a steroid glycoside that preferentially interacts with cholesterol in the membranes but does not promote oligomer formation. These results indicate that ecto-ATPase activity is regulated by a multitude of mechanisms, some of which may have physiological significance. Ecto-ATPase is also susceptible to transcriptional regulation. Ecto-ATPase gene expression is increased in a human hepatoma whereas it is undetectable in the normal liver.

Extracellular adenine nucleotides metabolism in astrocyte cultures from different brain regions

Primary astrocyte cultures from hippocampus, cortex and cerebellum presented different extracellular pattern of adenine nucleotide hydrolysis. The ATP/ADP hydrolysis ratio was 8:1 for hippocampal and cortical astrocytes and 5:1 for cerebellar astrocytes. The AMP hydrolysis in cerebellar astrocytes was seven-fold higher than in cortical or hippocampal cells. No accumulation of extracellular adenosine in all structures studied was observed. Dipyridamol increased significantly inosine levels in the extracellular medium of hippocampal and cortical, but not in cerebellar astrocytes medium. A higher expression of ecto-5'-nucleotidase was identified by RT-PCR in cerebellum. The differences observed may indicate functional heterogeneity of nucleotides in the brain.

Effect of drugs active at adenosine receptors upon chronic stress-induced hyperalgesia in rats

Hyperalgesia and altered activities of enzymes involved in nucleotide hydrolysis are observed after exposure to repeated restraint in rats. Here, we investigated the effect of an adenosine A(1) receptor agonist, N(6)-cyclopentyladenosine (CPA, 3.35 mg/kg, i.p.), adenosine A(1) receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 0.8 mg/kg, i.p.) as well the effect of an adenosine reuptake blocker, dipyridamole (5 mg/kg, i.p.), on nociception in chronically stressed and control rats. We repeatedly submitted rats to restraint for 40 days. Nociception was assessed with a tail-flick apparatus. The control group presented increased tail-flick latencies after administration of CPA and dipyridamole, but this effect was not observed in the stressed group. DPCPX by itself had no effect on nociception. The analgesic effect of CPA and dipyridamole observed in the control group was reverted by DPCPX. These results indicate the involvement of adenosine A(1) receptor in the antinociception observed in control animals and suggest that the pain signaling induced by chronic stress presents a different modulation involving the adenosinergic system.