Action of suramin upon ecto-apyrase activity and synaptic depression of Torpedo electric organ

Plasmodium falciparum GFP-E-NTPDase expression at the intraerythrocytic stages and its inhibition blocks the development of the human malaria parasite

Plasmodium falciparum is the causative agent of the most dangerous form of malaria in humans. It has been reported that the P. falciparum genome encodes for a single ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase), an enzyme that hydrolyzes extracellular tri- and di-phosphate nucleotides. The E-NTPDases are known for participating in invasion and as a virulence factor in many pathogenic protozoa. Despite its presence in the parasite genome, currently, no information exists about the activity of this predicted protein. Here, we show for the first time that P. falciparum E-NTPDase is relevant for parasite lifecycle as inhibition of this enzyme impairs the development of P. falciparum within red blood cells (RBCs). ATPase activity could be detected in rings, trophozoites, and schizonts, as well as qRT-PCR, confirming that E-NTPDase is expressed throughout the intraerythrocytic cycle. In addition, transfection of a construct which expresses approximately the first 500 bp of an E-NTPDase-GFP chimera shows that E-NTPDase co-localizes with the endoplasmic reticulum (ER) in the early stages and with the digestive vacuole (DV) in the late stages of P. falciparum intraerythrocytic cycle.

Effects of ATP and adenosine on contraction amplitude of rat soleus muscle at different temperatures

INTRODUCTION

The aim of this study was to evaluate the effects of adenosine 5'-triphosphate (ATP) and adenosine on the contractility of mammalian skeletal muscle under hypothermic conditions.

METHODS

Contractions of isolated rat soleus muscle were induced by either electrical stimulation (ES) or carbachol at physiological temperatures (37°C) and hypothermic conditions (30-14°C) and recorded in the presence of ATP, adenosine, suramin, and 8-(p-sulfophenyl)-theophylline (8-SPT).

RESULTS

At 37°C, incubation of the muscles with ATP inhibited ES-induced contractions; the inhibitory effect of ATP disappeared at 14°C. Adenosine inhibited ES-induced contractions at all temperature levels; 8-SPT fully prevented the action of adenosine. ATP and adenosine did not significantly affect carbachol-induced contractions at 37°C, while at lower temperatures ATP potentiated them. Suramin fully prevented effects of ATP.

CONCLUSIONS

ATP is involved in both pre- and postsynaptic regulation of rat soleus muscle contractility, and these processes are significantly more pronounced at low temperatures. Muscle Nerve 55: 417-423, 2017.

Evolutionary origins of the purinergic signalling system

Purines appear to be the most primitive and widespread chemical messengers in the animal and plant kingdoms. The evidence for purinergic signalling in plants, invertebrates and lower vertebrates is reviewed. Much is based on pharmacological studies, but important recent studies have utilized the techniques of molecular biology and receptors have been cloned and characterized in primitive invertebrates, including the social amoeba Dictyostelium and the platyhelminth Schistosoma, as well as the green algae Ostreococcus, which resemble P2X receptors identified in mammals. This suggests that contrary to earlier speculations, P2X ion channel receptors appeared early in evolution, while G protein-coupled P1 and P2Y receptors were introduced either at the same time or perhaps even later. The absence of gene coding for P2X receptors in some animal groups [e.g. in some insects, roundworms (Caenorhabditis elegans) and the plant Arabidopsis] in contrast to the potent pharmacological actions of nucleotides in the same species, suggests that novel receptors are still to be discovered.

Removal from the membrane affects the interaction of rat osseous plate ecto-nucleosidetriphosphate diphosphohydrolase-1 with substrates and ions

We have characterized the kinetic properties of ectonucleoside triphosphate diphosphohydrolase 1 (E-NTPDase1) from rat osseous plate membranes. A novel finding of the present study is that the solubilized enzyme shows high- and low-affinity sites for the substrate in contrast with a single substrate site for the membrane-bound enzyme. In addition, contrary to the Michaelian chraracteristics of the membrane-bound enzyme, the site-site interactions after solubilization with 0.5% digitonin plus 0.1% lysolecithin resulted in a less active ectonucleoside triphosphate diphosphohydrolase, showing activity of about 398.3 nmol Pi min(-1) mg(-1). The solubilized enzyme has M (r) of 66-72 kDa, and its catalytic efficiency was significantly increased by magnesium and calcium ions; but the ATP/ADP activity ratio was always <2.0. Partial purification and kinetic characterization of the rat osseous plate E-NTPDase1 in a solubilized form may lead to a better understanding of a possible function of the enzyme as a modulator of nucleotidase activity or purinergic signaling in matrix vesicle membranes. The simple procedure to obtain the enzyme in a solubilized form may also be attractive for comparative studies of particular features of the active sites from this and other ATPases.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Cloning, molecular characterization and expression of ecto-nucleoside triphosphate diphosphohydrolase-1 from Torpedo electric organ

During synaptic transmission large amounts of ATP are released from pre- and post-synaptic sources of Torpedo electric organ. A chain reaction sequentially hydrolyses ATP to adenosine, which inhibits acetylcholine secretion. The first enzyme implicated in this extracellular ATP hydrolysis is an ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase) that dephosphorylates both ATP and ADP to AMP. This enzyme has been biochemically characterized in the synaptosomal fraction of Torpedo electric organ, having almost equal affinity for ATP as for ADP, a fact that pointed to the type-1 NTPDase enzyme. In the present work we describe the cloning and molecular characterization of the cDNA for an NTPDase from Torpedo marmorata electric organ. The clone, obtained using the RACE-PCR technique, contains and open-reading frame of 1506bp and encodes a 502 amino acids protein that exhibits high homology with other NTPDases1 from vertebrates previously identified, including those of zebrafish and Xenopus, as well as human, rat and mouse. Topology analyses revealed the existence of two transmembrane regions, two short cytoplasmic tails and a long extracellular domain containing five apyrase-conserved regions. Gene expression studies revealed that this gene is expressed in all the Torpedo tissues analyzed. Finally, activity and cellular localization of the protein encoded by this newly cloned cDNA was assessed by heterologous expression experiments involving COS-7 and HeLa cells.

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.

Gadolinium inhibition of ecto-nucleoside triphosphate diphosphohydrolase activity in Torpedo electric organ

Ecto-nucleoside triphosphate diphosphohydrolases (E-NTPDases) are widely expressed enzymes implicated in the modulation of nucleotide cell signaling. They dephosphorylate either ATP or ADP in the presence of divalent cations, and efforts have been made to identify efficient inhibitors. E-NTPDase activity has been described in Torpedo electric organ electrocytes. We show here that gadolinium, an established blocker of stretch-activated channels, efficiently inhibits E-NTPDase activity of Torpedo electric organ (Ki = 3 microM for ATPase) as well as apyrase from potato tuber, frequently used in inhibition experiments. To our knowledge, gadolinium is the most potent inhibitor described to date for both membrane-bound and soluble E-NTPDases.

Characterization of an ectonucleoside triphosphate diphosphohydrolase 1 activity in alkaline phosphatase-depleted rat osseous plate membranes: possible functional involvement in the calcification process

An ectonucleoside triphosphate diphosphohydrolase 1 (NTPDase1) activity present in alkaline phosphatase-depleted rat osseous plate membranes, obtained 14 days after implantation of demineralized bone particles in the subcutaneous tissue of Wistar rats, was characterized. At pH 7.5, NTPDase1 hydrolyzed nucleotide triphosphates at rates 2.4-fold higher than those of nucleotide diphosphates, while the hydrolysis of nucleotide monophosphates and non-nucleotide phosphates was negligible. NTPDase 1 hydrolyzed ATP and ADP following Michaelis-Menten kinetics with V=1278.7+/-38.4 nmol Pi/min/mg and K(M)=83.3+/-2.5 microM and V=473.9+/-18.9 nmol Pi/min/mg and K(M)=150.6+/-6.0 microM, respectively, but in the absence of magnesium and calcium ions, ATP or ADP hydrolysis was negligible. The stimulation of the NTPDase1 by calcium (V=1084.7+/-32.5 nmol Pi/min/mg; and K(M)=377.8+/-11.3 microM) and magnesium (V=1367.2+/-41.0 nmol Pi/min/mg and K(M)=595.3+/-17.8 microM) ions suggested that each ion could replace the other during the catalytic cycle of the enzyme. Oligomycin, ouabain, bafilomycin A(1), theophylline, thapsigargin, ethacrynic acid, P(1),P(5)-(adenosine-5')-pentaphosphate and omeprazole had negligible effects on the hydrolysis of ATP and ADP by NTPDase1. However, suramin and sodium azide were effective inhibitors of ATP and ADP hydrolysis. To our knowledge this is the first report suggesting the presence of NTPDase1 in rat osseous plate membranes. Considering that the ectonucleoside triphosphate diphosphohydrolase family of enzymes participates in many regulatory functions, such as response to hormones, growth control, and cell differentiation, the present observations raise interesting questions about the participation of this activity in the calcification process.

ATP as a cotransmitter in sympathetic nerves and its inactivation by releasable enzymes

ATP and norepinephrine (NE) are cotransmitters released from many postganglionic sympathetic nerves. In this article, we review the evidence for ATP and NE cotransmission in the rodent vas deferens with special attention to the mechanisms involved in removing the cotransmitters from the neuroeffector junction. Although the clearance of NE is well understood (e.g., the primary mechanism being reuptake into the nerves), the clearance of ATP is just beginning to be explained. The general belief has been that ATP is metabolized by cell-fixed ecto-nucleotidases. It now seems, however, that when ATP is released from nerves as a transmitter there is a concomitant release of nucleotidases that rapidly degrade ATP sequentially to ADP, AMP, and adenosine, thereby terminating the action of ATP. In the guinea pig vas deferens, there appear to be at least two enzymes, one that converts ATP to ADP and ADP to AMP (an ATPDase) and a second enzyme that converts AMP to adenosine (an AMPase). An important feature of this process is that the transmitter-metabolizing nucleotidases are released into the synaptic space as opposed to being fixed to cell membranes. A preliminary characterization of these enzymes suggests that the releasable ATPDase exhibits some similarities to known ectonucleoside triphosphate/diphosphohydrolases, whereas the releasable AMPase exhibits some similarities to ecto-5'-nucleotidases.

Enzyme kinetics and pharmacological characterization of nucleotidases released from the guinea pig isolated vas deferens during nerve stimulation: evidence for a soluble ecto-nucleoside triphosphate diphosphohydrolase-like ATPase and a soluble ecto-5'-nucleotidase-like AMPase

Previously, we have demonstrated that stimulation of the sympathetic nerves of the guinea pig vas deferens evokes release not only of the cotransmitters ATP and norepinephrine but also of soluble nucleotidases that break down extracellular ATP, ADP, and AMP into adenosine. In this study we show that the apparent K(m) values of the releasable enzyme activity vary depending on which of these adenine nucleotides is used as initial substrate. The K(m) value for ATP was 33.6 +/- 2.3 microM, 21.0 +/- 2.3 microM for ADP, and 10.0 +/- 1.1 microM for AMP. The ratios of the V(max) values for each enzyme reaction were 4:2:3. We have also found a different sensitivity of the metabolism of ATP and AMP by releasable nucleotidases to known nucleotidase inhibitors. Suramin inhibited the breakdown of ATP by releasable nucleotidases in a noncompetitive manner and with a K(i) value of 53 microM, but had no effect on the breakdown of AMP. The 5'-nucleotidase inhibitor alpha,beta-methylene ADP inhibited the breakdown of AMP but not that of ATP. Concanavalin A inhibited the breakdown of AMP but had neither inhibitory nor facilitatory effects on the breakdown of ATP. 6-N,N-Diethyl-beta,gamma-dibromomethylene-D-ATP (ARL67156), an ecto-ATPase inhibitor, suppressed ATPase and AMPase activities, whereas NaN(3) (10 mM) affected neither reaction, but inhibited the ADP metabolism. Phosphatase- and phosphodiesterase inhibitors did not affect the activity of the releasable nucleotidases. This evidence suggests that the soluble nucleotidases released during neurogenic stimulation of the guinea pig vas deferens combine an ecto-5'-nucleotidase-like and an ecto-nucleoside triphosphate diphosphohydrolase-like activity.

ATP crossing the cell plasma membrane generates an ionic current in xenopus oocytes

The presence of ATP within cells is well established. However, ATP also operates as an intercellular signal via specific purinoceptors. Furthermore, nonsecretory cells can release ATP under certain experimental conditions. To measure ATP release and membrane currents from a single cell simultaneously, we used Xenopus oocytes. We simultaneously recorded membrane currents and luminescence. Here, we show that ATP release can be triggered in Xenopus oocytes by hyperpolarizing pulses. ATP release (3.2 +/- 0.3 pmol/oocyte) generated a slow inward current (2.3 +/- 0.1 microA). During hyperpolarizing pulses, the permeability for ATP(4-) was more than 4000 times higher than that for Cl(-). The sensitivity to GdCl(3) (0. 2 mm) of hyperpolarization-induced ionic current, ATP release and E-ATPase activity suggests their dependence on stretch-activated ion channels. The pharmacological profile of the current inhibition coincides with the inhibition of ecto-ATPase activity. This enzyme is highly conserved among species, and in humans, it has been cloned and characterized as CD39. The translation, in Xenopus oocytes, of human CD39 mRNA encoding enhances the ATP-supported current, indicating that CD39 is directly or indirectly responsible for the electrodiffusion of ATP.

Stimulation-dependent release, breakdown, and action of endogenous ATP in mouse hemidiaphragm preparation: the possible role of ATP in neuromuscular transmission

In this study the in vitro mouse phrenic nerve- hemidiaphragm preparation was utilized to study the release and extracellular catabolism of endogenous ATP and its action on the postsynaptic site, i.e. on the contraction force evoked by nerve stimulation. ATP, measured by the luciferin-luciferase assay, was released stimulation-dependently from the mouse hemidiaphragm in response to electrical field stimulation at 10 Hz. Blockade of the Na(+) channel activity by tetrodotoxin inhibited the majority of the release of ATP in response to stimulation, showing that it is related to neuronal activity. The nicotinic receptor antagonists d-tubocurarine, and alpha-bungarotoxin and cooling the bath temperature to 7 degrees C also reduced stimulation-induced ATP outflow, suggesting that nicotinic receptors are responsible for the part of the release of ATP that is released from postsynaptic sites in a carrier-mediated manner. Exogenous ATP (20-500 microM) added to the bath was degraded to ADP and AMP by the action of ectoATPase and ectoATPdiphosphohydrolase; the K(m) and v(max) values of these enzymes were 185.8 microM and 55.16 nmol/min.g respectively. However, the total amount of nucleotides ([ATP+ADP+AMP]) was increased after the addition of ATP, indicating that ATP itself promoted further adenine nucleotide release. Twitch contractions of the rat hemidiaphragm preparation evoked by low frequency electrical stimulation was blocked concentration-dependently by the non-depolarizing muscle relaxants d-tubocurarine and pancuronium. Suramin (100 microM-1 mM) reversed neuromuscular blockade by d-tubocurarine and pancuronium; i.e., it shifted their concentration-response curves to the right Taken together our data, that endogenous ATP is released by stimulation and subsequently catabolized in the hemidiaphragm preparation and that suramin inhibits ecto-ATPase activity could be interpreted as meaning that suramin prolongs the action of endogenous ATP to elicit twitch contraction, which points to a new, undefined role of ATP in neuromuscular transmission. The source of ATP is partly postsynaptic, released from the muscle in response to activation of nicotinic ACh receptors expressed on the muscle.

Effects of suramin on hippocampal apyrase activity and inhibitory avoidance learning of rats

The action of suramin on apyrase activity in hippocampal synaptosomes and its effects on retention of inhibitory avoidance learning were evaluated. Suramin, a P2-purinoceptor antagonist, significantly inhibited in a noncompetitive manner the ATP and ADP hydrolysis promoted by apyrase in hippocampal synaptosomes of adult rats. The Ki values obtained were 72.8 and 109 microM for ATP and ADP hydrolysis, respectively. Intrahippocampal infusion of suramin (0.01, 0.1, 1, and 10 microg) immediately posttraining, in a dose-dependent effect, significantly reduced the response latency during the retention test applied 24 h after the rats received step-down inhibitory avoidance training. The amnesic effects promoted by suramin probably occur by its antagonist action on hippocampal P2-purinoceptors and NMDA receptors. In view of the fact that ATP-metabolizing enzymes and P2-purinoceptors have similar binding domains, these results suggest that suramin can either alter ATP degradation and/or block purinergic neurotransmission.

8-Azido-nucleotides as substrates of Torpedo electric organ apyrase. effect of photoactivation on apyrase activity

Ecto-apyrase is a widespread enzymatic activity that hydrolyses tri- and diphosphonucleotides and consequently controls the amount of available extracellular ATP and ADP. In the nervous system, purines have important neuromodulatory actions, acting at pre- and postsynaptic sites, and consequently, ecto-apyrase may play an indirect role in the modulation of nucleotide- and nucleoside-mediated processes. The azido-nucleotides have been largely employed to characterize the nucleotide binding sites of several proteins. In the present work the azido-nucleotides are described as putative substrates for apyrase activity in a presynaptic plasma membrane preparation (PSPM) from the Torpedo electric organ. Both 8-N3-ATP and 8-N3-ADP were hydrolyzed in a calcium-dependent manner showing Vmax of 23.8 +/- 4.8 and 14.5 +/- 3 U/mg of protein, and Km values (in microM) of 116 +/- 39 and 119 +/- 4, respectively. Vmax for calcium-dependent hydrolysis of ATP and ADP were significantly higher: 59.2 +/- 3.9 and 32.9 +/- 3.5 U/mg of protein respectively, while Km values did not show any significant differences regarding azido-nucleotides: 83.8 +/- 12 microM for Ca2+-ATP and 121 +/- 34 microM for Ca2+-ADP. The photoactivation of the PSPM in the presence of the azido-derivatives results in an irreversible inactivation of apyrase activity, showing an IC50 of 10 microM and a maximal inhibitory effect of 38 and 60% on Ca2+-ATPase and Ca2+-ADPase activities. Apyrase was protected from inactivation by nucleotides that are natural substrates for this enzymatic activity and also by AMP while adenosine did not protect from apyrase inhibition.

Diadenosine polyphosphate hydrolase from presynaptic plasma membranes of Torpedo electric organ

The diadenosine polyphosphate hydrolase present in presynaptic plasma membranes from the Torpedo electric organ has been characterized using fluorogenic substrates of the form di-(1, N6-ethenoadenosine) 5',5'''-P1,Pn-polyphosphate. The enzyme hydrolyses diadenosine polyphosphates (ApnA, where n=3-5), producing AMP and the corresponding adenosine (n-1) 5'-phosphate, Ap(n-1). The Km values of the enzyme were 0.543+/-0.015, 0.478+/-0.043 and 0. 520+/-0.026 microM, and the Vmax values were 633+/-4, 592+/-18 and 576+/-45 pmol/min per mg of protein, for the etheno derivatives of Ap3A (adenosine 5',5'''-P1,P3-triphosphate), Ap4A (adenosine 5',5"'-P1,P4-tetraphosphate) and Ap5A (adenosine 5',5'''-P1,P5-pentaphosphate) respectively. Ca2+, Mg2+ and Mn2+ are enzyme activators, with EC50 values of 0.86+/-0.11, 1.35+/-0.24 and 0.58+/-0.10 mM respectively. The fluoride ion is an inhibitor with an IC50 value of 1.38+/-0.19 mM. The ATP analogues adenosine 5'-tetraphosphate and adenosine 5'-[gamma-thio]triphosphate are potent competitive inhibitors and adenosine 5'-[alpha,beta-methylene]diphosphate is a less potent competitive inhibitor, the Ki values being 0.29+/-0.03, 0.43+/-0.05 and 7.18+/-0.8 microM respectively. The P2-receptor antagonist pyridoxal phosphate 6-azophenyl-2',4'-disulphonic acid behaves as a non-competitive inhibitor with a Ki value of 29.7+/-3.1 microM, and also exhibits a significant inhibitory effect on Torpedo apyrase activity. The effect of pH on the Km and Vmax values, together with inhibition by diethyl pyrocarbonate, strongly suggests the presence of functional histidine residues in Torpedo diadenosine polyphosphate hydrolase. The enzyme from Torpedo shows similarities with that of neural origin from neurochromaffin cells, and significant differences compared with that from endothelial vascular cells.