Extracellular hydrolysis of diadenosine polyphosphates, ApnA, by bovine chromaffin cells in culture

Ectonucleotide pyrophosphatase/phosphodiesterase activity in Neuro-2a neuroblastoma cells: changes in expression associated with neuronal differentiation

Neuro-2a (N2a) neuroblastoma cells display an ectoenzymatic hydrolytic activity capable of degrading diadenosine polyphosphates. The Apn A-cleaving activity has been analysed with the use of the fluorogenic compound BODIPY FL guanosine 5'-O-(3-thiotriphosphate) thioester. Hydrolysis of this dinucleotide analogue showed a hyperbolic kinetic with a Km value of 4.9 ± 1.3 μM. Diadenosine pentaphosphate, diadenosine tetraphosphate, diadenosine triphosphate, and the nucleoside monophosphate AMP behaved as an inhibitor of BODIPY FL guanosine 5'-O-(3-thiotriphosphate) thioester extracellular degradation. Ectoenzymatic activity shared the typical characteristics of the ectonucleotide pyrophosphatase/phosphodiesterase family, as hydrolysis reached maximal activity at alkaline pH and was dependent on the presence of divalent cations, being strongly inhibited by EDTA and activated by Zn(2+) ions. Both NPP1 and NPP3 isozymes are expressed in N2a cells, their expression levels substantially changing when cells differentiate into a neuronal-like phenotype. In this sense, it is relevant to point the expression pattern of the NPP3 protein, whose levels were drastically reduced in the differentiated cells, being almost completely absent after 24 h of differentiation. Enzymatic activity assays carried out with differentiated N2a cells showed that NPP1 is the main isozyme involved in the extracellular degradation of dinucleotides in these cells, this enzyme reducing its activity and changing its subcellular location following neuronal differentiation. We described the presence of an ectoenzymatic activity able to hydrolyse diadenosine polyphosphates (ApnA) in N2a cells. This activity displays biochemical features that are typical of the ectonucleotide pyrophosphatase/phosphodiesterase (E-NPP) family members, as demonstrated by the use of the fluorogenic compound BODIPY-FL-GTPγS. Both NPP1 and NPP3 ectoenzymes are expressed in N2a cells, their levels dramatically changing when cells differentiate into a neuronal-like phenotype. Activity assays in differentiated cells showed that the ApnA-hydrolytic activity largely depends on the NPP1 isozyme.

Biochemical analysis of ecto-nucleotide pyrophosphatase phosphodiesterase activity in brain membranes indicates involvement of NPP1 isoenzyme in extracellular hydrolysis of diadenosine polyphosphates in central nervous system

Synaptosomes and plasma membranes obtained from rat brain display ectoenzymatic hydrolytic activity responsible for hydrolysis of the neurotransmitter/neuroregulatory nucleotides diadenosine polyphosphates. Intact synaptosomes and plasma and synaptic membranes isolated by sucrose-gradient ultracentrifugation from several brain regions (hypothalamus, hippocampus, temporal cortex, frontal cortex striatum and cerebellum) degraded the fluorogenic substrates diethenoadenosine polyphosphates up to ethenoadenosine as by-product. Purified ectoenzyme cleaved substrates always releasing the mononucleotide moieties ethenoadenosine 5'-monophosphate and the corresponding ethenoadenosine (n-1) 5'-phosphate. Ectoenzymatic hydrolysis reached maximal activity at pH 9.0 (pH range 6.5-9.0) and was activated by Ca(2+) and Mg(2+) ions, with maximal effects around 2.0 mM cation. EDTA drastically reduced activity and Zn(2+) was required for enzyme reactivation. Hydrolysis of substrates followed hyperbolic kinetics with K(m) values in the 3-10 microM range. Diadenosine polyphosphates and heparin behaved as competitive inhibitors in the enzymatic hydrolysis of diethenoadenosine polyphosphates and AMP, ATP, alpha,beta-methyleneADP, ADPbetaS ATPgammaS, beta,gamma-methyleneATP, suramin and diethyl pyrocarbonate were also inhibitors. Ectoenzymatic activity shared the typical characteristics of members of the ecto-nucleotide pyrophosphatase/phosphodiesterase (E-NPP) family and inhibition data suggest that NPP1 ectoenzyme is involved in the cleavage of extracellular diadenosine polyphosphates in brain. Synaptic membranes from cerebellum, hypothalamus and hippocampus presented the highest activities and no activity differences were observed between young and aged animals. However, plasma membranes showed a more homogeneous distribution of ectoenzymatic activity but a general increase was detected in aged animals. Enhancement of ectoenzymatic diadenosine polyphosphate cleaving activity found in plasma membranes from old animals could play a deleterious role in aged brain by limiting neuroprotective effects reported for extracellular diadenosine tetraphosphate.

Hydrolysis of diadenosine polyphosphates by nucleotide pyrophosphatases/phosphodiesterases

Diadenosine polyphosphates (ApnAs) act as extracellular signaling molecules in a broad variety of tissues. They were shown to be hydrolyzed by surface-located enzymes in an asymmetric manner, generating AMP and Apn-1 from ApnA. The molecular identity of the enzymes responsible remains unclear. We analyzed the potential of NPP1, NPP2, and NPP3, the three members of the ecto-nucleotide pyrophosphatase/phosphodiesterase family, to hydrolyze the diadenosine polyphosphates diadenosine 5',5"'-P1,P3-triphosphate (Ap3A), diadenosine 5',5"'-P1,P4-tetraphosphate (Ap4A), and diadenosine 5',5"'-P1,P5-pentaphosphate, (Ap5A), and the diguanosine polyphosphate, diguanosine 5',5"'-P1,P4-tetraphosphate (Gp4G). Each of the three enzymes hydrolyzed Ap3A, Ap4A, and Ap5A at comparable rates. Gp4G was hydrolyzed by NPP1 and NPP2 at rates similar to Ap4A, but only at half this rate by NPP3. Hydrolysis was asymmetric, involving the alpha,beta-pyrophosphate bond. ApnA hydrolysis had a very alkaline pH optimum and was inhibited by EDTA. Michaelis constant (Km) values for Ap3A were 5.1 micro m, 8.0 micro m, and 49.5 micro m for NPP1, NPP2, and NPP3, respectively. Our results suggest that NPP1, NPP2, and NPP3 are major enzyme candidates for the hydrolysis of extracellular diadenosine polyphosphates in vertebrate tissues.

Ryanodine receptor modulation by diadenosine polyphosphates in synaptosomal and microsomal preparations of rat brain

Diadenosine polyphosphates (Ap(n)As) are transmitter-like substances that act intracellularly via unclear mechanisms. Here we tested hypotheses that diadenosine tetraphosphate (Ap(4)A) modulates ryanodine binding in microsomal and synaptosomal fractions of rat brain, and that Ap(4)A affects modulation of ryanodine binding by divalent cations and caffeine. Using [3H]ryanodine-binding assays, we showed that Ap(4)A produced significant and concentration-dependent increases in [3H]ryanodine binding in microsomes and these actions were reduced by Mg(2+) and potentiated by caffeine. In synaptosomal subfractions, effects of Ap(4)A on [3H]ryanodine binding were most profound in subfractions enriched in synaptic vesicle-associated protein synaptophysin. These results suggest that Ap(n)As and ryanodine receptors are well placed to modulate Ca(2+)-dependent synaptic processes.

Ophidian envenomation strategies and the role of purines

Snake envenomation employs three well integrated strategies: prey immobilization via hypotension, prey immobilization via paralysis, and prey digestion. Purines (adenosine, guanosine and inosine) evidently play a central role in the envenomation strategies of most advanced snakes. Purines constitute the perfect multifunctional toxins, participating simultaneously in all three envenomation strategies. Because they are endogenous regulatory compounds in all vertebrates, it is impossible for any prey organism to develop resistance to them. Purine generation from endogenous precursors in the prey explains the presence of many hitherto unexplained enzyme activities in snake venoms: 5'-nucleotidase, endonucleases (including ribonuclease), phosphodiesterase, ATPase, ADPase, phosphomonoesterase, and NADase. Phospholipases A(2), cytotoxins, myotoxins, and heparinase also participate in purine liberation, in addition to their better known functions. Adenosine contributes to prey immobilization by activation of neuronal adenosine A(1) receptors, suppressing acetylcholine release from motor neurons and excitatory neurotransmitters from central sites. It also exacerbates venom-induced hypotension by activating A(2) receptors in the vasculature. Adenosine and inosine both activate mast cell A(3) receptors, liberating vasoactive substances and increasing vascular permeability. Guanosine probably contributes to hypotension, by augmenting vascular endothelial cGMP levels via an unknown mechanism. Novel functions are suggested for toxins that act upon blood coagulation factors, including nitric oxide production, using the prey's carboxypeptidases. Leucine aminopeptidase may link venom hemorrhagic metalloproteases and endogenous chymotrypsin-like proteases with venom L-amino acid oxidase (LAO), accelerating the latter. The primary function of LAO is probably to promote prey hypotension by activating soluble guanylate cyclase in the presence of superoxide dismutase. LAO's apoptotic activity, too slow to be relevant to prey capture, is undoubtedly secondary and probably serves principally a digestive function. It is concluded that the principal function of L-type Ca(2+) channel antagonists and muscarinic toxins, in Dendroaspis venoms, and acetylcholinesterase in other elapid venoms, is to promote hypotension. Venom dipeptidyl peptidase IV-like enzymes probably also contribute to hypotension by destroying vasoconstrictive peptides such as Peptide YY, neuropeptide Y and substance P. Purines apparently bind to other toxins which then serve as molecular chaperones to deposit the bound purines at specific subsets of purine receptors. The assignment of pharmacological activities such as transient neurotransmitter suppression, histamine release and antinociception, to a variety of proteinaceous toxins, is probably erroneous. Such effects are probably due instead to purines bound to these toxins, and/or to free venom purines.

Ecto-diadenosine polyphosphates hydrolase activity on human prostasomes

BACKGROUND

Ecto-diadenosine polyphosphates are ubiquitous compounds with several physiological roles. Ecto-diadenosine polyphosphates hydrolase control their actions by degrading and terminating their signaling. The present work deals with the identification and partial characterization of ecto-diadenosine polyphosphates hydrolase on human prostasomes.

METHODS

Reverse-phase and paired-ion HPLC techniques have been used.

RESULTS

Prostasomes have an ecto-diadenosine polyphosphates hydrolase that leads to the degradation of several diadenosine compounds. Kinetic parameters of the enzyme show that diadenosine tetraphosphate is the preferred substrate that is further metabolized by the prostasome-ecto-nucleotidases to adenosine. The ecto-enzyme is bound to the prostasome-membranes through a GPI-anchor and is activated by physiological concentration of Ca+2, Mg+2, and Mn+2. Its optimum pH is also in the slightly alkaline physiological range. Human spermatozoa do not possess this hydrolytic activity, but they can acquire it after fusion with prostasomes.

CONCLUSIONS

The existence of an enzyme capable of degrading diadenosine compounds and can be transferred to human spermatozoa suggests new physiological implications for the role of prostasomes in fertilization.

Comparative study of the contractile activity evoked by ATP and diadenosine tetraphosphate in isolated rat urinary bladder

The present study was designed to investigate the effect and possible mechanism(s) of action of ATP and diadenosine tetraphosphate (AP(4)A) on the isolated rat urinary bladder rings. ATP ( 0.1- 1 x 10(-3)M) or AP(4)A ( 0.01- 0.1 x 10(-3)M) produced contractions of the isolated bladder rings in a concentration-dependent manner. The contraction-induced by AP(4)A in the bladder rings was approximately ten times more potent than that produced by ATP. Addition of ATP prior to addition of AP(4)A or vice versa desensitized bladder tissue to the second agonist with great reduction in the contraction produced. Electrical field stimulation (EFS, 40 V, 0.5 ms, 2 Hz) produced contraction (79.8 +/-7.1 g tension x g(-1)tissue) in the bladder rings that can be greatly reduced by prior addition of ATP or AP(4)A. Theophylline, a P(1)-purinoceptor antagonist, significantly reduced the contraction-induced by AP(4)A and did altered that produced by ATP in bladder rings. Atropine, a non-selective muscarinic receptor antagonist, or indomethacin, a cyclo-oxygenase inhibitor, significantly suppressed the contractions of the bladder rings to ATP or AP(4)A. Similarly, nifedipine, an l -type Ca(2+)channel blocker, significantly attenuate the contractions induced by ATP and AP(4)A in the isolated rat urinary bladder rings. In conclusion, the results of the present study show that ATP, AP(4)A, and EFS evoked contractions in the rat urinary bladder rings and that the contractions induced by AP(4)A was more potent than that produced by ATP. Furthermore, the contractions evoked by ATP or AP(4)A were Ca(2+)-dependent and mediated at least in part through one of the cyclo-oxygenase products. Also, the present results suggested the involvement of the P(1)-purinoceptor in mediating the contractions evoked by AP(4)A but not ATP in the bladder rings.

Adenosine 5'-tetraphosphate (Ap(4)), a new agonist on rat midbrain synaptic terminal P2 receptors

The aim of this study was to see whether the compound adenosine 5'-tetraphosphate (Ap(4)) is active in the central nervous system by examining its effect on isolated rat brain synaptic terminals. Ap(4) proved to be more resistant to ecto-enzymatic hydrolysis than adenosine triphosphate (ATP), showing only 2% hydrolysis after a 2-min incubation, compared to 75% for ATP. In addition, Ap(4) was able to produce concentration-dependent increases in intracellular Ca(2+) when applied extracellularly. This action was dependent upon the presence of extracellular calcium. Ap(4) acts through ionotropic ATP receptors (P2X receptors) and not through diadenosine polyphosphate receptors, since ATP abolished the response elicited by Ap(4) whereas Ap(5)A did not. Ap(4), ATP and ATP-gamma-S were of similar potency (EC(50) approximately 20 microM) while 2MeSATP, alpha,beta-meATP and ADP-beta-S possessed slightly lower potency (EC(50) approximately 50 microM). The P2-purinoceptor antagonists suramin and PPADS blocked the Ap(4) effect. The IC(50) values for these compounds were 35.5 and 7.8 microM respectively. Diinosine polyphosphates and inosine tetraphosphate inhibited the response elicited by Ap(4) with IC(50) values that varied between approximately 40 and 50 microM. These results show that Ap(4) is as good an agonist as ATP on synaptosomal P2X receptors, being more resistant to extracellular hydrolysis by ecto-nucleotidases.

Biochemical evidence for an ecto alkaline phosphodiesterase I in human airways

Because dinucleotides are signaling molecules that can interact with cell surface receptors and regulate the rate of mucociliary clearance in lungs, we studied their metabolism by using human airway epithelial cells. A membrane-bound enzyme was detected on the mucosal surface of polarized epithelia that metabolized dinucleotides with a broad substrate specificity (diadenosine polyphosphates and diuridine polyphosphates [Up(n)U], n = 2 to 6). The enzymatic reaction yielded nucleoside monophosphates (NMP) and Np(n)(-)(1) (N = A or U), and was inhibited by nucleoside 5'-triphosphates (alpha,betamet adenosine triphosphate [ATP] > ATP >/= uridine triphosphate > guanidine triphosphate > cytidine triphosphate). The apparent Michaelis constant (K(m,app)) and apparent maximal velocity (V(max,app)) for [(3)H]Up(4)U were 22 +/- 4 microM and 0.24 +/- 0.05 nmoles. min(-)(1). cm(-)(2), respectively. Thymidine 5'-monophosphate p-nitrophenyl ester and adenosine diphosphate (ADP)- ribose, substrates of ecto alkaline phosphodiesterase I (PDE I) activities, were also hydrolyzed by the apical surface of airway epithelia. ADP-ribose competed with [(3)H]Up(4)U, with a K(i) of 23 +/- 3 microM. The metabolism of ADP-ribose and Ap(4)A was not affected by inhibitors of cyclic nucleotide phosphodiesterases (3-isobutyl-1-methylxanthine, Ro 20-1724, and 1,3-dipropyl-8-p-sulfophenylxanthine), but similarly inhibited by fluoride and N-ethylmaleimide. These results suggest that a PDE I is responsible for the hydrolysis of extracellular dinucleotides in human airways. The wide substrate specificity of PDE I suggests that it may be involved in several signaling events on the luminal surface of airway epithelia, including purinoceptor activation and cell surface protein ribosylation.

Ecto-diadenosine 5',5'''-P1,P4-tetraphosphate (Ap4A)-hydrolase is expressed as an ectoenzyme in a variety of mammalian and human cells and adds new aspects to the turnover of Ap4A

Ap4A and other dinucleotides participate in the regulation of hemostasis and blood pressure control. With the exception of two previously reported surface anchored ectoAp4A-hydrolases on bovine aortic endothelial and chromaffine cells, all Ap4A-hydrolases reported are intracellular or freely soluble. We demonstrated that ectoAp4A-hydrolases are present on a broad variety of cell types of different species: rat mesangial, bovine corneal epithelial, human Hep-G2 and peridontal cells. Ectoenzyme properties were evaluated on rat mesangium cells. Chromatography of purified plasma membranes on Sephacel 300 resulted in enrichment of ectoAp4A-hydrolase and in separation from ectoATPase. In contrast to ATPase, Ap4A-hydrolase was stable at room temperature. EctoAp4A-hydrolase also recognized ATP as substrate, and therefore is not highly specific. The molecular weight was 180 kD. Unlike ectoAMPase ectoAp4A-hydrolase was not attached via a glycosyl-phosphatidylinositol (GPI)-moiety. Concentrations of PI-PLC 10-100-fold higher than effective for ectoAMPase cleavage (10-100 mU/ml) plus extensively extended incubation times up to eight hours did not result in cleavage of ectoAp4A-hydrolase. The enzyme ectoAp4A-hydrolase might presage a direction for pharmaceutical manipulation in the control of blood pressure and hemostasis.

Catabolism of Ap4A and Ap5A by rat brain synaptosomes

Adenosine 5',5"'-P1,P4-tetraphosphate (Ap4A) and adenosine 5',5"'-P1,P5-pentaphosphate (Ap5A) are stored in and released from rat brain synaptic terminals. In the present study we investigated the hydrolysis of dinucleotides (Ap4A and Ap5A) in synaptosomes from the cerebral cortex of adult rats. Ap4A and Ap5A, but not Ap3A, were hydrolyzed at pH 7.5 in the presence of 20 mM Tris/HCl, 2.0 mM MgCl2, 10 mM glucose and 225 mM sucrose at 37 degrees C. The disappearance of the substrates measured by FPLC on a mono-Q HR column was both time and protein dependent. Since synaptosome integrity was at least 90% at the end of the assay, hydrolysis probably occurred by the action of an ecto-enzyme. Extracellular actions of adenine dinucleotides at central nervous system terminate due to the existence of ecto-nucleotidases which specifically cleave these dinucleotides. These enzymes in association with an ATP diphosphohydrolase and a 5'-nucleotidase are able to promote the complete hydrolysis of dinucleotides to adenosine in the synaptic cleft.

The hydrolytic activity of bovine adrenal medullary plasma membranes towards diadenosine polyphosphates is due to alkaline phosphodiesterase-I

A hydrolase activity directed against diadenosine 5',5"'-P1, P4-tetraphosphate (Ap4A) has been solubilised and partially purified from the plasma membrane fraction of bovine adrenal medullary chromaffin tissue in order to determine its relationship to alkaline phosphodiesterase-I/nucleotide pyrophosphatase (PDase-I, EC 3.1.4.1). Activity with the specific dinucleoside tetraphosphatase (EC 3.6.1. 17) substrate Ap4A and with the non-specific PDase-I substrate thymidine 5'-monophosphate p-nitrophenyl ester had Km and Vmax values of 2.0 microM and 600 pmol/min/mg protein and 0.2 mM and 26 nmol/min/mg protein respectively and co-chromatographed upon gel filtration and ion-exchange chromatography. Activity with the fluorescent substrates etheno-Ap4A and 4-methylumbelliferyl phenylphosphonate co-electrophoresed on native polyacrylamide gels. No activity was detected which exclusively hydrolysed Ap4A. Immunoblotting of the most purified fraction with an antibody against mouse PC-1, one of the major PDase-I family members, detected bands of 240, 120 and 62 kDa corresponding to PC-1 dimer, monomer and proteolytic fragment. Therefore, the activity previously described as bovine adrenal chromaffin cell ecto(diadenosine polyphosphate hydrolase) (ecto-ApnAase) is a PDase-I, probably bovine PC-1.

Ecto-enzymatic hydrolysis of diadenosine polyphosphates by cultured adrenomedullary vascular endothelial cells

We investigated the extracellular degradation of diadenosine polyphosphates (ApnA) by cultured adrenomedullary endothelial cells using fluorogenic analogs of ApnA, the di(1,N6-ethenoadenosine) 5',5"'-P1,Pn-polyphosphates [epsilon-(ApnA)]. Kinetic parameters of epsilon-(ApnA) cleavage and effects of pH, ions, and inhibitors were determined by continuous fluorometric assays, using suspensions of endothelial cells grown on Cytodex-1 microspheres. Ecto-enzyme kinetic parameters for epsilon-(Ap3A), epsilon-(Ap4A), and epsilon-(Ap5A) hydrolysis are as follows: Michaelis-Menten constants of 0.39 +/- 0.07, 0.42 +/- 0.09, and 0.37 +/- 0.05 microM respectively, and maximal velocities of 26.1 +/- 6.8, 74.2 +/- 16.4, and 24.4 +/- 3.4 pmol.min-1.10(6) cells-1, respectively. ApnA and guanosine 5',5"'-P1,P4-tetraphosphate behave as competitor substrates of epsilon-(Ap4A) hydrolysis. The ectoenzyme is activated by Mg2+ and Mn2+ and inhibited by Ca2+, F-, adenosine 5'-tetraphosphate, adenosine 5'-O-(3-thiotriphosphate), and suramin. Optimum pH is around 9.0. High-performance liquid chromatography analysis reveals that the ecto-enzyme hydrolyzes epsilon-(ApnA) to give epsilon-adenosine-5'(n-1)-phosphate and epsilon-AMP, which are then further catabolized up to epsilon-adenosine via the membrane-bound nucleotidase system ecto-ATPase, ecto-ADPase (or apyrase), and ecto-5'-nucleotidase. The endothelial ecto-diadenosine polyphosphate hydrolase studied here exhibits different kinetic parameters and sensitivity to ions with respect to the enzyme from the tissue-related neurochromaffin cells. These different properties may be important in the extracellular signaling by ApnA.

Diadenosine polyphosphates inhibit adenosine kinase activity but decrease levels of endogenous adenosine in rat brain

Findings in peripheral tissues that diadenosine polyphosphates (Ap(n)As) activate 5'-nucleotidase activity and inhibit adenosine kinase activity in vitro led us to test the hypothesis that Ap(n)As and analogues thereof, through such actions on purine enzymes, increase brain levels of endogenous adenosine in vivo. Accordingly, we tested Ap(n)As for their effects on the in vitro activities of adenosine kinase, adenosine deaminase, AMP deaminase and 5'-nucleotidase and, following unilateral microinjections in rat striatum, on in vivo levels of endogenous adenosine. Adenosine kinase activity was not affected significantly by 5',5'''-P1,P2-diadenosine pyrophosphate (Ap2A) or by 5',5'''-P1,P3-diadenosine triphosphate (Ap3A), but was inhibited by 5',5'''-P1,P4-diadenosine tetraphosphate (Ap4A), 5',5'''-P1,P5-diadenosine pentaphosphate (Ap5A) and 5',5'''-P1,P6-diadenosine hexaphosphate (Ap6A); apparent IC50 values were 5.0, 3.3 and 500 microM, respectively. Inhibition of adenosine kinase activity by Ap4A and the four metabolically stable analogues of Ap4A tested was uncompetitive. Following unilateral intrastriatal injections, adenosine levels, relative to uninjected contralateral striatum, were decreased significantly (P < 0.05) by 48% with Ap4A and by 37% with AppCH2ppA, a metabolically stable analogue of Ap4A. Striatal levels of adenosine were not affected significantly by Ap5A or Ap6A. Cytosolic, but not particulate 5'-nucleotidase activity was inhibited and AMP deaminase activity was increased by some Ap(n)As. Although adenosine kinase inhibitors increase levels of endogenous adenosine and we showed here that Ap(n)As were potent inhibitors of this enzyme, these particular actions of Ap(n)As were not consistent with their effects on levels of endogenous adenosine.

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.

Distribution of [3H]diadenosine tetraphosphate binding sites in rat brain

The distribution of the diadenosine tetraphosphate high-affinity binding sites has been studied in rat brain by an autoradiographic method using [3H]diadenosine tetraphosphate as the ligand. The binding characteristics are comparable to those described in studies performed on rat brain synaptosomes. White matter is devoid of specific binding. The range of binding site densities in gray matter varies from 3 to 15 fmol/mg of tissue, exhibiting a widespread but heterogeneous distribution. The highest densities correspond to the seventh cranial nerve, medial superior olive, pontine nuclei, glomerular and external plexiform layers of the olfactory bulb, and the granule cell layer of the cerebellar cortex. Intermediate density levels of binding correspond to different cortical areas, several nuclei of the amygdala, and the oriens and pyramidal layers of the hippocampal formation. The localization of diadenosine tetraphosphate binding sites in the brain may provide information on the places where diadenosine polyphosphate compounds can be expected to function in the central nervous system.

The effects of P2 purinoceptor agonists on the isolated portal vein of the guinea pig

UTP, ATP and several of its analogues enhanced contractions of the longitudinal smooth muscle layer of the guinea-pig portal vein. The rank order of potency was 2-methylthioATP > alpha, beta-methyleneATP > adenosine tetraphosphate > or = beta, gamma-methyleneATP > or = ATP = UTP > > adenosine. Suramin (100 microM) blocked the contractile effects of 2-methylthioATP and alpha,beta-methyleneATP, but not those of ATP and adenosine tetraphosphate. The P1 purinoceptor antagonist, 8-phenyltheophylline (10 microM), was without effect on the response to ATP. Field stimulation (5 s trains every 100 s, 1 ms, 55 V) caused frequency-dependent contractions that were partially reduced by the noradrenergic neurone blocking drug; BW 172C58 (4-benzoyl-xylocholine, 10 microM), but not by suramin. alpha,beta-MethyleneATP was more potent than beta,gamma-methyleneATP, UTP and adenosine tetraphosphate in partially inhibiting field stimulation-induced contractions of the portal vein; its effects, but not those of adenosine tetraphosphate, were reduced by suramin. These results indicate that the guinea-pig portal vein contains P2 purinoceptors; these include a P2x subtype, mediating contraction.

Extracellular metabolism of nucleotides in the nervous system

1. A variety of surface-located enzymes are involved in the metabolism of extracellular nucleotides. The biochemical properties of some of these are briefly discussed. 2. The molecular identity of ecto-diadenosine polyphosphate hydrolase has not yet been revealed. On neural cells the enzyme catalyses the hydrolysis of ApnA to Apn-1 and AMP. 3. The molecular structure of ATP-diphosphohydrolase has recently been identified. The enzyme occurs in essentially all tissues where it catalyses the extracellular hydrolysis of ATP and ADP with the formation of AMP. 4. Ecto-5'-nucleotidase is a GPI-anchored glycoprotein and catalyses the formation of AMP to adenosine. In the adult brain, and as revealed by immunocytochemistry, the enzyme is mainly associated with astrocytes. It is associated with developing nerve cells and cultured neural cells. In vitro its inhibition or suppression of its synthesis result in the inhibition of neurite formation and long-time survival of neural cells. Continued extracellular hydrolysis of AMP and formation of adenosine thus appear to be essential for neural differentiation and survival.

Vascular actions of diadenosine phosphates

1. Diadenosine phosphates were isolated from platelets, adrenal gland and autonomic nerves. The presence of diadenosine phosphates in storage pools releasable into the circulation suggests an important role in the control of blood pressure, and potentially to a modulation of the actions of catecholamines. 2. Besides a role of the diadenosine phosphates in platelet aggregation, these agents have potent vasoactive properties. Vasoactive actions of the diadenosine phosphates were demonstrated in numerous vascular models including most of the physiologically important elements of blood pressure regulation. Mostly, the vasoactive action depends on the number of phosphates in the diadenosine phosphates. Vasodilation can be observed in intact vessels after administration of Ap2A, Ap3A and Ap4A whereas contraction is affected by Ap5A and Ap5A and Ap6A. Vasocontraction induced by the diadenosine phosphates in vascular smooth muscle cells is mediated by an increase in intracellular free Ca2+. 3. In vivo, intravenous injection of Ap4A lowers blood pressure whereas injections of Ap5A and Ap6A caused a prolonged increase in blood pressure. In blood, in contrast to ATP, diadenosine phosphates are relatively long-lived molecules, suggesting that the action of the latter is of intermediate time span. In a similar manner to the vasoconstrictor angiotensin II, diadenosine phosphates also act as mitogens. It can be assumed that diadenosine phosphates may be involved in pathophysiological events of circulation including hypertension and atherosclerosis.

Negative chronotropic and inotropic effects exerted by diadenosine hexaphosphate (AP6A) via A1-adenosine receptors

1. Diadenosine hexaphosphate (AP6A) exerts vasoconstrictive effects. The purpose of this study was to investigate whether AP6A has any effect on cardiac function. 2. The effects of AP6A (0.1-100 microM) on cardiac contractility and frequency were studied in guinea-pig and human isolated cardiac preparations. Furthermore, the effects of AP6A on the amplitude of the L-type calcium current, on the adenosine 3':5'-cyclic monophosphate (cyclic AMP) content and on the phosphorylation of regulatory phosphoproteins, i.e. phospholamban and troponin inhibitor, were investigated in guinea-pig isolated ventricular myocytes. 3. In isolated spontaneously beating right atria of the guinea-pig AP6A exerted a negative chronotropic effect and reduced the rate of contraction maximally by 35% (IC20 = 35 microM). 4. In isolated electrically driven left atria of the guinea-pig AP6A exerted a negative inotropic effect and reduced force of contraction maximally by 23% (IC20 = 70 microM). 5. In isolated electrically driven papillary muscles of the guinea-pig AP6A alone was ineffective, but attenuated isoprenaline-stimulated force of contraction maximally by 23% (IC20 = 60 microM). Furthermore, AP6A attenuated the relaxant effect of isoprenaline. 6. In human isolated electrically driven ventricular preparations AP6A alone was ineffective, but attenuated isoprenaline-stimulated force of contraction by maximally 42% (IC20 = 18 microM). Moreover, AP6A attenuated the relaxant effect of isoprenaline. 7. All these effects of AP6A were abolished by the selective A1-adenosine receptor antagonist 1,3-dipropyl-cyclopentyl-xanthine (DPCPX, 0.3 microM), whereas the M-cholinoceptor antagonist atropine (10 microM) and the P2-purinoceptor antagonist suramin (300 microM) failed to abolish the effects of AP6A. 8. AP6A 100 microM had no effect on the amplitude of the L-type calcium current, but attenuated isoprenaline-stimulated L-type calcium current. The maximum of the current-voltage relationship (I-V curve) was shifted to the left by isoprenaline and additional application of AP6A shifted the I-V curve back to the right to the control value. The phosphorylation state of phospholamban and the troponin inhibitor was unchanged by AP6A alone, but was markedly attenuated by AP6A in the presence of isoprenaline. Cyclic AMP levels remained unchanged by AP6A, even after stimulation with isoprenaline. 9. In summary, AP6A exerts negative chronotropic and inotropic effects in guinea-pig and human cardiac preparations. These effects are mediated via A1-adenosine receptors as all effects were sensitive to the selective A1-adenosine receptor antagonist DPCPX. Furthermore, the effects of AP6A on cyclic AMP levels, protein phosphorylation and the L-type calcium current are in accordance with stimulation of A1-adenosine receptors.

Possible role of diadenosine polyphosphates as modulators of cardiac sensory-motor neurotransmission in guinea-pigs

1. Isolated guinea-pig atria were used to study the neuromodulatory effect of diadenosine polyphosphates (APnA) on cardiac capsaicin-sensitive sensory-motor neurotransmission. 2. In the presence of atropine, guanethidine and propranolol, electrical field stimulation (EFS) of the atrial preparations evoked a positive inotropic response which is known to be mediated by release of calcitonin gene-related peptide (CGRP) from sensory-motor nerves. P1,P2-diadenosine pyrophosphate (AP2A), P1,P3-diadenosine triphosphate (AP3A), P1,P4-diadenosine tetraphosphate (AP4A), P1,P5-diadenosine pentaphosphate (AP5A) and P1,P6-diadenosine hexaphosphate (AP6A) inhibited in a concentration-dependent way (0.1-30 microM) cardiac responses to EFS. The inhibitory effect of APnA was mimicked by adenosine. 3. All the APnA tested had a direct negative inotropic effect, by reducing in a concentration-dependent manner the basal contractile tension. The inotropism of APnA was comparable to that of adenosine. 4. Both inhibition of cardiac responses to EFS and negative inotropism of AP2A, AP3A and AP4A were sensitive to the antagonism by the A1 adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 0.1-1 nM). The extent of antagonism of DPCPX for the APnA tested was comparable to that for adenosine. 5. Despite the direct negative inotropism, AP4A tested at the highest concentration used did not affect the cardiac responses to the neurotransmitter CGRP, applied exogenously. 6. These results have demonstrated that in isolated guinea-pig atria APnA inhibited sensory-motor neurotransmission, without affecting cardiac responses to exogenous CGRP. The effect of APnA was sensitive to antagonism by DPCPX, which suggests it operates via the activation of prejunctional A1 adenosine receptors. A postjunctional negative inotropism was also shown, mediated by myocardial A1 adenosine receptors.

Biochemistry, localization and functional roles of ecto-nucleotidases in the nervous system

Nucleotides such as ATP, ADP, UTP or the diadenosine polyphosphates and possibly even NAD+ are extracellular signaling substances in the brain and in other tissues. Enzymes located on the cell surface catalyze the hydrolysis of these compounds and thus limit their spatio-temporal activity. As a final hydrolysis product they generate the nucleoside and phosphate. The paper discusses the biochemical properties, cellular localization and functional properties of surface-located enzymes that hydrolyse nucleotides released from nervous tissue. This is preceded by a brief discussion of nucleotide receptors, cellular storage and mechanisms of nucleotide release. In nervous tissue nucleoside 5'-triphosphates are hydrolysed by ecto-ATP-diphosphohydrolase and possibly in addition also by ecto-nucleoside triphosphatase and ecto-nucleoside diphosphatase. The molecular identity of the ATP-diphosphohydrolase has now been revealed. The hydrolysis of nucleoside 5'-monophosphates is catalysed by 5'-nucleotidase whose biochemical properties and molecular structure have been studied in detail. Little is known about the molecular properties of the diadenosine polyphosphatases. Surface located enzymes for the extracellular hydrolysis of NAD+ and also ecto-protein kinases are discussed briefly. The cellular localization of the ecto-nucleotidases is only partly defined. Whereas in adult mammalian brain activity for hydrolysis of ATP and ADP may be associated with nerve cells or glial cells 5'-nucleotidase appears to have a preferential glial allocation in the adult mammal. The extracellular hydrolysis of the nucleotides is of functional importance not only during synaptic transmission where it functions in signal elimination. It plays a crucial role also for the survival and differentiation of neural cells in vitro and presumably during neuronal development in vivo.

Specific dinucleoside polyphosphate cleaving enzymes from chromaffin cells: a fluorimetric study

This article presents a fluorimetric study of the main properties of the enzymes dinucleoside tetraphosphate (asymmetrical) hydrolase or dinucleoside tetraphosphatase (Ap4Aase, EC 3.6.1.17) and dinucleoside triphosphate hydrolase or dinucleoside triphosphatase (Ap3Aase, EC 3.6.1.29), both present in adrenal medulla cytosolic extracts. Diethenoadenosine polyphosphates, epsilon-(ApnA), are used as artificial fluorogenic substrates. Ap4Aase exhibits a molecular mass around 20 kDa and neutral optimum pH (7.0-7.5). It requires Mg2+ and preferentially hydrolyzes substrates with four phosphate groups. Km for epsilon-(Ap4A) is 1.3 microM and Ki for Ap4A and Gp4G are 1 and 0.2 microM respectively. Km for Ap4A determined by HPLC is 1.6 microM. epsilon-(Ap5A) and epsilon-(Ap6A) are hydrolyzed at reduced rates. This enzyme is inhibited by Zn2+, F- and very strongly by Ap4 and epsilon-Ap4. Ca2+ cannot replace Mg2+, but behaves as inhibitor in its presence. The substrate analogs dinucleoside triphosphates Ap3A, G;3G, m7Gp3G and m7Gp3A and the periodate-oxidized nucleotides o-(Ap4A), o epsilon-(Ap4A), o-Ap4 and o epsilon-Ap4 behave as inhibitors. Ap3Aase exhibits a molecular mass around 30 kDa and neutral optimum pH (7.0-7.5). It requires Mg2+ or Ca2+, but retains a low measurable activity around 10% in the absence of these divalent cations. It only hydrolyzes substrates with three phosphate groups. Km for epsilon-(Ap3A) is 11 microM and Ki for Ap3A and Gp3G are 20 and 22 microM, respectively. Km for Ap3A determined by HPLC is 16 microM. m7Gp3G and m7Gp3A are also good substrates for triphosphatase.

P2 purinergic receptors for diadenosine polyphosphates in the nervous system

1. The actions of diadenosine polyphosphates, diadenosine tetraphosphate (Ap4A), diadenosine pentaphosphate (Ap5A) and diadenosine hexaphosphate (Ap6A) in the nervous system have been reviewed. 2. In the peripheral nervous system, diadenosine polyphosphates bind to P2-purinergic receptors such as the P2Y in chromaffin cells and Torpedo synaptosomes, P2X in vas deferens and urinary bladder and also Torpedo synaptosomes and P2U in endothelial chromaffin cells. 3. In the central nervous system ApnA compounds can act through P2X-purinoceptors opening cation channels in nodose ganglion neurones. Diadenosine polyphosphates bind to a P2d-purinergic receptor in rat brain synaptic terminals and hippocampus, linked to protein kinase C (PKC) activation. 4. P4-purinoceptors are specific receptors for diadenosine polyphosphates, coupled to the Ca2+ influx, in the central synapses. This purinoceptor is not activated by ATP and synthetic analogs. The P4-purinoceptor could act as a positive modulator of the synaptic transmission, giving even more importance to diadenosine polyphosphates as neurotransmitters.

Regulation of purified hepatic PC-1 (phosphodiesterase-I/nucleotide pyrophosphatase) by threonine auto(de)phosphorylation and by binding of acidic fibroblast growth factor

The plasma cell differentiation antigen PC-1 was purified to homogeneity from rat liver membranes. Denaturing electrophoresis revealed polypeptides of 118 and 128 kDa, which were both recognized by antibodies against recombinant murine PC-1. During gel filtration PC-1 migrated as a protein of about 500 kDa, suggesting a tetrameric structure. Purified PC-1 displayed a phosphodiesterase-I/nucleotide pyrophosphatase activity that could be completely blocked by EDTA, dithiothreitol and acidic fibroblast growth factor (extrapolated Ki = 1.3 nM). Purified PC-1 was also capable of threonine autophosphorylation and of phosphorylation of histone IIa. The autophosphorylation of PC-1 was inhibited by addition of histone IIa, and it was blocked by phosphodiesterase-I inhibitors (acidic fibroblast growth factor, dithiothreitol), by nucleotides (ATP, ADP, AMP), and by vanadate. When added to autophosphorylated PC-1, these compounds caused a prompt dephosphorylation. However, the same agents did not affect the (de)phosphorylation of histone IIa, which is not a substrate for the PC-1 phosphatase. These data indicate that phosphodiesterase-I inhibitors, nucleotides and vanadate affect the (de)phosphorylation of PC-1 by stimulating the PC-1 phosphatase and/or by shielding the autophosphorylation site from the PC-1 kinase. The rate of dephosphorylation of PC-1 was independent of the dilution, suggesting an autocatalytic intramolecular process. We propose that the autophosphorylation of PC-1 serves to block its nucleotide pyrophosphatase activity when extracellular ATP becomes scarce.

Evidence that adenine nucleotides modulate nucleoside-transporter function. Characterization of uridine transport in chromaffin cells and plasma membrane vesicles

Uridine transport was investigated in cultured chromaffin cells and plasma membrane vesicles from chromaffin tissue. In intact cells, the kinetic parameters for uridine uptake were Km 150 +/- 45 microM, and Vmax 414 +/- 17 pmol . 10(6) cells-1 . min-1. This low affinity for uridine and its inhibition by low concentrations of nitrobenzylthioinosine (Ki 3 nM) and dipyridamole (Ki 54 nM) are consistent with a facilitated diffusion nucleoside transport system. The IC50 value for the adenosine transport inhibition by uridine was very high (240 microM), agreeing with the relative affinities of these nucleosides in the chromaffin cell nucleoside transport system, which was 150-fold higher for adenosine than for uridine. Uridine was significantly metabolized in chromaffin cells but not in plasma membrane vesicles. The affinity of uridine transport measured in these membrane vesicles was reproducible and similar to the affinity found for intact cells with a Km value of 185 +/- 11 microM and a Vmax value of 4.24 +/- 0.10 pmol . mg protein-1 . s-1. These membrane preparations were employed to investigate the regulatory action of ATP and other nucleotide analogues on nucleoside transport. ATP increased the Vmax value but the Km value was not significantly modified. Adenosine 5'-[beta,gamma-imino]triphosphate, 1,N6-ethenoadenosine 5'-triphosphate, and adenosine(5')-tetraphospho(5')adenosine(Ap4A) at 100 microM were able to mimic the ATP effect. These results agree with a regulatory role of ATP, and the uridine transport on chromaffin plasma membrane vesicles is a good model for analyzing the nucleoside-transporter function and regulation.

Single-cell fura-2 microfluorometry reveals different purinoceptor subtypes coupled to Ca2+ influx and intracellular Ca2+ release in bovine adrenal chromaffin and endothelial cells

ATP and adenosine(5')tetraphospho(5')adenosine (Ap4A), released from adrenal chromaffin cells, are potent stimulators of endothelial cell function. Using single-cell fura-2 fluorescence recording techniques to measure free cytosolic Ca2+ concentration ([Ca2+]i), we have investigated the role of purinoceptor subtypes in the activation of cocultured chromaffin and endothelial cells. ATP evoked concentration-dependent [Ca2+]i rises (EC50 = 3.8 microM) in a subpopulation of chromaffin cells. Both ATP-sensitive and -insensitive cells were potently activated by nicotine, bradykinin and muscarine. Reducing extracellular free Ca2+ concentration to around 100 nM suppressed the [Ca2+]i transient evoked by ATP but not the [Ca2+]i response to bradykinin. ATP-sensitive chromaffin cells were also potently stimulated by 2-methylthioadenosine triphosphate (2MeSATP; EC50 = 12.5 microM) and UTP, but did not respond to either adenosine 5'-[beta-thio]diphosphate (ADP[beta S]), a P2Y receptor agonist, adenosine 5'-[alpha,beta-methylene]triphosphate (pp-[CH2]pA), a P2X agonist or AMP. Adrenal endothelial cells displayed concentration-dependent [Ca2+]i responses when stimulated with ATP (EC50 = 0.86 microM), UTP (EC50 = 1.6 microM) and 2MeSATP (EC50 = 0.38 microM). 2MeSATP behaved as a partial agonist. Ap4A and ADP[beta S] also raised the [Ca2+]i in endothelial cells, whereas AMP and pp[CH2]pA were ineffective. Lowering extracellular free Ca2+ to around 100 nM did not affect the peak ATP-evoked [Ca2+]i rise in these cells. It is concluded that different purinoceptor subtypes are heterogeneously distributed among the major cell types of the adrenal medulla. An intracellular Ca(2+)-releasing P2U-type purinoceptor is specifically localized to adrenal endothelial cells, while a subpopulation of chromaffin cells expresses a non-P2X, non-P2Y subtype exclusively coupled to Ca2+ influx.

Amphetamine-induced release of diadenosine polyphosphates--Ap4A and Ap5A--from caudate putamen of conscious rat

The release of diadenosine polyphosphates--diadenosine tetraphosphate (Ap4A) and diadenosine pentaphosphate (Ap5A)--was measured by intracerebral push-pull perfusion in conscious rats after systemic amphetamine injection. Samples were collected from the caudate putamen, and nucleotide compounds were analyzed by HPLC. The presence of Ap4A and Ap5A was demonstrated by their retention times and phosphodiesterase digestion. Dinucleotides were not detectable before amphetamine injection (5 mg/kg). The maximal levels were reached 20 min after the injection with values of 12.9 +/- 0.9 and 11.5 +/- 0.9 pmol/fraction for Ap4A and Ap5A, respectively. A slow and progressive decrease in their concentration followed. This study shows for the first time the amphetamine-induced release of diadenosine polyphosphates in conscious rats, and a role for Ap4A and Ap5A in the central nervous system is therefore suggested.