Ecto-Enzymes of the Guinea Pig Polymorphonuclear Leukocyte

Ectonucleoside triphosphate diphosphohydrolases and ecto-5'-nucleotidase in purinergic signaling: how the field developed and where we are now

Geoffrey Burnstock will be remembered as the scientist who set up an entirely new field of intercellular communication, signaling via nucleotides. The signaling cascades involved in purinergic signaling include intracellular storage of nucleotides, nucleotide release, extracellular hydrolysis, and the effect of the released compounds or their hydrolysis products on target tissues via specific receptor systems. In this context ectonucleotidases play several roles. They inactivate released and physiologically active nucleotides, produce physiologically active hydrolysis products, and facilitate nucleoside recycling. This review briefly highlights the development of our knowledge of two types of enzymes involved in extracellular nucleotide hydrolysis and thus purinergic signaling, the ectonucleoside triphosphate diphosphohydrolases, and ecto-5'-nucleotidase.

History of ectonucleotidases and their role in purinergic signaling

Ectonucleotidases are key for purinergic signaling. They control the duration of activity of purinergic receptor agonists. At the same time, they produce hydrolysis products as additional ligands of purinergic receptors. Due to the considerable diversity of enzymes, purinergic receptor ligands and purinergic receptors, deciphering the impact of extracellular purinergic receptor control has become a challenge. The first group of enzymes described were the alkaline phosphatases - at the time not as nucleotide-metabolizing but as nonspecific phosphatases. Enzymes now referred to as nucleoside triphosphate diphosphohydrolases and ecto-5'-nucleotidase were the first and only nucleotide-specific ectonucleotidases identified. And they were the first group of enzymes related to purinergic signaling. Additional research brought to light a surprising number of ectoenzymes with broad substrate specificity, which can also hydrolyze nucleotides. This short overview traces the development of the field and briefly highlights important results and benefits for therapies of human diseases achieved within nearly a century of investigations.

Tissue Non-specific Alkaline Phosphatase (TNAP) in Vessels of the Brain

The microvessels of the brain represent around 3-4 % of the brain compartment but constitute the most important length (400 miles) and surface of exchange (20 m(2)) between the blood and the parenchyma of brain. Under influence of surrounding tissues, the brain microvessel endothelium expresses a specific phenotype that regulates and restricts the entry of compounds and cells from blood to brain, and defined the so-called blood-brain barrier (BBB). Evidences that alkaline phosphatase (AP) is a characteristic feature of the BBB phenotype that allows differentiating capillary endothelial cells from brain to those of the periphery have rapidly emerge. Thenceforth, AP has been rapidly used as a biomarker of the blood-brain barrier phenotype. In fact, brain capillary endothelial cells (BCECs) express exclusively tissue non-specific alkaline phosphatase (TNAP). There are several lines of evidence in favour of an important role for TNAP in brain function. TNAP is thought to be responsible for the control of transport of some compounds across the plasma membrane of the BCECs. Here, we report that levamisole-mediated inhibition of TNAP provokes an increase of the permeability to Lucifer Yellow of the endothelial monolayer. Moreover, we illustrate the disruption of the cytoskeleton organization. Interestingly, all observed effects were reversible 24 h after levamisole removal and correlated with the return of a full activity of the TNAP. This reversible effect remains to be studied in details to evaluate the potentiality of a levamisole treatment to enhance the entry of drugs in the brain parenchyma.

Enzymes involved in metabolism of extracellular nucleotides and nucleosides: functional implications and measurement of activities

Extracellular nucleotides and nucleosides mediate diverse signaling effects in virtually all organs and tissues. Most models of purinergic signaling depend on functional interactions between distinct processes, including (i) the release of endogenous ATP and other nucleotides, (ii) triggering of signaling events via a series of nucleotide-selective ligand-gated P2X and metabotropic P2Y receptors as well as adenosine receptors and (iii) ectoenzymatic interconversion of purinergic agonists. The duration and magnitude of purinergic signaling is governed by a network of ectoenzymes, including the enzymes of the nucleoside triphosphate diphosphohydrolase (NTPDase) family, the nucleotide pyrophosphatase/phosphodiesterase (NPP) family, ecto-5'-nucleotidase/CD73, tissue-nonspecific alkaline phosphatase (TNAP), prostatic acid phosphatase (PAP) and other alkaline and acid phosphatases, adenosine deaminase (ADA) and purine nucleoside phosphorylase (PNP). Along with "classical" inactivating ectoenzymes, recent data provide evidence for the co-existence of a counteracting ATP-regenerating pathway comprising the enzymes of the adenylate kinase (AK) and nucleoside diphosphate kinase (NDPK/NME/NM23) families and ATP synthase. This review describes recent advances in this field, with special emphasis on purine-converting ectoenzymes as a complex and integrated network regulating purinergic signaling in such (patho)physiological states as immunomodulation, inflammation, tumorigenesis, arterial calcification and other diseases. The second part of this review provides a comprehensive overview and basic principles of major approaches employed for studying purinergic activities, including spectrophotometric Pi-liberating assays, high-performance liquid chromatographic (HPLC) and thin-layer chromatographic (TLC) analyses of purine substrates and metabolites, capillary electrophoresis, bioluminescent, fluorometric and electrochemical enzyme-coupled assays, histochemical staining, and further emphasizes their advantages, drawbacks and suitability for assaying a particular catalytic reaction.

Purinergic signalling and immune cells

This review article provides a historical perspective on the role of purinergic signalling in the regulation of various subsets of immune cells from early discoveries to current understanding. It is now recognised that adenosine 5'-triphosphate (ATP) and other nucleotides are released from cells following stress or injury. They can act on virtually all subsets of immune cells through a spectrum of P2X ligand-gated ion channels and G protein-coupled P2Y receptors. Furthermore, ATP is rapidly degraded into adenosine by ectonucleotidases such as CD39 and CD73, and adenosine exerts additional regulatory effects through its own receptors. The resulting effect ranges from stimulation to tolerance depending on the amount and time courses of nucleotides released, and the balance between ATP and adenosine. This review identifies the various receptors involved in the different subsets of immune cells and their effects on the function of these cells.

CrATP interferes in the promastigote-macrophage interaction in Leishmania amazonensis infection

Recent have shown the relationship between Ecto-Nucleoside-Triphosphate-Diphosphohydrolases (Ecto-NTPDases or ecto-nucleotidases) and virulence and infectivity in trypanosomatids. In this work, the inhibition of the ecto-ATPase activities and promastigote growth of Leishmania amazonensis by CrATP was characterized. Furthermore, this compound was used to investigate the role of ecto-nucleotidase in the interaction of L. amazonensis with resident peritoneal macrophages obtained from BALB/c mice. CrATP partially inhibits the ecto-ATPase activity, presenting Ki values of 575·7±199·1 and 383·5±79·0 μm, in the presence or absence of 5 mm MgCl2, respectively. The apparent Kms for ATP (2·9±0·5 mm to Mg2+-dependent ecto-ATPase and 0·4±0·2 mm to Mg2+-independent ecto-ATPase activities) are not significantly altered by CrATP, suggesting a reversible non-competitive inhibition of both enzymes. When CrATP was added to the cultivation medium at 500 μm, it drastically inhibited the cellular growth. The interaction of promastigote forms of L. amazonensis with BALB/c peritoneal macrophages is strongly affected by CrATP. When the parasites were treated with 500 μm CrATP before interacting with macrophages, the adhesion and endocytic indices were strongly reduced to 53·0±14·8% and 39·8±1·1%, respectively. These results indicate that ecto-nucleotidase plays an important role in the infection process caused by Leishmania amazonensis.

Hydrolysis of dinitrobenzamide phosphate prodrugs: the role of alkaline phosphatase

Phosphate prodrugs which undergo hydrolysis in vivo have been used to improve the solubility and pharmacokinetic properties of a number of drugs. Dinitrobenzamide mustards (DNBM) are examples of such drugs. We investigated the ability of purified alkaline phosphatase isoforms to dephosphorylate three DNBM phosphate prodrugs. In addition, the relative rate of dephosphorylation of these phosphate prodrugs in a number of tissues was determined. These phosphate prodrugs are indeed substrates for alkaline phosphatase, with time dependent formation of the hydrolysis product. Intestinal alkaline phosphatase (IAP) and placental alkaline phosphatase (PLAP) had the highest activity for these substrates and compound P2 was the most rapidly metabolised. Similarly, compound P2 had the shortest half life in mouse serum (t1/2 = 1.15 h) compared with P1 (t1/2 = 13.34 h) and P3 (t1/2 = 4.4 h). However, serum has very low dephosphorylase activity for these substrates compared with intestine and liver homogenates. In addition, there is little or no difference in the relative rate of dephosphorylation of each of the three compounds in mouse tissues in contrast to the pattern observed with purified alkaline phosphatase and mouse serum. Hence additional phosphatase enzymes may be involved in the metabolism of phosphate prodrugs in vivo.

Acute effect of tea, wine, beer, and polyphenols on ecto-alkaline phosphatase activity in human vascular smooth muscle cells

Alkaline phosphatase (ALP) is an ecto-enzyme widely distributed across species. It modulates a series of transmembranar transport systems, has an important role in bone mineralization, and can also be involved in vascular calcification. Polyphenol-rich diets seem to have protective effects on human health, namely, in the prevention of cardiovascular diseases. We aimed to investigate the effects of polyphenols and polyphenol-rich beverages upon membranar alkaline phosphatase (ecto-ALP) activity in intact human vascular smooth muscle cells (AALTR). The ecto-ALP activity was determined at pH 7.8, with p-nitrophenyl phosphate as the substrate, by absorbance spectrophotometry at 410 nm. Cell viability was assessed by the lactate dehydrogenase (LDH) method, and the polyphenol content of beverages was assessed using the Folin-Ciocalteu reagent. All polyphenols tested inhibited ecto-ALP activity, in a concentration-dependent way. Teas, wines, and beers also inhibited ecto-ALP activity, largely according to their polyphenol content. All tested compounds and beverages improved or did not change AALTR cell viability. Stout beer was an exception to the described behavior. Although more studies must be done, the inhibition of AALTR ecto-ALP activity by polyphenolic compounds and polyphenol-containing beverages may contribute to their cardiovascular protective effects.

Sesquiterpene lactone from Wunderlichia crulsiana inhibits the respiratory burst of leukocytes triggered by distinct biochemical pathways

The sesquiterpene lactone tubiferin was chemically purified from the brazilian native plant Wunderlichia crulsiana and identified by NMR and GC/MS data. Its ability to inhibit the respiratory burst of peritoneal inflammatory polymorphonuclear leukocytes (PMN) stimulated upon addition of phorbol miristate acetate (PMA), opsonized zymosan (OZ), and N-formyl-methionyl-leucyl-phenylalanine (fMLP) was evaluated. The tubiferin inhibition was more pronounced when PMN were stimulated through the protein kinase C pathway (PMA) compared to the alternative complement pathway (OZ). The inhibition when PMN were triggered by a chemoattractant stimulus (fMLP) was similar to that achieved with OZ-stimulated phagocytes. Tubiferin showed dose-dependent effects on the PMN respiratory burst triggered by the three different substances, and also decreased substantially the carrageenan-induced mice paw edema.

Modulation of uptake of organic cationic drugs in cultured human colon adenocarcinoma Caco-2 cells by an ecto-alkaline phosphatase activity

Alkaline phosphatase (ALP) refers to a group of nonspecific phosphomonoesterases located primarily in cell plasma membrane. It has been described in different cell lines that ecto-ALP is directly or indirectly involved in the modulation of organic cation transport. We aimed to investigate, in Caco-2 cells, a putative modulation of 1-methyl-4-phenylpyridinium (MPP(+)) apical uptake by an ecto-ALP activity. Ecto-ALP activity and (3)H-MPP(+) uptake were evaluated in intact Caco-2 cells (human colon adenocarcinoma cell line), in the absence and presence of a series of drugs. The activity of membrane-bound ecto-ALP expressed on the apical surface of Caco-2 cells was studied at physiological pH using p-nitrophenylphosphate as substrate. The results showed that Caco-2 cells express ALP activity, characterized by an ecto-oriented active site functional at physiological pH. Genistein (250 micro M), 3-isobutyl-1-methylxanthine (1 mM), verapamil (100 micro M), and ascorbic acid (1 mM) significantly increased ecto-ALP activity and decreased (3)H-MPP(+) apical transport in this cell line. Orthovanadate (100 micro M) showed no effect on (3)H-MPP(+) transport and on ecto-ALP activity. On the other hand, okadaic acid (310 nM) and all trans-retinoic acid (1 micro M) significantly increased (3)H-MPP(+) uptake and inhibited ecto-ALP activity. There is a negative correlation between the effect of drugs upon ecto-ALP activity and (3)H-MPP(+) apical transport (r = -0.9; P = 0.0014). We suggest that apical uptake of organic cations in Caco-2 cells is affected by phosphorylation/dephosphorylation mechanisms, and that ecto-ALP activity may be involved in this process.

Modulation of insulin transport in rat brain microvessel endothelial cells by an ecto-phosphatase activity

The physiological function of alkaline phosphatase (ALP) remains controversial. It was recently suggested that this membrane-bound enzyme has a role in the modulation of transmembranar transport systems into hepatocytes and Caco-2 cells. ALP activity expressed on the apical surface of blood-brain barrier cells, and its relationship with (125)I-insulin internalization were investigated under physiological conditions using p-nitrophenylphosphate (p-NPP) as substrate. For this, an immortalized cell line of rat capillary cerebral endothelial cells (RBE4 cells) was used. ALP activity and (125)I-insulin internalization were evaluated in these cells. The results showed that RBE4 cells expressed ALP, characterized by an ecto-oriented active site which was functional at physiological pH. Orthovanadate (100 microM), an inhibitor of phosphatase activities, decreased both RBE4-ALP activity and (125)I-insulin internalization. In the presence of L-arginine (1 mM) or adenosine (100 microM) RBE4-ALP activity and (125)I-insulin, internalization were significantly reduced. However, D-arginine (1 mM) had no significant effect. Additionally, RBE4-ALP activity and (125)I-insulin internalization significantly increased in the presence of the bioflavonoid kaempferol (100 microM), of the phorbol ester PMA (80 nM), IBMX (1 mM), progesterone (200 microM and 100 microM), beta-estradiol (100 microM), iron (100 microM) or in the presence of all-trans retinoic acid (RA) (10 microM). The ALP inhibitor levamisole (500 microM) was able to reduce (125)I-insulin internalization to 69.1 +/- 7.1% of control. Our data showed a positive correlation between ecto-ALP activity and (125)I-insulin incorporation (r = 0.82; P < 0.0001) in cultured rat brain endothelial cells, suggesting that insulin entry into the blood-brain barrier may be modulated through ALP.

A Mg-dependent ecto-ATPase in Leishmania amazonensis and its possible role in adenosine acquisition and virulence

The plasma membrane of cells contains enzymes whose active sites face the external medium rather than the cytoplasm. The activities of these enzymes, referred to as ectoenzymes, can be measured using living cells. In this work we describe the ability of living promastigotes of Leishmania amazonensis to hydrolyze extracellular ATP. In these intact parasites whose viability was assessed before and after the reactions by motility and by trypan blue dye exclusion, there was a low level of ATP hydrolysis in the absence of any divalent metal (5.39 +/- 0.71 nmol P(i)/h x 10(7) cells). The ATP hydrolysis was stimulated by MgCl(2) and the Mg-dependent ecto-ATPase activity was 30.75 +/- 2.64 nmol P(i)/h x 10(7) cells. The Mg-dependent ecto-ATPase activity was linear with cell density and with time for at least 60 min. The addition of MgCl(2) to extracellular medium increased the ecto-ATPase activity in a dose-dependent manner. At 5 mM ATP, half-maximal stimulation of ATP hydrolysis was obtained with 1.21 mM MgCl(2). This stimulatory activity was also observed when MgCl(2) was replaced by MnCl(2), but not by CaCl(2) or SrCl(2). The apparent K(m) for Mg-ATP(2-) was 0.98 mM and free Mg(2+) did not increase the ecto-ATPase activity. In the pH range from 6.8 to 8.4, in which the cells were viable, the acid phosphatase activity decreased, while the Mg(2+)-dependent ATPase activity increased. This ecto-ATPase activity was insensitive to inhibitors of other ATPase and phosphatase activities, such as oligomycin, sodium azide, bafilomycin A(1), ouabain, furosemide, vanadate, molybdate, sodium fluoride, tartrate, and levamizole. To confirm that this Mg-dependent ATPase was an ecto-ATPase, we used an impermeant inhibitor, 4,4'-diisothiocyanostylbene 2',2'-disulfonic acid as well as suramin, an antagonist of P(2) purinoreceptors and inhibitor of some ecto-ATPases. These two reagents inhibited the Mg(2+)-dependent ATPase activity in a dose-dependent manner. A comparison between the Mg(2+)-dependent ATPase activity of virulent and avirulent promastigotes showed that avirulent promastigotes were less efficient than the virulent promastigotes in hydrolyzing ATP.

Ectonucleotide diphosphohydrolase activities in hemocytes of larval Manduca sexta

In this work, we describe the ability of living hemocytes from an insect (Manduca sexta, Lepidoptera) to hydrolyze extracellular ATP. In these intact cells, there was a low level of ATP hydrolysis in the absence of any divalent metal (8.24 +/- 0.94 nmol of Pi/h x 10(6) cells). The ATP hydrolysis was stimulated by MgCl2 and the Mg2+-dependent ecto-ATPase activity was 15.93 +/- 1.74 nmol of Pi/h x 10(6) cells. Both activities were linear with cell density and with time for at least 90 min. The addition of MgCl2 to extracellular medium increased the ecto-ATPase activity in a dose-dependent manner. At 5 mM ATP, half-maximal stimulation of ATP hydrolysis was obtained with 0.33 mM MgCl2. This stimulatory activity was not observed when Ca2+ replaced Mg2+. The apparent Km values for ATP-4 and Mg-ATP2- were 0.059 and 0.097 mM, respectively. The Mg2+-independent ATPase activity was unaffected by pH in the range between 6.6 and 7.4, in which the cells were viable. However, the Mg2+-dependent ATPase activity was enhanced by an increase of pH. These ecto-ATPase activities were insensitive to inhibitors of other ATPase and phosphatase activities, such as oligomycin, sodium azide, bafilomycin A1, ouabain, furosemide, vanadate, sodium fluoride, tartrate, and levamizole. To confirm the observed hydrolytic activities as those of an ecto-ATPase, we used an impermeant inhibitor, DIDS (4,4'-diisothiocyanostilbene-2,2'-disulfonic acid), as well as suramin, an antagonist of P2-purinoreceptors and inhibitor of some ecto-ATPases. These two reagents inhibited the Mg2+-independent and the Mg2+-dependent ATPase activities to different extents. Interestingly, lipopolysaccharide, a component of cell walls of gram-negative bacteria that increase hemocyte aggregation and phagocytosis, increased the Mg2+-dependent ecto-ATPase activity in a dose-dependent manner but did not modify the Mg2+-independent ecto-ATPase activity.

Ascorbate-dependent protection of human erythrocytes against oxidant stress generated by extracellular diazobenzene sulfonate

Diazobenzene sulfonic acid (DABS) has been used to label thiols and amino groups on cell-surface proteins. However, we found that in addition to inhibiting an ascorbate-dependent trans-plasma membrane oxidoreductase in human erythrocytes, it also depleted alpha-tocopherol severely in the cell membrane. When erythrocytes were loaded with ascorbate, DABS-dependent loss of alpha-tocopherol was decreased, despite little change in intracellular ascorbate content. Sparing of alpha-tocopherol also was seen in erythrocyte ghosts resealed to contain ascorbate, although this was accompanied by loss of intravesicular ascorbate, probably due to the inability of ghosts to recycle ascorbate. A transmembrane transfer of electrons from ascorbate was confirmed by electron paramagnetic resonance spectroscopy, in which extracellular DABS was found to generate the ascorbate free radical within cells. When the membrane content of alpha-tocopherol was decreased to 20% of the initial value by DABS treatment, lipid peroxidation ensued, manifest by generation of F(2)-isoprostanes in the cell membranes. Intracellular ascorbate also strongly protected against F(2)-isoprostane formation. These results show that DABS causes an oxidant stress at the membrane surface that is transmitted within the cell, in part by an alpha-tocopherol-dependent mechanism, and that ascorbate recycling of alpha-tocopherol can protect against loss of alpha-tocopherol and the ensuing lipid peroxidation.

Effect of polydeoxyribonucleotides on human fibroblasts in primary culture

The effects of a mixture of oligo- and polydeoxyribonucleotides (PDRN) on the growth and protein secretion of cultured human skin fibroblasts were investigated. Both intact and DNAase-digested PDRN stimulated cell proliferation to a similar extent. When cultured fibroblasts were incubated with radioactive amino acids in the presence of intact or digested PDRN the incorporation of the tracer into secreted proteins increased significantly. This stimulation appears to be specific for certain protein components, including fibronectin. These results are interpreted assuming that PDRN and the nucleotides and nucleosides resulting from its degradation, can act as signal transducers or, alternatively, can be internalized and utilized to provide purine and pyrimidine rings for the salvage pathways.

Phosphorylation of farnesol in rat liver microsomes: properties of farnesol kinase and farnesyl phosphate kinase

As farnesol may serve as a nonsterol endogenous regulator of the mevalonate pathway, the possibility that a kinase specific for its phosphorylation is present in the rat liver was investigated. In the 10,000 g supernatant of rat liver, farnesyl monophosphate was synthesized in the presence of ATP. The Km value for farnesol was 2.3 microM. Various detergents inhibited the activity of the enzyme. The farnesol kinase was present in rough and in smooth I microsomes, but not in smooth II microsomes, mitochondria, peroxisomes, Golgi, or plasma membranes. The enzyme was associated with the inner, luminal surface of the vesicles. Further analyses have demonstrated that an enzymatic mechanism exists which catalyzes the phosphorylation of farnesyl-P to farnesyl-PP. Activity of the farnesyl phosphate kinase resulted in the phosphorylation of the monophosphate by CTP but not by ATP, GTP, or UTP. This enzyme is activated by low concentrations of detergents. Treatment with proteases and chemical probes indicate that this second phosphorylation reaction probably takes place on the outer, cytoplasmic surface of microsomal vesicles. These results demonstrate that rat liver microsomes contain two enzymes for the consecutive phosphorylation of farnesol to farnesyl-PP.

Ecto-enzyme and signaling functions of lymphocyte CD73

CD73 or ecto-5'-nucleotidase (5'-NT) is a widely expressed ecto-enzyme which catalyzes the dephosphorylation of AMP and other nucleoside monophosphates. CD73 participates in purine salvage through this enzymatic activity, supplying cells with precursors for energy metabolism and nucleic acid biosynthesis. As an enzyme that produces adenosine, CD73 can also regulate adenosine receptor engagement in many tissues. However, CD73 also has functions independent of its enzyme activity. Like many glycosyl phosphatidylinositol (GPI)-anchored molecules, it transmits potent activation signals in T cells when ligated by antibodies. Less compelling evidence suggests that CD73 may function as a cell adhesion molecule. In the human immune system, CD73 is expressed on subsets of T and B cells, on germinal center follicular dendritic cells, and on thymic medullary reticular fibroblasts and epithelial cells. Many challenging areas remain to be explored before the role of CD73 in the immune system will be fully understood. These include an evaluation of the role of adenosine receptors in lymphoid development, the identification of physiological CD73 ligands, a functional assessment of the GPI anchor, and an analysis of the intricate cell-type-specific and developmental regulation of CD73 expression.

Mg-dependent ecto-ATPase activity in Leishmania tropica

ATPase activity has been located on the external surface of Leishmania tropica. Since Leishmania is known to have an ecto-acid phosphatase, in order to discard the possibility that the ATP hydrolysis observed was due to the acid phosphatase activity, the effect of pH in both activities was examined. In the pH range from 6.8 to 8.4, in which the cells were viable, the phosphatase activity decreased, while the ecto-ATPase activity increased. To confirm that the observed ATP hydrolysis was promoted by neither phosphatase nor 5'-nucleotidase activities, a few inhibitors for these enzymes were tested. Vanadate and NaF strongly inhibited the phosphatase activity; however, no effect was observed on ATPase activity. Neither levamizole nor tetramizole, two specific inhibitors of alkaline phosphatases, inhibited this activity. The lack of response to ammonium molybdate indicated that 5'-nucleotidase did not contribute to the ATP hydrolysis. Also, the lack of inhibition of the ATP hydrolysis by high concentrations of ADP at nonsaturating concentrations of ATP discarded the possibility of any ATP diphosphohydrolase activity. The ATPase here described was stimulated by MgCl2 but not by CaCl2. In the absence of divalent metal, a low level of ATP hydrolysis was observed, and CaCl2 varying from 0.1 to 10 mM did not increase the ATPase activity. At 5 mM ATP, half-maximal stimulation of ATP hydrolysis was obtained with 0.29 +/- 0.02 mM MgCl2. The apparent K(m) for Mg-ATP2- was 0.13 +/- 0.01 mM and free Mg2+ did not increase the ATPase activity. ATP was the best substrate for this enzyme. Other nucleotides such as ITP, CTP, GTP, UTP, and ADP produced lower reaction rates. To confirm that this Mg-dependent ATPase was an ecto-ATPase, an impermeant inhibitor, 4,4'-diisothiocyanostylbene-2,2'-disulfonic acid was used. This amino/sulfhydryl-reactive reagent did inhibit the Mg-ecto-ATPase activity in a dose-dependent manner (I0.5 = 27.5 +/- 1.8 microM).

Identification and partial characterization of ectoATPase expressed by immortalized B lymphocytes

EctoATPases are extracellular membrane-bound enzymes that catalyze the hydrolysis of the gamma phosphate from ATP. EctoATPase is expressed by activated and immortalized Epstein-Barr virus-transformed human peripheral blood B lymphocytes and murine B cell hybridomas. By contrast, ectoATPase activity is not expressed on nontransformed human peripheral blood B lymphocytes, murine spleen cells, or murine myeloma cells. The K(m) for ATP for the B cell ectoATPases ranged from 5 to 77 microM; the Vmax ranged from 48 to 129 pmol/ min/10(4) cells. The enzyme required Mg2+ for maximal activity with little dependence on Ca2+. ADP and purine and pyrimidine nucleoside triphosphates were competitive inhibitors of the catalytic reaction. A putative ectoATPase protein has been identified by Western blot analysis of membrane proteins from the immortalized B cells. Under reducing conditions, antiectoATPase antibodies cross-reacted with a 66-kDa protein from murine B cell hybridoma membranes. By contrast a 200-kDa protein from the B cell hybridoma membranes cross-reacted with the antibodies under nonreducing conditions, suggesting a disulfide-linked trimer. The antibodies also cross-reacted with a 66-kDa protein from human B cell membranes under reducing conditions, but did not cross-react with membrane proteins under nonreducing conditions. This suggests that the antibody epitope(s) recognized on the reduced human protein is masked under nonreducing conditions. Thus, this work demonstrates: (1) that ectoATPase may serve as a marker for B cell activation; and (2) mammalian and avian ectoATPases have conserved interspecies immunological epitopes and kinetic properties.

Enzymatic activities affecting exogenous nicotinamide adenine dinucleotide in human skin fibroblasts

The fate of nicotinamide adenine dinucleotide (NAD), AMP, and ADP-ribose supplied to intact human skin fibroblasts was monitored, and the concentrations of intra- and extracellular pyridine and purine compounds were determined by HPLC analysis. Two enzymatic activities affecting extracellular NAD were detected on the plasma membrane, one hydrolyzing the pyrophosphoric bond and yielding nicotinamide mononucleotide (nucleotide pyrophosphatase) and the other cleaving the glycoside link and releasing nicotinamide (NAD-glycohydrolase). No AMP or ADP-ribose was found in the extracellular medium of cells incubated with NAD, the former being completely catabolized to hypoxanthine and the latter degraded to adenine and hypoxanthine.

A possible mechanism of increase in serum alkaline phosphatase activity in rats given granulocyte colony-stimulating factor

Recombinant human granulocyte colony-stimulating factor (G-CSF) at a dose of 1 to 300 micrograms/kg/day was administered intravenously to rats daily for 13 weeks. Serum alkaline phosphatase (ALP) activity increased dose-dependently with leukocytosis. Most of the increased leukocytes were segmented neutrophils, and neutrophil alkaline phosphatase (NAP) scores were elevated markedly. Serum ALP activity correlated very well with the segmented neutrophil counts, and the coefficient of correlation was more than 0.97 in both sexes. Pathological examinations revealed splenomegaly and a marked increase in neutrophils in the red pulp of the spleen. In the spleen, phagocytosis of neutrophils by macrophages was observed. These data indicate that the increased ALP was of neutrophil origin. Serum ALP activity may be increased by the direct release of ALP from the high number of neutrophils into the blood, or by the leakage of ALP into the blood mainly from the spleen where many neutrophils are pooled and destroyed by the macrophage system.

Divalent cation-activated ecto-ATPase activity of rat glomerular mesangial cells

Evidence is presented of a Mg2+ and Ca(2+)-activated ATPase at the surface of cultured rat glomerular mesangial cells. Substrate specificity was very low when different nucleoside-5'-triphosphates were examined. The apparent Km values were 0.46 and 0.32 mM for Mg(2+)-ATPase and Ca(2+)-ATPase, respectively. Inhibition studies have revealed that this enzyme is different from the (Na+ + K+)-ATPase and from the divalent cation activated mitochondrial ATPases. Culture in the serum-free medium markedly decreased ecto-ATPase activity. Con A treatment reduced at 50 micrograms/ml by 15% the Ca(2+)-ATPase activity. Endocytosis of serum-treated zymosan (STZ) did not significantly affect ecto-ATPase activity. In contrast, endocytosis of STZ by macrophages was accompanied by a biphasic response, an increase in Mg(2+)-ATPase and Ca(2+)-ATPase activities after ingestion of smaller amounts of STZ and a marked decrease after loading doses of STZ.

Inhibition of redox cycling of methoxatin (PQQ), and of superoxide release by phagocytic white cells

The iodonium compounds diphenyleneiodonium and diphenyliodonium, and the amine compounds, 4,5-dimethyl phenylene diamine, N,N-dimethyl 1,4-phenylene diamine, 1,2-diamino-4,5-methyleneoxybenzene, and aminomalononitrile inhibit methoxatin's (PQQ's) redox activity in vitro, that is, the methoxatin-coupled oxidation of glycine and reduction of nitroblue tetrazolium to formazan. The compounds mentioned above also inhibit phorbol myristate acetate (PMA) stimulated superoxide release by phagocytic white cells--determined mainly as the superoxide dismutase sensitive reduction of ferricytochrome C. Related compounds, 3,4-diaminopyridine and 4-dimethylamino-benzylamine, did not inhibit redox activity of PQQ in vitro, nor did they inhibit PMA stimulated superoxide production in monocytes or neutrophils. Thus, there is a correlation between an agent's ability to inhibit PQQ redox cycling and its ability to inhibit superoxide release by phagocytes. The findings are a further indication that PQQ is involved in the respiratory burst of phagocytic cells.

Ecto-ATPases: identities and functions

Ecto-ATPases are ubiquitous in eukaryotic cells. They hydrolyze extracellular nucleoside tri- and/or diphosphates, and, when isolated, they exhibit E-type ATPase activity, (that is, the activity is dependent on Ca2+ or Mg2+, and it is insensitive to specific inhibitors of P-type, F-type, and V-type ATPases; in addition, several nucleotide tri- and/or diphosphates are hydrolysed, but nucleoside monophosphates and nonnucleoside phosphates are not substrates). Ecto-ATPases are glycoproteins; they do not form a phosphorylated intermediate during the catalytic cycle; they seem to have an extremely high turnover number; and they present specific experimental problems during solubilization and purification. The T-tubule Mg2+-ATPase belongs to this group of enzymes, which may serve at least two major roles: they terminate ATP/ADP-induced signal transduction and participate in adenosine recycling. Several other functions have been discussed and identity to certain cell adhesion molecules and the bile acid transport protein was suggested on the basis of cDNA clone isolation and immunological work.

Methoxatin (PQQ) in guinea-pig neutrophils

PQQ, also called methoxatin, has been isolated from guinea-pig neutrophils. The organic cations diphenyleneiodonium (DPI) and diphenyliodonium (BPI) and the aromatic o-diamine 4,5-dimethylphenylenediamine (DIMPDA) sequester synthetic PQQ and inhibit its redox-cycling activity in a model system. Standards were made of adducts of tritiated PQQ with unlabeled DIMPDA and of unlabeled PQQ with tritiated DPI or DIMPDA. PQQ adducts were isolated from guinea-pig neutrophils with each of the tritiated inhibitors. They were separated and defined by high-performance liquid chromatography (HPLC). Tiron, a disodium benzene disulphonic acid, broke the DPI-PQQ adduct isolated from neutrophils and released free PQQ. Both DPI and DIMPDA, as well as BPI, blocked O2.- release by stimulated neutrophils. The blockade exerted by these inhibitors was released by the addition of PQQ to the cell suspensions. The data demonstrate the presence of PQQ in guinea-pig neutrophils and suggest that it has a possible role, direct or indirect, in the O2.(-)-producing respiratory burst.

The possibility that Ca(2+)-ATPase from the plasma membrane-rich fraction of bovine parotid gland is ecto-Ca(2+)-dependent nucleoside triphosphatase

1. Parotid plasma membrane nonpump low-affinity Ca(2+)-ATPase, which possesses high-affinity (Ca2+ + Mg2+)-ATPase activity, was characterized. 2. Purified Ca(2+)-ATPase hydrolyzed the nucleoside triphosphates, GTP, ITP, CTP, UTP, TTP (67-93% of ATP) and nucleoside diphosphates, ADP, GDP, IDP, CDP, TDP (12-40% of ATP) but not AMP and p-NPP. 3. The maximum activities of Ca(2+)- and (Ca2+ + Mg2+)-ATPases were obtained in the presence of 1 mM and 0.13 microM Ca2+, respectively. 4. The Km values for Ca2+ in Ca(2+)- and (Ca2+ + Mg2+)-ATPases were 0.2 mM and 22 nM, respectively. 5. The activities of both Ca(2+)- and (Ca2+ + Mg2+)-ATPases were found in the right-side-out-vesicles obtained from the plasma membrane-rich fraction. 6. These features suggest that Ca(2+)-ATPase is an ecto-Ca(2+)-dependent nucleoside triphosphatase.

Leukocyte-deactivating factor from macrophages: partial purification and biochemical characterization. A novel cytokine

A deactivating factor (MDF) is released from granuloma-like lesions of mice (giant and epithelioid macrophages) to the surrounding medium. Test cells incubated in the presence of MDF display dramatic inhibition of superoxide anion (O2-) release when stimulated. This failure to manifest O2 release is observed whether PMA, all-transretinal, or fMet-Leu-Phe is the stimulating agent. MDF acts on different cell types from different species; mouse macrophages as well as guinea pig, human, and mouse neutrophils. Such results suggest that it is a universal regulatory cytokine with high affinity for phagocytic lineages. The factor was subjected to various purification methods: ultrafiltration, gel chromatography, and reversed phase HPLC. A crude preparation that resulted from conditioning of medium by old macrophages (MCM) shows two peaks of activity when subjected to gel filtration. These correspond to molecular weights for the active principle of 3 and 11 kD. When the factor was obtained by extraction of the same cells after washing and sonication, only the former peak was seen. Fractions corresponding to a MW of 3 kD from several preparations were combined and subjected to HPLC. MDF activity then appeared in a single fraction. MDF is thus putatively a modulator of the cidal activity of phagocytic cells that utilize release of reactive oxygen species for cytocidal activity.

N-acetylaspartylglutamate catabolism is achieved by an enzyme on the cell surface of neurons and glia

N-Acetylaspartylglutamate (NAAG) is a nervous system-specific, acidic dipeptide which is released from neurons in a manner consistent with a function in synaptic neurotransmission. The hydrolysis of NAAG to produce glutamate and N-acetylaspartate was analyzed in cell cultures prepared from murine brain cells. Peptidase activity against NAAG was found in cultures which contained both neurons and glia, as well as in cultures of glia alone. Several lines of evidence were obtained in support of the hypothesis that this peptidase activity is predominantly bound to the extracellular face of the plasma membranes of these cells. Glutamate released from NAAG accumulated in the extracellular medium. Extracellular application of peptidase inhibitors effectively reduced NAAG hydrolysis. Peptidase activity was not secreted into the cell culture medium by intact cells, and lysed cells did not release detectable peptidase activity beyond that obtained with intact cells. Replacement of extracellular sodium with choline inhibited peptide uptake while stimulating apparent extracellular NAAG hydrolysis by intact cells in culture. Finally, the steady rise in extracellular glutamate as a consequence of NAAG hydrolysis by these brain cells, including glia, supports the conclusion that glutamate uptake is not tightly coupled to peptidase activity and thus that NAAG serves as a significant source of glutamate in the synaptic space following depolarization-induced peptide release.

Epidermal Langerhans cells are resistant to the permeabilizing effects of extracellular ATP: in vitro evidence supporting a protective role of membrane ATPase

Extracellular adenosine 5'-triphosphate (ATPo) can induce pore formation in cell membranes, leading to cell permeabilization and eventual cell death. In this study, we examined the sensitivity of human epidermal Langerhans cells to ATP-induced permeabilization and tested the possibility that the Mg(++)- or Ca(++)-dependent plasma membrane ectonucleotidase (mATPase) on Langerhans cells provides protection against the cytotoxic effects of ATPo. Membrane permeability was assessed by using the fluorescent tracer propidium iodide, which confers red nuclear fluorescence to permeabilized cells. Langerhans cells were identified within human epidermal cell suspensions with fluorescein isothiocyanate-conjugated MoAb against CD1a or human leukocyte antigen-DR (HLA-DR) antigens. Cultured human keratinocytes and J774 macrophages were both highly sensitive to permeabilization induced by incubation with ATP (0.5 to 20 mM at 37 degrees C), whereas Langerhans cells were relatively resistant. The non-hydrolyzable ATP analog, adenosine 5'-(beta,gamma-imido) triphosphate, but not other nucleotides such as ADP, AMP, GTP, or UTP, was also able to induce permeabilization comparable to that of ATP, thereby suggesting that ATP hydrolysis is not required for this effect. ATP4- is the moiety most likely responsible for permeabilization, because propidium iodide uptake occurred only when the pH of the medium was > or = 7.4. Permeabilization induced by ATP was augmented by chelation of divalent cations with ethylene-diamine-tetraacetic acid and by the addition of lanthanum or cerium (0.01 to 1 mM). Finally, incubation with the adenosine analog, 5'-p-fluorosulfonylbenzoyl-adenosine (1 mM), inhibited mATPase staining of Langerhans cells in human epidermal sheets, but markedly augmented ATP-induced permeabilization of Langerhans cells. The results indicate that epidermal LC are resistant to the lytic effects of ATPo and that mATPase is involved in such resistance.

Rapid changes of mitochondrial Ca2+ revealed by specifically targeted recombinant aequorin

Introduction of Ca2+ indicators (photoproteins, fluorescent dyes) that can be trapped in the cytosolic compartment of living cells has yielded major advances in our knowledge of Ca2+ homeostasis. Ca2+ however regulates functions not only in the cytosol but also within various organelles where indicators have not yet been specifically targeted. Here we present a novel procedure by which the free Ca2+ concentration of mitochondria, [Ca2+]m, can be monitored continuously at rest and during stimulation. The complementary DNA for the Ca2+ sensitive photoprotein aequorin was fused in frame with that encoding a mitochondrial presequence. The hybrid cDNA was transfected into bovine endothelial cells and stable clones were obtained expressing variable amounts of mitochondrially targeted apoaequorin. The functional photoprotein could be reconstituted in intact cells by incubation with purified coelenterazine and [Ca2+]m could thus be monitored in situ. This allowed the unprecedented direct demonstration that agonist-stimulated elevations of cytosolic free Ca2+, [Ca2+]i, (measured in parallel with Fura-2) evoke rapid and transient increases of [Ca2+]m, which can be prevented by pretreatment with a mitochondrial uncoupler. The possibility of targeting aequorin to cellular organelles not only offers a new and powerful method for studying aspects of Ca2+ homeostasis that up to now could not be directly approached, but might also be used in the future as a tool to report in situ a variety of apparently unrelated phenomena of wide biological interest.

Utility of immobilon-bound phosphoproteins as substrates for protein phosphatases from neutrophils

Immobilon-bound phosphoproteins labeled with 32P were utilized as substrates to study the enzymes in neutrophils that are active against the major products of protein kinase C. The labeled proteins were separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and transferred electrophoretically to immobilon-P membranes. Both particulate and soluble phosphatases were found to be active against the blotted phosphoproteins. Reactions were followed by autoradiography as the loss of 32P from individual protein bands. The tumor promoter okadaic acid and the hepatoxin microcystin-LR inhibited these reactions in a manner consistent with the enzymes being type 1 and/or 2A protein phosphatases.

A simple and sensitive radioimmunoassay for adenosine

We developed a simple and sensitive radioimmunoassay (RIA) for adenosine. The RIA is based on the double antibody method with adenosine 2', 3'-0-disuccinyl-3-[125I]-iodotyrosine methyl ester (diSc-adenosine-[125I]-TME) as a tracer. Anti-adenosine antiserum for the RIA was raised in rabbits immunized with diSc-adenosine conjugated to human serum albumin (diSc-adenosine-HSA). All samples and standards were succinylated prior to assay. The present immunoassay allows detection of 6.25-400 pmol/ml of adenosine in sample. Values obtained by the RIA and by a HPLC analysis showed a high correlation with correlation coefficient of 0.997. In order to determine adenosine in plasmas, blood cells must be separated in the presence of 6 mM EDTA, 0.006% dipyridamole (Dip) and 23 microM 2'-deoxycoformycin (dCF) at 2 degrees C. Adenosine in plasma could be accurately determined by the proposed method even without any pretreatments by deproteinizing. The adenosine levels with or without EDTA-treated normal human plasmas determined were 26.2 +/- 7.26 and 100 +/- 3.62 pmol/ml (mean +/- SEM), respectively.

Nucleotide hydrolytic activity of isolated intact rat mesenteric small arteries

Segments of isolated intact rat mesenteric small arteries were incubated in physiological bicarbonate buffer in the presence of nano- to millimolar concentrations of ATP. ATP was hydrolysed, and when the vessel was transferred from one incubation to another, the enzyme activity was transferred with the vessel, consistent with the presence of an ecto-ATPase. The substrate, ATP, was shown to induce a modification of the hydrolytic activity which occurred the more rapidly the higher the concentration of ATP. The modified system hydrolysed ATP with a decreased substrate affinity. As the substrate induced a modification of the hydrolytic activity, steady-state velocity measurements for determination of kinetic parameters could not be obtained. Nevertheless, it was possible to compare the modification caused by ATP and UTP, and to compare the hydrolysis rates measured with [32P]ATP, [32P]UTP and [32P]GTP. It was concluded that the hydrolytic activity of the vessels did not distinguish between the nucleoside triphosphates (NTPs). In a histidine buffer, the activity was shown to be activated by micromolar concentrations of either Ca2+ or Mg2+, and not to be influenced by inhibitors of P-type, F-type and V-type ATPases. Functional removal of the endothelium before assay did not reduce the measured NTP hydrolysis. At millimolar concentrations of trinucleotide the hydrolysis rate was 10-15 mumol per min per gram of tissue or 0.11-0.17 mumol per min per 10(6) vascular smooth muscle cells. This value is equivalent to the maximal velocity obtained for the Ca2+ or Mg(2+)-dependent NTPase released to the medium upon 2 s of sonication of the vessels (Plesner, L., Juul, B., Skriver, E. and Aalkjaer, C. (1991) Biochim. Biophys. Acta 1067, 191-200). Comparing the characteristics of the released NTPase to the characteristics of the activity of the intact vessel, they showed a strong resemblance, but the substrate-induced modification of the enzyme was seen only in the intact preparation.

Mg.ATP primes superoxide-generating responses in electropermeabilized neutrophils

The present study utilizes an electropermeabilized cell system to determine the effect of Mg.ATP on neutrophil superoxide (O2-)-generating responses stimulated by suboptimal concentrations of fMLP, GTP gamma S and PMA. Permeabilization in the presence of exogenously added Mg.ATP was neither sufficient to initiate O2- release nor necessary for stimulated O2- production. However, the inclusion of Mg.ATP in the permeabilization medium primed the O2(-)-generating responses mediated by suboptimal concentrations of these stimuli. The site of action of Mg.ATP is intracellular. Moreover, the fact that Mg.ATP primes responses stimulated by fMLP, GTP gamma S and PMA suggests that the modulatory effect is at the level of protein kinase C.

Expression and biochemical characterization of human immunodeficiency virus type 1 nef gene product

nef genes from human immunodeficiency virus type 1 isolates BH10 and LAV1 (lymphadenopathy-associated virus type 1) were expressed in Escherichia coli under the deo operon promoter. The two proteins found in the soluble compartment of the bacterial lysate were purified by ion-exchange column chromatography to apparent homogeneity. Determination of the amino-terminal sequence revealed glycine as the first amino acid in the Nef protein, indicating removal of the initiator methionine during expression in E. coli. Under native conditions, the recombinant Nef protein is a monomer of 23 kilodaltons. In denaturing polyacrylamide gels, however, BH10 and LAV1 Nef proteins migrate as 28 and 26 kilodaltons, respectively. GTP binding and GTPase activity were monitored during Nef protein purification. These activities did not copurify with the recombinant Nef protein from either the BH10 or the LAV1 isolate. Purified recombinant BH10 Nef protein was used as an immunogen to elicit mouse monoclonal antibodies. A series of monoclonal antibodies were obtained which reacted with sequences at either the amino or carboxy terminus of Nef. In addition, a conformational epitope reacting with native BH10, but not LAV1, Nef was isolated.

Ocular dominance plasticity and developmental changes of 5'-nucleotidase distributions in the kitten visual cortex

The distribution of the adenosine-producing ecto-enzyme 5'-nucleotidase was investigated histochemically in the visual cortex of normally reared and monocularly deprived kittens and cats. In normally reared kittens aged between 11 to 44 days, 5'-nucleotidase activity formed a band of intense neuropil staining throughout cortical layer IV of areas 17 and 18. The other layers were almost devoid of reaction product. Between the 4th and 6th week, this band had a patchy appearance in area 17, the center-to-center spacing of 5'-nucleotidase patches being approximately 1 mm. Monocular enucleation accentuated these patches of enhanced 5'-nucleotidase activity or made them reappear at developmental stages at which they had normally faded. Simultaneous visualization of ocular dominance columns by transneuronal transport of intraocularly injected 3H-proline showed that the patches of enhanced 5'-nucleotidase activity coincided with the territories of afferents from the intact eye. With increasing age and normal visual development, the patches disappeared and 5'-nucleotidase activity spread to the supra- and infragranular layers. The adult pattern was characterized by dense staining of all cortical laminae in both areas 17 and 18 and was established at about 8 weeks of age. At approximately 7 weeks of age, when the patches in layer IV had disappeared in the course of normal development, monocular enucleation caused a reappearance of the discontinuous pattern of 5'-nucleotidase activity in layer IV. These results reveal a close relation between the distribution of 5'-nucleotidase and the time course of the developmental phase during which the visual cortex is susceptible to experience-dependent alterations. As suggested by the correlation between sites of enzyme activity and eye dominance columns, the expression of 5'-nucleotidase patches in layer IV appears to be associated with the remodelling of ocular dominance territories that occurs both in normal development and after manipulation of afferent retinal input. Thus, 5'-nucleotidase is likely to serve a function in activity-dependent modifications of cortical circuitry. Moreover, 5'-nucleotidase activity is the only endogenous marker known to date that exhibits a columnar pattern in cat visual cortex.

A sensitive and specific assay for superoxide anion released by neutrophils or macrophages based on bioluminescence of polynoidin

Using the luminescent protein polynoidin, present in the bioluminescent system isolated from the marine annelid Harmothoe lunulata, we have developed a new method to measure, specifically, superoxide anion (O2-) released by macrophages or neutrophils. A small quantity of an aqueous crude extract of polynoidin is used to detect O2- released by stimulated cells. Light emission is linearly dependent on the number of cells over a wide range (10(3) to 10(7) cells), and the assay is thus more sensitive than either luminol or ferricytochrome c reduction. Luminescence is enhanced 20% by mannitol, 80% by catalase, and is totally quenched by superoxide dismutase. For the same number of cells, neutrophils showed a threefold higher release of O2- and a twofold faster first-order light decay than stimulated macrophages, in accordance with data obtained by other methods.