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Characterization of a mastoparan-stimulated nucleotidase from bovine brain.

Mastoparan is a 14-amino-acid peptide that stimulates secretion from several cell types. Secretion can be partially blocked by pertussis toxin and may be mediated by guanine-nucleotide-binding proteins (G-proteins). Mastoparan can act directly on G-proteins, probably at the hormone receptor-binding site, to stimulate guanosine 5'-[gamma-thio]triphosphate binding and GTPase activities of pertussis-toxin substrates Go and Gi [Higashijima, Uzu, Nakajima & Ross (1988) J. Biol. Chem. 263, 6491-6494]. We now describe a nucleotidase from bovine brain that is not a known G-protein whose GTPase and ATPase activities are stimulated by mastoparan. This nucleotidase hydrolyses ATP faster than GTP, but has similar affinities for both (0.4 microM). Mastoparan maximally stimulates both ATPase and GTPase activities by about 8-fold after insertion of the protein into phospholipid vesicles, but does not affect the EC50 (concentration at which half the maximal effect is observed) for ATP and GTP. The EC50 for mastoparan stimulation of GTPase and ATPase is 6 and 12 microM respectively. The native molecular mass of the partially purified mastoparan-stimulated nucleotidase is 87 kDa. This nucleotidase may be another receptor-activated enzyme, and its identification may be useful for understanding mastoparan-stimulated processes.

Adenosine Triphosphatases↗

Soluble low-Km 5'-nucleotidase from electric-ray (Torpedo marmorata) electric organ and bovine cerebral cortex is derived from the glycosyl-phosphatidylinositol-anchored ectoenzyme by phospholipase C cleavage.

Soluble and membrane-bound low-Km 5'-nucleotidase was isolated from high-speed supernatants and membrane fractions derived from the electric organ of the electric ray (Torpedo marmorata) or from bovine brain cerebral cortex. Purification of both enzymes included chromatography on concanavalin A-Sepharose and AMP-Sepharose. The contribution to the total of soluble enzyme activity was lower in electric organ (1.6%) than in bovine cerebral cortex (27.9%). Membrane-bound and soluble forms have very similar Km values for AMP and are inhibited by micromolar concentrations of ATP. Both forms cross-react with, and are inhibited by, an antibody against the membrane-bound surface-located (ecto-) 5'-nucleotidase from electric organ. The HNK-1 carbohydrate epitope is present on both forms of the Torpedo enzyme, but is entirely absent from bovine cerebral-cortex 5'-nucleotidase. An antibody specific for the inositol 1,2-(cyclic)monophosphate that is formed on phospholipase C cleavage of an intact glycosyl-phosphatidylinositol (GPI) anchor binds to the soluble, but not to the membrane-bound, form of the enzyme from both sources. Our results suggest that soluble low-Km 5'-nucleotidase in both electric organ and bovine brain is derived from the membrane-bound GPI-anchored form of the enzyme by the action of a phospholipase C and is not a soluble cytoplasmic enzyme.

5'-Nucleotidase↗

The mechanism of adenosine formation in cells. Cloning of cytosolic 5'-nucleotidase-I.

Adenosine increases blood flow and decreases excitatory nerve firing. In the heart, it reduces rate and force of contraction and preconditions the heart against injury by prolonged ischemia. Based on indirect kinetic arguments, an AMP-selective cytosolic 5'-nucleotidase designated cN-I has been implicated in adenosine formation during ATP breakdown. The molecular identity of cN-I is unknown, although an IMP/GMP-selective cytosolic 5'-nucleotidase (cN-II) and an ecto-5'-nucleotidase (e-N) have been cloned. We utilized the high abundance of cN-I in pigeon heart to purify a 40-kDa subunit for partial protein sequencing and subsequent cDNA cloning. We obtained a full-length clone encoding a novel 40-kDa peptide, unrelated to cN-II or e-N, that was most abundant in heart, brain, and breast muscle. Immunolocalization in heart showed a striated cytoplasmic location, suggesting association with contractile elements. Transient expression in COS-7 cells, generated a 5'-nucleotidase that catalyzed adenosine formation from AMP, which was increased during ATP catabolism. In conclusion, the cloning and expression of cN-I provides definitive evidence of its ability to produce adenosine during ATP breakdown.

5'-Nucleotidase↗

The possible importance of the cation-binding site for the oxidative modification of liver 5'-nucleotidase.

The oxidative damage of proteins and lipid peroxidation of membrane lipoproteins has already been described as a possible pathogenic mechanism for liver injury. The aim of the present study was to examine the mechanism that could be responsible for the oxidative modification of rat liver 5'-nucleotidase during exposure to different free radical generating systems: FeCl2/ascorbate, xanthine/xanthine oxidase and H2O2. The level of lipid peroxidation products malondialdehyde (MDA), as well as the level of protein carbonyl groups formation was measured in cells and extracellular medium. The activity of 5'-nucleotidase was linearly decreased in both hepatocytes and extracellular medium after exposure to the FeCl2/ascorbate system indicating that the possible mechanism for oxidative modification could be a metal-binding site of the enzyme. In xanthine/xanthine oxidase system the enzyme activity of hepatocytes had decreased in hepatocytes but increased in the extracellular medium indicating that proteolysis of membrane proteins could he responsible for enzyme release in the extracellular medium. When hepatocytes were exposed to a H2O2 free-radical generating system, the activity of 5'-nucleotidase tended to be decreased in cells and decreased in extracellular medium too, indicating that H2O2 could be less reactive in producing an oxidative modification of the enzyme. In order to support the hypothesis that the cation-binding site can be responsible for oxidative modification of the enzyme, the isolated hepatocytes were preincubated with a Ca(2+)-channel blocker (Verapamil) and then exposed to different radical-generating systems. Verapamil had only a slight effect in potentiating the inhibition in the FeCl2/ascorbate system. This probably means that the cellular cation flux and cation binding may be included as a vulnerable site with the greatest importance in the oxidative modification of 5'-nucleotidase.

5'-Nucleotidase↗

Cell cycle dependent regulation of deoxycytidine kinase, deoxyguanosine kinase, and cytosolic 5'-nucleotidase I activity in MOLT-4 cells.

Activation of nucleoside analogues is dependent on kinases and 5'-nucleotidases and the balance between the activity of these enzymes. The purpose of this study was to analyze deoxycytidine kinase, deoxyguanosine kinase, and 4 different 5'-nucleotidases during cell cycle progression in MOLT-4 cells. The activity of both kinases was cell cycle dependent and increased during proliferation while the activity of cytosolic 5'-nucleotidase I decreased. We could show that the kinase activity was higher than the total nucleotidase activity, which was unchanged or decreased during cell cycle progression. These data may be important in designing modern combination therapy with nucleoside analogues.

5'-Nucleotidase↗

Purification and cDNA cloning of bovine liver 5'-nucleotidase, a GPI-anchored protein, and its expression in COS cells.

A glycosylphosphatidylinositol (GPI)-anchored protein, 5'-nucleotidase [EC 3.1.3.5], was released from the membrane of bovine liver by use of phosphatidylinositol-specific phospholipase C (PI-PLC) of Bacillus thuringiensis and purified by several column chromatographies to a homogeneous state. The purified protein has an apparent molecular mass of 61 kDa, as estimated by SDS-polyacrylamide gel electrophoresis. From the partial amino acid sequence of a tryptic peptide, mixed oligonucleotides were synthesized and used to screen a lambda gt11 liver cDNA library, and one positive clone, pE1, was isolated. Since the insert of the clone lacked the NH2-terminal coding region, another lambda gt11 liver cDNA library was screened by using a synthetic probe corresponding to the 5' region of the insert of pE1. Three additional cDNA clones were obtained. Sequencing of these cDNAs revealed an open reading frame that encodes a 574-residue polypeptide with a calculated mass of 63,084 Da. The predicted structure showed two highly hydrophobic stretches at both ends of the protein, like those of rat and human 5'-nucleotidases. The NH2-terminal 26 residues comprise a signal peptide and the COOH-terminal hydrophobic stretch may serve as a signal for the posttranslational GPI modification. An expression vector of the cDNA, pSVNT, was constructed in a mammalian expression vector pSVL and the 5'-nucleotidase activity was transiently expressed in COS-1 cells. The expressed activity was about 8 times higher than the pSVL-transfected control activity. PI-PLC released 45% of the transiently expressed 5'-nucleotidase activity, indicating that the cDNA isolated here encodes this enzyme expressed as a GPI-anchored protein.

5'-Nucleotidase↗

Primary structure of human placental 5'-nucleotidase and identification of the glycolipid anchor in the mature form.

A cDNA was cloned coding for human placental 5'-nucleotidase. The 3547-bp cDNA contains an open reading frame that encodes a 574-residue polypeptide with calculated size of 63 375 Da. The NH2-terminal 26 residues comprise a signal peptide, which is followed by the NH2-terminal sequence of the purified protein. four potential N-linked glycosylation sites are found in the molecule, accounting for a larger mass of the mature form (71 kDa). The predicted structure contains a hydrophobic amino acid sequence at the COOH terminus, a possible signal for the post-translational modification by glycophospholipid. To confirm this possibility, we tried to isolate and characterize the membrane-anchoring domain of 5'-nucleotidase. BrCN-cleaved fragments of the protein were extracted with hexane and subjected to HPLC, resulting in purification of a single component of 2.3 kDa. Chemical analyses revealed that the purified fragment contains the tetradecapeptide Lys-Val-Ile-Tyr-Pro-Ala-Val-Glu-Gly-Arg-Ile-Lys-Phe-Ser, ethanolamine, glucosamine, mannose, inositol, palmitic acid, and stearic acid. The peptide sequence determined is identified at positions 510-523 in the primary structure deduced from the cDNA sequence, which predicts a further extension to position 548, containing the hydrophobic amino acid sequence. Thus, it is concluded that the mature 5'-nucleotidase lacks the predicted COOH-terminal peptide extension (524-548), which has been replaced by the glycophospholipid functioning as the membrane anchor of 5'-nucleotidase.

5'-Nucleotidase↗

Ecto-5'-nucleotidase is associated with cholinergic nerve terminals in the hippocampus but not in the cerebral cortex of the rat.

The extracellular catabolism of exogenously added AMP was studied in immunopurified cholinergic nerve terminals and in slices of the hippocampus and cerebral cortex of the rat. AMP (10 microM) was catabolized into adenosine and inosine in hippocampal cholinergic nerve terminals and in hippocampal slices, as well as in cortical slices. IMP formation from extracellular AMP was not detected. alpha, beta-Methylene ADP (100 microM) inhibited almost completely the extracellular catabolism of AMP in these preparations. The relative rate of catabolism of AMP was greater in hippocampal slices than in cortical slices. AMP was virtually not catabolized when added to immunopurified cortical cholinergic nerve terminals, although ATP could be catabolized extracellularly under identical conditions. The comparison of the relative rates of catabolism of exogenously added AMP, calculated from the amount of AMP catabolized after 5 min, in hippocampal cholinergic nerve terminals and in hippocampal slices revealed a nearly 50-fold enrichment in the specific activity of ecto-5'-nucleotidase upon immunopurification of the cholinergic nerve terminals from the hippocampus. The results suggest that there is a regional variation in the subcellular distribution of ecto-5'-nucleotidase activity in the rat brain, the ecto-5'-nucleotidase in the hippocampus being closely associated with the cholinergic nerve terminals, whereas in the cerebral cortex ecto-5'-nucleotidase activity seems to be located preferentially outside the cholinergic nerve terminals.

5'-Nucleotidase↗

Phosphorylation of ribavirin and viramidine by adenosine kinase and cytosolic 5'-nucleotidase II: Implications for ribavirin metabolism in erythrocytes.

Many nucleoside analog drugs, such as ribavirin and viramidine, are activated or metabolized in vivo through 5'-phosphorylation. In this report, we determined the steady-state kinetic parameters for 5'-monophosphorylation of ribavirin and viramidine by adenosine kinase. The apparent Km for ribavirin is 540 microM, and k(cat) is 1.8 min-1. Its catalytic efficiency of 3.3 x 10(-3) min-1 . microM-1 is 1,200-fold lower than that of adenosine. In contrast to the common belief that ribavirin is exclusively phosphorylated by adenosine kinase, cytosolic 5'-nucleotidase II was found to catalyze ribavirin phosphorylation in vitro. The reaction is optimally stimulated by the physiological concentration of ATP or 2,3-bisphosphoglycerate. In phosphate-buffered saline plus ATP and 2,3-bisphosphoglycerate, the apparent Km for ribavirin is 88 microM, and k(cat) is 4.0 min-1. These findings suggest that cytosolic 5'-nucleotidase II may be involved in ribavirin phosphorylation in vivo. Like ribavirin, viramidine was found to be phosphorylated by either adenosine kinase or cytosolic 5'-nucleotidase II, albeit with a much lower activity. The catalytic efficiency for viramidine phosphorylation is 10- to 330-fold lower than that of ribavirin, suggesting that other nucleoside kinase(s) may be involved in viramidine phosphorylation in vivo. Both ribavirin and viramidine are not phosphorylated by deoxycytidine kinase and uridine-cytidine kinase. The coincidence of presence of high concentrated 2,3-bisphosphoglycerate in erythrocytes suggests that cytosolic 5'-nucleotidase II could play an important role in phosphorylating ribavirin and contribute to anabolism of ribavirin triphosphate in erythrocytes. Elucidation of ribavirin and viramidine phosphorylation mechanism should shed light on their in vivo metabolism, especially the ribavirin-induced hemolytic anemia in erythrocytes.

5'-Nucleotidase↗

Acid-soluble degradation products of ribonucleic acid in Escherichia coli and the role of nucleotidases in their catabolism.

The fate of the internally formed nucleotides resulting from the degradation of ribonucleic acid was studied. Prelabeled Escherichia coli cells were submitted to carbon starvation, and the acid-soluble products were separated by thin-layer chromatography. It was determined that free bases constitute some 75% of the end product, the balance consisting of nucleoside diphosphates, 5'-nucleoside monophosphates, 3'-nucleoside monophosphates, and nucleosides. The majority of degradation products, including phosphorylated derivatives, were excreted into the medium. The amount of products in the pool remained constant. The soluble products formed by E. coli mutants lacking either 5'-nucleotidase (Ush-) or 3'-nucleotidase (Cpd-) were compared with those produced by the parental strain with both enzymes. The results obtained indicated that 5'-nucleotidase is involved in the degradation of internally foromed nucleotides but that 3'-nucleotidase takes no part in the process.

Carbon↗

Alterations in hepatic 5'-nucleotidase in the tumor-bearing rat.

The effect of the growth of the Walker 256 carcinoma on the level of 5'-nucleotidase and alkaline phosphatase in the whole liver and in an isolated hepatocyte membrane preparation of its host was investigated. Alkaline phosphatase activities of whole liver and plasma membrane were increased approximately 5-fold by tumor growth. A 50% decrease in whole liver 5'-nucleotidase activity was observed in tumor-bearing rats while the 5'-nucleotidase activity per milligram membrane protein was unaltered. Tumor growth would therefore appear to affect a pool of 5'-nucleotidase which is not associated with the plasma membrane.

Alkaline Phosphatase↗

5'-Nucleotidase in rat liver lysosomal membranes is anchored via glycosyl-phosphatidylinositol.

We have previously demonstrated that 5'-nucleotidase, known as a plasma membrane enzyme, is also distributed both in rat liver tritosomal membranes and contents (J. Biochem. 101, 1077-1085, 1987). When the lysosomal membranes isolated from rat livers were incubated with phosphatidylinositol-specific phospholipase C purified from B. thuringiensis, about 70% of 5'-nucleotidase activity was released from the membranes. Judging from the result by phase separation with Triton X-114, the enzyme solubilized by the phospholipase C digestion showed a hydrophilic nature such as that of the tritosomal contents. Immunoblot analysis showed that the molecular weight of 5'-nucleotidase released from the lysosomal membranes by the phospholipase C digestion was almost identical with that of the enzymes from the Tritosomal contents. The above results showed that the phosphatidylinositol-specific phospholipase C-like enzyme in the lysosomes may be responsible for the conversion of the lysosomal membrane-bound 5'-nucleotidase to the soluble form present in the lysosomal matrix.

5'-Nucleotidase↗

Phosphatase-inhibitory activity and activation of murine macrophages by new 5'-nucleotidase inhibitors, NPF-86IA, NPF-86IB, NPF-86IIA and NPF-86IIB.

New 5'-nucleotidase-inhibitory polyphenols named NPF-86IA, NPF-86IB, NPF-86IIA and NPF-86IIB were isolated from the seeds of Areca catechu L. The ability of the inhibitors to precipitate gelatin was investigated by microturbidimetry. These inhibitors produced weak turbidity. As 5'-nucleotidase is a kind of phosphatase, we examined the effects of these inhibitors on alkaline and acidic phosphatases. While they showed moderate inhibitory effects on the activity of acidic phosphatases, they did not have any significant effect on the activity of alkaline phosphatase. Therefore, they showed a higher inhibitory effect on the 5'-nucleotidase than the other phosphatases, Murine macrophages were directly stimulated by the 5'-nucleotidase inhibitors.

5'-Nucleotidase↗

New polyphenolic 5'-nucleotidase inhibitors isolated from the wine grape "Koshu" and their biological effects.

New 5'-nucleotidase inhibitors designated as NPF-88BU-IA, NPF-88BU-IB, NPF-88BU-IIA and NPF-88BU-IIB, respectively, were isolated from the seeds and skin of the wine grape "Koshu". They were purified by solvent extraction, dialysis, and reversed-phase high performance liquid chromatography (HPLC). Their physico-chemical properties revealed these compounds to be polyphenolic substances. The average relative molecular masses of the four were estimated by gel permeation chromatography (GPC) analysis to be 7850, 5950, 11900, and 11300, respectively. They strongly inhibited 5'-nucleotidase activities from snake venom and rat liver membrane, and displayed significant therapeutic activity against Ehrlich ascites carcinoma. They also showed inhibitory effects on the growth of Streptococcus mutans MT8148(c), a primary cariogenic bacterium. Furthermore, these 5'-nucleotidase inhibitors inhibited the glucan formation from sucrose. These results suggest that the 5'-nucleotidase inhibitors can prevent the cause of caries of tooth.

5'-Nucleotidase↗

Relation between reticulocyte count and characteristics of erythrocyte 5'-nucleotidase in dogs, cats, cattle and humans.

To examine substrate specificity and susceptibility to lead, erythrocyte 5'-nucleotidase was measured in dogs, cats, cattle and humans, and its relationship to the reticulocyte count in these species was determined. The reticulocyte count in dogs was similar to that in humans, but the count in cats was higher than that in humans. Reticulocytes were not observed in cattle. The activities of canine erythrocyte 5'-nucleotidase measured using cytidine and uridine 5'-monophosphates, which are preferentially catalyzed by one of the human pyrimidine 5'-nucleotidase isozymes (P5N-I), were similar to those of the human enzyme. The canine enzyme preferentially catalyzed thymidine 3'-monophosphate, which is catalyzed only by human P5N-II, more strongly than the human enzyme. This suggests that canine erythrocytes have two isozymes similar to human P5N-I and P5N-II, and a higher P5N-II-like activity than human erythrocytes. Feline erythrocytes had the highest level of P5N-I-like activity among the species examined, and the bovine enzymic activities including those of P5N-I and II were the lowest among these species. According to these observations, the reticulocyte count was approximately proportional to the P5N-I-like activity in these species. Therefore, the P5N-I-like activity may be involved in the morphological maturation of mammalian erythrocytes. The canine and feline erythrocytes had markedly high activity and preferentially catalyzed purine 5'-monophosphate suggesting the presence of a purine-specific 5'-nucleotidase as in human erythrocytes. In addition, the canine and feline P5N-I-like activity showed less susceptibility to lead than the human P5N-I. This may be a reason why there are few case reports of lead-induced anemia in dogs and cats.

5'-Nucleotidase↗

Glycosylation and processing of carbohydrate side chains of ecto-5'-nucleotidase in cultured human chorionic cells.

Glycosylation and carbohydrate processing of ecto-5'-nucleotidase were studied in cultured human chorionic cells using metabolic labelling and immunoprecipitation with monoclonal antibodies. Tunicamycin blocks glycosylation altogether leading to a reduction in molecular mass of 9,500 Da. The same result is obtained by digesting the mature 72,000-Da protein with endoglycosidase F. Using various inhibitors of the carbohydrate-trimming reactions like deoxynojirimycin, deoxymannojirimycin and swainsonine smaller molecular mass reductions are observed and the oligosaccharide side chains are kept in a configuration sensitive to endoglycosidase H digestion. Digestion of mature 5'-nucleotidase with endoglycosidase H leads to a much smaller (2,000 Da) reduction in molecular mass. It is calculated that, in addition to the phosphatidylinositol-glycan anchor structure, ecto-5'-nucleotidase of human chorionic cells should carry 4 oligosaccharide side chains per subunit, 3 of which should be of the complex and one of the high mannose type. Interference with carbohydrate processing by various inhibitors does not seem to influence the distribution of ecto-5'-nucleotidase between the cell surface and intracellular membranes nor does it block the transfer of the enzyme to the phosphatidylinositol glycan anchor.

1-Deoxynojirimycin↗

Superoxide dismutase enhances both adenosine release and coronary hyperemic flow through protection of 5'-nucleotidase against its degradation during reperfusion following ischemia in dogs.

Oxygen-derived free radicals are thought to inactivate ectosolic enzymes that regulate myocardial cellular functions. One such enzyme is ecto-5'-nucleotidase, which is responsible for adenosine production during coronary hyperemic flow. In the present study, we measured both reactive and hyperemic flow and adenosine release during reperfusion with and without superoxide dismutase. In 10 open chest dogs, the left anterior descending coronary artery was perfused through an extracorporeal bypass tube from the carotid artery. In the control hearts, a five-minute occlusion of the coronary artery caused the hyperemic flow with increased release of adenosine. In the superoxide dismutase-treated hearts, although administration of superoxide dismutase altered neither adenosine release nor coronary blood flow in the baseline, it augmented both peak coronary blood flow and repayment, and adenosine release during reperfusion. Therefore, we hypothesized that superoxide dismutase reduces generation of oxygen-derived free radicals during ischemia and reperfusion and attenuates the degradation of ecto-5'-nucleotidase. The administration of superoxide dismutase enhanced the increase in ecto-5'-nucleotidase activity at 10 minutes after reperfusion over the untreated group. Thus, we conclude that superoxide dismutase enhances reactive hyperemic flow and adenosine release during reperfusion following ischemia, which may be attributable to the protection of ecto-5'-nucleotidase by superoxide dismutase during ischemia and reperfusion.

5'-Nucleotidase↗

Correlation between the release of the sympathetic neurotransmitter ATP and soluble nucleotidases from the guinea pig vas deferens.

Recently, we have shown that by releasing specific nucleotidases the sympathetic nerves of the guinea pig vas deferens may regulate the metabolism of extracellular adenine nucleotides and consequently, the inactivation of neurotransmitter ATP. Based on the evidence for tetrodotoxin sensitivity and calcium dependence of the nerve stimulation-evoked overflow of enzyme activity, we have suggested that soluble nucleotidases may be stored in synaptic vesicles within the sympathetic nerves and released upon arrival of nerve action potentials by a mechanism similar to that for release of neurotransmitters. To further test this hypothesis we studied the time course of nerve stimulation-evoked overflow of ATP, norepinephrine (NE), releasable ATPase (r-ATPase) activity, and releasable AMPase (r-AMPase) activity under control conditions and in the presence of drugs known to selectively modulate sympathetic neurotransmission. The results show that the time course of overflow of r-ATPase and r-AMPase activities resembles the transient pattern of overflow of ATP but not the tonic pattern of overflow of NE. Vasa deferentia dissected from animals treated with reserpine release ATP, r-ATPase, and r-AMPase, whereas the overflow of NE is completely abolished. Guanethidine, on the other hand, inhibits equally well the overflow of the two neurotransmitters and the releasable nucleotidase activities. Agonists of the alpha(2)-adrenergic receptors abolish the overflow of ATP, r- ATPase, and r-AMPase but not the overflow of NE. This evidence supports the idea that the sympathetic nerves of the guinea pig vas deferens store and release ATP together with specific nucleotidases responsible for the inactivation of this neurotransmitter.

Adenosine Triphosphatases↗