Human placental cytoplasmic 5'-nucleotidase. Kinetic properties and inhibition

Glucose decreases extracellular adenosine levels in isolated mouse and rat pancreatic islets

The pancreatic islets of Langerhans are responsible for the regulated release of the endocrine hormones insulin and glucagon that participate in the control of glucose homeostasis. Abnormal regulation of these hormones can result in glucose intolerance and lead to the development of diabetes. Numerous efforts have been made to better understand the physiological regulators of insulin and glucagon secretion. One of these regulators is the purine nucleoside, adenosine. Though exogenous application of adenosine has been demonstrated to stimulate glucagon release and inhibit insulin release, the physiological significance of this pathway has been unclear. We used a novel 7 µm enzyme-coated electrode biosensor to measure adenosine levels in isolated rodent islets. In the mouse islets, basal adenosine levels in the presence of 3 mM glucose were estimated to be 5.7 ± 0.6 µM. As glucose was increased, extracellular adenosine diminished. A 10-fold increase of extracellular KCl increased adenosine levels to 16.4 ± 2.0 µM. This release required extracellular Ca (2+) suggesting that it occurred via an exocytosis-dependent mechanism. We also found that while rat islets were able to convert exogenous ATP into adenosine, mouse islets were unable to do this. Our study demonstrates for the first time the basal levels of adenosine and its inverse relationship to extracellular glucose in pancreatic islets.

Expression patterns of AMP-deaminase and cytosolic 5'-nucleotidase genes in human term placenta

Background AMP-deaminase (EC 3.5.4.6) and 5'-nucleotidase (EC 3.1.3.5) are enzymes responsible for the maintenance of cellular adenine nucleotides pool. Both exist in several isoforms that differ in kinetic properties and tissue distribution. Profile of isoforms of these enzymes in human placenta has not been analyzed so far while this could be important for understanding of pathology of placental ischemia such as in preeclampsia. Our aim was therefore to analyze expression of AMPD and CN-I genes in human term placenta. Methods RT-PCR analysis was used for determine expression of AMPD1, AMPD2, AMPD3 and CN-I. Results and conclusion The experimental results presented here indicate that genes coding "AMP-preferring", cytosolic isozyme of 5'-nucleotidase (cN-I) as well as "muscle-type" isozyme of AMP-deaminase (AMPD1) are not expressed in human term placenta. Among other AMPD family genes, only these coding "liver-type" isozyme (AMPD2) and, in lesser degree, "erythrocyte-type" isozyme (AMPD3) of AMP-deaminase are expressed in this organ. The expression level of AMPD3 was a half of that presented by AMPD2. We conclude that high abundance of AMP-deaminase 2 transcript suggest that this particular isoform is a predominant pathway of adenine nucleotides degradation in human term placenta that follows liver-type regulation of this process.

NMDA receptor-mediated extracellular adenosine accumulation is blocked by phosphatase 1/2A inhibitors

We have previously demonstrated that NMDA receptor-mediated extracellular adenosine accumulation in neuronal cultures is receptor-mediated and requires calcium influx. Because protein kinase C (PKC) is a calcium-dependent enzyme, we hypothesized that activation of PKC might be involved in NMDA-mediated adenosine accumulation. PKC inhibitors, however, did not block NMDA-evoked adenosine accumulation, but rather, stimulated basal adenosine accumulation. These data suggested the possibility that NMDA receptor-mediated adenosine accumulation involves net dephosphorylation rather than phosphorylation of one or more substrates. Thus, inhibition of kinases would be expected to increase adenosine accumulation and inhibition of phosphatases would be expected to block adenosine accumulation. To test this hypothesis, we used the phosphatase 1/2A inhibitors calyculin A and okadaic acid. Both inhibitors significantly reduced NMDA-evoked adenosine accumulation. In contrast phosphatase 2B inhibitors did not block NMDA-evoked adenosine accumulation. These data suggest that NMDA-evoked adenosine accumulation is mediated by activation of phosphatase 1/2A. We have established previously that NMDA-mediated adenosine accumulation is associated with adenosine kinase inhibition. However, adenosine kinase is not a direct substrate for phosphatase 1/2A because inhibition of phosphatase 1/2A did not abolish NMDA-evoked adenosine kinase inhibition. Okadaic acid also had no effect on NO donor-evoked adenosine accumulation, which previously has been shown to be associated with adenosine kinase inhibition. Dephosphorylation of one or more proteins other than adenosine kinase as a consequence of NMDA receptor activation might play an important role in extracellular adenosine regulation, with important consequences for the regulation of excitatory synaptic transmission, plasticity, epileptogenesis, and excitotoxicity.

General enzymatic screens identify three new nucleotidases in Escherichia coli. Biochemical characterization of SurE, YfbR, and YjjG

To find proteins with nucleotidase activity in Escherichia coli, purified unknown proteins were screened for the presence of phosphatase activity using the general phosphatase substrate p-nitrophenyl phosphate. Proteins exhibiting catalytic activity were then assayed for nucleotidase activity against various nucleotides. These screens identified the presence of nucleotidase activity in three uncharacterized E. coli proteins, SurE, YfbR, and YjjG, that belong to different enzyme superfamilies: SurE-like family, HD domain family (YfbR), and haloacid dehalogenase (HAD)-like superfamily (YjjG). The phosphatase activity of these proteins had a neutral pH optimum (pH 7.0-8.0) and was strictly dependent on the presence of divalent metal cations (SurE: Mn(2+) > Co(2+) > Ni(2+) > Mg(2+); YfbR: Co(2+) > Mn(2+) > Cu(2+); YjjG: Mg(2+) > Mn(2+) > Co(2+)). Further biochemical characterization of SurE revealed that it has a broad substrate specificity and can dephosphorylate various ribo- and deoxyribonucleoside 5'-monophosphates and ribonucleoside 3'-monophosphates with highest affinity to 3'-AMP. SurE also hydrolyzed polyphosphate (exopolyphosphatase activity) with the preference for short-chain-length substrates (P(20-25)). YfbR was strictly specific to deoxyribonucleoside 5'-monophosphates, whereas YjjG showed narrow specificity to 5'-dTMP, 5'-dUMP, and 5'-UMP. The three enzymes also exhibited different sensitivities to inhibition by various nucleoside di- and triphosphates: YfbR was equally sensitive to both di- and triphosphates, SurE was inhibited only by triphosphates, and YjjG was insensitive to these effectors. The differences in their sensitivities to nucleotides and their varied substrate specificities suggest that these enzymes play unique functions in the intracellular nucleotide metabolism in E. coli.

Rabbit liver dCMP phosphohydrolase: a pyrimidine 5'-nucleotidase I-like enzyme in non-erythrocytic cells

A nucleotide phosphomonoesterase activity that preferably hydrolyzed dCMP was detected in rabbit liver and purified approximately 20-fold. The enzyme was similar in the catalytic and molecular properties to pyrimidine 5'-nucleotidase subclass I (P5N-I), which distributed specifically in vertebrate erythrocytes. In addition to liver, the activity was found in rabbit kidney, spleen, heart, intestine, but was not detected in any rat or chicken tissues tested. The rabbit enzyme protein reacted with antibodies against chicken P5N-I. Its pI was estimated to be approximately 5.3, and the enzyme was concluded to consist of single polypeptide of an approximately 38 kDa based on gel filtration and Western blot analysis. The partially purified enzyme preferentially hydrolyzes dCMP, UMP and CMP, K(m) values for these substrates are approximately 0.3 mM, the optimal pH is approximately 7, and the enzyme requires Mg(2+). This nucleotidase may contribute to the regulation of intracellular pyrimidine nucleotides in the rabbit.

Catabolism of pyrimidine nucleotides in the deep-sea tube worm Riftia pachyptila

The present study describes the distribution and properties of enzymes of the catabolic pathway of pyrimidine nucleotides in Riftia pachyptila, a tubeworm living around deep-sea hydrothermal vents and known to be involved in a highly specialized symbiotic association with a bacterium. The catabolic enzymes, 5'-nucleotidase, uridine phosphorylase, and uracil reductase, are present in all tissues of the worm, whereas none of these enzymatic activities were found in the symbiotic bacteria. The 5'-nucleotidase activity was particularly high in the trophosome, the symbiont-harboring tissue. These results suggest that the production of nucleosides in the trophosome may represent an alternative source of carbon and nitrogen for R. pachyptila, because these nucleosides can be delivered to other parts of the worm. This process would complement the source of carbon and nitrogen from organic metabolites provided by the bacterial assimilatory pathways. The localization of the enzymes participating in catabolism, 5'-nucleotidase and uridine phosphorylase, and of the enzymes involved in the biosynthesis of pyrimidine nucleotides, aspartate transcarbamylase and dihydroorotase, shows a non-homogeneous distribution of these enzymes in the trophosome. The catabolic enzymes 5'-nucleotidase and uridine phosphorylase activities increase from the center of the trophosome to its periphery. In contrast, the anabolic enzymes aspartate transcarbamylase and dihydroorotase activities decrease from the center toward the periphery of the trophosome. We propose a general scheme of anatomical and physiological organization of the metabolic pathways of the pyrimidine nucleotides in R. pachyptila and its bacterial endosymbiont.

Chicken erythrocyte pyrimidine 5'-nucleotidase: purification and characterization of the subclass I enzyme

Nucleotidase activities resembling subclass I and subclass II of human pyrimidine 5'-nucleotidases (P5N) were detected in chicken red blood cells (RBCs). In chicken RBCs from untreated controls, the activity of the subclass II enzyme was about one third of that of subclass I enzyme, whereas that ratio was approximately 5:1 in rat or human RBCs. The subclass I activity in chicken RBCs was increased 5- to 6-fold upon erythropoietic induction by phenylhydrazine administration, but the subclass II activity did not increase under these conditions. The subclass I enzyme was purified to near homogeneity. Its molecular mass was about 35 kDa as estimated by gel filtration and SDS-polyacrylamide gel electrophoresis. Its N-terminal 12 amino acids, PEFQKKTVHIKD, were also determined. The catalytic properties of the subclass I enzyme were very similar to those of the human enzyme with regard to substrate (preferential hydrolysis of CMP, dCMP, UMP), Km values, optimum pH, and metal ion requirements. Antibodies against chicken P5N subclass I were raised in rats. The chicken P5N-I as well as the rat P5N-I proteins could be detected by antibodies in Western blot analyses, but not the P5N-II proteins. These findings indicate that P5N subclass I may have an important function in chicken erythropoiesis.

Human high-Km 5'-nucleotidase effects of overexpression of the cloned cDNA in cultured human cells

5'-Nucleotidases participate, together with nucleoside kinases, in substrate cycles involved in the regulation of deoxyribonucleotide metabolism. Three major classes of nucleotidases are known, one on the plasma membrane and two in the cytosol. The two cytosolic classes have been named high-Km nucleotidases and 5'(3')-nucleotidases. Starting from two plasmids with partial sequences (Oka, J., Matsumoto, A., Hosokawa, Y. & Inoue, S. (1994) Biochem. Biophys. Res. Commun. 205, 917-922) we cloned the complete cDNA of the human high-Km nucleotidase into vectors suitable for transfection of Escherichia coli or mammalian cells. After transfection, E. coli overproduced large amounts of the enzyme. Most of the enzyme was present in inclusion bodies that also contained many partially degraded products of the protein. Part of the enzyme, corresponding to approximately 2% of the soluble proteins, was in a soluble active form. Stably transfected human 293 cells were obtained with a vector where the 3'-end of the nucleotidase coding sequence is linked to the 5'-end of the green fluorescent protein coding sequence. Several green clones overproduced both mRNA and fusion protein. Two clones with 10-fold higher enzyme activity were analyzed further. The nucleotidase activity of cell extracts showed the same substrate specificity and allosteric regulation as the high-Km enzyme. The growth rate of the two clones did not differ from the controls. The cells were not resistant to deoxyguanosine or deoxyadenosine, and did not show an increased ability to phosphorylate dideoxyinosine. Both ribonucleoside and deoxyribonucleoside triphosphate pools were decreased slightly, suggesting participation of the enzyme in their regulation.

Presence of two enzymes, different from the F1F0-ATPase, hydrolyzing nucleotides in human term placental mitochondria

The hydrolysis of ATP, ADP or GTP was characterized in mitochondria and submitochondrial particles since a tightly-bound ATPase associated with the inner mitochondrial membrane from the human placenta has been described. Submitochondrial particles, which are basically inner membranes, were used to define the location of this enzyme. Mitochondria treated with trypsin and specific inhibitors were also used. The oxygen consumption stimulated by ATP or ADP was 100% inhibited in intact mitochondria by low concentrations of oligomycin (0.5 microgram/mg) or venturicidine (0.1 microgram/mg), while the hydrolysis of ATP or ADP was insensitive to higher concentrations of these inhibitors but it was inhibited by vanadate. Oligomycin or venturicidine showed a different inhibition pattern in intact mitochondria in relation to the hydrolysis of ATP, ADP or GTP. When submitochondrial particles were isolated from mitochondria incubated with oligomycin or venturicidine, no further inhibition of the nucleotide hydrolysis was observed, contrasting with the partial inhibition observed in the control. By incubating the placental mitochondria with trypsin, a large fraction of the hydrolysis of nucleotides was eliminated. In submitochondrial particles obtained from mitochondria treated with trypsin or trypsin plus oligomycin, the hydrolysis of ATP was 100% sensitive to oligomycin at low concentrations, resembling the oxygen consumption; however, this preparation still showed some ADP hydrolysis. Native gel electrophoresis showed two bands hydrolyzing ADP, suggesting at least two enzymes involved in the hydrolysis of nucleotides, besides the F1F0-ATPase. It is concluded that human placental mitochondria possesses ADPase and ATP-diphosphohydrolase activities (247).

Biochemical properties of 5'-nucleotidase from mouse skeletal muscle

Ecto-5'-nucleotidase (eNT) from mouse muscle has been purified after extraction with detergent followed by chromatography on concanavalin A- and AMP-Sepharose. Three fractions were recovered: UF was NT non-retained in immobilised AMP; F-I was bound enzyme eluted with beta-glycerophosphate, and F-II was bound NT released with AMP. eNT was 80000-fold purified in F-II, this fraction showing proteins of 74, 68 and 51 kDa after immunoblotting. NT in UF migrated at 6.7S after centrifugation in sucrose gradients with Triton X-100, the peak being split into two of 6.7S and 4.4S in gradients with Brij 96. Ecto-NT in F-I or F-II migrated at 5.8S in Triton X-100-, or 4.4S in Brij 96-containing gradients. The hydrodynamic behaviour, concentration in Triton X-114, binding to phenyl-agarose, and sensitivity to phosphatidylinositol-specific phospholipase C revealed that enzyme forms in F-I or F-II were amphiphilic dimers with linked phosphatidylinositol residues, whilst most of NT forms in UF were hydrophilic dimers. A zinc/protein molar ratio of 2.2 was determined for eNT in F-II. NT activity was decreased in assays made in imidazole buffer, and was partly restored with 10 microM Zn2+ or 100 microM Mn2+. In assays with Tris buffer, NT showed a Km for AMP of 12 microM, and was competitively inhibited by ATP or ADP.

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.

Developmental disorder associated with increased cellular nucleotidase activity

Four unrelated patients are described with a syndrome that included developmental delay, seizures, ataxia, recurrent infections, severe language deficit, and an unusual behavioral phenotype characterized by hyperactivity, short attention span, and poor social interaction. These manifestations appeared within the first few years of life. Each patient displayed abnormalities on EEG. No unusual metabolites were found in plasma or urine, and metabolic testing was normal except for persistent hypouricosuria. Investigation of purine and pyrimidine metabolism in cultured fibroblasts derived from these patients showed normal incorporation of purine bases into nucleotides but decreased incorporation of uridine. De novo synthesis of purines and cellular phosphoribosyl pyrophosphate content also were moderately decreased. The distribution of incorporated purines and pyrimidines did not reveal a pattern suggestive of a deficient enzyme activity. Assay of individual enzymes in fibroblast lysates showed no deficiencies. However, the activity of cytosolic 5'-nucleotidase was elevated 6- to 10-fold. Based on the possibility that the observed increased catabolic activity and decreased pyrimidine salvage might be causing a deficiency of pyrimidine nucleotides, the patients were treated with oral pyrimidine nucleoside or nucleotide compounds. All patients showed remarkable improvement in speech and behavior as well as decreased seizure activity and frequency of infections. A double-blind placebo trial was undertaken to ascertain the efficacy of this supplementation regimen. Upon replacement of the supplements with placebo, all patients showed rapid regression to their pretreatment states. These observations suggest that increased nucleotide catabolism is related to the symptoms of these patients, and that the effects of this increased catabolism are reversed by administration of uridine.

Effects of adenine nucleotides on low Km 5' nucleotidase from human seminal plasma

1. Low Km 5' nucleotidase purified from human seminal plasma has been used in this study to investigate the response of the enzyme ot adenine nucleoside di- and triphosphates in the presence of AMP and IMP as substrates. 2. In the presence of AMP, the addition of 0.5 mM ATP to the enzyme Mg-free results into the highest Vmax/Km ratio value and other experimental combinations of effectors tested cause variation of the kinetic parameters of the enzyme, indicating a control of AMP dephosphorylation by adenine nucleotides. 3. In the presence of IMP, ATP and ADP activate the enzyme but the response to various experimental combinations of effectors shows no significant difference in the kinetic properties of the enzyme, indicating a different control of the dephosphorylation of IMP.

Subcellular localization and properties of adenosine diphosphatase in human placenta

It was found that mitochondria from human placenta exhibited an ADPase activity with the following characteristics. The enzyme responsible for this activity was associated with the inner mitochondrial membrane. It was not released by treatment of the submitochondrial particles with solutions of high ionic strength. Maximal ADP hydrolysis was reached at pH 8. Specific inhibitors for alkaline phosphatase (L-phenylalanine), myokinase (P1,P5-di(adenosine-5')pentaphosphate), or 5'-nucleotidase (concanavalin A) did not decrease ADP hydrolysis. ATP synthesis from ADP by myokinase was about 13 nmol/mg/min, whereas ADP hydrolysis reached values around 500 to 550 nmol/mg/min, indicating that a myokinase-H+ATPase combination could not account for the observed rates of ADP hydrolysis. The activity was stimulated by Mg2+, but high concentrations of this cation produced inhibition. High ADP concentrations did not inhibit ADPase activity. Kinetic measurements of the activity in the submitochondrial particles showed that the true substrate was ADP-Mg. The kinetic studies showed V(app) values of 476 and 270 nmol/mg/min, and Kmapp values of 416 and 8.7 microM.

A novel human phosphotransferase highly specific for adenosine

A novel nucleoside phosphotransferase, referred to as adenosine phosphotransferase (Ado Ptase), was partially purified 1230-fold from human placenta. This enzyme differed from other known nucleoside phosphotransferases in its substrate specificity. Using AMP as the phosphate donor, it readily phosphorylated Ado. Changes in the sugar moiety were tolerated. dAdo and ddAdo were phosphate acceptors and dAMP was a donor. No other nucleotide or nucleoside common in nature displayed appreciable activity as donor or acceptor substrate, respectively. In the absence of nucleoside, the enzyme catalyzed the hydrolysis of AMP, typical of other nucleoside phosphotransferases. However, in the presence of Ado, little, if any, hydrolysis occurred. Ado Ptase had an absolute requirement for a metal cation, with Mg2+ and, to a lesser extent, Mn2+ fulfilling this requisite. The apparent Km for Ado was 0.2 mM. However, the donor AMP displayed cooperativity in both transfer and hydrolytic reactions. This cooperativity was eliminated by nucleotides, 2,3-diphosphoglycerate, and inorganic phosphate. ADP and 2,3-diphosphoglycerate were especially potent. In the presence of these effectors, the apparent Km for AMP was 3.0 mM in the transfer reaction and 4.0 mM in the hydrolytic reaction. Kinetic data suggest that there are two nucleotide binding sites on Ado Ptase, one for the donor, the other for an effector. AMP appeared to bind to both sites. Although this novel enzyme might play a role in the anabolism of nucleoside analogues, the normal physiological role of this nucleoside phosphotransferase is not understood.

Purification and partial characterization of the soluble low Km 5'-nucleotidase from human seminal plasma

Soluble low Km 5'-nucleotidase from human seminal plasma has been purified to homogeneity by one affinity and two gel-filtration chromatographic steps. The pure enzyme had a specific activity of 2000 nmol min-1 mg-1. Sodium dodecyl sulphate polyacrylamide gel electrophoresis of purified low Km 5'-nucleotidase revealed a single polypeptide band of 40 +/- 7 kDa and a tetrameric structure of 160 +/- 10 kDa has been proposed for the native enzyme. The kinetic properties of low Km 5'-nucleotidase have been determined and rather unique characteristics have been found for this soluble low Km 5'-nucleotidase: the substrate efficiency was slightly higher for IMP with an optimum pH at 7.5; the enzyme showed an absolute dependence on Mg2+ ions. Ca2+ could replace Mg2+ ions for activity while other divalent cations could not substitute for Mg2+; the enzymes were equally activated by ATP and ADP up to 0.1 mM concentrations. At higher concentrations up to 1 mM, ADP was still an activator while ATP caused a gradual decrease of activation to the native activity. This effect could not be related to the Mg-ATP = complexes since the enzymic preparation Mg(2+)-free still showed the same biphasic pattern of activation.

Synovial fluid 5'-nucleotidase activity. Relationship to other purine catabolic enzymes and to arthropathies associated with calcium crystal deposition

We measured 5'-nucleotidase (5NT) activity in synovial fluid from 159 patients with various diagnoses. The activity of 5NT was compared with activities of nucleotide pyrophosphohydrolase, alkaline and neutral phosphatases, and adenosine deaminase, in the same samples. Higher levels of 5NT activity occurred in synovial fluid from osteoarthritic joints than from joints of patients with gout, pseudogout, or rheumatoid arthritis. The highest levels of 5NT activity were found in synovial fluid from patients with Milwaukee shoulder syndrome and from osteoarthritis patients in whom deposition of calcium-containing crystals was also present.

Deoxyribonucleotide metabolism in hydroxyurea-resistant V79 hamster cells

V79 hamster cells were made resistant against hydroxyurea by continuous culture at stepwise increasing drug concentrations. Two cell lines were cloned, resistant to 0.4 mM (V79/H0.4) and 4 mM (V79/H4) hydroxyurea, with a fivefold and a 20-fold increase in soluble ribonucleotide reductase activity. We investigated how the increased amount of enzyme affected the in situ activity of ribonucleotide reductase and deoxyribonucleotide metabolism, in particular substrate cycles between pyrimidine deoxyribonucleosides and their 5'-phosphates. The in situ activity of the reductase was only moderately elevated (1.3-fold in V79/H4 cells). In the fully resistant line, the steady-state level of dATP was increased fourfold, and that of dTTP twofold. These nucleotides are negative allosteric effectors of the reductase and we propose that the increased pools inhibit the enzyme and thereby maintain the in situ activity of the reductase at only a slightly increased level. The surplus deoxyribonucleotides was excreted from the cells as thymidine and deoxycytidine via substrate cycles. The data support and extend our previous model for the regulation of deoxyribonucleotide synthesis via the allosteric properties of ribonucleotide reductase and substrate cycles that link salvage and degradation of deoxyribonucleotides.

A comparison of the properties of 5'-nucleotidase purified from the cytosolic and synaptic plasma membrane fractions of rat forebrain

1. 5'-Nucleotidase was purified 1247-fold from the post-microsomal supernatant (I) and 3862-fold from the synaptic plasma membrane (II) of rat brain homogenates. 2. The apparent molecular masses of I and II were 131 and 72 kDa respectively by polyacrylamide gel electrophoresis in the presence of sodium dodecylsulphate and 268 and 286 kDa respectively by Sephacryl S-300 chromatography. 3. The activities of both I and II were strongly inhibited by concanavalin A but were affected differently by digestion with glycosidases. for II, these were 0.083 and 0.056 mM respectively. 5. Activities of both I and II were strongly inhibited by ATP and ADP.

Soluble 5'-nucleotidase activities in rat brain

5'-Nucleotidase activity was assayed in 105,000-g supernatants from rat brain by following conversion of [3H]AMP into adenosine. The effect of ATP on this process was complex and suggested the presence of at least two soluble 5'-nucleotidase activities: one inhibited by ATP and another activated by ATP. The relative proportions of these activities differed considerably among brain regions. Activity changes induced by hypothyroidism also suggested that these activities may be regulated independently. These findings may have consequences for the regional regulation of adenosine formation in the brain.

Nucleotidase activities in soluble and membrane fractions of three different mammalian cell lines

Soluble cytoplasmic and membrane fractions were prepared from three cultured mammalian cell lines: 3T3 mouse fibroblasts, V79 hamster lung cells, and human "Cherry" B-lymphoblastoid cells. By using relatively specific nucleotidase assays, together with a phosphotransferase assay, the activities of three different enzymes (low-Km nucleotidase, high-Km nucleotidase, and 5'(3')-nucleotidase) capable of dephosphorylating deoxyribonucleoside 5'-monophosphates were determined in these fractions. The three nucleotidases exist simultaneously in all cell lines, but their relative amounts showed large variations. The 5'(3')-nucleotidase dominated Cherry and 3T3 cells, while in V79 cells equal amounts of this enzyme and the high-Km nucleotidase were recovered. In the membrane fractions, the low-Km nucleotidase was the predominant enzyme. We found no evidence for cell-cycle control of any nucleotidase. We postulated earlier that substrate cycles, involving 5'-nucleotidases and deoxyribonucleoside kinases, provide a mechanism for the regulation of deoxyribonucleotide pools. We suggest that both the low-Km nucleotidase and the 5'(3)-nucleotidase are candidate enzymes for such cycles.

Nucleotide ectoenzyme activities of human and Chinese hamster fibroblasts in tissue culture

We have previously assigned human ecto-5'-nucleotidase (NT) to chromosome 6 on the basis of conversion of exogenously supplied [14C]AMP to adenosine by whole cells of human and Chinese hamster hybrids carrying chromosome 6. In this paper we demonstrate that the activity on human MRC-5 fibroblasts is typical of previously described and purified ecto-5'-nucleotidases. In contrast to MRC-5 cells, Chinese hamster V79A2 cells weakly express an AMPase activity that is not NT. The cytosolic form of NT in human and hybrid fibroblasts is similar to the ectoenzyme in substrate specificity. Hybrids that lack chromosome 6 express neither the ecto- nor the cytosolic enzyme, suggesting that both forms may be coded by the same gene on chromosome 6. Ecto-ATPase, ecto-ADPase, and ecto-ADP kinase activities are each expressed at similar levels in MRC-5 and V79A2. The ATPase, ADPase and NT activities of MRC-5 cells act sequentially to generate adenosine. A similar cascade acts on V79A2 cells but the lack of NT causes the accumulation of AMP.

AMP and IMP dephosphorylation by soluble high- and low-Km 5'-nucleotidases

Three distinct 5'-phosphomonoesterase activities were isolated from soluble fractions of human placenta, cultured human T and B lymphoblasts, and rat liver using 5'-AMP-sepharose 4B affinity chromatography. We define these activities as "low-Km" 5'-nucleotidase, "high-Km" 5'-nucleotidase, and nonspecific phosphatase. High-Km 5'-nucleotidase was eluted with 0.5 M NaCl, low-Km 5'-nucleotidase was eluted with 10 mM ADP, and nonspecific phosphatase was not retained on the column. We have found significant variability in the relative content of high- to low-Km activities in the tissues studied with the ratios ranging from 5.5 to 264. The properties were studied after further purification. The molecular mass of the low-Km enzymes ranged from 72.5 to 209 kDa, optimum pH ranged from 7.4 to 9.0, Km for AMP ranged from 7 to 15 microM, and Km for IMP ranged from 10 to 26 microM. The molecular mass of the high-Km enzymes ranged from 182 to 210 kDa, pH optimum was at 6.5, Km for AMP ranged from 3.0 to 9.4 mM, and the Km for IMP ranged from 0.3 to 0.5 mM. The data indicate that the soluble low- and high-Km 5'-nucleotidase coexist in the mammalian cells and tissues studied. These observations suggest a complex system for the regulation of nucleoside 5'-monophosphate dephosphorylation.

Assignment of ecto-5'-nucleotidase to human chromosome 6

Ecto-5'-nucleotidase activity (5NT) was measured on whole cells of 26 human x Chinese hamster hybrids. Concordance analysis showed 100% correlation between enzyme activity and inheritance of human chromosome 6. This observation was confirmed by a segregation analysis in which cells of a hybrid containing chromosome 6 were stained by indirect immunofluorescence for HLA Class 1 antigen and sorted by a fluorescence-activated cell sorter (FACS). Cells in the HLA- compartment were cloned and expression of HLA and 5NT was determined. Of nine clones, three were HLA-, 5NT- and six were HLA+, 5NT+, supporting the linkage of 5NT to chromosome 6.

Purine and pyrimidine metabolism in human muscle and cultured muscle cells

Using radiochemical methods, we determined the activities of various enzymes of purine and pyrimidine metabolism in homogenates of human skeletal muscle and of cultured human muscle cells. Results show a large discrepancy between the enzyme activities in muscle and cultured cells. With regard to purine metabolism, adenylate (AMP) deaminase activity was only 1-3% in cultured cells compared to that in muscle, whereas the activity of adenosine deaminase, purine-nucleoside phosphorylase, adenosine kinase, adenine phosphoribosyltransferase and hypoxanthine phosphoribosyltransferase was 7-15-fold higher in the cultured cells. The enzymes of pyrimidine metabolism, orotate phosphoribosyltransferase, orotidine 5'-monophosphate decarboxylase and uridine kinase showed activity of 100-200-fold higher in cultured cells than in adult muscle. The differences in enzyme activity are probably related to the low differentiation stage and the absence of contractile activity in the cultured muscle cells. Care must be taken when using these cells as a model for studying purine and pyrimidine metabolism of adult myofibers.

A reappraisal of the subunit molecular mass of chicken liver cytosol 5'-nucleotidase

The subunit molecular mass of chicken liver cytosol 5'-nucleotidase was earlier reported to be 51 kDa upon sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (Naito, Y. and Tsushima, K. (1976) Biochim. Biophys. Acta 438, 159-168). By immunoblot analyses after SDS-gel electrophoresis, however, a fraction from the liver homogenized in the presence of leupeptin showed multiple bands with molecular masses around 57 kDa, and SDS-extracted proteins directly from the liver exhibited a single 70 kDa component. These results indicate that the 51 kDa peptide observed in the cytosol 5'-nucleotidase preparation might, in fact, be a proteolytic artifact.

Purification of bovine liver cytosolic 5'-nucleotidase. Kinetic and structural studies as compared to the membrane isoenzyme

Cytosolic 5'-nucleotidase from bovine liver has been purified to homogeneity. Two affinity chromatographies on concanavalin A and 5'AMP-Sepharose columns result in a 12,000-fold purification. The sequential elution of glycoproteins from the concanavalin-A-Sepharose column with methyl alpha-D-glucoside and methyl alpha-D-mannoside greatly increases the degree of purification of the enzyme. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate shows two subunits having apparent molecular masses of 65 kDa and 57 kDa respectively, while only one band at 70 kDa is observed in the case of the membrane-bound 5'-nucleotidase. Both the Stokes radii, measured by gel exclusion HPLC, and the sedimentation coefficient, determined by density gradient ultracentrifugation, indicate that the cytosolic enzyme is a heterodimer of about 130 kDa. This contrasts with the membrane-bound 5'-nucleotidase which is a homodimer of 140 kDa. Moreover, the antibodies raised against the membrane 5'-nucleotidase inhibited the cytosolic form indicating that a common antigenic determinant(s) exists between the two isoenzymes. However, structural differences are revealed by immunoblotting. In the same way, the effect of lectins suggests that differences in the structure of the carbohydrate chains exist between the two isoenzymes. The purified cytosolic enzyme has lower affinity for the nucleotides than does the membrane enzyme. In addition, while ADP, [alpha,beta-CH2]ADP and ATP were strong competitive inhibitors of the membrane enzyme, ADP and ATP activate the cytosolic form and [alpha,beta-CH2]ADP has no effect. Moreover, two pH optima at 7.5 and 9.5 are observed in the cytosolic enzyme while only one at 7.5 occurred in the membrane form. Finally the exogenous cations, MgCl2 and MnCl2, are necessary for the maximal activity of the cytosolic but not of the membrane 5'-nucleotidase. All these observations indicate that the two isoenzymes are different.

Enzymes of nucleotide metabolism: the significance of subunit size and polymer size for biological function and regulatory properties

The 72 enzymes in nucleotide metabolism, from all sources, have a distribution of subunit sizes similar to those from other surveys: an average subunit Mr of 47,900, and a median size of 33,300. The same enzyme, from whatever source, usually has the same subunit size (there are exceptions); enzymes having a similar activity (e.g., kinases, deaminases) usually have a similar subunit size. Most simple enzymes in all EC classes (except class 6, ligases/synthetases) have subunit sizes of less than 30,000. Since structural domains defined in proteins tend to be in the Mr range of 5,000 to 30,000, it may be that most simple enzymes are formed as single domains. Multifunctional proteins and ligases have subunits generally much larger than Mr 40,000. Analyses of several well-characterized ligases suggest that they also have two or more distinct catalytic sites, and that ligases therefore are also multifunctional proteins, containing two or more domains. Cooperative kinetics and evidence for allosteric regulation are much more frequently associated with larger enzymes: such complex functions are associated with only 19% of enzymes having a subunit Mr less than or equal to 29,000, and with 86% of all enzymes having a subunit Mr greater than 50,000. In general, larger enzymes have more functions. Only 20% of these enzymes appear to be monomers; the rest are homopolymers and rarely are they heteropolymers. Evidence for the reversible dissociation of homopolymers has been found for 15% of the enzymes. Such changes in quaternary structure are usually mediated by appropriate physiological effectors, and this may serve as a mechanism for their regulation between active and less active forms. There is considerable structural organization of the various pathways: 19 enzymes are found in various multifunctional proteins, and 13 enzymes are found in different types of multienzyme complexes.

Metabolism of nicotinamide mononucleotide in beef liver

Nicotinamide mononucleotide (NMN) is not only an intermediate for the biosynthesis but also a degradation product of pyridine cofactors in animal tissues. Among the animal tissues tested, the highest NMN catabolizing activity was detected in beef liver (5.6 mumol/min/g tissue). This activity was 16 times higher than the NAD hydrolysis catalyzed by the liver NAD glycohydrolase. As a result of enzymatic analysis of the NMN splitting process, two types of enzyme responsible for this catabolism were partially purified and identified as a membrane-bound 5'-nucleotidase and a cytoplasmic nicotinamide riboside (NR) phosphorylase. No specific NMN glycohydrolase could be found in contrast to results observed in bacterial systems. The 5'-nucleotidase and NR phosphorylase constitute an obligatory process of the pyridine nucleotide cycle. The dephosphorylation and phosphorolysis catalyzed suggest that these enzymes could serve as an important mechanism for salvaging the ribose and nicotinamide moieties of NMN and pyridine nucleotides in the cell and a process that could be regulated at the mononucleotide level by this "NMN cycle" rather than by a NAD glycohydrolase cycle. In addition to the enzymatic properties of these enzymes, a regulatory mechanism by nucleotides such as ATP was also demonstrated.