Nucleoside phosphotransferase activity of human colon carcinoma cytosolic 5'-nucleotidase

Label-free liquid chromatography mass spectrometry analysis of changes in broiler liver proteins under transport stress

Transportation duration and distance are significant concerns for animal welfare, particularly in the poultry industry. However, limited proteomic studies have investigated the impact of transport duration on poultry welfare. In this study, mass spectrometry based bottom up proteomics was employed to sensitively and impartially profile the liver tissue proteome of chickens, addressing the issue of animal stress and welfare in response to transportation before slaughter. The liver exudates obtained from Ross 508 chickens exposed to either short or long road transportation underwent quantitative label-free LC-MS proteomic profiling. This method identified a total of 1,368 proteins, among which 35 were found to be significantly different (p < 0.05) and capable of distinguishing between short and long road transportation conditions. Specifically, 23 proteins exhibited up-regulation in the non stressed group, while 12 proteins showed up-regulation in the stressed group. The proteins identified in this pilot study encompassed those linked to homeostasis and cellular energetic balance, including heat shock proteins and the 5'-nucleotidase domain-containing family. These results contribute to a deeper understanding of the proteome in broiler liver tissues, shedding light on poultry adaptability to transport stress. Furthermore, the identified proteins present potential as biomarkers, suggesting promising approaches to enhance poultry care and management within the industry.

Screening the Thermotoga maritima genome for new wide-spectrum nucleoside and nucleotide kinases

Enzymes from thermophilic organisms are interesting biocatalysts for a wide variety of applications in organic synthesis, biotechnology and molecular biology. Next to an increased stability at elevated temperatures, they were described to show a wider substrate spectrum than their mesophilic counterparts. To identify thermostable biocatalysts for the synthesis of nucleotide analogs, we performed a database search on the carbohydrate and nucleotide metabolism of T. maritima. After expression and purification of 13 enzyme candidates involved in nucleotide synthesis, these enzymes were screened for their substrate scope. We found that the synthesis of 2'-deoxynucleoside 5'-monophosphates (dNMPs) and uridine 5'-monophosphate from nucleosides was catalyzed by the already known wide-spectrum thymidine kinase (TK) and the ribokinase. In contrast, no NMP-forming activity was detected for adenosine-specific kinase, uridine kinase or nucleotidase. The NMP kinases (NMPKs) and the pyruvate-phosphate-dikinase of T. maritima exhibited a rather specific substrate spectrum for the phosphorylation of NMPs, while pyruvate kinase, acetate kinase and three of the NMPKs showed a broad substrate scope with (2'-deoxy)nucleoside 5'-diphosphates as substrates. Based on these promising results, TmNMPKs were applied in enzymatic cascade reactions for nucleoside 5'-triphosphate synthesis using four modified pyrimidine nucleosides and four purine NMPs as substrates, and we determined that base- and sugar-modified substrates were accepted. In summary, besides the already reported TmTK, NMPKs of T. maritima were identified to be interesting enzyme candidates for the enzymatic production of modified nucleotides.

Cytosolic 5'-Nucleotidase II Silencing in Lung Tumor Cells Regulates Metabolism through Activation of the p53/AMPK Signaling Pathway

Cytosolic 5′-nucleotidase II (cN-II) is an allosteric catabolic enzyme that hydrolyzes IMP, GMP, and AMP. The enzyme can assume at least two different structures, being the more active conformation stabilized by ATP and the less active by inorganic phosphate. Therefore, the variation in ATP concentration can control both structure and activity of cN-II. In this paper, using a capillary electrophoresis technique, we demonstrated that a partial silencing of cN-II in a pulmonary carcinoma cell line (NCI-H292) is accompanied by a decrease in adenylate pool, without affecting the energy charge. We also found that cN-II silencing decreased proliferation and increased oxidative metabolism, as indicated by the decreased production of lactate. These effects, as demonstrated by Western blotting, appear to be mediated by both p53 and AMP-activated protein kinase, as most of them are prevented by pifithrin-α, a known p53 inhibitor. These results are in line with our previous observations of a shift towards a more oxidative and less proliferative phenotype of tumoral cells with a low expression of cN-II, thus supporting the search for specific inhibitors of this enzyme as a therapeutic tool for the treatment of tumors.

Cytosolic 5'-Nucleotidase II Is a Sensor of Energy Charge and Oxidative Stress: A Possible Function as Metabolic Regulator

Cytosolic 5'-nucleotidase II (NT5C2) is a highly regulated enzyme involved in the maintenance of intracellular purine and the pyrimidine compound pool. It dephosphorylates mainly IMP and GMP but is also active on AMP. This enzyme is highly expressed in tumors, and its activity correlates with a high rate of proliferation. In this paper, we show that the recombinant purified NT5C2, in the presence of a physiological concentration of the inhibitor inorganic phosphate, is very sensitive to changes in the adenylate energy charge, especially from 0.4 to 0.9. The enzyme appears to be very sensitive to pro-oxidant conditions; in this regard, the possible involvement of a disulphide bridge (C175-C547) was investigated by using a C547A mutant NT5C2. Two cultured cell models were used to further assess the sensitivity of the enzyme to oxidative stress conditions. NT5C2, differently from other enzyme activities, was inactivated and not rescued by dithiothreitol in a astrocytoma cell line (ADF) incubated with hydrogen peroxide. The incubation of a human lung carcinoma cell line (A549) with 2-deoxyglucose lowered the cell energy charge and impaired the interaction of NT5C2 with the ice protease-activating factor (IPAF), a protein involved in innate immunity and inflammation.

ADNP differentially interact with genes/proteins in correlation with aging: a novel marker for muscle aging

Activity-dependent neuroprotective protein (ADNP) is essential for embryonic development with ADNP mutations leading to syndromic autism, coupled with intellectual disabilities and motor developmental delays. Here, mining human muscle gene-expression databases, we have investigated the association of ADNP transcripts with muscle aging. We discovered increased ADNP and its paralogue ADNP2 expression in the vastus lateralis muscle of aged compared to young subjects, as well as altered expression of the ADNP and the ADNP2 genes in bicep brachii muscle of elderly people, in a sex-dependent manner. Prolonged exercise resulted in decreased ADNP expression, and increased ADNP2 expression in an age-dependent manner in the vastus lateralis muscle. ADNP expression level was further correlated with 49 genes showing age-dependent changes in muscle transcript expression. A high degree of correlation with ADNP was discovered for 24 genes with the leading gene/protein being NMNAT1 (nicotinamide nucleotide adenylyl transferase 1). Looking at correlations differentiating the young and the old muscles and comparing protein interactions revealed an association of ADNP with the cell division cycle 5-like protein (CDC5L), and an aging-muscle-related interactive pathway in the vastus lateralis. In the bicep brachii, very high correlation was detected with genes associated with immune functions as well as mitochondrial structure and function among others. Taken together, the results suggest a direct association of ADNP with muscle strength and implicate ADNP fortification in the protection against age-associated muscle wasting.

Genetics and mechanisms of NT5C2-driven chemotherapy resistance in relapsed ALL

Mutations in the cytosolic 5' nucleotidase II (NT5C2) gene drive resistance to thiopurine chemotherapy in relapsed acute lymphoblastic leukemia (ALL). Mechanistically, NT5C2 mutant proteins have increased nucleotidase activity as a result of altered activating and autoregulatory switch-off mechanisms. Leukemias with NT5C2 mutations are chemoresistant to 6-mercaptopurine yet show impaired proliferation and self-renewal. Direct targeting of NT5C2 or inhibition of compensatory pathways active in NT5C2 mutant cells may antagonize the emergence of NT5C2 mutant clones driving resistance and relapse in ALL.

Cytosolic 5'-Nucleotidase II Silencing in a Human Lung Carcinoma Cell Line Opposes Cancer Phenotype with a Concomitant Increase in p53 Phosphorylation

Purine homeostasis is maintained by a purine cycle in which the regulated member is a cytosolic 5'-nucleotidase II (cN-II) hydrolyzing IMP and GMP. Its expression is particularly high in proliferating cells, indeed high cN-II activity or expression in hematological malignancy has been associated to poor prognosis and chemoresistance. Therefore, a strong interest has grown in developing cN-II inhibitors, as potential drugs alone or in combination with other compounds. As a model to study the effect of cN-II inhibition we utilized a lung carcinoma cell line (A549) in which the enzyme was partially silenced and its low activity conformation was stabilized through incubation with 2-deoxyglucose. We measured nucleotide content, reduced glutathione, activities of enzymes involved in glycolysis and Krebs cycle, protein synthesis, mitochondrial function, cellular proliferation, migration and viability. Our results demonstrate that high cN-II expression is associated with a glycolytic, highly proliferating phenotype, while silencing causes a reduction of proliferation, protein synthesis and migration ability, and an increase of oxidative performances. Similar results were obtained in a human astrocytoma cell line. Moreover, we demonstrate that cN-II silencing is concomitant with p53 phosphorylation, suggesting a possible involvement of this pathway in mediating some of cN-II roles in cancer cell biology.

Structure and Mechanisms of NT5C2 Mutations Driving Thiopurine Resistance in Relapsed Lymphoblastic Leukemia

Activating mutations in the cytosolic 5'-nucleotidase II gene NT5C2 drive resistance to 6-mercaptopurine in acute lymphoblastic leukemia. Here we demonstrate that constitutively active NT5C2 mutations K359Q and L375F reconfigure the catalytic center for substrate access and catalysis in the absence of allosteric activator. In contrast, most relapse-associated mutations, which involve the arm segment and residues along the surface of the inter-monomeric cavity, disrupt a built-in switch-off mechanism responsible for turning off NT5C2. In addition, we show that the C-terminal acidic tail lost in the Q523X mutation functions to restrain NT5C2 activation. These results uncover dynamic mechanisms of enzyme regulation targeted by chemotherapy resistance-driving NT5C2 mutations, with important implications for the development of NT5C2 inhibitor therapies.

Interplay between adenylate metabolizing enzymes and AMP-activated protein kinase

Purine nucleotides are involved in a variety of cellular functions, such as energy storage and transfer, and signalling, in addition to being the precursors of nucleic acids and cofactors of many biochemical reactions. They can be generated through two separate pathways, the de novo biosynthesis pathway and the salvage pathway. De novo purine biosynthesis leads to the formation of IMP, from which the adenylate and guanylate pools are generated by two additional steps. The salvage pathways utilize hypoxanthine, guanine and adenine to generate the corresponding mononucleotides. Despite several decades of research on the subject, new and surprising findings on purine metabolism are constantly being reported, and some aspects still need to be elucidated. Recently, purine biosynthesis has been linked to the metabolic pathways regulated by AMP-activated protein kinase (AMPK). AMPK is the master regulator of cellular energy homeostasis, and its activity depends on the AMP : ATP ratio. The cellular energy status and AMPK activation are connected by AMP, an allosteric activator of AMPK. Hence, an indirect strategy to affect AMPK activity would be to target the pathways that generate AMP in the cell. Herein, we report an up-to-date review of the interplay between AMPK and adenylate metabolizing enzymes. Some aspects of inborn errors of purine metabolism are also discussed.

[NT5C2 expression in children with acute leukemia and its clinical significance]

OBJECTIVE

To investigate the expression level and analyze the clinical significance of NT5C2, which is an nucleoside analogues metabolism related gene, in children with acute leukemia (AL).

METHODS

Real-time PCR and immunohistochemistry were presented to detect the level of NT5C2 mRNA and its protein product cN- Ⅱ in bone marrow samples of 63 patients initially diagnosed with AL, 15 patients who achieved complete remission, 7 patients who relapsed and 16 non- hematologic malignancie controls. The expression of NT5C2 mRNA in different groups of AL and its relevance with clinical indicators were analyzed.

RESULTS

①The expression of NT5C2 mRNA in newly diagnosed B-ALL, TALL, AML and controls were 1.16 (0.89-2.25, 0.96 (0.74-1.25, 1.66 (0.84-3.15) and 0.88 (0.61-1.21), respectively. NT5C2 mRNA expression in AML (P<0.01) and B-ALL (P<0.05) cases were higher than that in controls; NT5C2 mRNA expression in T- ALL and in controls showed no significant difference (P>0.05). Changes of NT5C2 mRNA level were observed between preliminary diagnosis and complete remission in 15 patients. NT5C2 mRNA levels were significantly decreased in complete remission stage than that in newly diagnosis AL (P<0.01). NT5C2 mRNA levels of relapsed-refractory group were higher than that of complete remission group and controls (P<0.01). ② Immunohistochemical staining results revealed that NT5C2 protein levels were consistent with the trend of mRNA levels. ③NT5C2 mRNA levels in AML (r=0.434) and T-ALL (r=0.389) were positively correlated with risk classification (P<0.05). ④ During chemotherapy of patients with AML, the NR rate of bone marrow in NT5C2 high expression group was higher than that of low expression group after 9 days induction chemotherapy (35.2% vs 0) and before consolidation therapy (25.0% vs 0); The positive rate of minimal-residual disease (36.4% vs 14.3%) and relapse rate of AL (38.5% vs 28.6%) were increased in NT5C2 high expressed patients than that in low expressed patients, but all the differences were insignificant (P>0.05).

CONCLUSION

High expression of NT5C2 was found to be a related risk factor of AL children with unfavourable prognosis. NT5C2 promises a new target for guiding individualized chemotherapy and evaluating the prognosis of childhood acute leukemia and monitoring recurrence.

IMP-GMP specific cytosolic 5'-nucleotidase regulates nucleotide pool and prodrug metabolism

BACKGROUND

Type II cytosolic 5'-nucleotidase (cN-II) catalyzes the hydrolysis of purine and, to some extent, of pyrimidine monophosphates. Recently, a number of papers demonstrated the involvement of cN-II in the mechanisms of resistance to antitumor drugs such as cytarabine, gemcitabine and fludarabine. Furthermore, cN-II is involved in drug resistance in patients affected by hematological malignancies influencing the clinical outcome. Although the implication of cN-II expression and/or activity appears to be correlated with drug resistance and poor prognosis, the molecular mechanism by which cN-II mediates drug resistance is still unknown.

METHODS

HEK 293 cells carrying an expression vector coding for cN-II linked to green fluorescent protein (GFP) and a control vector without cN-II were utilized. A highly sensitive capillary electrophoresis method was applied for nucleotide pool determination and cytotoxicity exerted by drugs was determined with 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay.

RESULTS

Over-expression of cN-II causes a drop of nucleoside triphosphate concentration and a general disturbance of nucleotide pool. Over-expressing cells were resistant to fludarabine, gemcitabine and cytarabine independently of cN-II ability to hydrolyze their monophosphates.

CONCLUSIONS

An increase of cN-II expression is sufficient to cause both a general disturbance of nucleotide pool and an increase of half maximal inhibitory concentration (IC50) of the drugs. Since the monophosphates of cytarabine and gemcitabine are not substrates of cN-II, the protection observed cannot be directly ascribed to drug inactivation.

GENERAL SIGNIFICANCE

Our results indicate that cN-II exerts a relevant role in nucleotide and drug metabolism through not only enzyme activity but also a mechanism involving a protein-protein interaction, thus playing a general regulatory role in cell survival.

SENTENCE

Resistance to fludarabine, gemcitabine and cytarabine can be determined by an increase of cN-II both through dephosphorylation of active drugs and perturbation of nucleotide pool.

Cytosolic 5'-nucleotidase II interacts with the leucin rich repeat of NLR family member Ipaf

IMP/GMP preferring cytosolic 5'-nucleotidase II (cN-II) is a bifunctional enzyme whose activities and expression play crucial roles in nucleotide pool maintenance, nucleotide-dependent pathways and programmed cell death. Alignment of primary amino acid sequences of cN-II from human and other organisms show a strong conservation throughout the entire vertebrata taxon suggesting a fundamental role in eukaryotic cells. With the aim to investigate the potential role of this homology in protein-protein interactions, a two hybrid system screening of cN-II interactors was performed in S. cerevisiae. Among the X positive hits, the Leucin Rich Repeat (LRR) domain of Ipaf was found to interact with cN-II. Recombinant Ipaf isoform B (lacking the Nucleotide Binding Domain) was used in an in vitro affinity chromatography assay confirming the interaction obtained in the screening. Moreover, co-immunoprecipitation with proteins from wild type Human Embryonic Kidney 293 T cells demonstrated that endogenous cN-II co-immunoprecipitated both with wild type Ipaf and its LRR domain after transfection with corresponding expression vectors, but not with Ipaf lacking the LRR domain. These results suggest that the interaction takes place through the LRR domain of Ipaf. In addition, a proximity ligation assay was performed in A549 lung carcinoma cells and in MDA-MB-231 breast cancer cells and showed a positive cytosolic signal, confirming that this interaction occurs in human cells. This is the first report of a protein-protein interaction involving cN-II, suggesting either novel functions or an additional level of regulation of this complex enzyme.

Relapse-specific mutations in NT5C2 in childhood acute lymphoblastic leukemia

Relapsed childhood acute lymphoblastic leukemia (ALL) carries a poor prognosis, despite intensive retreatment, owing to intrinsic drug resistance. The biological pathways that mediate resistance are unknown. Here, we report the transcriptome profiles of matched diagnosis and relapse bone marrow specimens from ten individuals with pediatric B-lymphoblastic leukemia using RNA sequencing. Transcriptome sequencing identified 20 newly acquired, novel nonsynonymous mutations not present at initial diagnosis, with 2 individuals harboring relapse-specific mutations in the same gene, NT5C2, encoding a 5'-nucleotidase. Full-exon sequencing of NT5C2 was completed in 61 further relapse specimens, identifying additional mutations in 5 cases. Enzymatic analysis of mutant proteins showed that base substitutions conferred increased enzymatic activity and resistance to treatment with nucleoside analog therapies. Clinically, all individuals who harbored NT5C2 mutations relapsed early, within 36 months of initial diagnosis (P = 0.03). These results suggest that mutations in NT5C2 are associated with the outgrowth of drug-resistant clones in ALL.

Structural basis for the allosteric regulation and substrate recognition of human cytosolic 5'-nucleotidase II

Cytosolic 5'-nucleotidase II (cN-II) catalyzes the dephosphorylation of 6-hydroxypurine nucleoside 5'-monophosphates and participates in the regulation of purine nucleotide pools within the cell. It interferes with the phosphorylation-dependent activation of nucleoside analogues used in the treatment of cancer and viral diseases. It is allosterically activated by a number of phosphate-containing cellular metabolites such as ATP, diadenosine polyphosphates, and 2,3-bisphosphoglycerate, which couple its activity with the metabolic state of the cell. We present seven high-resolution structures of human cN-II, including a ligand-free form and complexes with various substrates and effectors. These structures reveal the structural basis for the allosteric activation of cN-II, uncovering a mechanism where an effector-induced disorder-to-order transition generates rearrangements within the catalytic site and the subsequent coordination of the catalytically essential magnesium. Central to the activation is the large transition of the catalytically essential Asp356. This study also provides the structural basis for the substrate specificity of cN-II, where Arg202, Asp206, and Phe157 seem to be important residues for purine/pyrimidine selectivity. These structures provide a comprehensive structural basis for the design of cN-II inhibitors. They also contribute to the understanding of how the nucleotide salvage pathway is regulated at a molecular level.

Active and regulatory sites of cytosolic 5'-nucleotidase

Cytosolic 5'-nucleotidase (cN-II), which acts preferentially on 6-hydroxypurine nucleotides, is essential for the survival of several cell types. cN-II catalyses both the hydrolysis of nucleotides and transfer of their phosphate moiety to a nucleoside acceptor through formation of a covalent phospho-intermediate. Both activities are regulated by a number of phosphorylated compounds, such as diadenosine tetraphosphate (Ap₄A), ADP, ATP, 2,3-bisphosphoglycerate (BPG) and phosphate. On the basis of a partial crystal structure of cN-II, we mutated two residues located in the active site, Y55 and T56. We ascertained that the ability to catalyse the transfer of phosphate depends on the presence of a bulky residue in the active site very close to the aspartate residue that forms the covalent phospho-intermediate. The molecular model indicates two possible sites at which adenylic compounds may interact. We mutated three residues that mediate interaction in the first activation site (R144, N154, I152) and three in the second (F127, M436 and H428), and found that Ap₄A and ADP interact with the same site, but the sites for ATP and BPG remain uncertain. The structural model indicates that cN-II is a homotetrameric protein that results from interaction through a specific interface B of two identical dimers that have arisen from interaction of two identical subunits through interface A. Point mutations in the two interfaces and gel-filtration experiments indicated that the dimer is the smallest active oligomerization state. Finally, gel-filtration and light-scattering experiments demonstrated that the native enzyme exists as a tetramer, and no further oligomerization is required for enzyme activation.

Knockdown of cytosolic 5'-nucleotidase II (cN-II) reveals that its activity is essential for survival in astrocytoma cells

IMP preferring cytosolic 5'-nucleotidase (cN-II) is an ubiquitous nucleotide hydrolysing enzyme. The enzyme is widely distributed and its amino acid sequence is highly conserved among vertebrates. Fluctuations of cN-II activity have been associated with the pathogenesis of neurological disorders. The enzyme appears to be involved in the regulation of the intracellular availability of the purine precursor IMP and also of GMP and AMP, but the contribution of this activity and of its regulation to cell metabolism and to CNS cell functions remains uncertain. To address this issue, we used a vector based short hairpin RNA (shRNA) strategy to knockdown cN-II activity in human astrocytoma cells. Our results demonstrated that 53 h after transduction, cN-II mRNA was reduced to 17.9+/-0.03% of control cells. 19 h later enzyme activity was decreased from 0.7+/-0.026 mU/mg in control ADF cells to 0.45+/-0.046 mU/mg, while cell viability (evaluated by the MTT reduction assay) decreased up to 0.59+/-0.01 (fold vs control) and caspase 3 activity increased from 136+/-5.8 pmol min(-1) mg(-1) in control cells to 639+/-37.5 pmol min(-1) mg(-1) in silenced cells, thus demonstrating that cN-II is essential for cell survival. The decrease of enzyme activity causes apoptosis of the cultured cells without altering intracellular nucleotide and nucleoside concentration or energy charge. Since cN-II is highly expressed in tumour cells, our finding offers a new possible therapeutical approach especially against primary brain tumours such as glioblastoma, and to ameliorate chemotherapy against leukemia.

Identification of the nucleotidase responsible for the AMP hydrolysing hyperactivity associated with neurological and developmental disorders

Nucleoside monophosphate phosphohydrolases comprise a family of enzymes dephosphorylating nucleotides both in intracellular and extracellular compartments. Members of this family exhibit different sequence, location, substrate specificity and regulation. Besides the ectosolic 5'-nucleotidase, several cytosolic and one mitochondrial enzymes have been described. Nevertheless, researchers refer any AMP-dephosphorylating activity to as 5'-nucleotidase, lacking a more accurate identification. Increase of AMP hydrolysing activity has been associated with neurological and developmental disorders. The identification of the specific enzyme involved in these pathologies would be fundamental for the comprehension of the linkage between the enzyme activity alteration and brain functions. We demonstrate that the described neurological symptoms are associated with increased ectosolic 5'-nucleotidase activity on the basis of radiochemical assays and immunoblotting analysis. Furthermore, present data evidence that the assay conditions normally applied for the determination of cytosolic 5'-nucleotidases activity in crude extracts are affected by the presence of solubilised ectosolic nucleotidase.

Crystal structure of human cytosolic 5'-nucleotidase II: insights into allosteric regulation and substrate recognition

Cytosolic 5'-nucleotidase II catalyzes the dephosphorylation of 6-hydroxypurine nucleoside 5'-monophosphates and regulates the IMP and GMP pools within the cell. It possesses phosphotransferase activity and thereby also catalyzes the reverse reaction. Both reactions are allosterically activated by adenine-based nucleotides and 2,3-bisphosphoglycerate. We have solved structures of cytosolic 5'-nucleotidase II as native protein (2.2 Angstrom) and in complex with adenosine (1.5 Angstrom) and beryllium trifluoride (2.15 Angstrom) The tetrameric enzyme is structurally similar to enzymes of the haloacid dehalogenase (HAD) superfamily, including mitochondrial 5'(3')-deoxyribonucleotidase and cytosolic 5'-nucleotidase III but possesses additional regulatory regions that contain two allosteric effector sites. At effector site 1 located near a subunit interface we modeled diadenosine tetraphosphate with one adenosine moiety in each subunit. This efficiently glues the tetramer subunits together in pairs. The model shows why diadenosine tetraphosphate but not diadenosine triphosphate activates the enzyme and supports a role for cN-II during apoptosis when the level of diadenosine tetraphosphate increases. We have also modeled 2,3-bisphosphoglycerate in effector site 1 using one phosphate site from each subunit. By comparing the structure of cytosolic 5'-nucleotidase II with that of mitochondrial 5'(3')-deoxyribonucleotidase in complex with dGMP, we identified residues involved in substrate recognition.

Recent advances in structure and function of cytosolic IMP-GMP specific 5'-nucleotidase II (cN-II)

Cytosolic 5′nucleotidase II (cN-II) catalyses both the hydrolysis of a number of nucleoside monophosphates (e.g., IMP + H₂O→inosine + Pi), and the phosphate transfer from a nucleoside monophosphate donor to the 5′position of a nucleoside acceptor (e.g., IMP + guanosine →inosine + GMP). The enzyme protein functions through the formation of a covalent phosphoenzyme intermediate, followed by the phosphate transfer either to water (phosphatase activity) or to a nucleoside (phosphotransferase activity). It has been proposed that cN-II regulates the intracellular concentration of IMP and GMP and the production of uric acid. The enzyme might also have a potential therapeutic importance, since it can phosphorylate some anti-tumoral and antiviral nucleoside analogues that are not substrates of known kinases. In this review we summarise our recent studies on the structure, regulation and function of cN-II. Via a site-directed mutagenesis approach, we have identified the amino acids involved in the catalytic mechanism and proposed a structural model of the active site. A series of in vitro studies suggests that cN-II might contribute to the regulation of 5-phosphoribosyl-1-pyrophosphate (PRPP) level, through the so-called oxypurine cycle, and in the production of intracellular adenosine, formed by ATP degradation.

The 5'-nucleotidases as regulators of nucleotide and drug metabolism

The 5'-nucleotidases are a family of enzymes that catalyze the dephosphorylation of nucleoside monophosphates and regulate cellular nucleotide and nucleoside levels. While the nucleoside kinases responsible for the initial phosphorylation of salvaged nucleosides have been well studied, many of the catabolic nucleotidases have only recently been cloned and characterized. Aside from maintaining balanced ribo- and deoxyribonucleotide pools, substrate cycles that are formed with kinase and nucleotidase activities are also likely to regulate the activation of nucleoside analogues, a class of anticancer and antiviral agents that rely on the nucleoside kinases for phosphorylation to their active forms. Both clinical and in vitro studies suggest that an increase in nucleotidase activity can inhibit nucleoside analogue activation and result in drug resistance. The physiological role of the 5'-nucleotidases will be covered in this review, as will the evidence that these enzymes can mediate resistance to nucleoside analogues.

Mechanistic studies on bovine cytosolic 5'-nucleotidase II, an enzyme belonging to the HAD superfamily

Cytosolic 5'-nucleotidase/phosphotransferase specific for 6-hydroxypurine monophosphate derivatives (cN-II), belongs to a class of phosphohydrolases that act through the formation of an enzyme-phosphate intermediate. Sequence alignment with members of the P-type ATPases/L-2-haloacid dehalogenase superfamily identified three highly conserved motifs in cN-II and other cytosolic nucleotidases. Mutagenesis studies at specific amino acids occurring in cN-II conserved motifs were performed. The modification of the measured kinetic parameters, caused by conservative and nonconservative substitutions, suggested that motif I is involved in the formation and stabilization of the covalent enzyme-phosphate intermediate. Similarly, T249 in motif II as well as K292 in motif III also contribute to stabilize the phospho-enzyme adduct. Finally, D351 and D356 in motif III coordinate magnesium ion, which is required for catalysis. These findings were consistent with data already determined for P-type ATPases, haloacid dehalogenases and phosphotransferases, thus suggesting that cN-II and other mammalian 5'-nucleotidases are characterized by a 3D arrangement related to the 2-haloacid dehalogenase superfold. Structural determinants involved in differential regulation by nonprotein ligands and redox reagents of the two naturally occurring cN-II forms generated by proteolysis were ascertained by combined biochemical and mass spectrometric investigations. These experiments indicated that the C-terminal region of cN-II contains a cysteine prone to form a disulfide bond, thereby inactivating the enzyme. Proteolysis events that generate the observed cN-II forms, eliminating this C-terminal portion, may prevent loss of enzymic activity and can be regarded as regulatory phenomena.

Regulation and activity of cytosolic 5′-nucleotidase II

In many vertebrate tissues, cytosolic 5'-nucleotidase II (cN-II) either hydrolyses or phosphorylates a number of purine (monophosphorylated) nucleosides through a scheme common to the Haloacid Dehalogenase superfamily members. It possesses a pivotal role in purine cellular metabolism and it acts on anti-tumoural and antiviral nucleoside analogues, thus being of potential therapeutic importance. cN-II is Mg2+-dependent, regulated and stabilised by several factors such as allosteric effectors ATP and 2,3-DPG, although these are not directly involved in the reaction stoichiometry. We review herein the experimental knowledge currently available about this remarkable enzymatic activity.

Identification of the 5'-nucleotidase activity altered in neurological syndromes

5'-Nucleotidases comprise a family of enzymes involved in the regulation of intracellular and extracellular nucleotide concentration. There is increasing knowledge about an involvement of these activities in the aetiology of neurological disorders. In this paper we present a protocol for the identification of the altered enzyme in fibroblasts primary culture from patients and controls.

Role of IMP-selective 5'-nucleotidase (cN-II) in hematological malignancies

Cytotoxic nucleoside analogs (NA) are important in the treatment of hematologic malignancies. The NA in routine clinical use include the pyrimidine analog cytosine arabinoside (ara-c), which is extensively used in the treatment of acute leukemias, and the purine analogs, cladribine and fludarabine. These drugs have mostly been used in the treatment of low grade hematological malignancies. NA become therapeutically effective only after phosporylation to the triphosphate level. The 5'-nucleotidases (5'-NTs) dephosphorylate the monophosphate form of NA and, therefore, may affect the pharmacological activity of these antimetabolites in the clinic. Several 5'-NTs attached to membranes or present in the cytosol or in mitochondria are present in mammalian cells. cN-II, an IMP-selective 5'-NT, participates in the regulation of purine deoxyribonucleotide metabolism. cN-II opposes the action of the salvage enzymes by dephosphorylating purine nucleoside mononphosphates to purine nucleosides. Due to its phosphotransferase activity, cN-II can also phosphorylate inosine and 2',3'-dideoxyribonucleosides utilizing IMP as a phosphate donor. The observation that cytosolic cN-II is able to phosphorylate purine nucleosides has initiated studies on its potential participation in the metabolism of anticancer agents and in the development of cN-II inhibitory substances. In this review, we highlight the current knowledge concerning cN-II activity and regulation of intracellular deoxyribonucleotide pools and it role in hematological malignancies.

Pathways for alpha-D-ribose utilization for nucleobase salvage and 5-fluorouracil activation in rat brain

Recently, interest has increased in the use of alpha-D-ribose (Rib) as a naturally occurring nutriceutical for enhancement of cardiac and muscular performance. Most likely the elevation of available PRPP, following Rib administration, plays a key role in the salvage of purine nucleobases, thus, accelerating the restitution of ATP pool. In addition, administration of Rib improves some of the neurological symptoms in patients with adenylosuccinase deficiency. In this paper, we show that rat brain extract can catalyze the Rib-mediated salvage of both adenine and uracil, as well as the activation of the pyrimidine pro-drug, 5-fluorouracil (5-FU). The results strongly support that the pentose may be converted to both PRPP and Rib1-P for the salvage of the adenine and uracil, respectively. Most likely two-reaction pathway, composed of ribokinase and PRPP synthetase, is responsible of the PRPP formation, needed to salvage adenine to adenine nucleotides. A two-reaction pathway, composed of ribokinase and phosphopentomutase, appears to be responsible of the Rib1-P formation, needed to salvage uracil to uracil-nucleotides and to activate 5-FU to cytotoxic 5-FU-ribonucleotides. alpha-D-2'-Deoxyribose (deoxyRib) has a negligible effect on both the salvage of natural nucleobases to deoxyribonucleotides and on the activation of 5-FU to cytotoxic 5-FU-deoxynucleotides.

Bovine cytosolic 5'-nucleotidase acts through the formation of an aspartate 52-phosphoenzyme intermediate

Cytosolic 5'-nucleotidase/phosphotransferase (cN-II), specific for purine monophosphates and their deoxyderivatives, acts through the formation of a phosphoenzyme intermediate. Phosphate may either be released leading to 5'-mononucleotide hydrolysis or be transferred to an appropriate nucleoside acceptor, giving rise to a mononucleotide interconversion. Chemical reagents specifically modifying aspartate and glutamate residues inhibit the enzyme, and this inhibition is partially prevented by cN-II substrates and physiological inhibitors. Peptide mapping experiments with the phosphoenzyme previously treated with tritiated borohydride allowed isolation of a radiolabeled peptide. Sequence analysis demonstrated that radioactivity was associated with a hydroxymethyl derivative that resulted from reduction of the Asp-52-phosphate intermediate. Site-directed mutagenesis experiments confirmed the essential role of Asp-52 in the catalytic machinery of the enzyme and suggested also that Asp-54 assists in the formation of the acyl phosphate species. From sequence alignments we conclude that cytosolic 5'-nucleotidase, along with other nucleotidases, belong to a large superfamily of hydrolases with different substrate specificities and functional roles.

In vitro recycling of alpha-D-ribose 1-phosphate for the salvage of purine bases

In this paper, we extend our previous observation on the mobilization of the ribose moiety from a purine nucleoside to a pyrimidine base, with subsequent pyrimidine nucleotides formation (Cappiello et al., Biochim. Biophys. Acta 1425 (1998) 273-281). The data show that, at least in vitro, also the reverse process is possible. In rat brain extracts, the activated ribose, stemming from uridine as ribose 1-phosphate, can be used to salvage adenine and hypoxanthine to their respective nucleotides. Since the salvage of purine bases is a 5-phosphoribosyl 1-pyrophosphate-dependent process, catalyzed by adenine phosphoribosyltransferase and hypoxanthine guanine phosphoribosyltransferase, our results imply that Rib-1P must be transformed into 5-phosphoribosyl 1-pyrophosphate, via the successive action of phosphopentomutase and 5-phosphoribosyl 1-pyrophosphate synthetase; and,in fact, no adenosine could be found as an intermediate when rat brain extracts were incubated with adenine, Rib-1P and ATP, showing that adenine salvage does not imply adenine ribosylation, followed by adenosine phosphorylation. Taken together with our previous results on the Rib-1P-dependent salvage of pyrimidine nucleotides, our results give a clear picture of the in vitro Rib-1P recycling, for both purine and pyrimidine salvage.

ATP and phosphate reciprocally affect subunit association of human recombinant High Km 5'-nucleotidase. Role for the C-terminal polyglutamic acid tract in subunit association and catalytic activity

IMP-specific, High Km 5'-nucleotidase (EC 3.1.3.5) is an ubiquitous enzyme, the activity of which is highly regulated by substrate, ATP, and inorganic phosphate. The cDNA encoding this enzyme has recently been cloned and found to contain a unique stretch of nine glutamic and four aspartic acid residues at the C-terminus. To study the effects of this acidic tail, and of ATP and inorganic phosphate on enzyme function, we generated several structural modifications of the 5'-nucleotidase cDNA, expressed the corresponding proteins in Escherichia coli and compared their molecular and kinetic properties. As with the enzyme purified from human placenta, all recombinant proteins were activated by ATP and inhibited by inorganic phosphate. Although the S0.5-values were higher, the specific activities of the purified protein variants (except that truncated at the C-terminus) were similar. The molecular mass of the full-length enzyme subunit has been estimated at 57.3 kDa and the molecular mass of the native protein, as determined by gel-filtration chromatography, was estimated to be 195 kDa. Increasing the concentration of NaCl to 0.3 M promoted oligomerization of the protein and the formation of aggregates of 332 kDa. ATP induced further oligomerization to 715 kDa, while inorganic phosphate reduced the estimated molecular mass to 226 kDa. In contrast to the truncation of 30 amino acids at the N-terminus, which did not alter enzyme properties, the removal of the polyglutamic/aspartic acid tail of 13 residues at the C-terminus caused profound kinetic and structural changes, including a 29-fold decrease in specific activity and a significant increase in the sensitivity to inhibition by inorganic phosphate in the presence of AMP. Structurally, there was a dramatic loss of the ability to form oligomers at physiological salt concentration which was only partially restored by the addition of NaCl or ATP. These data suggest an important function of the polyglutamic acid tract in the process of association and dissociation of 5'-nucleotidase subunits.

Bovine cytosolic IMP/GMP-specific 5'-nucleotidase: cloning and expression of active enzyme in Escherichia coli

A cDNA coding for bovine cytosolic IMP/GMP-specific 5'-nucleotidase endowed with phosphotransferase activity was cloned from calf thymus RNA, by 5' and 3' rapid amplification of cDNA ends protocols (5' and 3' RACE). Two products were isolated: a 5' RACE 1.6 kb fragment and a 3' RACE 2.0 kb fragment, with an overlapping region of 505 bp, leading to a total length of approx. 2951 bp. The similarity in the coding region to that of the human 5'-nucleotidase cDNA sequence [Oka, Matsumoto, Hosokawa and Inoue (1994) Biochem. Biophys. Res. Commun. 205, 917-922], indirectly identified as a 5'-nucleotidase, was 94% and the deduced amino acid sequences were 99.5% identical. The bovine cDNA sequence included the sequences codifying for six peptides obtained from 5'-nucleotidase/phosphotransferase purified from calf thymus. Northern blots of human mRNA species from different tissues showed a 3.6 kb mRNA expressed at equal levels in most tissues. The cDNA was cloned into a pET-28c expression vector and the protein obtained after induction had a molecular mass of 61 kDa under SDS/PAGE. It exhibited both 5'-nucleotidase and phosphotransferase activity, as well as immunological and kinetic properties similar to those of the enzyme purified from calf thymus. This is the first time that a fully active recombinant 5'nucleotidase has been described.

Mechanism of the reaction catalysed by cytosolic 5'-nucleotidase/phosphotransferase: formation of a phosphorylated intermediate

Cytosolic 5'-nucleotidase preferentially catalysing the hydrolysis of IMP, GMP and their deoxy derivatives, and endowed with phosphotransferase activity, was purified from calf thymus and its reaction mechanism was studied. In the presence of [32P]IMP, ATP and MgCl2, a covalent enzyme-phosphate intermediate was trapped by mixing with an SDS solution. Heart or acid treatment of the enzyme before incubation with radiolabelled substrate prevented formation of the intermediate. Furthermore, on the basis of studies on the kinetic parameters of the enzyme as function of pH, and of experiments on thiol oxidation and photo-oxidation, we suggest the involvement of cysteine and histidine residue(s) in the reaction mechanism.

Characterization of the F1F0-ATPase and the tightly-bound ATPase activities in submitochondrial particles from human term placenta

In a previous study we demonstrated the existence of a tightly-bound ATPase in the human placental mitochondria (Martínez et al., 1993). The current study characterizes the ATP hydrolysis produced by the F1F0-ATPase and the tightly-bound ATPase in submitochondrial particles from the human term placenta. Both enzymes were not differentiated by pH. Inhibitors were necessary to distinguish the activity of each enzyme. The kinetic of the total ATP hydrolysis fitted into a model of two enzymes. During the characterization, it was observed that the tightly-bound ATPase activity was partially inhibited by vanadate and Mg2+, whereas the F1F0-ATPase was totally inhibited by Mg2+. Different nucleotides were hydrolyzed by the tightly-bound ATPase; the F1F0-ATPase hydrolyzed exclusively ATP. Glucose-6-phosphate, p-nitrophenylphosphate, or pyrophosphate were not hydrolyzed by the F1F0-ATPase, although some hydrolysis was observed with the tightly-bound ATPase. It is concluded that the tightly-bound ATPase activity corresponded to a 5'-nucelotidase, and that the human placental mitochondria could participate in the metabolism of nucleotides.

Synergistic action of ADP and 2,3-bisphosphoglycerate on the modulation of cytosolic 5'-nucleotidase

Cytosolic 5'-nucleotidase, acting preferentially on IMP, GMP and their deoxyderivatives, can also behave as a phosphotransferase, operating a transfer of phosphate from a nucleoside monophosphate donor to a nucleoside acceptor which, besides a natural nucleoside, can be also an analog. The enzyme activity is stimulated by ADP, ATP and 2,3-bisphosphoglycerate (BPG). The concentration of effector required to attain half maximal activation (A0.5) for the bisphosphorylated compound is in the millimolar range, so that BPG seems to act as a physiological activator of 5'-nucleotidase only in erythrocytes. However, the combination of BPG and ADP brings about a significant increase of their respective affinity for the enzyme, lowering their A0.5 values approx. 4-times. The observation that BPG favors the phosphotransferase more than the hydrolase activity of 5'-nucleotidase stands for a key role of this metabolite in the regulation of the processes of activation of purine pro-drugs, in which this enzyme is involved.

Production of adenosine and nucleoside analogs by the exchange reaction catalyzed by rat liver adenosine kinase

We have previously shown [8] that rat liver adenosine kinase can produce [14C]AMP from [14C]adenosine (Ado) and unlabelled adenosine monophosphate (AMP), in the absence of ATP, by an exchange reaction. In this study, we investigated whether Ado or AMP could be replaced in this exchange reaction by other nucleosides or nucleoside monophosphates (NMP), respectively. In the presence of 1 mM of the unlabelled NMP analogs 7-deazaadenosine (tubercidin) 5'-monophosphate, 6-chloropurine riboside 5'-monophosphate, or N6-methyl-AMP, [14C]AMP was formed from 20 microM [14C]Ado at up to 50% of the rate recorded with 1 mM unlabelled AMP. In the presence of 0.2 mM of the unlabelled analog nucleosides tubercidin, N6-methyladenosine, or 6-methylmercaptopurine riboside, [14C]Ado was generated from 1 mM [14C]AMP at up to 60% of the rate recorded with 0.2 mM unlabeled Ado. Small amounts of [14C]Ado were also formed from the natural nucleosides 5-amino-4-imidazolecarboxamide (AICA) riboside or 2'-deoxyadenosine. Administration of therapeutic anticancer and antiviral nucleosides that can serve as substrates for the exchange reaction catalyzed by adenosine kinase might, thus, result in a net production of Ado, a potent autacoid with physiological effects in numerous tissues.

Hepatic purine and pyrimidine metabolism: implications for antiviral chemotherapy of viral hepatitis

The use of nucleoside analogues as antiviral agents is expanding. For most nucleoside analogues, intracellular phosphorylation is the major prerequisite for activity. Antiviral activity may be limited by poor uptake, absence of appropriate activating enzymes, catabolism, and competition from endogenous nucleotides. Appreciation of these factors, which are species-, tissue- and cell-specific is important in the understanding of the pharmacology and toxicology of nucleoside analogues. The use of nucleoside analogues against the agents of viral hepatitis is inherently problematic for many reasons including active hepatic nucleoside catabolism, probable absence of virus-specific activating enzymes, competition from endogenous nucleotides synthesised de novo or derived from RNA turnover, and factors related to mitochondrial toxicity. Despite these drawbacks, some nucleoside analogues have been found efficacious against hepatitis B virus and it is likely that as knowledge of their mechanism of action accumulates, their efficacy can be improved both by rational drug design and by use in combination with other drugs, including interferon.

Purine enzyme profile in human colon-carcinoma cell lines and differential sensitivity to deoxycoformycin and 2'-deoxyadenosine in combination

Different cell lines, 2 from human colon carcinoma (LoVo and HT29) and 1 from Chinese hamster ovary (CHO K-I), were examined to assess the effect of deoxycoformycin (dCF), an inhibitor of adenosine deaminase (ADA), and 2'-deoxyadenosine (dAdo) on their growth. When used alone, neither dCF or dAdo were cytotoxic for the 3 cell lines, while their combination caused inhibition of cell growth, with the following sensitivity: CHO K-I > LoVo > HT29. We studied the pattern of enzymatic activities involved in the metabolism of dAdo in the 3 cell lines. The phosphorylation of dAdo by adenosine kinase appears to play a central role in the toxicity of the deoxynucleoside in combination with dCF. In fact, CHO K-I cells, which are the most sensitive, possess the highest level of this enzyme. Moreover, the cytotoxic effect was almost completely reversed in the 3 cell lines when inhibitors of adenosine kinase, such as 5'-amino-5'-deoxyadenosine and iodotubercidine, were added to the culture medium together with dCF and dAdo. In addition, baby hamster kidney (BHK) adenosine-kinase-deficient (AK-) cells were highly resistant to this treatment. Uptake inhibition of dAdo using dipyridamole also caused reversal of the toxicity. The AMP and deoxyAMP dephosphorylating activities, much lower in the CHO K-I cells, also appear to play a central role in the toxicity of dAdo when adenosine deaminase is inhibited. However, our data suggest that other factors may modulate the toxic effect, such as S-adenosyl-homocysteine-hydrolase inhibition by dAdo at high concentrations.

Phosphorylation of adenosine in renal brush-border membrane vesicles by an exchange reaction catalysed by adenosine kinase

Uptake of [3H]adenosine in brush-border membrane (BBM) vesicles from either rat or pig kidney leads to an accumulation of intravesicular [3H]AMP. The lack of significant levels of ATP and the presence of AMP in BBM indicated that a phosphotransfer between [3H]adenosine and AMP occurs. The phosphotransfer activity is inhibited by iodotubercidin, which suggests that it is performed by adenosine kinase acting in an ATP-independent manner. The existence of a similar phosphotransferase activity was demonstrated in membrane-free extracts from pig kidney. From the compounds tested it was shown that a variety of mononucleotides could act as phosphate donors. The results suggest that phosphotransfer reactions may be physiologically relevant in kidney.

Cytosolic 5'-nucleotidase/nucleoside phosphotransferase: a nucleoside analog activating enzyme?

Nucleoside phosphotransferase acting on inosine and deoxyinosine has been partially purified from cultured Chinese hamster lung fibroblasts (V79). The activity is associated with a cytosolic 5'-nucleotidase acting on IMP and deoxyIMP. The transfer of the phosphate group from IMP to inosine catalyzed by this enzyme was activated by ATP and 2,3-bisphosphoglycerate. Inosine, deoxyinosine, guanosine, deoxyguanosine, and the nucleoside analogs 2',3'-dideoxyinosine and 8-azaguanosine are substrates, while adenosine and deoxyadenosine are not. IMP, deoxyIMP, GMP, and deoxyGMP are the best phosphate donors. The cytosolic 5'-nucleotidase/phosphotransferase substrate, 8-azaguanosine, was found to be very toxic for cultured fibroblasts (LD50 = 0.32 microM). Mutants resistant to either 8-azaguanosine and the correspondent base 8-azaguanine were isolated and characterized. Our results indicated that the 8-azaguanosine-resistant cells were lacking both cytosolic 5'-nucleotidase and hypoxanthine-guanine phosphoribosyltransferase, while 8-azaguanine resistant cells were lacking only the latter enzyme. Despite this observation, both mutants displayed 8-azaguanosine resistance, thus indicating that cytosolic 5'-nucleotidase is not essential for the activation of this nucleoside analog.

Cytosolic 5'-nucleotidase/nucleoside phosphotransferase: a single assay for a bifunctional enzyme

Cytosolic 5'-nucleotidase/nucleoside phosphotransferase has been purified from calf thymus. Since the same protein is able to catalyze both the hydrolysis and the interconversion of several nucleoside monophosphates, it is necessary to study the effect of different metabolites and assay conditions on both activities in order to elucidate their physiological roles. We describe herein a method which allowed us to follow both activities contemporaneously in the same assay mixture. The method takes advantage of the observation that deoxyGMP is both a good substrate for hydrolysis and a good phosphate donor for the phosphotransferase reaction, but its dephosphorylated product, deoxyguanosine, is not a phosphate acceptor. As a consequence, it is possible to follow both the deoxyguanosine production and the transfer of phosphate from deoxyGMP to the best phosphate acceptor, inosine, during the reaction, applying a method for the chromatographic separation on HPLC of both substrates (inosine and deoxyGMP) and both products (IMP and deoxyguanosine). The method was applied to the determination of the KM for inosine.

Membrane-bound 5'-nucleotidase/nucleoside phosphotransferase from Bacillus cereus

1. A search for nucleoside phosphotransferase activity in Bacillus cereus led to the following results: (i) The phosphotransferase activity was associated with a membrane bound 5'-nucleotidase. (ii) The enzyme phosphorylates both purine and pyrimidine nucleosides as well as 2',3'-dideoxyinosine. (iii) The enzyme was inhibited by adenylic nucleotide di- and triphosphates, and its nucleotidase activity was increased in the presence of inosine as phosphate acceptor. 2. Bacterial and vertebrate 5'-nucleotidases with phosphotransferase activity differ for several characteristics, such as cellular location, substrate specificity, magnesium requirement and regulation.

Different relationships between cellular adenosine or 3'-deoxyadenosine phosphorylation and cellular adenine ribonucleotide catabolism may be obtained

Treatment of BALB/c-3T3 mouse fibroblasts with 3'-deoxyadenosine led to a rapid accumulation of 3'-deoxyadenosine phosphates and the kinetics of this process has been determined. Concomitant with accumulation of these compounds, the adenine ribonucleotide pool was reduced. The kinetics of the two processes suggested that they were tightly coupled. The inhibitory effect of relatively high concentrations of coformycin indicated that IMP was an intermediate in the catabolic pathway. Similar experiments with Ehrlich ascites tumor cells were performed in Ringer-Hepes solution at pH 6.5 or 7.5 and with varying concentrations of orthophosphate. The experiments were performed with cells where ATP was [3H]-labeled. This allowed the determination of the catabolism of adenine ribonucleotides to labeled nucleosides under conditions where added adenosine was phosphorylated. The results showed that at low phosphate concentration (5.8 mM) at pH 6.5 adenosine may be phosphorylated at a rate that was completely balanced to the concomitant catabolism of adenine ribonucleotides; that is, there was apparently a tight kinetic coupling between anabolism of adenosine and catabolism of adenine ribonucleotides. With 3'-deoxyadenosine a corresponding effect was obtained although the apparent coupling between phosphorylation of 3'-deoxyadenosine and catabolism of adenine ribonucleotides was not complete. When experiments were performed at the same pH but at high concentration of phosphate (45 mM) there was in contrast no coupling between the two processes; that is, ATP was present in constant amounts while 3'-deoxyadenosine phosphates accumulated at a high rate. In experiments with adenosine under these conditions there was still some although a relatively limited degree of apparent coupling between phosphorylation of adenosine and catabolism of adenine ribonucleotides. In both lines of cells used and with both adenosine and 3'-deoxyadenosine, the main products of the catabolism of adenine ribonucleotides were inosine and hypoxanthine. With 3'-deoxyadenosine there was in addition (about 20%) formation of xanthosine, suggesting that IMP dehydrogenase had also been activated. These results lead to the suggestion that adenosine (or 3'-deoxyadenosine) may be phosphorylated in two ways. 1) Phosphorylation may depend on an adenosine kinase unrelated to catabolism of adenine ribonucleotides. 2) Phosphorylation may be tightly coupled to catabolism of adenine ribonucleotides. A nucleoside phosphotransferase may catalyze the transfer of a phosphoryl group from IMP to adenosine (or 3'-deoxyadenosine) to form AMP (or 3'-dAMP) and inosine, a process that may be tightly coupled to an AMP deaminase reaction. The IMP formed in the latter reaction may not be released but transferred to the phosphotransferase.(ABSTRACT TRUNCATED AT 400 WORDS)

A comparative study on tissue distribution and metabolic adaptation of IMP-GMP 5'-nucleotidase

1. Activity of a cytoplasmic 5'-nucleotidase which preferentially hydrolyzes IMP and GMP (IMP-GMP 5'-nucleotidase) was determined by a specific immunochemical method in two species of birds and two species of mammals. 2. The activity was markedly high in avian liver, and it increased two-fold in response to a high protein diet in chicken liver. 3. In mammals, the activity was high in testis and spleen. In the rat, the activities in liver, kidney and heart extracts increased by about 30% in response to the high protein diet, while they increased three-fold in regenerating liver. 4. Low activities were detected in skeletal muscles and in erythrocytes of all the species studied.

5'-Nucleotidase: molecular structure and functional aspects

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