Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

---

Query: EC:3.1.3.5 (5'-nucleotidase)
3,167 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The effect of increasing doses of GTP on agonist and antagonist binding to adenosine A1-receptors in different regions of rat brain was studied by autoradiography. A high concentration of GTP (100 microM) practically eliminated the binding of the agonist [3H]N6-cyclohexyladenosine in all regions. However, there were regional differences in the effects of low concentrations of GTP (0.1-10 microM). In some regions, for example the hippocampus, all concentrations of GTP decreased [3H]N6-cyclohexyladenosine binding, by decreasing the Bmax. In other structures, e.g. the superior colliculus, there was a biphasic response to GTP. Concentrations of 0.1-3 microM increased agonist binding, apparently due to a decrease in KD, whereas higher concentrations also decreased binding in these regions. The effects of GTP were mimicked by the stable GTP analogue guanosine-5'-O-(3-thiotriphosphate). GTP (0.5-100 microM) increased the binding of the antagonist [3H]8-cyclopentyl-1,3-dipropylxanthine in all regions, but most markedly in those where GTP had a biphasic effect on agonist binding. Decreasing the levels of endogenous adenosine by increasing the concentration of adenosine deaminase and adding the 5'-nucleotidase inhibitor alpha-beta-methylene adenosine-5'-diphosphate gave an increase in [3H]8-cyclopentyl-1,3-dipropylxanthine binding and diminished the response to GTP. In sections treated with adenosine deaminase and alpha-beta-methylene adenosine-5'-diphosphate, GTP steadily decreased [3H]N6-cyclohexyladenosine binding in all regions. Thus, the GTP-induced increase in both agonist and antagonist binding may be due to a displacement of endogenous adenosine. In the presence of 1 mM EDTA, GTP had a monophasic effect on the binding of [3H]N6-cyclohexyladenosine in all regions. In the presence of 2 mM MgCl2 a biphasic response to GTP was seen in all regions. In EDTA washed sections, the effect of MgCl2 on [3H]N6-cyclohexyladenosine binding was more pronounced in the superior colliculus, where we had observed a biphasic response to GTP. The results suggest that there are regional differences in the effects of GTP on adenosine A1-receptor binding in rat brain, that reflect regional differences in the magnesium-dependent binding of endogenous adenosine, which is bound to the receptor by tight binding, is very difficult to remove, and easily interferes with radioligand binding in in vitro experiments. There may be regional differences in the sensitivity of A1-receptor-G-protein complexes to magnesium, that reflect a heterogeneity of the G-proteins to which the A1-receptors are coupled.
...
PMID:Regional differences in the effect of guanine nucleotides on agonist and antagonist binding to adenosine A1-receptors in rat brain, as revealed by autoradiography. 235 51

Inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], arising from hydrolysis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2], is proposed as the link between membrane-receptor activation and mobilization of Ca2+ from intracellular sites in hormone-secreting cells. The location of Ins(1,4,5)P3-sensitive membranes was investigated in cultured neonatal beta-cells. Membranes were obtained after lysis of cells attached to positively charged Sephadex. After lysis the presence of the enzyme markers 5'-nucleotidase, glucose-6-phosphatase, NADH-cytochrome c reductase, UDP-galactosyltransferase and succinate dehydrogenase indicated the mixed nature of the preparation. After sonication, however, UDP-galactosyltransferase and succinate dehydrogenase activities were undetectable, but 4.8% of total cellular glucose-6-phosphatase and 3.4% of total cellular NADH-cytochrome c reductase remained with 5'-nucleotidase in the preparation, indicating endoplasmic-reticulum association. ATP-dependent 45Ca2+ accumulation was shown in this preparation (410 +/- 24 pmol/mg of protein at 150 nM free Ca2+) and was inhibited by vanadate (100 microM). Ca2+ release was effected by Ins(1,4,5)P3, with half-maximal release at 0.5 +/- 0.14 microM-Ins(1,4,5)P3, t1/2 11.2 +/- 1.1 s. GTP- and guanosine 5'-[beta gamma-imido]triphosphate (p[NH]ppG)-promoted release of 45Ca2+ was demonstrated in this preparation, but the kinetics of release (half-maximal Ca2+ release at 5.4 +/- 0.7 microM, with t1/2 77.3 +/- 6.9 s, and at 51.1 +/- 4.2 microM, with t1/2 19.0 +/- 2.2 s, for GTP and p[NH]ppG respectively), and the ability of neomycin sulphate to block p[NH]ppG-induced release only, are indicative of separate release mechanisms after treatment with these agents. A close association between plasma membrane and elements of the endoplasmic reticulum is indicated in this model, providing a possible mechanism for local alterations in free Ca2+ in the sub-plasma-membrane region.
...
PMID:GTP- and inositol 1,4,5-trisphosphate-induced release of 45Ca2+ from a membrane store co-localized with pancreatic-islet-cell plasma membrane. 245 19

The dnaK protein of Escherichia coli has been shown to possess both autophosphorylating and 5'-nucleotidase activities. The dnaK protein has been shown to bind avidly to ATP, but hydrolyzing it slowly. In vitro autophosphorylation occurs at a threonine residue when either ATP or GTP are used as phosphate donors. The extent of autophosphorylation is low; only a few percent of the molecules are phosphorylated. This activity is stimulated at least tenfold in the presence of Ca2+ ions with either ATP or GTP as the donor. The autophosphorylating activity of the mutant dnaK756 protein in the presence or absence of Ca2+ is reduced compared to that of the wild type.
...
PMID:Biochemical properties of the Escherichia coli dnaK heat shock protein and its mutant derivatives. 251 98

The flux rates through the metabolic pathways affecting the maintenance of GuRN pool in intact human RBC were studied. Normal RBC, incubated in KRBB, exhibited a markedly higher accumulation in nucleotides of Gu than of Hx. Addition of 8-AGuo, a potent inhibitor of PNP, resulted in a marked increase in the accumulation of label in the nucleosides, in Ino following incubation with Hx, and in Guo following incubation with Gu, indicating a very high rate of IMP and GMP degradation to bases through their respective nucleosides. Most of the degradation of GMP is by dephosphorylation to Guo, rather than through reductive deamination to IMP. The ultimate fate of IMP in RBC is its degradation to Ino and consequently to Hx. The contribution of AdRN or of IMP to the GuRN pool is negligible. The results indicate that concerning IMP and GMP, human RBC contain very active futile cycles, nucleotide----nucleoside----base----nucleotide, catalyzed by 5'-nucleotidase, PNP, and HGPRT. The operation of the complete cycles is essential for the maintenance of GuRN and the IMP pool size. These results may explain the finding of reduced GTP content in RBC from patients with an inborn deficiency of PNP or of HGPRT.
...
PMID:Guanine ribonucleotide metabolism in human red blood cells: evidence for a high rate of GMP dephosphorylation. 256 18

A purine 5'-nucleotidase has been separated by DEAE-Trisacryl chromatography from other 5'-nucleotidase activities present in human haemolysates and purified approx. 30,000-fold by subsequent chromatography on Blue Sepharose. The enzyme has an Mr of around 250,000, displays hyperbolic substrate-saturation kinetics and hydrolyses preferentially IMP, GMP and their deoxy counterparts. It is much less active with AMP and dAMP. The purine 5'-nucleotidase is inhibited by Pi, and is strongly stimulated by ATP, dATP and GTP, and by glycerate 2,3-bisphosphate. Stimulators decrease Km and increase Vmax. Glycerate 2,3-bisphosphate is the most potent stimulator of the enzyme and, under physiological conditions, over-rides the influence of the other effectors. Glycerate 2,3-bisphosphate also influences the binding of the enzyme to DEAE-Trisacryl, as evidenced by the different elution profile obtained with fresh as compared with outdated blood. It is concluded that the glycerate 2,3-bisphosphate-stimulated purine 5'-nucleotidase is responsible for the dephosphorylation of IMP and GMP, but not of AMP, in human erythrocytes.
...
PMID:5'-Nucleotidase activities in human erythrocytes. Identification of a purine 5'-nucleotidase stimulated by ATP and glycerate 2,3-bisphosphate. 283 44

A human placental soluble "high Km" 5'-nucleotidase has been separated from "low Km" 5'-nucleotidase and nonspecific phosphatase by AMP-Sepharose affinity chromatography. The enzyme was purified 8000-fold to a specific activity of 25.6 mumol/min/mg. The subunit molecular mass is 53 kDa, and the native molecular mass is 210 kDa, suggesting a tetrameric structure. Soluble high Km 5'-nucleotidase is most active with IMP and GMP and their deoxy derivatives. IMP is hydrolyzed 15 times faster than AMP. The enzyme has a virtually absolute requirement for magnesium ions and is regulated by them. Purine nucleoside 5'-triphosphates strongly activate the enzyme with the potency order dATP greater than ATP greater than GTP. 2,3-Diphosphoglycerate activates the enzyme as potently as ATP. Three millimolar ATP decreased the Km for IMP from 0.33 to 0.09 mM and increased the Vmax 12-fold. ATP activation was modified by the IMP concentration. At 20 microM IMP the ATP-dependent activation curve was sigmoidal, while at 2 mM IMP it was hyperbolic. The A0.5 values for ATP were 2.26 and 0.70 mM, and the relative maximal velocities were 32.9 and 126.0 nmol/min, respectively. Inorganic phosphate shifts the hyperbolic substrate velocity relationship for IMP to a sigmoidal one. With physiological concentrations of cofactors (3 mM ATP, 1-4 mM Pi, 150 mM KCl) at pH 7.4, the enzyme is 25-35 times more active toward 100 microM IMP than 100 microM AMP. These data show that: (a) soluble human placental high Km 5'-nucleotidase coexists in human placenta with the low Km enzyme; (b) under physiological conditions the enzyme favors the hydrolysis of IMP and is critically regulated by IMP, ATP, and Pi levels; and (c) kinetic properties of ATP and IMP are each modified by the other compound suggesting complex interaction of the associated binding sites.
...
PMID:High Km soluble 5'-nucleotidase from human placenta. Properties and allosteric regulation by IMP and ATP. 284 5

Of the various species of cellular 5'-nucleotidases, membranous, lysosomal and cytosolic, only the latter are likely to play a role in the physiologic dephosphorylation of the 5'-nucleoside monophosphates present in the cytoplasm. The necessity to preserve cellular ATP renders a strict control of the dephosphorylation as well as of the deamination of AMP mandatory, because both nucleotides are maintained in equilibrium by adenylate kinase. Our studies of cytosolic purine 5'-nucleotidases purified from rat liver and from human erythrocytes, reviewed in this presentation, have shown that both display complex kinetic properties. Both enzymes have markedly higher affinities for IMP and for GMP than for AMP. In addition, they are stimulated by nucleoside triphosphates, among them ATP and GTP, and inhibited by Pi. The erythrocytic purine 5'-nucleotidase is also stimulated by glycerate 2,3-bisphosphate. It could thus be expected that under conditions of ATP and GTP breakdown, particularly when accompanied by an increase in Pi, the dephosphorylation of AMP would be curtailed. To verify this hypothesis, experiments were performed with isolated rat hepatocytes and with human red blood cells. The rate of dephosphorylation of AMP was measured by following time-wise the production of adenosine in the presence of coformycin (or deoxycoformycin) and 5-iodotubercidin. The coformycins inhibit the deamination of adenosine into inosine by adenosine deaminase, and 5-iodotubercidin inhibits the recycling of adenosine into AMP by adenosine kinase. Upon induction of ATP catabolism by the addition of fructose to isolated rat hepatocytes, the dephosphorylation of AMP was nearly completely suppressed. In accordance with these results, the activity of the rat liver cytosolic 5'-nucleotidase, assayed in the presence of concentrations of substrate and effectors mimicking those measured in intact cells following the addition of fructose, was decreased as compared to control conditions. In hepatocytes in which ATP catabolism was induced by suppression of oxygen, the rate of dephosphorylation of AMP increased about 3-fold. However, in contradiction with these data, the activity of the cytosolic 5'-nucleotidase, measured under conditions mimicking anoxia, decreased markedly. In human erythrocytes, dephosphorylation of AMP did not occur under physiologic conditions, but proceeded when ATP catabolism was induced by glucose lack or by alkalinization. The rate of dephosphorylation of AMP was 3-fold higher during glucose deprivation than under alkaline conditions.(ABSTRACT TRUNCATED AT 400 WORDS)
...
PMID:Cytosolic purine 5'-nucleotidases of rat liver and human red blood cells: regulatory properties and role in AMP dephosphorylation. 285 49

The fluoropyrimidine deoxyribonucleotide 5-fluoro-2'-deoxyuridine 5'-monophosphate (FdUMP) was encapsulated in human erythrocytes by a procedure based on hypotonic hemolysis and isotonic resealing. Encapsulated FdUMP (up to 9 mumol/ml of packed erythrocytes) did not affect erythrocyte metabolism or morphology. Hemolysates were found to catalyze efficient dephosphorylation of FdUMP to yield nearly stoichiometric amounts of the corresponding deoxyribonucleoside 5-fluoro-2'-deoxyuridine (FdUrd), an antineoplastic drug showing selective cytotoxicity toward liver metastases from colorectal carcinomas. The dephosphorylation reaction had an apparent Km of 7.7 +/- 1.2 mM FdUMP at pH 7.4 and was remarkably slower at pH 8.2. ATP, GTP, and UTP inhibited both the disappearance of FdUMP and the formation of FdUrd in hemolysates. The enzyme responsible for the FdUMP-to-FdUrd conversion was identified with the deoxyribonucleotide-specific isozyme of erythrocyte pyrimidine 5'-nucleotidase (EC 3.1.3.5). Intracellular formation and subsequent release of FdUrd were observed in intact erythrocytes loaded with FdUMP. Inhibition of FdUrd release from these erythrocytes was obtained by raising the pH intracellularly and, alternatively, by coencapsulation of ATP. Autologous FdUMP-loaded erythrocytes might be used as endogenous bioreactors designed for time-programmed and liver-targeted delivery of FdUrd.
...
PMID:Conversion of encapsulated 5-fluoro-2'-deoxyuridine 5'-monophosphate to the antineoplastic drug 5-fluoro-2'-deoxyuridine in human erythrocytes. 296 99

Intact synaptosomes isolated from the electric organ of the electric ray Torpedo marmorata contain, at their surface, enzyme activities for the hydrolysis of externally applied nucleoside phosphates. The diazonium salt of sulfanilic acid, as a low-molecular-weight, slowly permeating, covalent inhibitory agent, selectively blocks these enzyme activities and leaves intracellular lactate dehydrogenase intact. The ectoenzymes comprise both a nucleoside triphosphate and diphosphate phosphohydrolase, as well as a 5'-nucleotidase. Activity of nonspecific ectophosphatases is absent. The nucleoside triphosphatase hydrolyzes almost equally well ATP, GTP, CTP, UTP, and ITP and is activated to a similar degree by Mg2+ or Ca2+. It has a high affinity for ATP (Km for ATP in the presence of Mg2+, 75 microM; in the presence of Ca2+, 66 microM). Maximal rates in the presence of Mg2+ and Ca2+ were very similar (34.8 and 32.5 nmol of Pi/min/mg of synaptosomal protein, respectively). Either Mg-ATP or Ca-ATP can act as a true substrate. ADP inhibits hydrolysis of ATP, but AMP is without effect. The nucleoside triphosphatase is not inhibited significantly by a number of inhibitors of mitochondrial Mg2+-ATPase or of Ca2+ + Mg2+-ATPases. It is, however, considerably inhibited by filipin and quercitin. The capacity of intact synaptosomes to hydrolyze also extracellular ADP, GDP, AMP, GMP, and IMP suggests that the nucleoside triphosphatase is part of an enzyme chain that causes complete hydrolysis of the respective nucleoside triphosphate to the nucleoside. We conclude that the cholinergic nerve terminals of the Torpedo electric organ can hydrolyze ATP released on coexocytosis with acetylcholine via an ectonucleoside triphosphatase activity that is different from known endogenous nerve terminal ATPases. The final product of the hydrolysis, adenosine, can then be salvaged by the nerve terminal for resynthesis of ATP. Other possible physiological functions of the ectonucleotidases are discussed.
...
PMID:Ectonucleotidase activities associated with cholinergic synaptosomes isolated from Torpedo electric organ. 301 88

A membrane fraction enriched in plasma membrane marker enzymes K+-dependent p-nitrophenyl phosphatase, 5'-nucleotidase and alkaline phosphatase was prepared from rat parotid glands using Percoll self-forming gradient. This fraction contained an ATP-dependent CA2+ transport system which was distinct from those located on the endoplasmic reticulum and mitochondria of parotid glands. The Km for ATP was 0.57 +/- 0.07 mM (n = 3). Nucleotides other than ATP such as ADP, AMP, GTP, CTP, UTP or ITP were unable to support significant Ca2+ uptake. ATP-dependent Ca2+ uptake displayed sigmoidal kinetics with respect to free Ca2+ concentration with a Hill coefficient of 2.02. The K0.5 for Ca2+ was 44 +/- 3.1 nM (n = 3) and the average Vmax was 13.5 +/- 1.1 nmol/min per mg of protein. The pH optimum was 7.2. Trifluorperazine inhibited Ca2+ transport with half maximal inhibition observed at 30.8 microM. Complete inhibition was observed at 70 microM trifluorperazine. Exogenous calmodulin however had no effect on the rate of transport. Na+ and K+ ions activated Ca2+ transport at 20 to 30 mM ion concentrations. Higher concentrations of Na+ or K+ were inhibitory.
...
PMID:Characterisation of an ATP-dependent Ca2+ transport system in a plasma membrane enriched fraction from rat parotid. 345 46


<< Previous 1 2 3 4 5 Next >>

---