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PubMed1996

Central nervous system depressant effects of N3-substituted derivatives of deoxyuridine in mice.

N3-Substituted derivatives of deoxyuridine (1) were synthesized and their pharmacological effects were evaluated by intracerebroventricular (i.c.v.) injection in mice. Eleven derivatives, including the methyl (2), ethyl (3), propyl (4), allyl (5), butyl (6), benzyl (7), o, m and p-xylyls (8, 9, 10), alpha-phenylethyl (11) and phenacyl (12) derivatives, of 1 were prepared and their pharmacological effects were evaluated by using hypnotic activity, pentobarbital-induced sleep prolongation, spontaneous activity and motor incoordination as indices of central nervous system (CNS) depressant effects. At a dose of 2.0 mumol/mouse, the values of mean sleeping time induced by 7, 8, 9 and 10 were 23, 35, 29 and 30 min, respectively. Although the alkyl (2-6) derivatives did not cause any hypnotic activity, some derivatives tested (3, 5, 6, 8-12) significantly prolonged the pentobarbital-induced sleeping time. When the CNS depressant effects of phenacyl substituted 1 were compared to that of other oxopyrimidine nucleosides, N3-phenacyluridine (13), N3-phenacylthymidine (14), N3-phenacyl-6-azauridine (15), compounds 12, 13 and 14 (1.0 mumol/mouse, i.c.v.) significantly decreased mouse spontaneous activity. Furthermore, 12-15 (1.0 mumol/mouse, i.c.v.) caused mouse motor incoordination. These results indicate that deoxyuridine derivatives have generally central depressant activity, and the benzyl and xylyl derivatives, but not alkyl derivatives, possess hypnotic activity.

Animals↗
PubMed2005

Synthesis of N3-substituted uridine and related pyrimidine nucleosides and their antinociceptive effects in mice.

Seventy eight N(3)-substituted derivatives of uridine (1), thymidine (2), 2'-deoxyuridine (3), 6-azauridine (4), 2',3'-O-isopropylideneuridine (5), and arabinofuranosyluracil (6) were synthesized and their antinociceptive effects were evaluated. N(3)-(2',4'-Dimethoxyphenacyl)uridine (1l), N(3)-(2',4'-dimethoxyphenacyl)2'-deoxyuridine (3l), and N(3)-(2',5'-dimethoxyphenacyl)arabinofuranosyluracil (6m) possessed 93, 86, and 82% of the antinociceptive effects tested by hot plate, respectively. The antinociceptive effects of three derivatives were 5.8, 5.4, and 5.1-folds of the effect of N(3)-phenacyluridine (1h) (16%), respectively. The structure-activity relationship of N(3)-substituted pyrimidine nucleosides was also discussed.

Analgesics↗
PubMed2003

Use of nucleotide analogs by class I and class II CCA-adding enzymes (tRNA nucleotidyltransferase): deciphering the basis for nucleotide selection.

We explored the specificity and nature of the nucleotide-binding pocket of the CCA-adding enzyme (tRNA nucleotidyltransferase) by using CTP and ATP analogs as substrates for a panel of class I and class II enzymes. Overall, class I and class II enzymes displayed remarkably similar substrate requirements, implying that the mechanism of CCA addition is conserved between enzyme classes despite the absence of obvious sequence homology outside the active site signature sequence. CTP substrates are more tolerant of base modifications than ATP substrates, but sugar modifications prevent incorporation of both CTP and ATP analogs by class I and class II enzymes. Use of CTP analogs (zebularine, pseudoisocytidine, 6-azacytidine, but not 6-azauridine) suggests that base modifications generally do not interfere with recognition or incorporation of CTP analogs by either class I or class II enzymes, and that UTP is excluded because N-3 is a positive determinant and/or O-4 is an antideterminant. Use of ATP analogs (N6-methyladenosine, diaminopurine, purine, 2-aminopurine, and 7-deaza-adenosine, but not guanosine, deoxyadenosine, 2'-O-methyladenosine, 2'-deoxy-2'-fluoroadenosine, or inosine) suggests that base modifications generally do not interfere with recognition or incorporation of ATP analogs by either class I or class II enzymes, and that GTP is excluded because N-1 is a positive determinant and/or the 2-amino and 6-keto groups are antideterminants. We also found that the 3'-terminal sequence of the growing tRNA substrate can affect the efficiency or specificity of subsequent nucleotide addition. Our data set should allow rigorous evaluation of structural hypotheses for nucleotide selection based on existing and future crystal structures.

Base Sequence↗
PubMed1999

Possible involvement of DNA methylation in 5-azacytidine-induced neuronal cell apoptosis.

Eight chemicals that are cytidine analogues or nucleosides (5-azacytidine (5AzC), 5-azadeoxycytidine, 6-azacytidine, 5-azacytosin, cytidine, 3-deazaadenine, 3-deazauridine and 6-azauridine) were examined for the ability to induce neuronal apoptosis. 5AzC and 5-azadeoxycytidine induced apoptosis in the brain and spinal cord of the fetuses at 24 hr after the injection to dams, while the other chemicals tested failed to induce apoptosis. In the system of PC12 cells, only 5AzC induced apoptosis, and other chemicals failed to provoke morphological and biochemical changes characteristic of apoptosis. 5AzC, 5-azadeoxycytidine and 6-azacytidine failed to induce apoptosis in C6 cells. Gel electrophoresis after MspI or HapII digestions revealed no apparent evidence of DNA demethylation after 5AzC-treatment in either fetal brains or PC12 cells. These results indicate that DNA demethylation is possibly involved in 5AzC-induced neuronal apoptosis although no direct evidence of DNA demethylation was obtained.

Animals↗
PubMed1990

Effects of pyrimidine salvage inhibitors on uracil incorporation of Toxoplasma gondii.

Metabolic inhibitors which act in the process of pyrimidine salvage influenced on the uracil incorporation into nucleic acids of Toxoplasma. Inhibitors of dihydrofolate reductase, pyrimethamine and methotrexate, and inhibitors of thymidylate synthase, fluoro-uridine, fluoro-dUMP and fluoro-uracil, diminished isotopic uracil uptake in dose-dependent manners. Azauridine which suppresses de novo pyrimidine biosynthesis did not affect the salvage even in a relatively high dose. These results suggested that the activation of uracil salvage should be closely related with the function of TMP biosynthetic enzymes. The pattern of thymidine uptake had no differences between control HL-60 cells and Toxoplasma infected cells, which did not reflect the specific proliferation of Toxoplasma. It can be exploited to characterize the effects of various compounds related with the proliferation of Toxoplasma, especially its DNA synthesis.

Animals↗
PubMed1976

Vitamin B12--folate interrelations.

Megaloblastic anaemia is due to a derangement of DNA synthesis caused by insufficient supply of one or other of the four deoxyribonucleoside triphosphate (dNTP) precursors of DNA synthesis or by direct inhibition of one or other DNA polymerase. Reduced supply of the pyrimidine deoxythymidine triphosphate (dTTP) may be caused by folate or vitamin B12 deficiencies or by the action of dihydrofolate reductase inhibitors (e.g. methotrexate, pyrimethamine or trimethoprim), all of which cause reduced supply of the coenzyme 5, 10 methylene tetrahydrofolate (pentaglutamate) needed for thymidylate synthetase. Reduced dTTP supply may also be caused by direct inhibition of thymidylate synthetase by 5-fluorouracil. Reduced supply of both purines, deoxyadenosine triphosphate (dATP) and deoxyguanosine triphosphate (dGTP), may be caused by hydroxyurea, 6-mercaptopurine (and probably by another purine antagonist azaserine), whilst reduced supply of both pyrimidine DNA precursors, dTTP and dCTP (deoxycytidine triphosphate) may be due to inherited orotic aciduria or to treatment with azauridine. Cytosine arabinoside directly inhibits DNA polymerase. DNA replication is a discontinuous process and a number of enzymes are concerned with different aspects of the process. The parental strands partly unwind and a large number of initiation points or origins are activated on both strands. A primer RNA is first synthesised using the parental strand of DNA as template. Fragments of new DNA are then synthesised on the parental DNA template, starting at the RNA primer, under the action of one or other DNA polymerase (probably gamma). The RNA primer is then removed and the gap left is filled by further DNA synthesis under the action of a different DNA polymerase (probably alpha). The fragments of new DNA are joined to give newly synthesised stretches of DNA (replicons) which are then liigated together to form bulk DNA of enormous molecular weight. It is suggested here that reduced supply of one or other of the four deoxyribonucleoside triphosphate (dNTP) during the 'S' phase of the cell cycle (due to vitamin B12 or folate deficiency, drug treatment or other congenital or acquired abnormality in synthesis of the dNTP) impairs the cell's ability to elongate newly initiated DNA fragments by preventing gap-filling, the polymerase needed for gap-filling requiring substantially greater concentrations of the deoxyribonucleoside triphosphates than the polymerase involved in chain initiation. Cytosine arabinoside, which also may cause megaloblastosis, may affect principally the synthesis of new DNA fragments. Since active protein synthesis is needed for the cell to enter the S phase and RNA synthesis is needed to prime new DNA synthesis, megaloblastic anaemia may be expected to occur only when DNA synthesis is inhibited but protein and RNA synthesis are relatively unimpaired...

Anemia, Megaloblastic↗
PubMed1991

Purification and characterization of yeast orotidine 5'-monophosphate decarboxylase overexpressed from plasmid PGU2.

Orotidine 5'-monophosphate decarboxylase (ODCase) has been overexpressed in yeast 15C cells transformed with a plasmid carrying the URA3 gene that encodes ODCase. Twenty g of cells having ODCase activity equal to 30 mg of pure enzyme per liter of cell culture were obtained after 9 h of galactose induction. To remove yeast proteases, a 60-90% ammonium sulfate fractionation step plus the addition of EDTA as an inhibitor of metallopeptidases was necessary. The purification protocol yielded ODCase that was protease-free and stable to storage at 4 degrees C for 16 months. The pure enzyme had a specific activity of 40 units/mg in 50 mM phosphate buffer, pH 6, and could be stored at -20 degrees C in 20% glycerol with retention of full activity for more than 2 years. The enzyme had a Km for orotidine 5'-monophosphate of 0.7 microM at pH 6 and 25 degrees C. The molecular weight of the plasmid-derived ODCase monomer determined by electrophoresis on denaturing polyacrylamide gels was 29,500. ODCase sedimented through sucrose density gradients as a monomer of about 30 kDa at low protein concentration and in the absence of ligands that bind at the catalytic site. An increase in the sedimentation rate could be induced by increasing the ODCase concentration or by adding ligands that are competitive inhibitors. ODCase sedimented in a single band typical of a protein of 46 kDa at the highest protein concentration studied or in the presence of 50 mM phosphate or 933 microM substrate (orotidine 5'-monophosphate) or product (UMP). A dimer sedimenting as a protein of about 64 kDa occurred in the presence of 50 microM 6-azauridine 5'-monophosphate or 2 microM 1-(5'-phospho-beta-D-ribofuranosyl) barbituric acid, competitive inhibitors of ODCase. These results resemble the ligand-induced subunit association of the ODCase domain of bifunctional UMP synthase and support the use of yeast ODCase as a model for ODCases from other species.

Binding, Competitive↗
PubMed1987

In situ regulation of mammalian CTP synthetase by allosteric inhibition.

The regulatory role of the allosteric site of CTP synthetase on flux through the enzyme in situ and on pyrimidine nucleotide triphosphate (NTP) pool balance was investigated using a mutant mouse T lymphoblast (S49) cell line which contains a CTP synthetase refractory to complete inhibition by CTP. Measurements of [3H]uridine incorporation into cellular pyrimidine NTP pools as a function of time indicated that CTP synthesis in intact wild type cells was markedly inhibited in a cooperative fashion by small increases in CTP pools, whereas flux across the enzyme in mutant cells was much less affected by changes in CTP levels. The cooperativity of the allosteric inhibition of the enzyme was greater in situ than in vitro. Exogenous manipulation of levels of GTP, an activator of the enzyme, indicated that GTP had a moderate effect on enzyme activity in situ, and changes in pools of ATP, a substrate of the enzyme, had small effects on CTP synthetase activity. The consequences of incubation with actinomycin D, cycloheximide, dibutyryl cyclic AMP, and 6-azauridine on the flux across CTP synthetase and on NTP pools differed considerably between wild type and mutant cells. Under conditions of growth arrest, an intact binding site for CTP on CTP synthetase was required to maintain a balance between the CTP and UTP pools in wild type cells. Moreover, wild type cells failed to incorporate H14CO3- into pyrimidine pools following growth arrest. In contrast, mutant cells incorporated the radiolabel at a high rate indicating loss of a regulatory function. These results indicated that uridine nucleotides are important regulators of pyrimidine nucleotide synthesis in mouse S49 cells, and CTP regulates the balance between UTP and CTP pools.

Animals↗
PubMed1989

[Plasma homocysteine, a risk factor for premature vascular disease. Plasma levels in healthy persons; during pathologic conditions and drug therapy].

Homocysteine is a branch-point metabolite, the biological fate of which is linked to vitamin B12, reduced folates and vitamin B6. Various inborn defects in homocysteine metabolism, among which cystathionine beta-synthase deficiency is most common, lead to the clinical condition homocystinuria. A central feature of this clinical state is premature arteriosclerosis. These patients benefit from agents serving as cofactors in homocysteine metabolism which both reduce the homocysteine levels in plasma and the incidence of vascular episodes. Experimental data point to homocysteine as an arteriosclerotic agent. Homocysteine in human plasma exists mainly as mixed disulfides with albumin (70 per cent) and cysteine. New methods determine total plasma homocysteine which includes all these species. Normal values for plasma homocysteine are lower in premenopausal women than in men and postmenopausal women. Impaired homocysteine metabolism seems to exist in 15-30 per cent of patients with premature cardiovascular disease. Moderate homocysteinemia is as a risk factor for cardiovascular disease, independent of conventional risk factors. Apart from homocystinuria, vitamin B12 deficiency causes the most extreme elevations of plasma homocysteine, and it has been established that plasma homocysteine is a more responsive parameter to impaired vitamin B12 function than serum cobalamin. Massive increase in plasma homocysteine level is also observed in folate deficiency, whereas renal failure, some malignant states and psoriasis cause a moderate homocysteinemia. High doses of folic acid reduce plasma homocysteine, and this innocuous mean should be considered as an intervention in patients with increased plasma level. Drugs like methotrexate, some anticonvulsants and 6-azauridine triacetate induce moderate elevation of plasma homocysteine, whereas a reduction is observed after penicillamine administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Arteriosclerosis↗
PubMed1987

2',3'-Dideoxycytidine: regulation of its metabolism and anti-retroviral potency by natural pyrimidine nucleosides and by inhibitors of pyrimidine nucleotide synthesis.

The antiretroviral action of 2',3'-dideoxycytidine (ddCyd) depends on its intracellular conversion to the 5'-triphosphate metabolite ddCTP. The effect of natural pyrimidines and pyrimidine nucleosides, as well as of a number of inhibitors of pyrimidine nucleotide synthesis (i.e., N-(phosphonacetyl)-L-aspartate, 6-azauridine, pyrazofurin, 3-deazauridine, and hydroxyurea) on the metabolism of the potent anti-human immunodeficiency virus drug ddCyd has been investigated in human and murine cell lines. Deoxycytidine (dCyd) and cytidine (Cyd) effectively blocked the intracellular phosphorylation of ddCyd: dCyd by competition with ddCyd for 2'-deoxycytidine kinase, and Cyd probably by competition with the higher nucleoside mono- and diphosphate kinases. These conclusions are supported by the observations that (i) the cytostatic effects of ddCyd against human Molt/4F cells are significantly reversed by dCyd; (ii) the antiviral effects of ddCyd against hman immunodeficiency virus-infected human ATH8 cells are reversed by dCyd and Cyd; (iii) phosphorylated metabolites of ddCyd could not be detected in a 2'-deoxycytidine kinase-deficient murine leukemia (L1210)/araC cell line; and (iv) ddCyd lacked any cytostatic effect against this araC-resistant L1210 cell line. In contrast to dCyd and Cyd, thymidine (dThd) stimulated formation of phosphorylated ddCyd metabolites. The degree of this stimulation proved dependent on preincubation time and dThd concentration. There was a correlation between the increased ddCTP levels upon preincubation of the cells with dThd, and decreased dCyd-5'-triphosphate pools, presumably caused by inhibition of cytidine-5' -diphosphate reductase by dThd-5'-triphosphate. In an attempt to discover compounds other than dThd that are able to stimulate ddCTP formation, a number of inhibitors of pyrimidine nucleotide metabolism were also studied. Under our experimental conditions, 3-deazauridine and hydroxyurea proved equally as effective as dThd in stimulating ddCyd phosphorylation. Finally, we could demonstrate that dThd significantly enhanced the protective effect of ddCyd against human immunodeficiency virus-infected ATH8 cells.

Animals↗
PubMed1985

Uridine kinase from Ehrlich ascites carcinoma. Purification and properties of homogeneous enzyme.

Uridine kinase from Ehrlich ascites tumor cells has been purified about 60,000-fold to apparent homogeneity and with an overall recovery of about 40%. This purification was achieved using phosphocellulose and adenosine 5'-triphosphate-agarose affinity chromatography. The subunit molecular mass as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis was 31,000 daltons. With two-dimensional electrophoresis, only one spot was observed, indicating the absence of isoenzymes. Multiple peaks of activity are routinely observed on ion exchange chromatography or gel filtration, for both crude preparations or homogeneous uridine kinase, in agreement with our earlier results that this enzyme exists as multiple interconvertible oligomeric forms (Payne, R. C., and Traut, T. W. (1982) J. Biol. Chem. 257, 12485-12488). The purified enzyme has a specific activity of 283 mumol/min/mg of protein at 22 degrees C. Initial velocity studies using uridine and ATP are consistent with a sequential mechanism. Km values for uridine, cytidine, and ATP are 40, 57, and 450 microM, respectively. CTP and UTP are competitive inhibitors with respect to ATP, with Ki values for CTP and UTP of 10 and 61 microM, respectively. The enzyme was active with several nucleoside analogs, the Km values being 69 microM (5-fluorouridine), 200 microM (3-deazauridine), and 340 microM (6-azauridine). The pure enzyme is very sensitive to freezing, but can be maintained at O degrees C for 8 weeks with only 20% loss of activity. For long-term storage, enzyme in 50% glycerol can be maintained at -20 degrees C for many months with no detectable loss of activity.

Animals↗
PubMed1986

Homogeneous uridine kinase from Ehrlich ascites tumor: substrate specificity and inhibition by bisubstrate analogs.

Uridine kinase has been purified to homogeneity from Ehrlich ascites tumor cells. For the phosphate acceptor site, the enzyme shows substrate specificity only for ribopyrimidine nucleosides and is active with various analogs that have limited structural alterations; both endocyclic and exocyclic substituents can be acceptable. Of nucleosides that have been used in the chemotherapy of cancer, 5-fluorouridine, 6-azauridine, and 3-deazauridine are good substrates, whereas arabinosylcytosine is a poor substrate. No analogs are better substrates than the physiological substrates uridine and cytidine. 5', 5''' -P1, P4-Bisnucleoside oligophosphate bisubstrate analogs (e.g., Ap4U, Ap5U) were synthesized and tested as inhibitors. The most effective compound was Ap4U; with a Ki of 197 microM, it bound more tightly than ATP but no better than uridine. Ap3A, Ap4A, and Ap5A were also tested, with the result that both Ap4A and Ap4U were most effective, suggesting that this size of bisubstrate analog most closely approaches the spacing of the catalytic site.

Adenine Nucleotides↗
PubMed1986

[Effect of chemotherapy on the nucleoside phosphate kinase activity in experimental tumors].

In animals with experimental transplantable tumors, an effective treatment with antitumor compounds brought about interrelated changes in nucleoside phosphate kinase activity. The decrease in thymidine phosphate kinase activity was as a rule matched by an increase in that of uridine phosphate kinase. Consequently, "thymidine kinase-uridine kinase" shunt responsible for thymidine-uridine interconversion was suggested, which is switched on when the homeostasis of tumor cells is disturbed. In treatment of tumor-bearing animals, the effective dosage of antitumor drugs was considerably decreased due to a combination of the said compounds (inhibiting nucleoside kinases) or with azauridine which inhibits uridine monophosphate synthesis from orotidine-5'-phosphate.

Animals↗
PubMed1987

Study of the kinetic and physical properties of the orotidine-5'-monophosphate decarboxylase domain from mouse UMP synthase produced in Saccharomyces cerevisiae.

In mammals, the bifunctional protein UMP synthase contains the final two enzymatic activities, orotate phosphoribosyltransferase and orotidine-5'-monophosphate decarboxylase (ODCase), for de novo biosynthesis of UMP. The plasmid pMEJ contains a cDNA for the ODCase domain of mouse Ehrlich ascites UMP synthase. The cDNA from pMEJ was joined to the Saccharomyces cerevisiae iso-1-cytochrome c (CYC1) promoter and the first four CYC1 coding nucleotides in the plasmid pODCcyc. ODCase-deficient yeast cells (HF200x1) transformed with pODCcyc expressed an active ODCase domain with a specific activity of 20 nmol/min/mg in cell extracts. The expressed ODCase domain has a lower affinity for the substrate orotidine 5'-monophosphate and the inhibitor 6-azauridine 5'-monophosphate than intact UMP synthase or an ODCase domain isolated after proteolysis of homogenous UMP synthase. Sucrose density gradient sedimentation experiments showed that the expressed ODCase domain forms a dimer in the presence of ligands which bind at the catalytic site. These studies support the existence of an ODCase structural domain which contains the ODCase catalytic site and a dimerization surface of UMP synthase, but the domain may not have the regulatory site required to form the altered dimer form.

Base Sequence↗
PubMed1987

The purification and preliminary characterization of UMP synthase from human placenta.

Orotate phosphoribosyltransferase (EC 2.4.2.10) and orotidine 5'-monophosphate decarboxylase (EC 4.1.1.23) are the final two of six enzymatic steps required in the de novo biosynthesis of uridine 5'-monophosphate (UMP). Earlier work of this laboratory showed that, in the mouse Ehrlich ascites carcinoma, both of these enzymatic activities were contained on the single multifunctional polypeptide chain, UMP synthase. We report here that the placenta provided an available human source for UMP synthase with 40-fold higher orotate phosphoribosyltransferase and orotidine 5'-monophosphate decarboxylase specific activities than erythrocytes, a human source previously used by others. By using the placenta as a source of UMP synthase and by developing a novel purification procedure different from that used in the purification of UMP synthase from the Ehrlich ascites carcinoma (the only other homogeneous preparation of a mammalian UMP synthase), we achieved the purification of human UMP synthase to apparent homogeneity. This represents the first publication to homogeneity of UMP synthase from a human source as well as from a source other than malignant cell lines. Contrary to earlier reports human placental UMP synthase was found to be a multifunctional protein containing both enzymatic activities on a single polypeptide of 51,000 molecular weight. Preliminary characterization of the human placental UMP synthase showed it to be similar to UMP synthase from the Ehrlich ascites carcinoma in subunit molecular weight, native molecular weight, isozyme pattern (although not absolute pI values), pH optima of enzymatic activities, and kinetic constants for orotidine 5'-monophosphate (Km) and 6-azauridine 5'-monophosphate (Ki) at the decarboxylase site.

Carboxy-Lyases↗
PubMed1979

The purification of orotidine-5'-phosphate decarboxylase from yeast by affinity chromatography.

We have prepared an affinity column for the purification of orotidine-5'-phosphate decarboxylase from yeast. The column effects a 3200-fold purification from yeast homogenate in one pass; simple additional steps produce enzyme that has been purified 6700-fold and is not contaminated by any other protein that can be detected by sodium dodecyl sulfate-acrylamide gel electrophoresis. Overall, 35% of the activity present in the yeast is recovered as pure enzyme. The resin for the column is synthesized by attaching the ethylenediamine amide of 5-(2-carboxyethyl)-6-azauridine 5'-phosphate to carboxymethyl-agarose.

Carboxy-Lyases↗
PubMed1985

De novo pyrimidine biosynthesis in isolated rat hepatocytes. Quantitative aspects of the regulation by UTP.

Quantitative aspects of de novo pyrimidine biosynthesis in rat hepatocytes were monitored. A reduction of intracellular UTP contents by different concentrations of D-galactosamine led to a dose-dependent increase of 14CO2 incorporation into the sum of all acid-soluble uracil nucleotides. In controls the rate of de novo synthesis which was calculated from the incorporation rate of 14CO2 into the sum of all acid-soluble uracil nucleotides was 0.014 mumol X h-1 X g-1 compared to 0.056 mumol X h-1 X g-1 wet weight of liver in situations of a maximally stimulated de novo synthesis. Incubation of hepatocytes with uridine led to a dose-dependent reduction of 14CO2 incorporation to less than 25% of the amount incorporated in the controls. Alterations of the CTP content had no influence on the 14CO2 incorporation. In the presence of high D-galactosamine concentrations the increase of the total amount of acid-soluble uracil nucleotides exceeded the rate of the de novo synthesis derived from the incorporation of 14CO2 into the sum of the acid-soluble uracil nucleotide pool. It was also greater than the increase of the total amount of intra- and extracellular orotate after acidic hydrolysis--even in the presence of 6-azauridine, which stimulated de novo pyrimidine biosynthesis by itself.

Adenosine Triphosphate↗
PubMed1984

Effect of uridine on response of 5-azacytidine-resistant human leukemic cells to inhibitors of de novo pyrimidine synthesis.

A uridine-cytidine kinase-deficient human promyelocytic leukemic subline (HL-60-5-aza-Cyd) has been isolated which is highly resistant to the antileukemic agent 5-azacytidine. Resistant cells exposed to 10(-5) M 5-azacytidine for 2 hr exhibit a marked reduction in both the total intracellular accumulation of 5-azacytidine (11.9 versus 156.0 pmol/10(6) cells) as well as its incorporation into RNA (3.1 versus 43.4 pmol/micrograms D-ribose) compared to the parent line. These biochemical changes are associated with nearly a 100-fold decrease in sensitivity to the growth inhibitory effects of 5-azacytidine (concentration of drug associated with a 50% reduction in cell growth, 3.5 X 10(-5) versus 5.0 X 10(-7) M). 5-Azacytidine-resistant cells exhibit cross-resistance to 3-deazauridine, 6-azauridine, and 5-fluorouridine, but not to 1-beta-D-arabinofuranosylcytosine, 2'-deoxyazacytidine, or 5-aza-1-beta-D-arabinofuranosylcytosine. Coadministration of 50 microM uridine prevented depletion of pyrimidine nucleoside triphosphates and inhibition of colony formation of HL-60 cells exposed to 3 mM N-(phosphonacetyl)-L-aspartate) or 5 X 10(-6) M pyrazofurin but was not capable of protecting HL-60-5-azacytidine under the same conditions. This uridine concentration was also able to restore control colony formation to normal human bone marrow progenitor cells (granulocyte-macrophage colony-forming units) exposed to de novo pyrimidine biosynthetic blockade. These in vitro studies suggest that 5-azacytidine resistant cells lacking the pyrimidine salvage pathway enzyme uridine-cytidine kinase may be selectively vulnerable to a regimen using pyrimidine antagonists administered in conjunction with the naturally occurring nucleoside uridine.

Azacitidine↗