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PubMed1997

Synthesis and stereochemical structure-activity relationships of 1,3-dioxoperhydropyrido[1,2-c]pyrimidine derivatives: potent and selective cholecystokinin-A receptor antagonists.

The synthesis and stereochemical structure--activity relationships of a new class of potent and selective non-peptide cholecystokinin-A (CCK-A) receptor antagonists based on the 1,3-dioxoperhydropyrido[1,2-c]pyrimidine skeleton are described. The most potent member of this series of eight diastereoisomers, (4aS,5R)-2-benzyl-5-[N-[(tert-butoxycarbonyl)-L-tryptophyl]-amino] - 1,3-dioxoperhydropyrido[1,2-c]pyrimidine, displays nanomolar CCK-A receptor affinity and higher than 8000-fold potency at the CCK-A than at the CCK-B receptor. As CCK-A antagonist, this compound inhibits the CCK-8-evoked amylase release from pancreatic acinar cells at a low concentration, similar to that of the typical antagonist Devazepide. Highly strict stereochemical requirements for CCK-A receptor binding and selectivity have been found. The L-Trp and the 4a,5-trans disposition of the bicyclic perhydropyrido[1,2-c]pyrimidine are essential for binding potency and selectivity.

Amylases↗

PubMed1998

Inhibition of human O6-alkylguanine-DNA alkyltransferase and potentiation of the cytotoxicity of chloroethylnitrosourea by 4(6)-(benzyloxy)-2,6(4)-diamino-5-(nitro or nitroso)pyrimidine derivatives and analogues.

A series of 4(6)-(benzyloxy)-2,6(4)-diamino-5-(nitro or nitroso)pyrimidine derivatives and analogues of which 4(6)-benzyloxy groups were replaced with a (2-, 3-, or 4-fluorobenzyl)oxy or (2-, 3-, or 4-pyridylmethyl)oxy group, was synthesized. The abilities of these compounds to inhibit human O6-alkylguanine-DNA alkyltransferase (AGAT) in vitro and to potentiate the cytotoxicity of 1-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-3-(2-chloroethyl) -3-nitrosourea (ACNU) toward HeLa S3 cells were evaluated. 2,4-Diamino-6-[(2-fluorobenzyl)oxy]-5-nitropyrimidine (3) and 2,4-diamino-5-nitro-6-(2-pyridylmethoxy)pyrimidine (6), whose ortho positions of the 6-substituent are modified, were much weaker in terms of these abilities than the corresponding meta- or para-modified compounds. These results are consistent with those of our previous study using a series of O6-benzylguanine derivatives. All 5-nitrosopyrimidine derivatives examined exerted both stronger AGAT-inhibition and ACNU-enhancement abilities than the corresponding 5-nitro derivatives. Among a variety of compounds that we have examined to date, 2,4-diamino-6-[(4-fluorobenzyl)oxy]-5-nitrosopyrimidine (10) exhibited the strongest ability to inhibit AGAT, and its magnitude was 2.5 and 50 times those of 4-(benzyloxy)-2,6-diamino-5-nitrosopyrimidine (9) and O6-benzylguanine (1), respectively. A strong positive correlation was observed between the ability to inhibit AGAT and to potentiate the cytotoxicity of ACNU. This strongly indicates that 4(6)-(benzyloxy)pyrimidine derivatives and their analogues potentiate ACNU cytotoxicity by inhibiting AGAT activity. To characterize the reactivity of test compounds, alkyl-transfer reactions were also carried out using the biomimetic alkyl-transfer system.

Cell Survival↗

PubMed1998

2,4-Diamino-6,7-dihydro-5H-cyclopenta[d]pyrimidine analogues of trimethoprim as inhibitors of Pneumocystis carinii and Toxoplasma gondii dihydrofolate reductase.

Three previously unreported (R,S)-2,4-diamino-5-[(3,4,5-trimethoxyphenyl) alkyl]-6,7-dihydro-5H-cyclopenta[d]pyrimidines 15a-c were synthesized as analogues of trimethoprim (TMP) and were tested as inhibitors of Pneumocystis carinii, Toxoplasma gondii, and rat liver dihydrofolate reductase (DHFR). The length of the alkyl bridge between the cyclopenta[d]pyrimidine and trimethoxyphenyl moiety ranged from one in 15a to three carbons in 15c. The products were tested as competitive inhibitors of the reduction of dihydrofolate by Pneumocystis carinii, Toxoplasma gondii, and rat liver DHFR. Compounds 15a-c had IC50 values of > 32, 1.8 and 1.3 microM, respectively, against P. carinii DHFR, as compared to 12 microM for TMP. Against the T. gondii enzyme, 15a-c had IC50 values of 21, 0.14 and 0.14 microM, respectively, as compared to 2.7 microM for TMP. Inhibitors 15b and 15c with two- and three-carbon bridges were significantly more potent than 15a against all three enzymes. Unlike TMP, 15b and 15c were better inhibitors of the rat liver enzyme than of the microbial enzymes. The potency of 15b and 15c against rat liver DHFR was less than has been reported for the corresponding 6,7-dihydro-5H-cyclopenta[d]pyrimidines with a classical p-aminobenzoyl-L-glutamate side chain as inhibitors of bovine, murine, and human DHFR.

Animals↗

PubMed1998

Pyrimidine thioethers: a novel class of HIV-1 reverse transcriptase inhibitors with activity against BHAP-resistant HIV.

A series of pyrimidine thioethers was synthesized and evaluated for inhibitory properties against wild-type HIV-1 reverse transcriptase (RT) and an RT carrying the resistance-conferring mutation P236L. Modifications of both the pyrimidine and the functionality attached through the thioether yielded several analogues, which demonstrated activity against both enzyme types, with IC50 values as low as 190 nM against wild-type and 66 nM against P236L RT. Evaluation of a select number of pyrimidine thioethers in cell culture showed that these compounds have excellent activity against HIV-1IIIB-WT and retain good activity against a laboratory-derived HIV-1MF delavirdine-resistant variant.

Amino Acid Substitution↗

PubMed1999

Synthesis of classical and a nonclassical 2-amino-4-oxo-6-methyl-5-substituted pyrrolo[2,3-d]pyrimidine antifolate inhibitors of thymidylate synthase.

Compounds 2-5 were designed as potential antifolate nonpolyglutamatable inhibitors of thymidylate synthase (TS). These analogues are structurally related to 2-amino-4-oxo-5-substituted quinazolines and 2-amino-4-oxo-5-substituted pyrrolo[2, 3-d]pyrimidines which have shown excellent inhibition of TS and, for the quinazoline, significant promise as clinically useful antitumor agents. Compounds 2-4 were synthesized by appropriate amine exchange reactions on pivaloyl-protected 5-dimethylaminomethyl-substituted 6-methyl pyrrolo[2,3-d]pyrimidine 7 which in turn was obtained from the Mannich reaction of pivaloylated-6-methyl pyrrolo[2, 3-d]pyrimidine 6. In instances where the amine exchange reaction was sluggish, the Mannich base was quaternized with methyl iodide which afforded much faster exchange reaction with improved yields. For compound 5, 4-mercaptopyridine was used as the nucleophile and reacted with 7. The analogues 2-4 inhibited Lactobacillus casei (lc) TS and recombinant human (h) TS with IC50 in the 10(-4) to 10(-5) M range. Compound 5 inhibited lcTS and hTS 20% at 26 and 25 microM, respectively. In addition, compound 5 inhibited the growth of Pneumocystis carinii and Toxoplasma gondii cells in culture by 76% at 32 x 10(-6) M and 50% at 831 x 10(-6) M, respectively.

Animals↗

PubMed1999

Tyrosine kinase inhibitors. 15. 4-(Phenylamino)quinazoline and 4-(phenylamino)pyrido[d]pyrimidine acrylamides as irreversible inhibitors of the ATP binding site of the epidermal growth factor receptor.

A series of 6- and 7-acrylamide derivatives of the 4-(phenylamino)quinazoline and -pyridopyrimidine classes of epidermal growth factor receptor (EGFR) inhibitors were prepared from the corresponding amino compounds by reaction with either acryloyl chloride/base or acrylic acid/1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride. All of the 6-acrylamides, but only the parent quinazoline 7-acrylamide, were irreversible inhibitors of the isolated enzyme, confirming that the former are better-positioned, when bound to the enzyme, to react with the critical cysteine-773. Quinazoline, pyrido[3,4-d]pyrimidine, and pyrido[3,2-d]pyrimidine 6-acrylamides were all irreversible inhibitors and showed similar high potencies in the enzyme assay (likely due to titration of the available enzyme). However the pyrido[3,2-d]pyrimidine analogues were 2-6-fold less potent than the others in a cellular autophosphorylation assay for EGFR in A431 cells. The quinazolines were generally less potent overall toward inhibition of heregulin-stimulated autophosphorylation of erbB2 (in MDA-MB-453-cells), whereas the pyridopyrimidines were equipotent. Selected compounds were evaluated in A431 epidermoid and H125 non-small-cell lung cancer human tumor xenografts. The compounds showed better activity when given orally than intraperitoneally. All showed significant tumor growth inhibition (stasis) over a dose range. The poor aqueous solubility of the compounds was a drawback, requiring formulation as fine particulate emulsions.

Acrylamides↗

PubMed1999

Synthesis of 2,8-disubstituted imidazo[1,5-a]pyrimidines with potent antitumor activity.

Seventeen 1,2,3,4-tetrahydroimidazo[1,5-a]pyrimidine derivatives bearing electron-withdrawing substituents were designed and synthesized by novel ring closure as potential antitumor agents. They were screened for their activities against mouse leukemia L1210 and human oral epidermoid carcinoma KB cell lines, and relationships of structure and antitumor activity in vitro are discussed. It was found that 8-thiocarbamoyl-1,2,3,4-tetrahydroimidazo[1, 5-a]pyrimidin-2(1H)-thione (8c) exhibited activity comparable to that of 5-fluorouracil against both L1210 and KB cells. The existence of both 2-thioxo and 8-substituent with a thioxo group in the molecule is crucial for the cytotoxicity against L1210 and KB cells. A novel procedure for introduction of a double bond between C-3 and C-4 in 8c was developed. Introduction of the 3,4-double bond increased the activity against L1210, but against KB cells the activity decreased by 4-fold. Cytotoxicity of compounds 8c and 8-thiocarbamoyl-1,2-dihydroimidazo[1,5-a]pyrimidin-2(1H)-thione (11c) against human solid tumor and leukemia cell lines was further evaluated. The saturation of the 3,4-double bond led to a significant increase in cytotoxicity against tumor cell lines tested.

Animals↗

PubMed1999

Structure-antiviral activity relationship in the series of pyrimidine and purine N-[2-(2-phosphonomethoxy)ethyl] nucleotide analogues. 1. Derivatives substituted at the carbon atoms of the base.

A series of dialkyl esters of purine and pyrimidine N-[2-(phosphonomethoxy)ethyl] derivatives substituted at position 2, 6, or 8 of the purine base or position 2, 4, or 5 of the pyrimidine base were prepared by alkylation of the appropriate heterocyclic base with 2-chloroethoxymethylphosphonate diester in the presence of sodium hydride, cesium carbonate, or 1,8-diazabicyclo[5,4, 0]undec-7-ene (DBU) in dimethylformamide. Additional derivatives were obtained by the transformations of the bases in the suitably modified intermediates bearing reactive functions at the base moiety. The diesters were converted to the corresponding monoesters by sodium azide treatment, while the free acids were obtained from the diester by successive treatment with bromotrimethylsilane and hydrolysis. None of the PME derivatives in the pyrimidine series, their 6-aza or 3-deaza analogues, exhibited any activity against DNA viruses or retroviruses tested, except for the 5-bromocytosine derivative. Substitution of the adenine ring in PMEA at position 2 by Cl, F, or OH group decreased the activity against all DNA viruses tested. PMEDAP was highly active against HSV-1, HSV-2, and VZV in the concentration range (EC50) of 0.07-2 microg/mL. Also the 2-amino-6-chloropurine derivative was strongly active (EC50 = 0.1-0. 4 microg/mL) against herpes simplex viruses and (EC50 = 0.006-0.3 microg/mL) against CMV and VZV. PMEG was the most active compound of the whole series against DNA viruses (EC50 approximately 0.01-0.02 microg/mL), though it exhibited significant toxicity against the host cells. The base-modified compounds did not show any appreciable activity against DNA viruses except for 7-deazaPMEA (IC50 approximately 7.5 microg/mL) against HIV-1 and MSV. The neutral (diisopropyl, diisooctyl) diesters of PMEA were active against CMV and VZV, while the corresponding monoesters were inactive. The diisopropyl ester of the 2-chloroadenine analogue of PMEA showed substantially (10-100x) higher activity against CMV and VZV than the parent phosphonate. Also, the diisopropyl and diisooctyl ester of PMEDAP inhibited CMV and VZV, but esterification of the phosphonate residue did not improve the activity against either MSV or HIV.

Animals↗

PubMed1999

Pneumocystis carinii and Toxoplasma gondii dihydrofolate reductase inhibitors and antitumor agents: synthesis and biological activities of 2,4-diamino-5-methyl-6-[(monosubstituted anilino)methyl] pyrido[2,3-d]pyrimidines.

Thirteen 2,4-diamino-5-methyl-6-[(monosubstituted anilino)methyl]pyrido[2,3-d]pyrimidines 5-17 were synthesized as potential Pneumocystis carinii (pc) and Toxoplasma gondii (tg) dihydrofolate reductase (DHFR) inhibitors and as antitumor agents. Compounds 5-17 were designed to investigate the structure-activity relationship of monomethoxy and monohalide substitution in the phenyl ring and N10-methylation of the C9-N10 bridge. The synthetic route to compounds 5-12 involved the reductive amination of a common intermediate, 2,4-diamino-5-methylpyrido[2, 3-d]pyrimidine-6-carbonitrile (18), with the appropriate anilines. N10-Methylation was achieved by reductive methylation. In contrast to previous reports of trimethoprim, the removal of methoxy and chloro groups from the phenyl ring in the 2, 4-diamino-5-methyl-6-[(substituted anilino)methyl]pyrido[2, 3-d]pyrimidine series generally did not decrease DHFR inhibitory activity. The monosubstituted phenyl analogues 5-12 were as potent against pcDHFR and tgDHFR as the previously reported disubstituted phenyl analogues. N10-Methylation generally resulted in a marginal increase in potency against both pcDHFR and tgDHFR. Compounds 5, 7, and 9 were evaluated and shown to inhibit the growth of T. gondii cells in culture at nanomolar concentrations. Compounds 6-8, 9, 11, and 16 were selected by the National Cancer Institute for evaluation in an in vitro preclinical antitumor screening program. All six compounds showed GI50 values in the 10(-7)-10(-9) M range in more than 20 cell lines.

Aniline Compounds↗

PubMed2001

Synthesis of bicyclic pyrimidine derivatives as ATP analogues.

A highly efficient and general solid-phase synthesis of bicyclic pyrimidine derivatives that target purine dependent proteins is reported. The synthesis of the key intermediate, 4,6-disubstituted-5-amino-pyrimidine, involved reduction of the corresponding nitro derivatives using 1,1'-dioctyl-viologen in a triphasic milieu. The mild reduction conditions enable the use of any acid labile solid support as well as a wide range of combinatorial substituents, thus enabling the synthesis of large libraries of highly diverse bicyclic pyrimidines. Alternative reduction conditions with tin(II) chloride and structure-reactivity studies are discussed as well.

Adenosine Triphosphate↗

PubMed2006

Simple synthesis of some pentafluoropropenyl derivatives of pyrimidine and purine based on addition-elimination reaction.

Various pentafluoropropenyl derivatives of pyrimidine and purine bases have been obtained in good to high yield. The procedure involves the reaction of appropriate lithium derivatives prepared from both electron-rich and electron-poor pyrimidines, with the hexafluoropropene at a low temperature, via an addition-elimination process. Organolithiums of pyrimidine and purine bases give addition-elimination products as E/Z mixtures, whereas the products of the reaction of lithium amide of protected inosine with hexafluoropropene contain traces of an addition product as well as the stable perfluoroenamine. The methodology proposed allows a series of perfluorovinyl nucleobases to be obtained quickly and conveniently.

Molecular Structure↗

PubMed2005

Molecular recognition kinetics of nonsurface active pyrimidine derivatives dissolved in the aqueous subphase by an amphiphilic melamine type monolayer: A theoretical approach.

Experimental studies show a drastic change of the characteristic features (Pi-A isotherms, morphology of the condensed phase domains) of the 2C11H23-melamine monolayers by molecular recognition of the pyrimidine derivatives uracil and thymine, largely affected by the reaction kinetics of the recognition process. A new approximate additive theoretical model is introduced for describing the molecular recognition kinetics. The theoretical approach is based on two assumptions: (i) first-order reaction kinetics for the molecular recognition of the nonsurface active pyrimidine derivatives dissolved in the aqueous subphase by the melamine type monolayer and (ii) applicability of the Pi-A isotherm equations derived previously for the description of melamine type monolayers in the fluid (gaseous, LE) state and the phase transition region to the condensed state. The rate constants for the molecular recognition reaction between the melamine type monolayer and the dissolved thymine and uracil are estimated, which are roughly proportional to the bulk concentrations of the pyrimidine derivatives and depend only slightly on the temperature. The theoretical estimates agree satisfactorily with the experimental results obtained under various conditions (Pi = const and dA/dt = const). This fact supports the validity of the first model proposed for the interfacial molecular recognition kinetics with nonsurface active substrates dissolved in the aqueous subphase by complementary hydrogen bonding.

Kinetics↗

PubMed1997

Two new puriniums and three new pyrimidines from Heterostemma brownii.

Two new puriniums, heteromines D (4) and E (5), and three new pyrimidines, heteromines F (6), G (7), and H (8), were isolated from the aerial parts of Heterostemma brownii Hay. Their structures were determined as 7,9-dimethyl-2-(N,N-dimethylamino)guaninium chloride, 7,9-dimethyl-2-(N-methylamino)guaninium chloride, 6-methoxy-4-(N-methylamino)-2-(N,N-dimethylamino)-5- (N-methylformamido)pyrimidine, 6-methoxy-2,4-bis(N-methylamino)-5-(N-methylformamido)pyrimidine, and 2-amino-6-methoxy-4-(N-methylamino)-5-(N-methylformamido)-pyrimidi ne, respectively.

Animals↗

PubMed1991

Quantitative structure-activity relationships (QSARs) of pyrimidine nucleosides as HIV-1 antiviral agents.

The structural requirements for the antiviral activity of pyrimidine nucleosides against HIV-1 virus was evaluated with the Hansch SAR analysis. Antiviral activity is best related to the hydrophobicity and steric (L and B3) properties of the substituent at the C5 of pyrimidine ring. Further, the antiviral activity is related to B4 of the substituent at position 3' of the sugar ring with a positive slope. The activity of both uracil and cytosine derivatives can be related to their structure by the same equations, which indicates that the SARs are similar in these two groups of congeners. These results suggest that compounds with a small substituent at the 5 position of the pyrimidine ring and a flat substituent at the 3' position of the sugar ring will be the most active compounds against HIV-1 virus.

Antiviral Agents↗

PubMed1978

The influence of ammonia on purine and pyrimidine nucleotide biosynthesis in rat liver and brain in vitro.

1. The effect of ammonia on purine and pyrimidine nucleotide biosynthesis was studied in rat liver and brain in vitro. The incorporation of NaH(14)CO(3) into acid-soluble uridine nucleotide (UMP) in liver homogenates and minces was increased 2.5-4-fold on incubation with 10mm-NH(4)Cl plus N-acetyl-l-glutamate, but not with either compound alone. 2. The incorporation of NaH(14)CO(3) into orotic acid was increased 3-4-fold in liver homogenate with NH(4)Cl plus acetylglutamate. 3. The 5-phosphoribosyl 1-pyrophosphate content of liver homogenate was decreased by 50% after incubation for 10min with 10mm-NH(4)Cl plus acetylglutamate. 4. Concomitant with this decrease in free phosphoribosyl pyrophosphate was a 40-50% decrease in the rates of purine nucleotide synthesis, both de novo and from the preformed base. 5. Subcellular fractionation of liver indicated that the effects of NH(4)Cl plus acetylglutamate on pyrimidine and purine biosynthesis required a mitochondrial fraction. This effect of NH(4)Cl plus acetylglutamate could be duplicated in a mitochondria-free liver fraction with carbamoyl phosphate. 6. A similar series of experiments carried out with rat brain demonstrated a significant, though considerably smaller, effect on UMP synthesis de novo and purine base reutilization. 7. These data indicate that excessive amounts of ammonia may interfere with purine nucleotide biosynthesis by stimulating production of carbamoyl phosphate through the mitochondrial synthetase, with the excess carbamoyl phosphate in turn increasing pyrimidine nucleotide synthesis de novo and diminishing the phosphoribosyl pyrophosphate available for purine biosynthesis.

Ammonia↗

PubMed1982

Uridylate trapping, induction of UTP deficiency, and stimulation of pyrimidine synthesis de novo by D-galactosone.

d-Galactosone (d-lyxo-2-hexosulose) is phosphorylated and metabolized to the uridine diphosphate derivative in AS-30D hepatoma cells and rat liver. These reactions were catalysed in vitro by galactokinase and hexose-1-phosphate uridylyltransferase. Nucleotide analyses by high-performance liquid chromatography and enzymic assays revealed that this galactose analogue interferes with cellular pyrimidine nucleotide metabolism leading to a deficiency of UTP. [(14)C]Uridine labelling of hepatoma cells indicated a division of [(14)C]uridylate from UTP into UDP-galactosone; the latter was formed at a rate of more than 1.7mmolxh(-1)x(kg AS-30D or liver wet wt.)(-1). As a consequence of UTP deficiency, d-galactosone (1mmol/1 or 1mmol/kg body wt.) strongly enhanced the rate of pyrimidine synthesis de novo as evidenced by incorporation of (14)CO(2) into uridylate and by an expansion of the uridylate pool. This resulted in a doubling of the total acid-soluble uridylate pool within 70min in the hepatoma cells and within 110min in rat liver. Combined treatment of hepatoma cells with d-galactosone and N-(phosphonoacetyl)-l-aspartate, an inhibitor of aspartate carbamoyltransferase, prevented the expansion of the uridylate pool and led to a synergistic reduction of UTP to 10% of the content in control cells. Hepatic UTP deficiency was selective with respect to other nucleotide 5'-triphosphates but was associated with reduced contents of UDP-glucose, UDP-glucuronate, and UDP-N-acetylhexosamines. Isolation of the UDP derivative of d-galactosone revealed an extremely alkali-labile UDP-sugar, probably an isomerization product of UDP-galactosone, that was degraded by elimination of UDP with a half-life of 45min at pH7.5 and 37 degrees C. The instability of UDP-galactosone may contribute in vivo to limit the time period of severe uridine phosphate deficiency in addition to the compensatory role of pyrimidine synthesis de novo. During the initial time period, however, d-galactosone is effective as a powerful uridylate-trapping sugar analogue.

Animals↗

PubMed1989

Glutamine-dependent carbamoyl-phosphate synthetase and other enzyme activities related to the pyrimidine pathway in spleen of Squalus acanthias (spiny dogfish).

The first two steps of urea synthesis in liver of marine elasmobranchs involve formation of glutamine from ammonia and of carbamoyl phosphate from glutamine, catalysed by glutamine synthetase and carbamoyl-phosphate synthetase, respectively [Anderson & Casey (1984) J. Biol. Chem. 259, 456-462]; both of these enzymes are localized exclusively in the mitochondrial matrix. The objective of this study was to establish the enzymology of carbamoyl phosphate formation and utilization for pyrimidine nucleotide biosynthesis in Squalus acanthias (spiny dogfish), a representative elasmobranch. Aspartate carbamoyltransferase could not be detected in liver of dogfish. Spleen extracts, however, had glutamine-dependent carbamoyl-phosphate synthetase, aspartate carbamoyltransferase, dihydro-orotase, and glutamine synthetase activities, all localized in the cytosol; dihydro-orotate dehydrogenase, orotate phosphoribosyltransferase, and orotidine-5'-decarboxylase activities were also present. Except for glutamine synthetase, the levels of all activities were very low. The carbamoyl-phosphate synthetase activity is inhibited by UTP and is activated by 5-phosphoribosyl 1-pyrophosphate. The first three enzyme activities of the pyrimidine pathway were eluted in distinctly different positions during gel filtration chromatography under a number of different conditions; although complete proteolysis of inter-domain regions of a multifunctional complex during extraction cannot be excluded, the evidence suggests that in dogfish, in contrast to mammalian species, these three enzymes of the pyrimidine pathway exist as individual polypeptide chains. These results: (1) establish that dogfish express two different glutamine-dependent carbamoyl-phosphate synthetase activities, (2) confirm the report [Smith, Ritter & Campbell (1987) J. Biol. Chem. 262, 198-202] that dogfish express two different glutamine synthetases, and (3) provide indirect evidence that glutamine may not be available in liver for biosynthetic reactions other than urea formation.

Animals↗

PubMed1993

Pyrimidine nucleotide metabolism in rat hepatocytes: evidence for compartmentation of nucleotide pools.

Pyrimidine nucleotide metabolism in rat hepatocytes was studied by measurement of the labelling kinetics of the various intermediates after double labelling with [14C]orotic acid and [3H]cytidine, the precursors for the de novo and the salvage pathways respectively. For the uridine nucleotides, differences were found for the 14C/3H ratios in the UDP-sugars, in UMP (of RNA) and in their precursor UTP, suggesting the existence of separated flows of the radioactive precursors through the de novo and the salvage pathways. Higher ratios in the UDP-sugars, which are synthesized in the cytoplasm, and a lower ratio in UMP (of RNA) relative to the 14C/3H ratio in UTP indicated that UTP derived from orotic acid is preferentially used for the cytoplasmic biosynthesis of the UDP-sugars. Uridine, derived from cytidine, is preferentially used for the nuclear-localized synthesis of RNA. In contrast to these findings, the 14C/3H ratios in the cytidine derivatives CMP-NeuAc and CMP (of RNA), and in the liponucleotides CDP-choline and CDP-ethanolamine, were all lower than that in the precursor CTP. This indicates a preferential utilization of the salvage-derived CTP for the synthesis of the liponucleotides as well as for RNA and CMP-NeuAc. Similar conclusions could be drawn from experiments in which the intracellular amounts of several uridine- and cytidine-nucleotide-containing derivatives were increased by preincubating the hepatocytes with unlabelled pyrimidine nucleotides or ethanolamine. Based on these data, we propose a refined model for the intracellular compartmentation of pyrimidine nucleotide biosynthesis in which three pools of UTP are distinguished: a pool of de novo-derived molecules and a pool of salvage-derived molecules, both of which are channelled to the site of utilization; in addition an 'overflow' pool exists, consisting of molecules having escaped from channelling. An overflow pool could also be distinguished for CTP, but no discrimination between de novo and salvage-derived molecules could be made.

Animals↗