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

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Query: EC:2.4.2.8 (hypoxanthine-guanine phosphoribosyltransferase)
2,527 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Chronic perturbations of intracellular deoxyribonucleoside triphosphate (dNTP) pools have been associated with a mutator phenotype and increased mutation rates at several genetic loci. We have examined the specific effects of transient pharmacological purine dNTP pool perturbations on mutations induced at the hypoxanthine-guanine phosphoribosyltransferase (HPRT) locus in a cultured human T-lymphoblast cell line. Incubation of CEM cells with 50 microM 2'-deoxyguanosine for 6 h increased intracellular dGTP levels 43-fold and induced a 40-fold increase in mutation frequency at the HPRT locus. Six-h incubations with 5, 10, and 20 microM 2'-deoxyadenosine increased dATP pools 4.8-, 8-, and 14.5-fold, respectively, with 59-, 34-, and 43-fold increases in HPRT mutant fractions. In contrast, 24-h incubations with hydroxyurea at concentrations which inhibited cell growth to similar extents did not induce HPRT mutations. Sequencing of HPRT complementary DNA derived from mutant cell lines revealed that the mutations induced by transient purine dNTP pool perturbations exhibited no significant misincorporation of the nucleotide in excess or next-nucleotide effect, and were similar in nature and location to spontaneous HPRT mutations. We conclude that mutations caused by transient purine dNTP pool elevations in these dividing cells are most likely induced by inhibition of DNA repair processes.
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PMID:Mutations induced at the hypoxanthine-guanine phosphoribosyltransferase locus of human T-lymphoblasts by perturbations of purine deoxyribonucleoside triphosphate pools. 236 32

The molecular correlation concept proposed that IMP dehydrogenase activity should be a sensitive target of chemotherapy. This hypothesis received support from an array of evidence. IMP dehydrogenase has the lowest activity in purine biosynthesis; it is the rate-limiting enzyme in GTP production; the enzymic activity is transformation-and progression-linked; it is elevated in all examined animal and human neoplastic cells. The activity of GMP synthetase and the concentrations of GMP and dGTP were increased in cancer cells. Whereas guanine salvage has a high potential activity, the low guanine content may well curtail actual salvage capacity. Ribonucleotide reductase activity was two orders of magnitude lower than that of IMP dehydrogenase. Tiazofurin, a C-nucleoside, had marked cytotoxicity on hepatoma cells in vitro and was the first drug that as a single agent profoundly inhibited the proliferation of the subcutaneously inoculated solid hepatoma 3924A in the rat. The impact of tiazofurin administration in hepatoma cells was revealed in a cascade of biochemical alterations involving primary, secondary and tertiary targets and markers of this drug action. The primary target was IMP dehydrogenase where the active metabolite of tiazofurin, TAD, was thought to be absorbed to the NADH site of the enzyme. As a consequence, the enzymic activity declined rapidly to about 30-40% and returned to normal range by 36 to 48 hr after injection. The secondary targets and markers are the profoundly decreased pools of guanylates (GMP, GDP, GTP). Concurrently, the concentrations of IMP and PRPP were increased 8- to 15-fold. The elevated IMP pools were attributed to the de-inhibition of the AMP deaminase activity subsequent to the decline in GTP concentration. The rise in PRPP pools was attributed to the selective inhibition of GPRT and HPRT activities by the high IMP pool which did not affect APRT activity. This interpretation is supported by the 6- to 8-fold increase in the concentrations of guanine and hypoxanthine and the lack of change in the adenine pools inthe hepatomas after tiazofurin administration. The marked drop in NAD concentration which was drug dose- and time-dependent is attributed to the competition for NAD pyrophosphorylase activity by the precursors of NAD and tiazofurin monophosphate. The tertiary targets were dominated by the profound alterations in the concentrations of the dNTPs. This was characterized by a rapid and persistent drop (for 3 days) of the dGTP pool. The concentrations of dATP and dCTP also declined, but these alterations were less pronounced and the pools returned to normal after 2 days.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:Targets and markers of selective action of tiazofurin. 242 86

We have studied the kinetics of guanine incorporation into DNA in mouse T-lymphoma (S-49) mutant cells [PNPase (purine-nucleoside phosphorylase)- and HGPRTase (hypoxanthine: guanine phosphoribosyltransferase)-deficient] that are incapable of converting dGuo (deoxyguanosine) to Gua (guanine) ribonucleotides. Of the two possible pathways for an exogenous guanine source to reach DNA, firstly: dGuo----dGMP----dGDP----dGTP and secondly: Gua----GMP----GDP----dGDP----dGTP only the second pathway was found to be functional in providing guanine for DNA replication, although deoxyguanosine readily produced toxic cellular dGTP levels via the first pathway. The functional guanine-nucleotide-precursor pools for DNA are rather small; further, the depletion of the small GMP pool, but not that of GDP, GTP and dGTP, correlated well with the inhibition of DNA synthesis by mycophenolic acid, an IMP dehydrogenase inhibitor. These results support the hypothesis that guanine-nucleotide incorporation into DNA is highly compartmentalized and that a small functional guanine-nucleotide pool, e.g., the GMP pool, may serve a crucial role in limiting the availability of DNA precursor substrate.
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PMID:Compartmentation of guanine nucleotide precursors for DNA synthesis. 242 29

This paper compares erythrocyte nucleotide levels in patients with eight different inherited purine or pyrimidine enzyme defects identified amongst a variety of patients referred predominantly for investigation of severe neurological abnormalities, or immunodeficiency syndromes. Characteristic nucleotide patterns were identified only in the six disorders (four involving purine and two pyrimidine metabolism) where there was clinical evidence of cellular toxicity. They were frequently related to the accumulation of abnormal metabolites in body fluids. These erythrocyte studies have demonstrated the following. 1. ATP depletion is not an invariable feature of adenosine deaminase (ADA) deficiency, but the accumulation of the deoxyribonucleotides dATP, or dGTP, is diagnostic of ADA, or purine nucleoside phosphorylase (PNP) deficiency, respectively. The early accumulation of dATP in foetal blood is a valuable aid to prenatal diagnosis of ADA deficiency. 2. GTP depletion appears to reflect the degree of CNS involvement in hypoxanthine-guanine phosphoribosyltransferase and PNP deficiency, as well as PP-ribose-P synthetase superactivity. Other diagnostic changes involving increased pyrimidine sugars and increased or decreased NAD levels, or ZTP in Lesch Nyhan erythrocytes, show no consistent correlation with the clinical manifestations. 3. These altered nucleotide levels afford a novel means for carrier detection of the X-linked defect associated with aberrant PP-ribose-P synthetase activity, where no other test is yet available. Measurement of erythrocyte nucleotide levels thus provides a simple and rapid aid to diagnosis and may sometimes be essential for determining prognosis, carrier detection, or monitoring therapy. These characteristic 'fingerprints' may give some insight into the mechanism by which the abnormal gene product produces disease. Such grossly altered nucleotide levels could also result in loss of erythrocyte flexibility, increased destruction and hence the anaemia, or other clinical manifestations, observed in some disorders.
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PMID:Altered erythrocyte nucleotide patterns are characteristic of inherited disorders of purine or pyrimidine metabolism. 337 Aug 20

Two protocols have been developed, both of which utilize the thymidine analog 5-bromodeoxyuridine (BrdUrd) to induce mutations in mammalian cells in culture (E. R. Kaufman and R. L. Davidson, Proc. Natl. Acad. Sci. USA 75:4982-4986, 1978; E. R. Kaufman, Mol. Cell. Biol. 4:2449-2454, 1984). The first protocol, termed incorporational (INC) mutagenesis, utilizes high concentrations of BrdUrd in the culture medium to generate a high intracellular ratio of BrdUTP/dCTP. The second protocol, termed replicational (REP) mutagenesis, entails the incorporation of BrdUrd into DNA under nonmutagenic conditions, the removal of all BrdUrd from the culture medium, and the subsequent replication of the bromouracil-containing DNA in the presence of high intracellular levels of dTTP and dGTP. Genetic studies using reversion analysis at the hypoxanthine-guanine phosphoribosyltransferase locus were used to determine whether the mechanisms of these two BrdUrd mutagenesis protocols had enough specificity to be distinguishable by their ability to revert various mutants. The results of these studies indicated that (i) mutants induced by INC mutagenesis were induced to revert only by REP mutagenesis and not by INC mutagenesis, (ii) mutants induced by REP mutagenesis were more efficiently reverted by INC mutagenesis than by REP mutagenesis, and (iii) both spontaneous mutants and mutants induced by the chemical mutagen ethyl methanesulfonate showed a high degree of specificity when tested for reversion by the BrdUrd mutagenesis protocols.
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PMID:Reversion analysis of mutations induced by 5-bromodeoxyuridine mutagenesis in mammalian cells. 383 41

Tiazofurin, a C-nucleoside, was cytotoxic in hepatoma 3924A cells grown in culture with an LC50 = 7.5 microM. In the culture, a closely linked dose-related response of tumor cell-kill and depletion of GTP pools was observed after tiazofurin treatment. In rats carrying subcutaneously transplanted hepatoma 3924A solid tumors, a single intraperitoneal injection of tiazofurin (200 mg/kg) caused a rapid inhibition of IMP dehydrogenase (EC 1.2.1.14) activity and depleted GDP, GTP, and dGTP pools in the tumor; concurrently, the 5-phosphoribosyl 1-pyrophosphate (PRPP) and IMP pools expanded 8- and 15-fold, respectively. Tiazofurin decreased tumoral IMP dehydrogenase activity and dGTP pools in a dose-dependent manner over a range of 50-200 mg/kg; by contrast, the depletion of GTP and the accumulation of IMP and PRPP pools were near maximum at 50 mg/kg. The increase in PRPP pools may be attributed to an inhibition by IMP of the activity of hypoxanthine-guanine phosphoribosyltransferase (EC 2.4.2.8). The IMP dehydrogenase activity and the pools of ribonucleotides returned to the normal range by 24-48 h after the single injection of tiazofurin. However, the markedly depleted dGTP pools remained low for 72 h. Tiazofurin treatment resulted in significant anti-tumor activity in rats inoculated with hepatoma 3924A. The decrease in GTP levels and particularly the sustained depletion in the dGTP pools may explain, in part at least, the chemo-therapeutic action of tiazofurin on hepatoma 3924A. This is the first report showing that a marked therapeutic response was achieved against rapidly growing hepatoma 3924A by treatment with a single anti-metabolite.
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PMID:Modulation of IMP dehydrogenase activity and guanylate metabolism by tiazofurin (2-beta-D-ribofuranosylthiazole-4-carboxamide). 614 52

The mechanism of action of acivicin and tiazofurin was compared in hepatoma 3924A. The results were evaluated by assessing the impact of these drugs on primary targets, the activities of key enzymes, and on secondary and tertiary targets, the concentrations of pools of ribonucleotides and deoxyribonucleotides. The action of acivicin entails inhibition and inactivation of the key enzymes of glutamine utilization in the biosynthesis of purines and pyrimidines. As a result, the GTP and CTP pools were markedly depleted, whereas those of ATP and UTP were unaffected. Acivicin also markedly decreased the concentrations of all 4 deoxynucleoside triphosphates. The nucleotide pools returned to normal or near normal range within 2 to 3 days after a single acivicin injection. The pharmacologic targets of acivicin in anticancer chemotherapy include prominently the activities of glutamine-utilizing enzymes and the pools of GTP and CTP and all 4 dNTP's. These biochemical targets also serve as indicators of acivicin action in cancer cells. The action of tiazofurin in hepatoma cells entails the primary target, IMP dehydrogenase. The subsequent effects include marked enlargement of IMP and PRPP pools and depletion of the pools of GDP and GTP. The increased IMP concentration selectively inhibited the activities of hypoxanthine-guanine phosphoribosyltransferase, but did not affect that of adenine phosphoribosyltransferase. The markedly decreased GTP pool de-inhibited the activity of AMP deaminase which permitted the channeling of AMP to IMP. An important indicator of tiazofurin action is the prolonged depletion of dGTP pools and similar but less pronounced declines in the pools of dCTP and dATP. In contrast, dTTP pools were increased. The crucial biochemical targets and indicators of tiazofurin action in sensitive cancer cells include inhibition of IMP dehydrogenase, a decrease in the concentrations of GDP, GTP, dGTP, dCTP, dATP and marked rise in the pools of IMP, PRPP and dTTP. Measurements of the molecular targets and indicators of drug action should be helpful in identifying cancer cells and tissues sensitive or resistant to the action of acivicin or tiazofurin. Identification of the targets and indicators should also be helpful in the design of frequency of administration of the drugs in combatting animal and human neoplasia.
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PMID:Control of enzymic programs and nucleotide pattern in cancer cells by acivicin and tiazofurin. 620 92

Acyclovir [9-(2-hydroxyethoxymethyl)guanine], a clinically useful anti-herpesvirus agent, was a weak inhibitor (Ki = 190 microM) of hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) from human erythrocytes. Nevertheless, this acyclic nucleoside analog was a more effective inhibitor than were its natural counterparts, guanosine (Ki = 1400 microM) and deoxyguanosine (Ki = 570 microM). The two oxidized metabolites of acyclovir, 9-carboxymethoxymethylguanine (Ki = 720 microM) and 8-hydroxy-9-(2-hydroxyethoxymethyl)guanine (Ki greater than 2000 microM), were less inhibitory than was the parent drug. None of the phosphorylated metabolites of acyclovir was as potent an inhibitor of HGPRTase as was GMP (Ki = 4 microM). However, the Ki value for acyclovir monophosphate was similar to that of dGMP (12 microM). The Ki values for acyclovir diphosphate (8.3 microM) and triphosphate (30 microM) were less than those for dGDP (110 microM) and dGTP (140 microM). The levels of these phosphate esters of acyclovir in cultured monkey kidney (Vero) and human embryo fibroblast (WI38) cells exposed to therapeutic levels of the drug were well below the observed Ki values. However, in herpesvirus-infected WI38 cells the levels of the phosphate esters of acyclovir were high enough potentially to inhibit the enzyme. Although inhibition of this enzyme by the phosphorylated metabolites of acyclovir may occur in these infected cells, concentrations of the drug very much higher than the EC50 concentration were required to achieve inhibitory levels. It is, therefore, unlikely that this inhibition contributes significantly to the antiviral activity.
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PMID:Effects of acyclovir and its metabolites on hypoxanthine-guanine phosphoribosyltransferase. 663 69

Incubation of mouse T lymphoma (S-49) cells with the inosinate dehydrogenase inhibitor mycophenolic acid produced a depletion of both GTP and dGTP, and resulted in growth inhibition, partial reduction in RNA synthesis, and drastic inhibition of DNA synthesis. Similar results suggested to others that the depletion of dGTP is primarily responsible for toxicity. However, guanosine was as effective as deoxyguanosine at preventing mycophenolic acid toxicity although deoxyguanosine was more effective at elevating dGTP levels. Moreover, in hypoxanthine-guanine phosphoribosyltransferase-deficient mutants of S-49 (6MPR-3-3) deoxyguanosine was unable to prevent mycophenolic acid toxicity or to re-establish normal DNA synthesis, although it returned cellular dGTP but not GTP levels to normal. No other nucleotide levels changed in a way which could account for the toxicity. Incubation of cells with a combination of deoxyadenosine, deoxycytidine, and erythro-9-(2-hydroxy-3-nonyl)adenine produced a selective depletion of dGTP to levels similar to that produced by mycophenolic acid, but did not affect cell growth. Studies with cells synchronized by centrifugal elutriation show that the toxicity of mycophenolic acid is specific to the S-phase of the cell cycle. Addition of actinomycin D at a concentration that inhibited RNA synthesis increased the availability of GTP and re-established normal DNA synthesis in mycophenolic acid-treated S-49 cells. These results suggest that the depletion of GTP rather than that of dGTP produces toxic effects in S-49 cells and that GTP is required for DNA synthesis.
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PMID:Guanine nucleotide depletion and toxicity in mouse T lymphoma (S-49) cells. 726 80

5,10-Dideazatetrahydrofolic acid (DDATHF) is an inhibitor of glycinamide ribonucleotide transformylase, the first of two tetrahydrofolate requiring enzymes in the de novo purine nucleotide biosynthetic pathway, and is a potent inducer of the maturation of HL-60 promyelocytic leukemia cells. The inhibition of cellular growth by DDATHF was effectively prevented by adenosine or deoxyadenosine, whereas guanosine or deoxyguanosine only partially prevented the growth inhibition produced by this folate antimetabolite, implying that the depletion of both ATP and GTP, which occurs with this agent, was responsible for its growth inhibitory effects. In contrast, the induction of differentiation by DDATHF was completely abolished by the presence of guanosine or deoxyguanosine, suggesting that the depletion of intracellular guanine nucleotides by DDATHF represents the event that is essential to the induction of differentiation by this folate analog. This possibility was supported by the observation that the concentration of dGTP was not decreased in cells treated with DDATHF under the conditions employed. Both guanine nucleosides selectively restored intracellular GTP pools depleted by the treatment with DDATHF to their normal level, whereas only adenine nucleosides completely restored the levels of both ATP and GTP to their normal intracellular concentrations. The relationship between guanine nucleotide pools and the induction of HL-60 differentiation by DDATHF was further supported by the finding that maturation and the depletion of intracellular GTP by DDATHF were not reversed by guanine nucleosides in HL-60 cells deficient in hypoxanthine-guanine phosphoribosyltransferase activity. The findings provide support for the hypothesis that the terminal differentiation of these leukemic cells by DDATHF is the result of the depletion of intracellular GTP pools.
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PMID:Evidence for a relationship between intracellular GTP levels and the induction of HL-60 leukemia cell differentiation by 5,10-dideazatetrahydrofolic acid (DDATHF). 801 61


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