The biological significance of purine salvage

Evidence for the involvement of cytosolic 5'-nucleotidase (cN-II) in the synthesis of guanine nucleotides from xanthosine

In this paper, we show that in vitro xanthosine does not enter any of the pathways known to salvage the other three main natural purine nucleosides: guanosine; inosine; and adenosine. In rat brain extracts and in intact LoVo cells, xanthosine is salvaged to XMP via the phosphotransferase activity of cytosolic 5'-nucleotidase. IMP is the preferred phosphate donor (IMP + xanthosine --> XMP + inosine). XMP is not further phosphorylated. However, in the presence of glutamine, it is readily converted to guanyl compounds. Thus, phosphorylation of xanthosine by cytosolic 5'-nucleotidase circumvents the activity of IMP dehydrogenase, a rate-limiting enzyme, catalyzing the NAD(+)-dependent conversion of IMP to XMP at the branch point of de novo nucleotide synthesis, thus leading to the generation of guanine nucleotides. Mycophenolic acid, an inhibitor of IMP dehydrogenase, inhibits the guanyl compound synthesis via the IMP dehydrogenase pathway but has no effect on the cytosolic 5'-nucleotidase pathway of guanine nucleotides synthesis. We propose that the latter pathway might contribute to the reversal of the in vitro antiproliferative effect exerted by IMP dehydrogenase inhibitors routinely seen with repletion of the guanine nucleotide pools.

Quantification of sugar phosphate intermediates of the pentose phosphate pathway by LC-MS/MS: application to two new inherited defects of metabolism

We describe a liquid chromatography tandem mass spectrometry (LC-MS/MS) method to quantify pentose phosphate pathway intermediates (triose-3-phosphates, tetrose-4-phosphate, pentose-5-phosphate, pentulose-5-phosphates, hexose-6-phosphates and sedoheptulose-7-phosphate (sed-7P)) in bloodspots, fibroblasts and lymphoblasts. Liquid chromatography was performed using an ion pair loaded C(18) HPLC column and detection of the sugar phosphates was carried out by tandem mass spectrometry using an electron ion spray source operating in the negative mode and multiple reaction monitoring. Reference values for the pentose phosphate pathway intermediates in blood spots, fibroblasts and lymphoblasts were established. The method was applied to cells from patients affected with a deficiency of transaldolase. The transaldolase-deficient cells showed an increased concentration of sedoheptulose-7-phosphate. (Bloodspots: 5.19 and 5.43 micromol/L [0.49-3.33 micromol/L]; fibroblasts 7.43 and 26.46 micromol/mg protein [0.31-1.14 micromol/mg protein]; lymphoblasts 16.03 micromol/mg protein [0.61-2.09 micromol/mg protein].) The method was also applied to study enzymes of the pentose phosphate pathway by incubating fibroblasts or lymphoblasts homogenates with ribose-5-phosphate or 6-phosphogluconate and the subsequent analysis of the formed sugar phosphates.

Quantification of DNA synthesis from different pathways in cultured human fibroblasts and myocytes

We have quantified DNA synthetic rates from different pathways in cultured cells using a new stable isotope technique. Human fibroblasts and myocytes were grown in culture media supplemented with [U-(13)C(6)]glucose and [(15)N]glycine. The cells were sampled daily from day 1 to day 5. A portion of the cells harvested at day 5 was subcultured for an additional 3 passages to reach isotopic plateau. In both cell types total DNA fractional synthetic rate (FSR) was found to agree closely with the rate of cell proliferation determined by cell counting (FSR = 0.94%. h(-1) v 0.92%. h(-1) for DNA synthesis and cell count, respectively, in myocytes and 0.85%. h(-1) v 0.91%. h(-1) for DNA synthesis and cell count, respectively, in fibroblasts). In fibroblasts the deoxyribonucleoside salvage pathway accounted for over 70% of total DNA synthesis. In myocytes the deoxyribonucleoside salvage pathway was minimal, whereas the de novo base synthesis pathway accounted for almost 80% of total DNA synthesis. We conclude that the contributions of various pathways to DNA synthesis are highly dependent on cell type. This new stable isotope technique can be modified for application in in vivo studies.

Nucleoside anticancer drugs: the role of nucleoside transporters in resistance to cancer chemotherapy

The clinical efficacy of anticancer nucleoside drugs depends on a complex interplay of transporters mediating entry of nucleoside drugs into cells, efflux mechanisms that remove drugs from intracellular compartments and cellular metabolism to active metabolites. Nucleoside transporters (NTs) are important determinants for salvage of preformed nucleosides and mediated uptake of antimetabolite nucleoside drugs into target cells. The focus of this review is the two families of human nucleoside transporters (hENTs, hCNTs) and their role in transport of cytotoxic chemotherapeutic nucleoside drugs. Resistance to anticancer nucleoside drugs is a major clinical problem in which NTs have been implicated. Single nucleotide polymorphisms (SNPs) in drug transporters may contribute to interindividual variation in response to nucleoside drugs. In this review, we give an overview of the functional and molecular characteristics of human NTs and their potential role in resistance to nucleoside drugs and discuss the potential use of genetic polymorphism analyses for NTs to address drug resistance.

Equilibrative nucleoside transporters of Arabidopsis thaliana. cDNA cloning, expression pattern, and analysis of transport activities

Equilibrative nucleoside transporters (ENTs) occur in diverse organisms. In the model plant Arabidopsis thaliana, eight potential ENTs (AtENTs) have been predicted by genome sequencing. We here report the cloning of the cDNAs for AtENTs 2, 3, 4, 6, 7, and 8. Conceptual translation of the cDNAs of AtENTs 2, 3, 4, 6, 7, and 8 yielded polypeptides possessing strong similarities to ENTs characterized previously. Eleven putative transmembrane domains were identified in each of the six AtENTs. In suspension cells, the transcription of AtENTs 1, 3, 4, 6, and 8 was increased by two treatments (nitrogen deprivation, application of 5-fluorouracil and methotrexate) that inhibited the de novo pathway of nucleotide synthesis, indicating that multiple members of the Arabidopsis ENT family may function in the salvage pathway of nucleotide synthesis. Except for AtENT1, the transcription of the remaining six AtENTs showed varying degrees of organ specificity. However, all seven AtENTs were expressed in the leaf and flower. In plant, insect, and yeast cells, ectopically expressed AtENT3 was targeted to the plasma membrane. AtENT3 expressed in yeast cells transported adenosine and uridine with high affinity. Furthermore, the activities of AtENT3 appear not to require a transmembrane proton gradient because protonophores did not abolish adenosine or uridine transport. In competition experiments, the transport of [3H]adenosine by AtENT3 was most significantly inhibited by a number of different purine and pyrimidine nucleosides and 2'-deoxynucleosides, although certain nucleobases and nucleotides were also found to have some inhibitory effect. This indicates that AtENT3 may possess broad substrate specificity. Adenosine and uridine transport by AtENT3, although partly sensitive to the vasodilator drugs dilazep and dipyridamole, was resistant to the nucleoside analogue nitrobenzylmercaptopurine ribonucleoside. We conclude that AtENT3 represents the first ei type ENT characterized from higher plants. The potential functions of ENTs in the biology of A. thaliana are discussed.

Molecular biology and regulation of nucleoside and nucleobase transporter proteins in eukaryotes and prokaryotes

The molecular cloning of cDNAs encoding nucleoside transporter proteins has greatly advanced understanding of how nucleoside permeants are translocated across cell membranes. The nucleoside transporter proteins identified thus far have been categorized into five distinct superfamilies. Two of these superfamilies, the equilibrative and concentrative nucleoside transporters, have human members and these will be examined in depth in this review. The human equilibrative nucleoside transporters translocate nucleosides and nucleobases bidirectionally down their concentration gradients and are important in the uptake of anticancer and antiviral nucleoside drugs. The human concentrative nucleoside transporters cotranslocate nucleosides and sodium unidirectionally against the nucleoside concentration gradients and play a vital role in certain tissues. The regulation of nucleoside and nucleobase transporters is being studied more intensely now that more tools are available. This review provides an overview of recent advances in the molecular biology and regulation of the nucleoside and nucleobase transporters.

Functional and molecular characterization of nucleobase transport by recombinant human and rat equilibrative nucleoside transporters 1 and 2. Chimeric constructs reveal a role for the ENT2 helix 5-6 region in nucleobase translocation

The human (h) and rat (r) equilibrative (Na(+)-independent) nucleoside transporters (ENTs) hENT1, rENT1, hENT2, and rENT2 belong to a family of integral membrane proteins with 11 transmembrane domains (TMs) and are distinguished functionally by differences in sensitivity to inhibition by nitrobenzylthioinosine and coronary vasoactive drugs. Structurally, the proteins have a large glycosylated loop between TMs 1 and 2 and a large cytoplasmic loop between TMs 6 and 7. In the present study, hENT1, rENT1, hENT2, and rENT2 were produced in Xenopus laevis oocytes and investigated for their ability to transport pyrimidine and purine nucleobases. hENT2 and rENT2 efficiently transported radiolabeled hypoxanthine, adenine, guanine, uracil, and thymine (apparent K(m) values 0.7-2.6 mm), and hENT2, but not rENT2, also transported cytosine. These findings were independently confirmed by hypoxanthine transport experiments with recombinant hENT2 produced in purine-cytosine permease (FCY2)-deficient Saccharomyces cerevisiae and provide the first direct demonstration that the ENT2 isoform is a dual mechanism for the cellular uptake of nucleosides and nucleobases, both of which are physiologically important salvage metabolites. In contrast, recombinant hENT1 and rENT1 mediated negligible oocyte fluxes of hypoxanthine relative to hENT2 and rENT2. Chimeric experiments between rENT1 and rENT2 using splice sites at rENT1 residues 99 (end of TM 2), 171 (between TMs 4 and 5), and 231 (end of TM 6) identified TMs 5-6 of rENT2 (amino acid residues 172-231) as a determinant of nucleobase transport activity, suggesting that this domain forms part(s) of the ENT2 substrate translocation channel.

Subcellular Distribution and Membrane Topology of the Mammalian Concentrative Na+-Nucleoside Cotransporter rCNT1

The rat transporter rCNT1 is the archetype of a family of concentrative nucleoside transporters (CNTs) found both in eukaryotes and in prokaryotes. In the present study we have used antibodies to investigate the subcellular distribution and membrane topology of this protein. rCNT1 was found to be expressed predominantly in the brush-border membranes of the polarized epithelial cells of rat jejunum and renal cortical tubules and in the bile canalicular membranes of liver parenchymal cells, consistent with roles in the absorption of dietary nucleosides, of nucleosides in the glomerular filtrate, or of nucleosides arising from the action of extracellular nucleotidases, respectively. The effect of endoglycosidase F treatment on wild-type and mutant rCNT1 expressed in Xenopus oocytes revealed that the recombinant transporter could be glycosylated at either or both of Asn605 and Asn643, indicating that its C terminus is extracellular. In contrast, potential N-glycosylation sites introduced near the N terminus, or between putative transmembrane (TM) helices 4 and 5, were not glycosylated. The deduced orientation of the N terminus in the cytoplasm was confirmed by immunocytochemistry on intact and saponin-permeabilized Chinese hamster ovary cells expressing recombinant rCNT1. These results, in conjunction with extensive analyses of CNT family protein sequences using predictive algorithms, lead us to propose a revised topological model, in which rCNT1 possesses 13 TM helices with the hydrophilic N-terminal and C-terminal domains on the cytoplasmic and extracellular sides of the membrane, respectively. Furthermore, we show that the first three TM helices, which are absent from prokaryote CNTs, are not essential for transporter function; truncated proteins lacking these helices, derived either from rCNT1 or from its human homolog hCNT1, were found to retain significant sodium-dependent uridine transport activity when expressed in oocytes.

Nucleotide pyrophosphatases/phosphodiesterases on the move

Nucleotide pyrophosphatases/phosphodiesterases (NPPs) release nucleoside 5'-monophosphates from nucleotides and their derivatives. They exist both as membrane proteins, with an extracellular active site, and as soluble proteins in body fluids. The only well-characterized NPPs are the mammalian ecto-enzymes NPP1 (PC-1), NPP2 (autotaxin) and NPP3 (B10; gp130(RB13-6)). These are modular proteins consisting of a short N-terminal intracellular domain, a single transmembrane domain, two somatomedin-B-like domains, a catalytic domain, and a C-terminal nuclease-like domain. The catalytic domain of NPPs is conserved from prokaryotes to mammals and shows remarkable structural and catalytic similarities with the catalytic domain of other phospho-/sulfo-coordinating enzymes such as alkaline phosphatases. Hydrolysis of pyrophosphate/phosphodiester bonds by NPPs occurs via a nucleotidylated threonine. NPPs are also known to auto(de)phosphorylate this active-site threonine, a process accounted for by an intrinsic phosphatase activity, with the phosphorylated enzyme representing the catalytic intermediate of the phosphatase reaction. NPP1-3 have been implicated in various processes, including bone mineralization, signaling by insulin and by nucleotides, and the differentiation and motility of cells. While it has been established that most of these biological effects of NPPs require a functional catalytic site, their physiological substrates remain to be identified.

Effect of partial outlet obstruction on 14C-adenine incorporation in the rabbit urinary bladder

Bladder outlet obstruction induces severe changes in urinary bladder function and metabolism. These changes are characterized by significant reductions in the ability of the in vitro whole bladder to generate pressure and to empty. Metabolically, partial outlet obstruction induces a shift from oxidative to anaerobic metabolism. The decreased oxidative metabolism is mediated in part by significant decreases in mitochondrial substrate metabolism, which in turn is correlated with decreased activity of 2 important mitochondrial enzymes: citrate synthase and malate dehydrogenase. The present study was designed to evaluate mitochondrial function by studying the incorporation of 14C-adenine into high-energy phosphates (ATP, AMP, and ADP). Mild partial outlet obstructions were created by surgically placing silk ligatures loosely around the bladder neck. The results of these studies demonstrate that after 60 min incubation in oxygenated medium containing glucose + 1uCi14C-adenine, 1) There was no significant differences in the total AMP, ADP, and ATP concentrations measured in bladders taken from controls, 7- and 14-day obstructed rabbits; 2) there was no effect of obstruction on either the concentration of 14C-AMP in the tissue or in the ratio of hot to cold AMP; and 3) there was a 50% decrease in the concentration of 14C-ADP and a 70% decrease in the concentration of 14C-ATP in the bladder smooth muscle obtained from obstructed tissue (from both 7- and 14-day obstructions) compared to concentration in the control bladder smooth muscle. These results confirm the previous finding that obstruction did not reduce the rate of incorporation of adenine to AMP within the obstructed bladder smooth muscle and extends these studies to identify a significant reduction in the synthesis of both ADP and ATP. These results support the hypothesis that partial outlet obstruction induce a major dysfunction in mitochondrial function, both in the ability to oxidize substrates and in the ability to generate ATP.

Identification and characterization of a novel hepatic canalicular ATP diphosphohydrolase

We have identified and characterized a novel ATP diphosphohydrolase (ATPDase) with features of E-type ATPases from porcine liver. Immunoblotting with a specific monoclonal antibody to this ectoenzyme revealed high expression in liver with lesser amounts in kidney and duodenum. This ATPDase was localized by immunohistochemistry to the bile canalicular domain of hepatocytes and to the luminal side of the renal ductular epithelium. In contrast, ATPDase/cd39 was detected in vascular endothelium and smooth muscle in these organs. We purified the putative ATPDase from liver by immunoaffinity techniques and obtained a heavily glycosylated protein with a molecular mass estimated at 75 kDa. This enzyme hydrolyzed all tri- and diphosphonucleosides but not AMP or diadenosine polyphosphates. There was an absolute requirement for divalent cations (Ca(2+) > Mg(2+)). Biochemical activity was unaffected by sodium azide or other inhibitors of ATPases. Kinetic parameters derived from purified preparations of hepatic ATPDase indicated V(max) of 8.5 units/mg of protein with apparent K(m) of 100 microM for both ATP or ADP as substrates. NH(2)-terminal amino acid sequencing revealed near 50% identity with rat liver lysosomal (Ca(2+)-Mg(2+))-ATPase. The different biochemical properties and localization of the hepatic ATPDase suggest pathophysiological functions that are distinct from the vascular ATPDase/cd39.

Maintenance of differential methotrexate toxicity between cells expressing drug-resistant and wild-type dihydrofolate reductase activities in the presence of nucleosides through nucleoside transport inhibition

Methotrexate (MTX), a potent inhibitor of dihydrofolate reductase (DHFR), has been used widely as a chemotherapeutic agent and as a selective agent for cells expressing drug-resistant DHFR activity. MTX deprives rapidly dividing cells of reduced folates that are necessary for thymidylate synthesis and de novo purine nucleotide synthesis. However, MTX toxicity can be circumvented by salvaging thymidine (TdR) and purine nucleosides. Here we have investigated conditions under which nucleoside transport inhibition can be used to maintain differential MTX toxicity between unmodified cells and cells expressing drug-resistant DHFR activity in the presence of exogenous nucleosides. PA317 cells (a 3T3 derivative cell line) were rescued from the toxicity of 0.1 microM MTX by 1.0 microM TdR in the presence of 100 microM inosine. The nucleoside transport inhibitor dipyridamole (DP) resensitized these cells to MTX, even in the presence of exogenous nucleosides. Furthermore, PA317 cells transduced with any of three retroviruses encoding drug-resistant DHFRs remained resistant to MTX over all concentrations tested (up to 10.0 microM) in the presence of DP. Similar results were obtained in transduced HuH7 and K562 cell lines, a human hepatoma and a human leukemia cell line, respectively. We conclude that nucleoside transport inhibition increases the toxicity and selectivity of MTX in cultured cells, and therefore is an effective way to maintain differential MTX toxicity between unmodified and DHFR-modified cells. Our results support the use of nucleoside transport inhibition in in vivo selection protocols involving the liver and hematopoietic systems.

Relation between changes in serum hypoxanthine levels by exercise and daily physical activity in the elderly

Effect of exercise at mild intensity on the serum levels of hypoxanthine was studied in eleven healthy elderly subjects. They were divided into the active and sedentary groups according to their daily physical activity. They performed exercise testing to walk for 5 minutes keeping heart rate at approximately 70% of the maximum heart rate. Mean intensity of exercise estimated according to Karvonen's formula in the active or sedentary group was 41.8 +/- 9.6% or 34.1 +/- 6.1%, respectively. In the sedentary group, the serum hypoxanthine levels at 10 minutes after completion of walk load was significantly higher than that before exercise. Changes in the serum hypoxanthine levels in the active and sedentary groups were -0.97 +/- 1.36 and 0.80 +/- 0.57 micromol/liter, respectively (p < 0.05). This result suggests that mild intensity exercise increases the serum hypoxanthine concentration in the elderly leading inactive daily life, and physical activity suppresses an increase in the serum hypoxanthine levels by mild exercise.

Nucleoside transporters: molecular biology and implications for therapeutic development

The uptake of nucleosides (or nucleobases) is essential for nucleic acid synthesis in many human cell types and in parasitic organisms that cannot synthesize nucleotides de novo. The transporters responsible are also the route of entry for many cytotoxic nucleoside analogues used in cancer and viral chemotherapy. Moreover, by regulating adenosine concentrations in the vicinity of its cell-surface receptors, nucleoside transporters profoundly affect neurotransmission, vascular tone and other processes. The recent molecular characterization of two families of human nucleoside transporters has provided new insights into the mechanisms of natural nucleoside and drug uptake and into future developments of improved therapies.

Changes in serum hypoxanthine levels after walk loads at mild to high intensity in healthy humans

Effect of mild intensity exercise on the serum levels of hypoxanthine was studied. Eighteen healthy subjects performed 2 to 4 bouts of 5 minutes walk load at different intensities. At the beginning, thirteen of them walked at intensity more than 80% of the maximum. The serum levels of hypoxanthine increased to the levels of more than 6 times of resting values showing a peak at 10 to 20 minutes after the completion of the walk load. In 62 bouts of the walk load by 18 subjects, statistically significant relationship was demonstrated between intensity of the walk load and increase in serum concentration of hypoxanthine at 10 minutes after the completion of the walk load with correlation coefficient of 0.556. The serum hypoxanthine levels were significantly increased by the walk load even at mild intensity between 41 and 60%. Increment in the serum hypoxanthine concentration also showed positive and statistically significant correlation with physiological cost index. These results suggest that the serum levels of hypoxanthine increase following mild as well as moderate to submaximal intensity of exercise, and its increment may be used as an indicator of energy balance in the muscle during exercise at mild to high intensity.

Bone marrow transplantation does not ameliorate the neurologic symptoms in mice deficient in hypoxanthine guanine phosphoribosyl transferase (HPRT)

The use of bone marrow transplantation (BMT) for the treatment of genetic diseases with neurologic involvement has yielded mixed results. We have employed a mouse model of Lesch-Nyhan disease (LND) to assess the efficacy of BMT in ameliorating the neurologic manifestations of the disease. Adult HPRT-deficient mice exhibit a measurable decrease in striatal dopamine levels and a hypersensitivity to amphetamine. Marrow-ablated adult HPRT-deficient mice were transplanted with marrow from congenic HPRT-expressing mice. BMT altered neither the neurochemical nor the behavioral phenotypes in either HPRT-positive or HPRT-deficient mice. Barring any important species differences, these results suggest that BMT in its present form may not be an effective therapy for Lesch-Nyhan syndrome.

PORPHOBILINOGEN DEAMINASE AND ADENOSINE DEAMINASE ACTIVITY AS A POSSIBLE DIAGNOSTIC AID IN LYMPHATIC LEUKEMIAS

It has been demonstrated recently that in addition to morphologic and immunologic methods, enzyme characterisation of lymphoid cells is useful in the study and classification of lympho proliferative disorders. Heme biosynthetic pathway and purine metabolism are reported to be disturbed in such patients, this study was an attempt to estimate two enzymes Adenosine Deaminase (ADA) and Porphobilinogen Deaminase (PBGD) and correlate them with cytological and morphological parameters in lymphatic leukemias. Fasting heparinized venous blood samples were collected from 40 patients and 20 controls. Porphobilinogen Deaminase (PBGD) and Adenosine Deaminase (ADA) assays were carried out in peripheral mononuclear cells. Both enzyme activities were significantly elevated in ALL. In CLL, while PBGD activity was elevated, ADA remained in normal limits. Our study indicates that PBGD and ADA Assays will be useful adjuncts to cytomorphological studies in differentiating ALL from CLL.

The importance of the measurement of ATP depletion and subsequent cell damage with an estimate of size and nature of the market for a practicable method: a review designed for technology transfer

ATP is the energy currency of cells. ATP depletion is a central process in pathogenesis, in particular ischaemia, hypoxia and hypoglycaemia. ATP depletion in cells can be indirectly measured from the increased concentrations of extracellular hypoxanthine, a central intermediate in the metabolism of ATP. Cell damage secondary to ATP depletion can also be measured from extracellular hypoxanthine. The relevant biochemistry and physiology is briefly reviewed. Since market size is needed for investment decisions that would allow technology transfer, the numbers of hypoxanthine analyses that are clinically justified from the extensive published evidence are calculated per million population from UK, Norwegian and other evidence. The concentration of oxygen in blood is measured to estimate whether mitochondrial oxidative phosphorylation is adequate. Measurements of bicarbonate are used to estimate anaerobic glycolysis. Since the indirect estimation of ATP depletion is a major objective of blood gas and acid-base analyses, the number of such analyses per million population provides a good estimate of potential market size for a more direct method of estimating ATP depletion. A method is required for the rapid, dispersed emergency analyses needed clinically. Routes for method development are indicated. Competition, risks, acceptability, consumer motivation and timetables are indicated for the development phase. There are medicolegal pressures, especially in the USA, for the proposed advances to be widely used.

Primary structure and functional expression of a cDNA encoding the bile canalicular, purine-specific Na(+)-nucleoside cotransporter

We previously characterized a purine-specific Na(+)-nucleoside cotransport system in bile canalicular membrane. The function of this transport system may be related to conserving nucleosides and preventing cholestasis. We report here the isolation of a cDNA encoding a Na(+)-dependent nucleoside transporter from rat liver using an expression cloning strategy. The substrate specificities and kinetic characteristics of the cloned cotransporter are consistent with the properties of the Na(+)-dependent, purine-selective nucleoside transporter in bile canalicular membranes. The nucleotide sequence predicts a protein of 659 amino acids (72 kDa) with 14 putative membrane-spanning domains. Northern blot analysis showed that the transcripts are present in liver and several other tissues. Data base searches indicate significant sequence similarity to the pyrimidine-selective nucleoside transporter (cNT1) of rat jejunum. Although these two subtypes of Na(+)-nucleoside cotransporter have different substrate specificities and tissue localizations, they are members of a single gene family.

Effect of long-term zidovudine exposure on salvage and de novo purine and pyrimidine nucleotide syntheses

Salvage and de novo purine and pyrimidine nucleotide syntheses were studied in H9 (a human lymphoid cell line) and H9-AZT cells (chronically zidovudine-exposed H9 cells). H9-AZT cells incorporated 18% and 27% more hypoxanthine and uridine, respectively, than H9 cells. The incorporation of the formate and bicarbonate was similar in both cell lines. Purine and pyrimidine de novo synthesis was inhibited by hypoxanthine and uridine, respectively. Hypoxanthine and uridine salvage pathways, however, were not affected by formate or bicarbonate. Short-term AZT exposure of cells had no effect on nucleotide synthesis. Some of the problems encountered in the studies of purine and pyrimidine synthesis are also discussed.

A re-evaluation of the tissue distribution and physiology of xanthine oxidoreductase

Xanthine oxidoreductase is an enzyme which has the unusual property that it can exist in a dehydrogenase form which uses NAD+ and an oxidase form which uses oxygen as electron acceptor. Both forms have a high affinity for hypoxanthine and xanthine as substrates. In addition, conversion of one form to the other may occur under different conditions. The exact function of the enzyme is still unknown but it seems to play a role in purine catabolism, detoxification of xenobiotics and antioxidant capacity by producing urate. The oxidase form produces reactive oxygen species and, therefore, the enzyme is thought to be involved in various pathological processes such as tissue injury due to ischaemia followed by reperfusion, but its role is still a matter of debate. The present review summarizes information that has become available about the enzyme. Interpretations of contradictory findings are presented in order to reduce confusion that still exists with respect to the role of this enzyme in physiology and pathology.

Lesch-Nyhan syndrome: carrier and prenatal diagnosis

We report the results of carrier and prenatal diagnosis for hypoxanthine guanine phosphoribosyltransferase (HPRT) deficiency, Lesch-Nyhan syndrome, by carrier testing of 83 women and prenatal analysis of 26 pregnancies. Our diagnostic methodologies include mutation detection and linkage analysis for probands and their families and biochemical measurement of HPRT enzyme activity for at-risk pregnancies. Identification of the mutation in the index case of each family permits precise carrier diagnosis using polymerase chain reaction (PCR) amplification of HPRT gene sequences and automated DNA sequencing. We demonstrate 100 per cent sensitivity for the detection of mutations in the HPRT gene of affected males and highly efficient carrier testing of at-risk females. Two other molecular methods proven to have high utility include PCR-based dosage analysis and linkage analysis by PCR amplification of a short tandem repeat (STR) in intron 3 of the HPRT gene. As a result, 45 at-risk women, 56 per cent of those tested, were identified not to be carriers of their family's HPRT gene mutation. Seven of these women were the mothers of affected males and prenatal testing for future pregnancies was recommended because of the possibility of gonadal mosaicism. Thirty-eight of these women were more distant relatives of affected males, thereby eliminating the need for future prenatal procedures. These studies illustrate the utility and precision of molecular methodologies for carrier and prenatal diagnosis of Lesch-Nyhan syndrome. These studies also illustrate that molecular diagnostic studies of affected males and carrier testing prior to pregnancy can clarify genetic risk predictions and eliminate unnecessary prenatal procedures.

Endogenous purine and pyrimidine derivative excretion in pregnant sows

The present experiment was carried out to study the endogenous losses of purine and pyrimidine derivatives from pregnant sows. Three pregnant and three non-pregnant Large White x Landrace sows were fed on a purine-free diet composed of starch, glucose, sucrose and vegetable oil, with casein as the protein source. The experiment began, for the six animals, after diagnosis of pregnancy and was divided into six 12 d periods. Urine was collected during the first 3 d of each experimental period by means of a urethral catheter for determination of allantoin, uric acid, xanthine, hypoxanthine and pseudouridine concentrations. In the absence of dietary nucleic acids (NA), allantoin and, as a consequence, excretion of total purine derivatives (PD) decreased significantly to a constant value (128.3 (SE 7.07) mumol/kg metabolic live weight (W0.75) per d), an amount assumed to represent endogenous excretion. Excretion of uric acid (38.7 (SE 2.15) mumol/kg W0.75 per d), hypoxanthine (21.0 (SE 2.58) mumol/kg W0.75 per d) and xanthine (11.2 (SE 0.83) mumol/kg W0.75 per d) were not affected by the experimental treatment, although there was a significant decrease in hypoxanthine excretion in pregnant sows (from 25.5 to 5.2 mumol/kg W0.75 per d) compared with non-pregnant sows (from 26.7 to 44.8 mumol/kg W0.75 per d). Creatinine excretion was not affected by pregnancy and was used as an internal urinary marker. Purine excretion, either expressed as mumol/kg W0.75 per d or as the ratio PD: creatinine, was not affected by experimental treatment, although an apparent increase in pseudouridine excretion, a modified unsalvageable catabolite of RNA-pyrimidine, was found in late pregnancy (3.6 v. 5.2 mol/100 mol creatinine in non-pregnant sows compared with pregnant sows at 102 d collection.

Age-related changes of AMP breakdown in chicken heart

The activity of adenylate deaminase, adenylate phosphatase and adenosine deaminase, as well as the endogenous content of adenine nucleotides, was examined in the heart of ageing chickens. In new-born (1-day-old) and young (20-day-old) chickens, AMP degradation in the heart seems to proceed preferentially through deamination, while in adult (1-year-old) through dephosphorylation. Compared with the adult heart, a 2-year-old one exhibits a decline of AMP catabolism. The total adenine nucleotide content and the concentration of ATP are higher in adult and aged chicken hearts, than in new-born and young ones. Adaptive mechanisms might occur in the heart of ageing chickens to ensure an adequate availability of adenine nucleotides.

Purine metabolism by intracellular Chlamydia psittaci

Purine metabolism was studied in the obligate intracellular bacterium Chlamydia psittaci AA Mp in the wild type and a variety of mutant host cell lines with well-defined deficiencies in purine metabolism. C. psittaci AA Mp cannot synthesize purines de novo, as assessed by its inability to incorporate exogenous glycine into nucleic acid purines. C. psittaci AA Mp can take ATP and GTP, but not dATP or dGTP, directly from the host cell. Exogenous hypoxanthine and inosine were not utilized by the parasite. In contrast, exogenous adenine, adenosine, and guanine were directly salvaged by C. psittaci AA Mp. Crude extract prepared from highly purified C. psittaci AA Mp reticulate bodies contained adenine and guanine but no hypoxanthine phosphoribosyltransferase activity. Adenosine kinase activity was detected, but guanosine kinase activity was not. There was no competition for incorporation into nucleic acid between adenine and guanine, and high-performance liquid chromatography profiles of radiolabelled nucleic acid nucleobases indicated that adenine, adenosine, and deoxyadenosine were incorporated only into adenine and that guanine, guanosine, and deoxyguanosine were incorporated only into guanine. Thus, there is no interconversion of nucleotides. Deoxyadenosine and deoxyguanosine were cleaved to adenine and guanine before being utilized, and purine (deoxy)nucleoside phosphorylase activity was present in reticulate body extract.

Brain purines in a genetic mouse model of Lesch-Nyhan disease

Mice carrying a mutation in the gene encoding the purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT) have recently been produced to provide an animal model for Lesch-Nyhan disease. The current studies were conducted to characterize the consequences of the mutation on the expression of HPRT and to characterize potential changes in brain purine content in these mutants. Our results indicate that the mutant animals have no detectable HPRT-immunoreactive material on western blots and no detectable HPRT enzyme activity in brain tissue homogenates, confirming that they are completely HPRT deficient (HPRT-). Despite the absence of HPRT-mediated purine salvage, the animals have apparently normal brain purine content. However, de novo purine synthesis, as measured by [14C]formate incorporation into brain purines, is accelerated four- to fivefold in the mutant animals. This increase in the synthesis of purines may protect the HPRT- mice from potential depletion of brain purines despite complete impairment of HPRT-mediated purine salvage.

Effect of partial urinary outlet obstruction in the rabbit on the incorporation of adenine into adenine nucleotides in bladder smooth muscle

Bladder outlet obstruction induces marked morphological, functional, and metabolic changes within the urinary bladder. Recent studies indicate that there is a close correlation between the contractile dysfunction induced by partial outlet obstruction and a marked decrease in mitochondrial oxidative activity of the hypertrophied bladder tissue. The current study investigates the effect of partial outlet obstruction on adenine metabolism within the bladder tissue. After transport into the cell, adenine becomes available as a substrate for adenine phosphoribosyl transferase (APRT), the enzyme that catalyses the non-mitochondrial conversion of adenine into AMP. Subsequently, AMP is phosphorylated to ADP, the phosphate acceptor in mitochondrial oxidative phosphorylation. The results of these studies demonstrate that partial outlet obstruction induces a significant increase in 14C-adenine uptake into the urinary bladder smooth muscle which in turn provides substrate for APRT and results in an increase in 14C-AMP synthesis. In contrast, the rate of incorporation of adenine into ATP+ADP was similar for both control and obstructed tissue. The activity of APRT was not significantly different in control and obstructed tissue.

Molecular and biochemical elucidation of a cellular phenotype characterized by adenine analogue resistance in the presence of high levels of adenine phosphoribosyltransferase activity

A mouse embryonal carcinoma cell line isolated for resistance to the adenine analogue 2,6-diaminopurine (DAP) was found to have near-wild-type levels of adenine phosphoribosyltransferase (APRT) activity in a cell-free assay. This DAP-resistant (DAPr) cell line, termed H29D1, also exhibited near-wild-type levels of adenine accumulation and the ability to grow in medium containing azaserine and adenine. Growth in this medium requires high levels of intracellular APRT activity. Using the polymerase chain reaction (PCR) and the dideoxy chain termination sequencing technique, an A-->G transition was discovered in exon 3 of the aprt gene in H29D1. This mutation resulted in an Arg-to-Gln change at amino acid 87 of the APRT protein that, in turn, resulted in a decreased affinity for adenine. An increased sensitivity of APRT to inhibition by AMP was observed when comparing H29D1 to P19, the parental cell line. Using a transgene containing the A-->G mutation, we demonstrated that this mutation is responsible for the biochemical and cellular phenotypes observed for the H29D1 cell line. The approach used in this study provides a definitive method for linking a mutation to a specific cellular phenotype.

Adenine deaminase and adenine utilization in Saccharomyces cerevisiae

Compared with other purine salvage and nitrogen catabolism enzymatic activities, adenine deaminase (adenine aminohydrolase [AAH]; EC 3.5.4.2) activity in Saccharomyces cerevisiae is uniquely regulated. AAH specific activity is not induced by adenine and is reduced sevenfold when cells are cultivated in medium containing proline in place of ammonium as the sole nitrogen source. Exogenous adenine enters metabolic pathways primarily via the function of either AAH or adenine phosphoribosyltransferase (APRT; EC 2.4.2.7). Exogenous adenosine cannot normally be utilized as a purine source. Strains efficiently utilized adenosine or inosine when grown in pH 4.5 medium containing Triton X-100. A recessive mutation permitting utilization of adenosine or inosine in standard media was isolated. In both situations, growth of purine auxotrophs required either AAH or APRT activity. With medium containing either ammonium or proline as a nitrogen source, minimum doubling times of purine auxotrophs deficient in either APRT or AAH were measured. In proline-based medium, AAH and APRT permitted equal utilization of exogenous adenine. In ammonium-based medium, the absence of APRT increased the minimum doubling time by 50%. Similar experiments using sufficient exogenous histidine to feedback inhibit histidine biosynthesis failed to affect the growth rates of adenine auxotrophs blocked in AAH or APRT, indicating that the histidine-biosynthetic pathway does not play a significant role in adenine utilization. The gene that encodes AAH in S. cerevisiae was isolated by complementation using yeast strain XD1-1, which is deficient in AAH, APRT, and purine synthesis. A 1.36-kb EcoRI-SphI fragment was demonstrated to contain the structural gene for AAH by expressing this DNA in Escherichia coli under control of the trp promoter-operator. Northern (RNA) studies using the AAH-, APRT-, and CDC3-coding regions indicated that AAH regulation was not mediated at the level of transcription or mRNA degradation.

Purine metabolite concentrations in portal and peripheral blood of steers, sheep and rats

1. The concentration of purine derivatives in portal and peripheral blood of steers, sheep and rats was measured by reverse-phase high performance liquid chromatography. 2. Nucleotides, nucleosides (apart from inosine), adenine and guanine were not found in the plasma samples. Allantoin, uric acid, hypoxanthine and xanthine accounted for virtually all purine metabolites in plasma samples. 3. Non-oxidized derivatives (hypoxanthine and xanthine) were consistently detected in sheep but not in steer or rat plasma samples showing a differential availability of reutilizable purine derivatives between species.

The effect of nutritional state and allopurinol on nucleotide formation in enterocytes from the guinea pig small intestine

The uptake of purine nucleosides (guanosine and hypoxanthine) and bases (guanine, hypoxanthine and adenine) and their incorporation into nucleotides were studied in enterocytes isolated from fed and 3-day fasted guinea pig jejunum. Both total uptake and synthesis of nucleotides were greater for these purines in the fasted, as compared to the fed state for the first 5 min, when the initial substrate concentration in the medium was 10 microM. Increased uptake did not result from a change in the relative distribution of synthesized nucleotides between the fed and fasted states. Reduced catabolism was observed in the medium by enterocytes from fasted as compared to fed animals after 1 min of incubation with both inosine and guanosine. Preincubation of enterocytes with allopurinol (a xanthine oxidase inhibitor) decreased total uptake but increased the formation of IMP from hypoxanthine. Xanthine oxidase activity measured in mucosa from fasted guinea pigs was lower than that from fed animals (6.29 vs. 9.30 nmol/min per mg protein, respectively). However, activities of the salvage enzymes adenine phosphoribosyltransferase and hypoxanthine-guanine phosphoribosyltransferase were not significantly different between the fed and fasted states. These data show that allopurinol treatment, and mucosal atrophy resulting from fasting, decrease xanthine oxidase activity and increase nucleotide synthesis from exogenous substrates in enterocytes from the guinea-pig small intestine, suggesting a regulatory function of mucosal xanthine oxidase in purine salvage by the small intestine.

The pathogenesis of the Lesch-Nyhan syndrome: ATP use is positively related to hypoxanthine supply to hypoxanthine guanine phosphoribosyltransferase

In order to explain features of severe hypoxanthine guanine phosphoribosyltransferase (HPRT) deficiency, the Lesch-Nyhan syndrome, a continuous supply of substrate, hypoxanthine, for the enzyme must be generated. This supply must be increased in association with increased ATP turnover. We have shown that ATP turnover continuously supplies hypoxanthine for recycling by the enzyme HPRT and that this supply increases curvilinearly with increasing ATP turnover. The effects of increasing exercise on ATP turnover were examined using a Latin square experimental design. The outputs of hypoxanthine, xanthine, urate and creatinine were measured. The data were then examined statistically.

Studies on the energy metabolism of opossum (Didelphis virginiana) erythrocytes: V. Utilization of hypoxanthine for the synthesis of adenine and guanine nucleotides in vitro

High pressure liquid radiochromatography was used to test the ability of opossum erythrocytes to incorporate tracer amounts of [G-3H] hypoxanthine (Hy) into [3H] labelled triphosphates of adenine and guanine. In the presence of supraphysiologic (30 mM) phosphate which is optimal for PRPP synthesis, both ATP and GTP are extensively labelled. When physiologic (1 mM) medium phosphate is used, red cells incubated under an atmosphere of nitrogen accumulate [3H] ATP in a linear fashion suggesting ongoing PRPP synthesis in red cells whose hemoglobin is deoxygenated. In contrast, a lesser increase of labelled ATP is observed in cells incubated under oxygen, suggesting that conditions for purine nucleotide formation from ambient Hy are more favorable in the venous circulation.

Hypoxanthine phosphoribosyltransferase activity in tissues and hypoxanthine concentrations in plasma and CSF of the horse in comparison with other species

1. Plasma hypoxanthine and xanthine concentrations are very low in the horse and low in rat, mouse and greyhound compared to concentrations in beagles, man, sheep and rabbit. 2. Activities in erythrocytes of the main enzyme metabolizing hypoxanthine, hypoxanthine phosphori-bosyltransferase, show a similar pattern (Tax et al., 1976, Comp. Biochem. Physiol. 54B, 209-212); thus low activities have been found where plasma concentrations were low. 3. Hypoxanthine phosphoribosyltransferase activities in horse tissue other than erythrocytes are similar to those in man and rabbit with high activities in brain; this enzyme may therefore be functionally important in equine brain.

Purine metabolism as a target for leukemia chemotherapy

This article focuses on the chemotherapeutic agents which alter purine metabolism as a means to achieve selective killing of leukemic cells. We present an overview of purine metabolism in order to highlight enzymatic steps which are targeted by antileukemic drugs. Purine antimetabolites used in the treatment of leukemia can be grouped into three classes: (1) structural analogs of normal purines (6-mercaptopurine and 6-thioguanine); (2) inhibitors of de novo purine biosynthesis (methotrexate and hydroxyurea); and (3) inhibitors of purine salvage (2'-deoxycoformycin). In addition, a number of investigational drugs (trimetrexate, fludarabine and 2'-chlorodeoxyadenosine) have been recently introduced and show promise in early clinical trials. Purine antimetabolites are active in a variety of lymphoid and myeloid leukemias and represent an important component of the therapy of these disorders. Several of the drugs have been developed with the specific intent of perturbing enzymes involved in purine metabolism. Refinements in our understanding of purine biochemistry in normal and leukemic cells may aid future efforts to design more effective drugs.

Hypoxanthine, xanthine and uridine in body fluids, indicators of ATP depletion

Measurements of hyp, xan and urd in body fluids can provide evidence of energy, ATP, depletion in the body, in organs or in cells. Such information is clinically useful in the many diseases in which cellular energy supplies cannot be maintained like perinatal asphyxia, hydrocephalus and vascular insufficiency in brain, heart, limbs, kidneys or other organs. Similar HPLC methods using reversed-phase C18 columns and quantitation by UV absorption have been employed in a variety of centres to yield almost identical results. These have been assembled in this review to form a series of reference values. The current analytical problems are reviewed. Since concentrations of hyp and xan may alter independently situations are discussed in which separate measurements rather than their summed, total oxypurine concentrations are needed. The biochemistry and physiology underlying the use of such analyses is examined to guide sampling of the appropriate body fluid at a relevant time and to avoid oversimplified interpretation of results as well as unnecessary arguments. Specifically: (1) Intracellular concentrations of hyp and xan are inversely related to adenylate energy change and therefore to the energy currency of the cell ATP. Uridine in tissues is similarly 'controlled'. (2) There is extensive evidence that large increases in hyp, xan and urd in body fluids indicate ATP depletion. (3) Small changes in hyp probably reflect alterations of ATP turnover. (4) Xanthine arises mainly from guanine and can change independently of hyp. (5) Clinically useful information is obtainable from hyp and xan concentrations in CSF, amniotic fluid, urine and plasma. Extensive clinical correlations are reviewed. At present we are in a development phase for which HPLC is ideal but the most efficient way to perform and use such analyses in routine clinical practice remains to be established.

Purine de novo synthesis as the basis of synergism of methotrexate and 6-mercaptopurine in human malignant lymphoblasts of different lineages

Methotrexate (MTX) causes an inhibition of purine de novo synthesis (PDNS), resulting in increased intracellular availability of 5-phosphoribosyl-1-pyrophosphate (PRPP) in human malignant lymphoblasts with an active PDNS. Normal bone marrow cells and peripheral blood lymphocytes lack this capacity. The increased levels of PRPP can be used for enhanced incorporation of 6-mercaptopurine (6MP), indicating a potential time-, sequence- and dose-dependent synergism of both drugs. The effects of 0.02 microM and 0.2 microM MTX on the PDNS of MOLT-4 (T-), RAJI (B-) and KM-3 (non-B-non-T-) human malignant lymphoblasts were studied with respect to PRPP levels, aminoimidazolecarboxamide ribonucleosidemonophosphate (AICAR) levels and the incorporation of labeled glycine into purine metabolites. These results were correlated with the activity of the PDNS (labeled glycine incorporation) and the purine salvage pathway (labeled hypoxanthine incorporation) in untreated cells. Inhibition of PDNS by 0.02 microM MTX was complete in KM-3 cells with a moderately active PDNS and salvage pathway. RAJI cells, with a relatively low PDNS and high salvage pathway, demonstrated an incomplete, but increasing inhibition of PDNS, whereas inhibition of PDNS in MOLT-4 cells with both pathways active was minimal and recovered in time. Treatment with 0.2 microM MTX resulted in a complete inhibition of PDNS in all cell lines. After treatment with MTX an enhanced incorporation of labeled hypoxanthine and 6MP was noticed, confirming the potential rescue from MTX cytotoxicity by hypoxanthine and a potential synergism of MTX and 6MP on cytotoxicity. The enhanced incorporation of 6MP was more obvious in RAJI and KM-3 cells in comparison with MOLT-4 cells. These data demonstrate the important role of both the activities of the PDNS and the purine salvage pathway in malignant lymphoblasts of different subclasses with respect to the synergism of MTX and 6MP.