Membrane transport of purine and pyrimidine bases and nucleosides in animal cells

Integrative Application of Transcriptomics and Metabolomics Provides Insights into Unsynchronized Growth in Sea Cucumber (Stichopus monotuberculatus)

Ever-increasing consumer demand for sea cucumbers mainly leads to huge damage to wild sea cucumber resources, including Stichopus monotuberculatus, which in turn exerts negative impacts on marine environments due to the lack of ecological functions performed by sea cucumbers. Aquaculture of sea cucumbers is an effective way to meet consumer demand and restore their resources. Unsynchronous growth is a prominent problem in the aquaculture of sea cucumbers which has concealed unelucidated molecular mechanisms until now. In this study, we carried out an integrative analysis of transcriptomics and metabolomics on fast-growing (SMF) and slow-growing (SMS) groups of S. monotuberculatus cultured in the same environmental conditions. The results revealed that a total of 2054 significantly differentially expressed genes (DEGs) were identified, which are mainly involved in fat digestion and absorption, histidine metabolism, arachidonic acid metabolism, and glutathione metabolism. 368 differential metabolites (DMs) were screened out between the SMF group and the SMS group; these metabolites are mainly involved in glycerophospholipid metabolism, purine metabolism, biosynthesis of unsaturated fatty acids, pyrimidine metabolism, arachidonic acid metabolism, and other metabolic pathways. The integrative analysis of transcriptomics and metabolomics of S. monotuberculatus suggested that the SMF group had a higher capacity for lipid metabolism and protein synthesis, and had a more frequent occurrence of apoptosis events, which are likely to be related to coping with environmental stresses. The results of this study provide potential values for the aquaculture of sea cucumbers which may promote their resource enhancement.

The potential of nucleotide analogs as inhibitors of retroviruses and tumors

The biologically active form of most purine or pyrimidine analogs is the nucleoside 5'-mono, di- or triphosphate. The nucleoside form is most often administered because of the ease with which it penetrates cells by facilitated transport. However, many nucleoside derivatives fail to exhibit significant antiviral or antitumor activity because they are not phosphorylated by cellular enzymes to the active nucleotide form. In this review, the potential use of suitable nucleotide analogs as selective inhibitors of ribonucleotide reductase and viral reverse transcriptase is considered. Masked nucleotides such as phosphoramidates or methyl phosphates could be employed to allow transport across cellular membranes. Furthermore, phosphonocarboxamide, phosphonoformate or sulfamidophosphoramidate may mimic nucleotide di- and triphosphates. Tumor cells and virally infected cells are often more permeable to nucleotides and their analogs than normal cells, which could provide a therapeutic advantage. There could be considerable therapeutic potential for nucleotide analogs that can penetrate the tumor cell membranes and that are resistant to enzymatic hydrolysis and are non-incorporable into DNA or RNA.

Catabolism of pyrimidine nucleotides in the deep-sea tube worm Riftia pachyptila

The present study describes the distribution and properties of enzymes of the catabolic pathway of pyrimidine nucleotides in Riftia pachyptila, a tubeworm living around deep-sea hydrothermal vents and known to be involved in a highly specialized symbiotic association with a bacterium. The catabolic enzymes, 5'-nucleotidase, uridine phosphorylase, and uracil reductase, are present in all tissues of the worm, whereas none of these enzymatic activities were found in the symbiotic bacteria. The 5'-nucleotidase activity was particularly high in the trophosome, the symbiont-harboring tissue. These results suggest that the production of nucleosides in the trophosome may represent an alternative source of carbon and nitrogen for R. pachyptila, because these nucleosides can be delivered to other parts of the worm. This process would complement the source of carbon and nitrogen from organic metabolites provided by the bacterial assimilatory pathways. The localization of the enzymes participating in catabolism, 5'-nucleotidase and uridine phosphorylase, and of the enzymes involved in the biosynthesis of pyrimidine nucleotides, aspartate transcarbamylase and dihydroorotase, shows a non-homogeneous distribution of these enzymes in the trophosome. The catabolic enzymes 5'-nucleotidase and uridine phosphorylase activities increase from the center of the trophosome to its periphery. In contrast, the anabolic enzymes aspartate transcarbamylase and dihydroorotase activities decrease from the center toward the periphery of the trophosome. We propose a general scheme of anatomical and physiological organization of the metabolic pathways of the pyrimidine nucleotides in R. pachyptila and its bacterial endosymbiont.

Endogenous nucleosides in the guinea-pig eye: analysis of transport and metabolites

This study investigates the transport of endogenous nucleosides and deoxynucleosides from the capillaries of the eye into the aqueous humour and the lens using the in situ vascular eye perfusion technique in the guinea-pig. The transport of [3H] adenosine and [3H] thymidine across the blood-aqueous barrier proved to be very rapid with a volume of distribution after 4 minutes perfusion reaching 11.9+/-3.0% and 9.93+/-1.1%, respectively. However, the transport of [3H] guanosine and [3H] cytidine was slower, with volumes of distribution reaching only 3.38+/-0.58% and 4.8+/-1.41%. The values for the entry of deoxyadenosine and deoxyguanosine were not significantly different from the values obtained for corresponding ribonucleosides (adenosine and guanosine) so that a change in the pentose sugar does not change the affinity of the nucleoside for the transport protein. Perfusion with a low sodium medium inhibited the transport of [3H] adenosine and [3H] thymidine into the aqueous humour. The presence of 800 nM NBTI also caused a decrease in adenosine transport into the aqueous humour, so that the volume of distribution after 2 minutes reached only 3.78+/-1.87%. These findings suggest that the transfer of adenosine across the blood-aqueous barrier has both concentrative and equilibrative components. The presence of 0.1 mM thymidine had no effect on the [3H] adenosine transport, whereas 0.1 mM of adenosine resulted in a marked decrease on the [3H] thymidine transport which suggests that the concentrative nucleotide transport is probably mediated by both cif and cit transport systems. The cellular uptake of nucleosides into the lens was very rapid and the volume of distribution of purine nucleosides was within the range of 30-50% whereas that for thymidine uptake was somewhat lower, reaching 20-30%. HPLC analysis of the eye structures in the guinea-pig showed that lens, vitreous body and the rest of the eye do not contain either free nucleosides or purine bases in detectable quantities, except for xanthine which was detected in aqueous humour at a concentration of 2.51+/-0.51 mM. However, serum of the anaesthetised guinea-pig did not contain xanthine in detectable amount so it seems that the metabolic degradation of the nucleosides in the guinea-pig eye progresses as far as xanthine, which is then accumulated in the aqueous humour.

Influence of both salvage and DNA damage response pathways on resistance to chemotherapeutic antimetabolites

The resistance of 3 human embryo fibroblast cell lines to the antimetabolites methotrexate (MTX), N-phosphonacetyl-L-aspartate (PALA) and 5-fluorouracil (5-FU) has been studied. The cell lines were of common genetic origin, all originating from the normal KMS parental cell line, which was irradiated with 60Co to produce the immortalised derivative KMST which, in turn, was transfected with an activated N-ras oncogene to produce the tumourigenic KN-NM cell line. Previous work from this group, using dialysed versus nondialysed serum, has provided evidence for the involvement of salvage pathways of purine and pyrimidine biosynthesis in the increased resistance to antimetabolites of those cell lines (KMST and KN-NM) tending towards increased tumourigenicity. The present study has extended this work by using the nucleoside and nucleobase transport inhibitor dipyridamole, to further assess the contribution of the salvage pathways to the increased cellular resistance to the three antimetabolites. The salvage pathways were found to contribute to the resistance of cell lines to PALA and MTX, but had no effect on the resistance to 5-FU. The addition of excess uridine in the case of PALA, and hypoxanthine plus thymidine in the case of MTX, could be used to "rescue" cells from the effects of dipyridamole-induced salvage pathway inhibition. The data will be discussed in relation to 1. the effect of limited substrate availability, 2. the induction of DNA damage and DNA damage-response pathways, and 3. DNA-damage protection by the salvage pathways of purine and pyrimidine biosynthesis.

Differences in resistance to 5-fluorouracil as a function of cell cycle delay and not apoptosis

A series of human embryo fibroblasts has previously been shown to display increasing resistance to the antimetabolites methotrexate (MTX) and N-phosphonacetyl-L-aspartate (PALA) with increasing tumorigenicity. This increased resistance was found to be further increased as a result of salvage pathway activity for purine and pyrimidine biosynthesis. A similar pattern of increasing resistance paralleling increasing tumorigenicity has now been shown to occur with 5-fluorouracil (5-FU), which is independent of salvage pathway activity. The KMS normal cell line was found to be more sensitive to 5-FU than either the immortalised KMST or tumorigenic KN-NM cell lines. Immunohistochemical analysis of the three cell lines demonstrated high levels of p53 protein in the KMST and KN-NM cell lines, but undetectable p53 levels in the KMS cell line. From these data it was hypothesised that a difference in p53 function may be causing the difference in the patterns of sensitivity observed in the three cell lines. P53 is now believed to function as a regulator of a G1 to S cell cycle checkpoint and as an inducer of apoptosis following DNA damage to the cell. The differences in sensitivity of the cell lines could not be explained by differences in the levels of apoptosis but could be attributed to differences in cell cycle response. Our evidence suggests that loss of cell cycle control, possibly through loss of p53 function, is an important factor in increasing the drug resistance of fibroblast cell lines.

Vesnarinone inhibits nucleoside and nucleobase transport

Vesnarinone is a novel synthetic oral inotropic agent that has been successfully used for treatment of patients with congestive heart failure. In addition to its cardiotonic activity, the drug has been proposed to have mild cytostatic and anti-HIV-1 effects. We have observed that vesnarinone profoundly inhibits radiolabeled thymidine and uridine incorporation into cells despite its modest inhibitory effect on DNA synthesis, RNA synthesis or cell proliferation. Here we demonstrate that vesnarinone inhibits both nucleoside and nucleobase transport in mammalian cells. This pharmacological action may be involved in some of its multiple biological effects.

Azathioprine-related bone marrow toxicity and low activities of purine enzymes in patients with rheumatoid arthritis

OBJECTIVE

Azathioprine (AZA) metabolism largely parallels the endogenous purine pathways. To date, thiopurine methyltransferase (TPMT) deficiency has been reported as a cause of AZA-related bone marrow toxicity in 1 patient with rheumatoid arthritis (RA). We therefore studied purine enzyme activities in 3 patients with RA who experienced AZA-related bone marrow toxicity.

METHODS

Lymphocyte activity of purine nucleoside phosphorylase and 5'-nucleotidase (5NT) and erythrocyte activity of TPMT, key enzymes in thiopurine catabolism, were measured in 3 RA patients who had experienced AZA-related bone marrow toxicity and in 16 RA patients without signs of toxicity despite at least 6 months of treatment with AZA.

RESULTS

Two patients with AZA-related bone marrow toxicity were found to have a TPMT deficiency, 1 partial and 1 total. In the third patient, 5NT activity was found to be well below the lowest level observed in the control subjects.

CONCLUSION

All 3 patients with severe AZA-related bone marrow toxicity had abnormal purine enzyme activities. Deficiency of purine enzymes, including TPMT and 5NT, may be a cause of AZA-related bone marrow toxicity in patients with RA.

Changes in pial arteriolar diameter and CSF adenosine concentrations during hypoxia

We measured the changes in pial arteriolar diameter and CSF concentrations of adenosine, inosine, and hypoxanthine during hypoxia in the absence and presence of topically applied dipyridamole (10(-6) M) and erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA; 10(-5) M). Closed cranial windows were implanted in halothane-anesthetized adult male Sprague-Dawley rats for the observation of the pial circulation and collection of CSF. The mean resting arteriolar diameter in mock CSF was 31.2 +/- 5.9 microns. Topically applied dipyridamole and EHNA, in combination, caused a slight but significant (p < 0.05) increase in resting arteriolar diameter (33.8 +/- 4.3 microns). With mock CSF, moderate hypoxia caused a 22.1 +/- 9.7% increase in pial vessel diameter. Topically applied dipyridamole and EHNA significantly (p < 0.01) potentiated pial arteriolar vasodilation in response to hypoxia. Moreover, the potentiating effects of dipyridamole and EHNA during hypoxia were completely abolished by theophylline (0.20 mumol/g, i.p.; p < 0.05), an adenosine receptor antagonist. Resting concentrations of adenosine, inosine, and hypoxanthine in the subwindow CSF were 0.18 +/- 0.09, 0.35 +/- 0.21, and 0.62 +/- 0.12 microM, respectively. In the absence of dipyridamole and EHNA, these levels were not affected by sustained moderate hypoxia (PaO2 = 36 +/- 6 mm Hg). However, in the presence of dipyridamole and EHNA, the concentration of adenosine in the CSF during hypoxia was significantly (p < 0.05) increased. Our data indicate that dipyridamole and EHNA potentiate hypoxic vasodilation of pial arterioles while simultaneously increasing extracellular adenosine levels, thus supporting the hypothesis that adenosine is involved in the regulation of cerebral blood flow.

Simultaneous analysis of adenosine 3',5'-cyclic monophosphate accumulation and adenosine 5'-triphosphate metabolism in cultured cells preincubated with [2-3H]adenine

A simple and sensitive method based on metabolic labeling was developed for the simultaneous analysis of cyclic AMP accumulation and ATP metabolism in small numbers of cultured cells. Cells are preincubated overnight with [2-3H]adenine to label the ATP pool to a high specific activity. After cell stimulation the metabolites are extracted in a small volume of aqueous acetic acid and chloroform and separated without further manipulation by one-dimensional thin-layer chromatography and the radioactivity incorporated is determined by liquid scintillation counting. With ATP labeled to about 6 Ci/mmol, the lower limit of cyclic AMP detection is 2 fmol, a sensitivity that is comparable to the radioimmunoassay of acetylated cyclic AMP. In primary neurons and a neural cell line, for example, levels of ATP and its metabolites change when large amounts of cyclic AMP are generated, each with its unique pattern. ATP is also depleted when metabolic energy is consumed concomitantly with stimulation of cyclic AMP production by agonists, probably as a result of an increase in ion pump activity following cation influx. As ATP is utilized for cyclic AMP production and simultaneously for many other processes, an assessment of its metabolism in parallel with that of cyclic AMP is critical. We suggest that the method described here is particularly advantageous over other methods for this purpose.

Uridine uptake and RNA synthesis in the brain of torpid and awakened ground squirrels

1. Uptake of [3H]uridine into the nucleotide precursor pool after intraventricular injection occurs with the same intensity in the brain of torpid and normothermic awakened ground squirrels. This indicates that the membrane uridine transporters and uridine kinases operate in the hibernator's brain in a hypothermia-tolerant way. 2. Utilization of the [3H]uridine pool for synthesis of the rapidly labelled RNA in the brain of torpid ground squirrels falls more than eight times against RNA labelling in the brain of the active animals between bouts of hibernation. 3. Two hours from the beginning of the artificially provoked awakening, RNA uridine incorporation in the brain of ground squirrels has risen 6.5 times. 4. Drastic changes in [3H]uridine RNA labelling under the stable uridine uptake exclude the precursors and energy supply as the main factors determining changes in intensity of the brain RNA synthesis in the different stages of hibernation.

Is adenosine deaminase involved in adenosine transport?

The hypothesis that adenosine metabolizing enzymes may have a key role in the transport of adenosine is discussed. The enhancement of adenosine transport by inhibitors of adenosine deaminase (the enzyme which deaminates adenosine to inosine) and the ecto-localization of adenosine deaminase suggest a contribution of the enzyme in taking up nucleosides. Two possible mechanisms are suggested: 1) transport and deamination of adenosine as a coupled process, or 2) uptake of inosine after cleavage of adenosine by ecto-adenosine deaminase. In both cases, the so-called adenosine deaminase binding protein which is a membrane protein could be the real nucleoside transporter. This behaviour of adenosine deaminase as an ectoenzyme anchored to a membrane protein remembers the behaviour of periplasmic binding proteins of bacteria. Thus, adenosine deaminase as well as, for instance, adenosine kinase would be a kind of 'periplasmic proteins' of eukaryotic cells. The function of adenosine deaminase and adenosine kinase would then be to take adenosine and give it to the true transporters.

Affinity labeling of the folate-methotrexate transporter from Leishmania donovani

An affinity labeling technique has been developed to identify the folate-methotrexate transporter of Leishmania donovani promastigotes using "activated" derivatives of the ligands. These "activated" derivatives were synthesized by incubating folate and methotrexate with a 10-fold excess of 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide (EDC) for 10 min at ambient temperature in dimethyl sulfoxide. Preincubation of intact cells with nonradioactive "activated" folate or methotrexate at a concentration of 40 microM inhibited the capacity of wild-type cells to transport submicromolar concentrations of unmodified ligand. When intact wild-type (DI700) Leishmania donovani or preparations of their membranes were incubated with a 0.4 microM concentration of either "activated" [3H]folate or "activated" [3H]methotrexate, the radiolabeled ligands were covalently incorporated into a polypeptide with a molecular weight of approximately 46,000, as demonstrated by SDS-polyacrylamide gel electrophoresis. No affinity labeling of a 46,000-dalton protein was observed when equimolar concentrations of "activated" radiolabeled ligands were incubated with intact cells or membranes prepared from a methotrexate-resistant mutant clone of Leishmania donovani, MTXA5, that is genetically defective in folate-methotrexate transport capability [Kaur, K., Coons, T., Emmett, K., & Ullman, B. (1988) J. Biol. Chem. 263, 7020-7028]. However, some labeling of a 46,000-dalton protein was observed when MTXA5 cells were incubated with higher concentrations of "activated" ligands. Time course studies indicated that maximal labeling of the 46,000-dalton protein occurred within 5-10 min of incubation of intact cells with "activated" ligand.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective activity of 5-fluoroorotic acid against Plasmodium falciparum in vitro

Unlike mammalian cells, malarial parasites are completely dependent on de novo pyrimidine metabolism. Even though these parasites do not use external uracil or uridine, orotic acid, an intermediate of pyrimidine biosynthesis, is successfully transported into the parasite and incorporated into parasite nucleic acids. On this basis, it was hypothesized that 5-fluoroorotate, a cytotoxic derivative of orotic acid, may be a potent and selective antimalarial agent. In vitro, 5-fluoroorotate caused 50% inhibition of the growth of Plasmodium falciparum at a concentration of 6.0 nM. In contrast, 5-fluorouracil, 5-fluorouridine, and 5-fluoro 2'-deoxyuridine were much less effective against malarial parasites. Chloroquine-susceptible and chloroquine-resistant clones of P. falciparum were equally susceptible to 5-fluoroorotate. The toxicity of 5-fluoroorotate was evaluated on four human cell lines (HT-1080, IMR-90, HeLa S3, and HL-60) and one mouse cell line (L-1210). Compared with malarial parasites, the mammalian cells were relatively tolerant of 5-fluoroorotic acid (50% inhibitory concentration, 0.9 to 10 microM). Finally, in the presence of 1 mM uridine, all mammalian cells were partially protected from 5-fluoroorotate cytotoxicity, but uridine offered no protection to P. falciparum.

The role of nonparenchymal and parenchymal liver cells in the catabolism of extracellular purines

Adenosine-degrading enzymes within the liver lobule can modulate both vascular and metabolic effects of circulating adenosine in the liver. Since it has not been fully established whether nonparenchymal cells participate in the elimination of sinusoidal purines, isolated Kupffer cells and endothelial cells were tested for their capacity to degrade extracellular purines. After perfusion and digestion of rat livers by collagenase, the resulting mixed cell population was separated by centrifugal elutriation. The isolated parenchymal and nonparenchymal cells were incubated for up to 2 hr in the presence of [8(-14)C]adenosine, [8(-14)C]guanosine and [8(-14)C]hypoxanthine (50 mumoles per liter). In the deproteinized medium, adenosine, guanosine, inosine, adenine, guanine, xanthine, hypoxanthine, uric acid and allantoin were separated by reversed-phase high-performance liquid chromatography. Radioactive peaks were collected and counted. Nonparenchymal cells catalyzed the degradation of adenosine into inosine and hypoxanthine. However, the formation of xanthine, uric acid or allantoin from adenosine could only be detected in hepatocyte suspensions. Within 15 min, adenosine was completely eliminated from the medium by Kupffer cells, whereas endothelial cells catabolized only less than half of the initial amount of the adenine nucleoside during this time period. Accordingly, incubation of nonparenchymal cells in the presence of hypoxanthine did not result in the formation of further breakdown products of the purine, whereas its catabolites slowly accumulated in the medium of hepatocytes. Guanosine conversion into guanine and xanthine was much slower in endothelial cells as compared to Kupffer cells and hepatocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

The specificity of pyrimidine nucleoside transport and metabolism by rat jejunum in vitro

1. 5-Methyluridine perfused through the lumen of isolated loops of rat jejunum gave rise to serosal thymine as the major species transported across the epithelial layer; no serosal 5-methyluridine was detected. 2. At 0.1 mM-luminal 5-methyluridine there was enhanced transmural transport of thymine (P less than 0.001) when compared with either 0.1 mM-thymine or 0.1 mM-thymidine as the luminal substrate for thymine transport. 3. At low luminal concentrations (0.025 mM) thymine was a significantly better substrate (P less than 0.001) for the transport of the free pyrimidine than either of the two nucleosides (thymidine or 5-methyluridine). 4. Luminal deoxyuridine gave rise to uracil as the major species appearing in the serosal secretion. High luminal concentrations of deoxyuridine (0.5 and 1.0 mM) gave rise to low levels of the nucleoside in the serosal fluid. 5. As a consequence of the specificity of the mucosal phosphorolysis the ribonucleosides are favoured over the deoxyribonucleosides as substrates for transmural pyrimidine transport.

Dideoxycytidine metabolism in wild type and mutant CEM cells deficient in nucleoside transport or deoxycytidine kinase

The growth inhibitory effects and metabolism of 2',3'-dideoxycytidine (ddC) were examined in wild type human CEM T lymphoblasts and in mutant populations of CEM cells that were genetically deficient in either nucleoside transport or deoxycytidine kinase activity. Whereas ddC at a concentration of 4 uM inhibited growth of the wild type CEM parental strain by 50%, two nucleoside transport-deficient clones were four-fold resistant to the pyrimidine analog. The deoxycytidine kinase-deficient cell line was virtually completely resistant to growth inhibition by the dideoxynucleoside (ddN) at a concentration or 1024 uM. An 80% diminished rate of [3H]ddC influx into the two nucleoside transport-deficient lines could account for their resistance to the ddN, while the resistance of the deoxycytidine kinase deficient cells to ddC toxicity could be explained by a virtually complete failure to incorporate [3H]ddC in situ. Two potent inhibitors of mammalian nucleoside transport, 4-nitrobenzylthioinosine and dipyridamole, mimicked the effects of a genetic deficiency in nucleoside transport with respect to ddC toxicity and incorporation. These data indicate that the intracellular metabolism of ddC in CEM cells is initiated by the nucleoside transport system and the cellular deoxycytidine kinase activity.

Na+- and K+-dependent uridine transport in rat renal brush-border membrane vesicles

The transport of uridine into rat renal brush-border membrane vesicles was investigated using an inhibitor-stop filtration method. Uridine was not metabolized under these conditions. The rapid efflux of intravesicular uridine was prevented by adding 1 mM phloridzin to the ice-cold stop solution. In the presence of inwardly directed gradients of either Na+ or K+, zero-trans uridine uptake exhibited a transient overshoot phenomenon indicating active transport. The overshoot was much more pronounced with Na+ than K+ and it was not observed when either Na+ or K+ was at equilibrium across the membrane. The K+-induced overshoot was not due to the presence of a membrane potential alone, as an inwardly directed gradient of choline chloride failed to produce it. The amplitude of the overshoot was increased by raising either the Na+ or K+ concentration outside the membrane or by using more lipophilic anions (reactive order was NO3- greater than SCN- greater than Cl- greater than SO4(2-). Zero-trans efflux studies showed that the uridine transport is bidirectional. Li+ could substitute poorly for Na+ but not at all for K+. Stoichiometries of 1:1 and greater than 1:1 were observed for Na+: uridine and K+: uridine coupling, respectively. A preliminary analysis of the interactions between Na+ and K+ for uridine uptake showed complex interactions which can best be explained by the involvement of two different systems for nucleoside transport in the rat renal brush-border membrane, one requiring Na+ and the other K+ as transport coupler.

Lesch-Nyhan syndrome and its pathogenesis: purine concentrations in plasma and urine with metabolite profiles in CSF

Purine metabolism in the Lesch-Nyhan syndrome has been re-examined in 10 patients. Hypoxanthine and xanthine concentrations in plasma and CSF and urinary excretion have been studied, on and off allopurinol treatment, using high performance liquid chromatographic methods. Accumulation of the substrate, hypoxanthine, of the missing hypoxanthine guanine phosphoribosyltransferase (HPRT) enzyme, is more marked in urine and in CSF than in plasma. The greater increase in CSF is consistent with the most metabolically active tissue, brain, showing the most marked functional changes. The function of HPRT seems to be the recycling of hypoxanthine which is released from tissues in increasing quantities as energy use, ATP 'turnover', in the tissue increases. The existing screening method for HPRT deficiency, the ratio of the urinary concentration of urate to that of creatinine, shows overlap between the values in severe HPRT deficiency and in controls; this overlap is not found with a urinary hypoxanthine/creatinine molar concentration ratio.

Cardiac myocyte guanosine transport and metabolism

Guanosine transport and metabolism were examined in adult rat cardiac myocytes. Myocytes transported guanosine via saturable [Km = 18 microM, maximum velocity (Vmax) = 3.61 pmol.mg-1.s-1] and nonsaturable (rate constant = 1.47 X 10(-2] processes. The saturable process was inhibited by nitrobenzyl-thioinosine, inosine [inhibition constant (Ki) = 180 microM], and adenosine (Ki = 112 microM). Extracellular guanosine taken up by myocytes was slowly phosphorylated to guanine nucleotides. The majority of guanosine (98%) existed as free intracellular guanosine after 60 s. Countertransport of nucleosides could not be demonstrated in these cells at physiological concentrations in the presence of up to a 10-fold gradient of nucleoside. These studies indicate that adult rat cardiac myocytes can be used to assess myocardial guanosine transport separate from its metabolism. Comparable inhibition of guanosine and adenosine transport by each other and by inosine support the hypothesis that guanosine and adenosine are transported by a common carrier.

Synergistic inhibitory effects of dipyridamole and vincristine on the growth of human leukaemia and lymphoma cell lines

The effects of combinations of dipyridamole, an effective blocker of the salvage pathway of DNA synthesis, and 8 types of anti-cancer drugs on the growth of human T, B and myeloid leukaemia/lymphoma cell lines in vitro were examined. In combinations, dipyridamole and vincristine (VCR), and dipyridamole and vindesine had synergistic inhibitory effects. Dipyridamole reduced the efflux of VCR from cells and enhanced their VCR accumulation in a dose-dependent manner at concentrations of up to 10 microM in the lymphoid cell lines, MOLT-3 and BL-TH, and of up to at least 20 microM in the myeloid cell line, ML-1. Dipyridamole also enhanced the accumulation of VCR in PHA-stimulated and un-stimulated lymphocytes of normal donors, but efflux of VCR was more rapid from normal lymphocytes than from cultured cell lines. It is proposed that combination therapy with dipyridamole plus VCR should be effective in the treatment of leukaemia and lymphoma.

Expression of the high-affinity purine nucleobase transporter in mutant mouse S49 cells does not require a functional wild-type nucleoside-nucleobase transporter

A novel type of somatic mutation that causes the expression of a high-affinity purine base permease (B. Aronow, D. Toll, J. Patrick, P. Hollingsworth, K. McCartan, and B. Ullmann, Mol. Cell Biol. 6:2957-2962, 1986) has been inserted into nucleoside transport-deficient S49 cells. Two classes of mutants expressing this nucleobase permease were generated. The first, as exemplified by the AE1HADPAB2 cell line, possessed an augmented capacity to transport low concentrations of the three purine bases, hypoxanthine, guanine, and adenine. The second class of mutants, as typified by the AE1HADPAB5 clone, possessed an augmented capability to translocate low levels of hypoxanthine and guanine, but not adenine. Neither the AE1HADPAB2 nor the AE1HADPAB5 cells could transport nucleosides, suggesting that the expression of the high-affinity base transporter did not reverse the mutation in the nucleoside transport system. The transport of purine bases by both AE1HADPAB2 and AE1HADPAB5 cells was much less sensitive than that by wild-type cells to inhibition by dipyridamole, 4-nitrobenzylthionosine, and N-ethylmaleimide, potent inhibitors of nucleoside and nucleobase transport in wild-type S49 cells. Fusion of the AE1HADPAB2 and AE1HADPAB5 cell lines with wild-type cells indicated that the expression of the high-affinity base transporter behaved in a dominant fashion, while the nucleoside transport deficiency was a recessive trait. These data suggest that the high-affinity purine base transporter of mutant cells and the nucleoside transport function of wild-type cells are products of different genes and that expression of the former probably requires the unmasking or alteration of a specific genetic locus that is silent or different in wild-type cells.

Expression of a novel high-affinity purine nucleobase transport function in mutant mammalian T lymphoblasts

The single nucleoside transport function of mouse S49 lymphoblasts also transports purine bases (B. Aronow and B. Ullman, J. Biol. Chem. 261:2014-2019, 1986). This transport of purine bases by S49 cells is sensitive to inhibition by dipyridamole (DPA) and 4-nitrobenzylthioinosine, two potent inhibitors of nucleoside transport. Therefore, wild-type S49 cells cannot salvage low hypoxanthine concentrations in the presence of 10 microM DPA and 11 microM azaserine; the latter is a potent inhibitor of purine biosynthesis. Among a mutagenized wild-type population, a cell line, JPA2, was isolated which could proliferate in 50 microM hypoxanthine-11 microM azaserine-10 microM DPA. The basis for the survival of JPA2 cells under these selective conditions was expression of a unique, high-affinity purine nucleobase transport function not present in wild-type cells. JPA2 cells could transport 5 microM concentrations of hypoxanthine, guanine, and adenine 15- to 30-fold more efficiently than parental cells did. Kinetic analyses revealed that the affinity of the JPA2 transporter for all three purine bases was much greater than that of the wild-type nucleobase transport system. Moreover, nucleobase transport in JPA2 cells, unlike that in parental cells, was insensitive to inhibition by DPA, 4-nitrobenzylthioinosine, sulfhydryl reagents, and nucleosides. No alterations in nucleoside transport capability, phosphoribosylpyrophosphate levels, or purine phosphoribosyltransferase enzymes were detected in JPA2 cells. Thus, JPA2 cells express a novel nucleobase transport capability which can be distinguished from the nucleoside transport function by multiple biochemical parameters.

Incomplete nucleoside transport deficiency with increased hypoxanthine transport capability in mutant T-lymphoblastoid cells

From a mutagenized population of wild-type mouse (S49) T-lymphoma cells, a clone, 80-5D2, was isolated in a single step by virtue of its ability to survive in 80 nM 5-fluorouridine. Unlike previously isolated nucleoside transport-deficient cell lines (A. Cohen, B. Ullman, and D. W. Martin, Jr., J. Biol. Chem. 254:112-116, 1979), 80-5D2 cells were only slightly less sensitive to growth inhibition by a variety of cytotoxic nucleosides and were capable of proliferating in hypoxanthine-amethopterin-thymidine-containing medium. The molecular basis for the phenotype of 80-5D2 cells was incomplete deficiency in the ability of the mutant cells to translocate nucleosides across the plasma membrane. Interestingly, mutant cells were more capable than wild-type cells of transporting the nucleobase hypoxanthine. Residual transport of adenosine into 80-5D2 cells was just as sensitive to inhibition by nucleosides and more sensitive to inhibition by hypoxanthine than that in wild-type cells, indicating that the phenomena of ligand binding and translocation can be uncoupled genetically. The 80-5D2 cells lacked cell surface binding sites for the potent inhibitor of nucleoside transport p-nitrobenzylthioinosine (NBMPR) and, consequently, were largely resistant to the physiological effects of NBMPR. However, the altered transporter retained its sensitivity to dipyridamole, another inhibitor of nucleoside transport. The biochemical phenotype of the 80-5D2 cell line supports the hypothesis that the determinants that comprise the nucleoside carrier site, the hypoxanthine carrier site, the NBMPR binding site, and the dipyridamole binding site of the nucleoside transport function of mouse S49 cells are genetically distinguishable.

Two high affinity nucleoside transporters in Leishmania donovani

A rapid sampling kinetic technique has been used to evaluate the nucleoside transport functions of Leishmania donovani. The results indicated that L. donovani promastigotes possessed two independent purine nucleoside transporters with nonoverlapping substrate specificity. The first transported inosine, guanosine, and their analogs, while the second carried adenosine, analogs of adenosine, and the pyrimidine nucleosides, uridine, cytidine, and thymidine. The apparent Km values of the two nucleoside permeases for their purine nucleoside substrates were extraordinarily low, in the micromolar range. The organisms were capable of concentrating purine nucleosides from the medium and converting them to the nucleotide level with great efficiency and rapidity. Inosine and adenosine transport could be distinguished by different sensitivities to sulfhydryl reagents, suggesting structural differences between the two transporters. Finally, the two nucleoside transport systems of L. donovani were virtually refractory to inhibition by 4-nitrobenzylthioinosine and dipyridamole, two potent inhibitors of nucleoside entry into mammalian cells.

Expression of the high-affinity purine nucleobase transporter in mutant mouse S49 cells does not require a functional wild-type nucleoside-nucleobase transporter

A novel type of somatic mutation that causes the expression of a high-affinity purine base permease (B. Aronow, D. Toll, J. Patrick, P. Hollingsworth, K. McCartan, and B. Ullmann, Mol. Cell Biol. 6:2957-2962, 1986) has been inserted into nucleoside transport-deficient S49 cells. Two classes of mutants expressing this nucleobase permease were generated. The first, as exemplified by the AE1HADPAB2 cell line, possessed an augmented capacity to transport low concentrations of the three purine bases, hypoxanthine, guanine, and adenine. The second class of mutants, as typified by the AE1HADPAB5 clone, possessed an augmented capability to translocate low levels of hypoxanthine and guanine, but not adenine. Neither the AE1HADPAB2 nor the AE1HADPAB5 cells could transport nucleosides, suggesting that the expression of the high-affinity base transporter did not reverse the mutation in the nucleoside transport system. The transport of purine bases by both AE1HADPAB2 and AE1HADPAB5 cells was much less sensitive than that by wild-type cells to inhibition by dipyridamole, 4-nitrobenzylthionosine, and N-ethylmaleimide, potent inhibitors of nucleoside and nucleobase transport in wild-type S49 cells. Fusion of the AE1HADPAB2 and AE1HADPAB5 cell lines with wild-type cells indicated that the expression of the high-affinity base transporter behaved in a dominant fashion, while the nucleoside transport deficiency was a recessive trait. These data suggest that the high-affinity purine base transporter of mutant cells and the nucleoside transport function of wild-type cells are products of different genes and that expression of the former probably requires the unmasking or alteration of a specific genetic locus that is silent or different in wild-type cells.

Effect of adenosine on atrioventricular conduction. I: Site and characterization of adenosine action in the guinea pig atrioventricular node

Adenosine has a negative dromotropic effect and modulates hypoxia-induced atrioventricular (AV) conduction delay. To further characterize the negative dromotropic effect of adenosine in the guinea pig heart, we determined the site of adenosine-induced AV conduction block; the effect of uptake and deamination of adenosine on its concentration-negative dromotropic effect, and the adenosine receptor that mediates this action. In isolated AV node preparations (n = 16), adenosine in a dose-dependent manner decreased significantly the duration and amplitude of the action potential of atrionodal and nodal cells and, in addition, markedly depressed the maximum rate of rise of the action potential of nodal cells. At high concentrations (greater than 20 microM), adenosine rendered nodal cells inexcitable. In isolated perfused hearts (n = 7), adenosine (5.7 microM) prolonged total AV conduction time by 21 +/- 2 msec. Of this prolongation, 83% was due to an increase in the nodal-to-His-bundle interval and the remaining 17% to an increase in the atrionodal to nodal interval. Infusion of adenosine to cause a 50% increase (EC50) in atria-to-His bundle (AH) interval prolongation resulted in a perfusate (arterial) adenosine concentration of 5.0 +/- 0.6 microM and effluent (venous) adenosine concentrations of 2.8 +/- 0.4 microM, i.e., an arteriovenous difference of 44% (n = 4). When adenosine uptake and deamination were inhibited with dipyridamole (0.5 microM) plus erythro-9-(2-hydroxy-3-nonyl)adenine (5 microM), respectively, the EC50s were 0.28 +/- 0.02 (perfusate) and 0.32 +/- 0.03 microM (effluent). These data indicate that when nucleoside metabolism is inhibited, arterial and venous concentrations of adenosine reach equilibrium. In an additional 10 hearts, the following rank order of potency of adenosine agonists in causing AH interval prolongation was found: N6-cyclopentyladenosine greater than N6-(L-2-phenyl-isopropyl)adenosine greater than 5'-N-ethylcarboxyamidoadenosine greater than or equal to 2-chloroadenosine greater than adenosine, which is compatible with activation of an A1-type receptor. In summary: the site of adenosine-induced AV conduction block is the nodal zone of the AV node, when adenosine uptake and deamination are inhibited, adenosine in concentrations similar to that released by hypoxia causes significant AH interval prolongation, and the adenosine receptor mediating the negative dromotropic effect of adenosine is of the A1-type.

Expression of a novel high-affinity purine nucleobase transport function in mutant mammalian T lymphoblasts

The single nucleoside transport function of mouse S49 lymphoblasts also transports purine bases (B. Aronow and B. Ullman, J. Biol. Chem. 261:2014-2019, 1986). This transport of purine bases by S49 cells is sensitive to inhibition by dipyridamole (DPA) and 4-nitrobenzylthioinosine, two potent inhibitors of nucleoside transport. Therefore, wild-type S49 cells cannot salvage low hypoxanthine concentrations in the presence of 10 microM DPA and 11 microM azaserine; the latter is a potent inhibitor of purine biosynthesis. Among a mutagenized wild-type population, a cell line, JPA2, was isolated which could proliferate in 50 microM hypoxanthine-11 microM azaserine-10 microM DPA. The basis for the survival of JPA2 cells under these selective conditions was expression of a unique, high-affinity purine nucleobase transport function not present in wild-type cells. JPA2 cells could transport 5 microM concentrations of hypoxanthine, guanine, and adenine 15- to 30-fold more efficiently than parental cells did. Kinetic analyses revealed that the affinity of the JPA2 transporter for all three purine bases was much greater than that of the wild-type nucleobase transport system. Moreover, nucleobase transport in JPA2 cells, unlike that in parental cells, was insensitive to inhibition by DPA, 4-nitrobenzylthioinosine, sulfhydryl reagents, and nucleosides. No alterations in nucleoside transport capability, phosphoribosylpyrophosphate levels, or purine phosphoribosyltransferase enzymes were detected in JPA2 cells. Thus, JPA2 cells express a novel nucleobase transport capability which can be distinguished from the nucleoside transport function by multiple biochemical parameters.

Incomplete nucleoside transport deficiency with increased hypoxanthine transport capability in mutant T-lymphoblastoid cells

From a mutagenized population of wild-type mouse (S49) T-lymphoma cells, a clone, 80-5D2, was isolated in a single step by virtue of its ability to survive in 80 nM 5-fluorouridine. Unlike previously isolated nucleoside transport-deficient cell lines (A. Cohen, B. Ullman, and D. W. Martin, Jr., J. Biol. Chem. 254:112-116, 1979), 80-5D2 cells were only slightly less sensitive to growth inhibition by a variety of cytotoxic nucleosides and were capable of proliferating in hypoxanthine-amethopterin-thymidine-containing medium. The molecular basis for the phenotype of 80-5D2 cells was incomplete deficiency in the ability of the mutant cells to translocate nucleosides across the plasma membrane. Interestingly, mutant cells were more capable than wild-type cells of transporting the nucleobase hypoxanthine. Residual transport of adenosine into 80-5D2 cells was just as sensitive to inhibition by nucleosides and more sensitive to inhibition by hypoxanthine than that in wild-type cells, indicating that the phenomena of ligand binding and translocation can be uncoupled genetically. The 80-5D2 cells lacked cell surface binding sites for the potent inhibitor of nucleoside transport p-nitrobenzylthioinosine (NBMPR) and, consequently, were largely resistant to the physiological effects of NBMPR. However, the altered transporter retained its sensitivity to dipyridamole, another inhibitor of nucleoside transport. The biochemical phenotype of the 80-5D2 cell line supports the hypothesis that the determinants that comprise the nucleoside carrier site, the hypoxanthine carrier site, the NBMPR binding site, and the dipyridamole binding site of the nucleoside transport function of mouse S49 cells are genetically distinguishable.

Hypoxanthine and adenine metabolism in bovine thyroid tissue

Hypoxanthine was converted primarily to uric acid by thyroid tissue slices and homogenates with little inosine 5'-monophosphate formation while adenine was essentially all salvaged to adenosine 5'-monophosphate by similar tissue preparations. The ratio of hypoxanthine/guanine phosphoribosyltransferase activity to adenine phosphoribosyltransferase activity was 0.15 in the thyroid homogenates.

Pre- and postjunctional modulation of cholinergic neuroeffector transmission by adenine nucleotides. Experiments with agonist and antagonist

Adenosine, 2-chloroadenosine, AMP, AMPNH2, ADP, ATP, AMPPNP and beta, gamma meATP dose-dependently and reversibly inhibited contractile responses to nerve stimulation in longitudinal plexus-containing muscle preparations of guinea pig ileum. All the purines, except for 2-chloroadenosine, were virtually equipotent and elicited maximal inhibition within 2 min after application. Adenosine deaminase abolished inhibition by adenosine but did not block the effect of 2-chloroadenosine or the adenine nucleotides. During transmural nerve stimulation, the nucleotidase-resistant analogues alpha, beta meADP and AMPPNP significantly depressed acetylcholine release measured by gas chromatography-mass spectrometry. The inhibitory effect of all the purines was competitively, reversibly and at comparable pA2 values antagonized by 8-p-sulphophenyltheophylline. The results indicate that nucleotides per se can inhibit neurotransmission via a prejunctional receptor common to nucleosides and nucleotides. An excitatory effect by all the di- and triphosphate nucleotides was also observed. The excitation was unaltered by atropine, tetrodotoxin and 8-p-sulphophenyltheophylline, thus suggesting an action at postjunctional ADP- and ATP-like receptive sites.

Cytochemical localization of 5'-nucleotidase in the enteric ganglia and in smooth muscle cells of the guinea-pig

Cytochemical techniques were used to study the localization of 5'-nucleotidase in the enteric ganglia and in smooth muscle cells of the guinea-pig ileum, iris and vas deferens. Enzymatic activity was revealed in plasma membranes and caveolae of smooth muscle cells and in neurons and neuroglia of the enteric ganglia. The strongest activity was seen in the membrane of the smooth muscle cells, especially the caveolae intracellulares, and this was interpreted to indicate a high level of purine utilization and involvement in calcium translocation by these cells. The ganglia displayed enzymatic activity in the membranes of non-specialized neuron-to-glia boundaries as well as at some synaptic specializations. This finding is consistent with a possible release of adenine nucleotides within the ganglia.

Uphill transport of pyrimidine nucleosides in renal brush border vesicles

The uptake of cytidine, of thymidine and of uridine in brush border vesicles prepared from the cortex of rat kidney has been studied by the technique of rapid filtration. The nucleosides were not metabolized in the vesicles. The time-courses of uptake in the presence of inwardly directed gradients of Na+ and of K+ showed an overshoot, indicating uphill transport. The overshoot was much more pronounced with Na+ than with K+; it was not observed when Na+ was at equilibrium across the membrane. The uptake of the nucleosides was stimulated by an inside negative potential in the presence of Na+. These results provide evidence for a cotransport of pyrimidine nucleosides with Na+. The apparent Km's for the uptake of cytidine, of thymidine and of uridine were 3.76 mumol X 1(-1), 4.18 mumol X 1(-1) and 7.21 mumol X 1(-1) respectively. The uptake of the pyrimidine nucleosides was insensitive to 6-nitrobenzylthioinosine. This insensitivity as well as the high affinity for the nucleosides and the capacity for uphill transport indicate that the nucleoside carrier(s) is renal brush border is (are) different from the carriers found in most other cell types.

Concentrative transport of purine nucleosides in brush border vesicles of the rat kidney

Nephrotoxicity has been reported in humans and in animals during treatment with nucleosides. We described recently a Na+ gradient-dependent concentrative transport of adenosine in brush border vesicles isolated from the cortex of the rat kidney. The results of the present study suggest that that transport is shared by other purine nucleosides. The transport of adenosine is inhibited by several purines at micromolar concentrations. The transports of deoxyadenosine, of inosine and of guanosine are concentrative in the presence of a Na+ gradient and they are inhibited by adenosine. We conclude that there exists a concentrative system for reabsorption of various purine nucleosides in the proximal tubule. The transport of nucleosides is non-concentrative in all other cell types studied so far. The concentrative capacity of the transport of nucleosides in the kidney might explain the particular sensitivity of that organ during treatment with nucleosides.

Dipyridamole decreases glomerular filtration in the sodium-depleted dog. Evidence for mediation by intrarenal adenosine

To determine the renal effects of inhibiting the uptake and subsequent metabolism of endogenous adenosine, dipyridamole, a nucleoside transport inhibitor, was infused intrarenally into anesthetized dogs. Dipyridamole (24 micrograms/kg per min) inhibited the cellular extraction of [14C]adenosine (72 +/- 3% vs. 9 +/- 3%) and elevated the excretion of endogenous adenosine (0.60 +/- 0.08 to 1.70 +/- 0.21 nmol/min, P less than 0.05). The action of exogenous adenosine to decrease glomerular filtration rate is known to be enhanced by sodium depletion, and is minimal or absent in sodium-loaded animals. To ascertain whether dietary sodium intake alters the renal effects of elevated endogenous adenosine, dipyridamole was infused into sodium-depleted and sodium-loaded dogs. In the sodium-depleted dogs (n = 9), dipyridamole infusion decreased the glomerular filtration rate by 59 +/- 7% (20 +/- 1 to 8 +/- 2 ml/min, P less than 0.05) which returned to control levels within 30 minutes after stopping infusion of dipyridamole. Renal vascular resistance was unchanged during dipyridamole infusion. In the sodium-loaded dogs (n = 5), dipyridamole had no effect on glomerular filtration rate (22 +/- 4 vs. 25 +/- 3 ml/min) or renal vascular resistance. In a separate series of sodium-depleted dogs (n = 8), the dipyridamole-induced decrease in glomerular filtration rate was completely reversed or inhibited by theophylline, an adenosine receptor antagonist. These experiments demonstrate that inhibition of cellular uptake of adenosine elevates adenosine levels, that dipyridamole decreases glomerular filtration rate in sodium-depleted but not sodium-loaded dogs, and that the decrease in glomerular filtration rate is inhibited by theophylline. We conclude that the decrease in glomerular filtration rate during dipyridamole administration is mediated by increased endogenous adenosine.

A possible explanation for a greater discrepant suppression by adenosine and N6-cyclohexyl adenosine of the guinea-pig ileal contraction

Adenosine and N6-cyclohexyl adenosine (CHA) reduced dose-dependently a twitch contraction of guinea-pig ileum and their IC50 values were 1.0 X 10(-5) and 1.1 X 10(-8) M, respectively. By exposure to adenosine uptake and deaminase inhibitors, dipyridamole and erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA), the adenosine-induced inhibition was enhanced, whereas the CHA-induced one was almost unchanged. Therefore, outstanding potency difference of the suppression by adenosine and CHA was minimized by concurrent addition of both the inhibitors. Uptakes of [3H]CHA into the guinea-pig ileum and taenia-coli were considerably lower than those of [3H]adenosine and were insensitive to dipyridamole. In the rat connective tissue segment, uptake of [3H]adenosine was smaller than that in the guinea-pig taenia-coli and ileal segments (especially, in the latter), and was virtually unaffected by 10(-5) M dipyridamole. These findings suggest that a greater discrepancy of the suppression by adenosine and CHA of the ileal twitch may substantially result from their different uptake and deamination in the synaptic region rather than their dissimilar intrinsic activity for the purinoceptor.

[3H]nitrobenzylthioinosine binding to the guinea pig CNS nucleoside transport system: a pharmacological characterization

The binding of [3H]nitrobenzylthioinosine (NBMPR) to specific membrane sites in guinea pig brain was rapid, reversible, and saturable, and was dependent upon protein concentration, pH, and temperature. Mass law analysis of the binding data for cortical membranes indicated that NBMPR bound with high affinity to a single class of sites at which the equilibrium dissociation constant (KD) for NBMPR was 0.10-0.25 nM and which possessed a maximum binding capacity (Bmax) per mg of protein of 300 fmol of NBMPR. Kinetic analysis of the site-specific binding of NBMPR yielded an independent estimate of the KD of 0.16 nM. A relatively homogeneous subcellular distribution of the sites for NBMPR was found in cortical tissue. Recognized inhibitors of nucleoside transport were potent, competitive inhibitors of the binding of NBMPR in guinea pig CNS membranes whereas benzodiazepines and phenothiazines have low affinity for the sites. NBMPR sites in guinea pig cortical membranes have characteristics similar to those for NBMPR in human erythrocytes, the occupation of which is associated with inhibition of nucleoside transport. The comparable affinities for a range of agents for sites in human erythrocytes and guinea pig CNS membranes suggest that NBMPR also binds to transport inhibitory elements of the guinea pig CNS nucleoside transport system. It is proposed that the study of the binding of NBMPR provides an effective method by which to examine drug interactions with the membrane-located nucleoside transport system in CNS membranes.

Specific increase in thymidine transport at a permissive temperature in the rat kidney cells infected with srcts-Rous sarcoma virus

Thymidine transport was found to be increased two-fold at a permissive temperature in the cells of a normal rat kidney (NRK) line which was infected with a temperature-sensitive Rous sarcoma virus. This increase in thymidine transport was independent of cell density and coincided with the changes in cellular morphology that result from this temperature shift. A double reciprocal plot of the data demonstrated two saturable components (Km values of 60 microM and 250 microM at 39 degrees, non-permissive temperature) of the uptake of thymidine, and the drop of temperature (at 33 degrees, the permissive temperature for transformation) decreased Km to one half, but did not change Vmax. These results indicate that a qualitative alteration of thymidine transport took place in the presence of the active src gene product.

Structural modifications at the 2'- and 3'-positions of some pyrimidine nucleosides as determinants of their interaction with the mouse erythrocyte nucleoside transporter

Modifications in the sugar moiety of pyrimidine nucleosides may affect their ability to function as permeants of the mouse erythrocyte nucleoside transporter. In this investigation, a number of synthetic uracil and thymine nucleosides which differ from the physiological nucleosides, uridine, deoxyuridine and thymidine, through structural changes at the 2'- and 3'-positions were studied. Interaction of the analogs with the transporter has been assessed in terms of their affinities for an external site on the transporter as well as their abilities to effect trans-acceleration of thymidine efflux. 1-(beta-D-Arabinofuranosyl) uracil (araU) and 1-(beta-D-arabinofuranosyl)thymine (araT) were comparable to thymidine as permeants while nucleosides in which the 3'-hydroxyl was replaced with hydrogen or a halogen had a decreased affinity for the transporter. 3'-Fluoro-3'-deoxy-araU weakly accelerated thymidine efflux while its ribo-isomer and the other 3'-halogeno-3' deoxy-arabino analogs as well as dideoxythymidine inhibited efflux. The absence of 2'- and 3'-carbons in acyclothymidine and acyclouridine strongly decreased the affinities of these nucleosides for the transporter; efflux of thymidine was not accelerated in the presence of these compounds. The conformationally constrained cyclic nucleoside 2,2'-anhydro-araU had a very low affinity for the transporter, and influx of the radiolabeled compound could not be demonstrated. The results suggest that modification at the 3'-position, loss of a portion of the sugar ring, and lack of conformational flexibility are factors which decrease the abilities of some pyrimidine nucleosides to function as permeants. It is suggested that combined effects of substituents which play a role in determining nucleoside conformation should be considered in assessing structural requirements for permeants of the transporter.

Purine metabolism and urate biosynthesis in isolated chicken hepatocytes

The metabolism of some purine compounds to urate and their effects on de novo urate synthesis in chicken hepatocytes were investigated. The purines, listed in descending order of rates of catabolism to urate, were hypoxanthine, xanthine, inosine, guanosine, guanine, IMP, GMP, adenosine, AMP, and adenine. During a 1-h incubation period, conversion to urate accounted for more than 80% of the total quantities of guanine, guanosine, and inosine metabolized, but only 42% of the adenosine and 23% of the adenine metabolism. Adenine, adenosine, and AMP inhibited de novo urate synthesis [( 14C]formate incorporation into urate), whereas the other purines, especially guanine, guanosine, and GMP, stimulated de novo urate synthesis. When hepatocytes were incubated with glutamine and adenosine, AMP, guanine, guanosine, or GMP, the rates of de novo urate synthesis were lower than the additive effects of glutamine and the purine in separate incubations. Increasing phosphate concentrations had no effect on urate synthesis in the absence of added purines but, in combination with adenosine, AMP, guanosine, or GMP, increased urate synthesis. These results indicate that the ratio of adenine to guanine nucleotides and the interaction between substrates and purine nucleotides are involved in the regulation of urate biosynthesis in chicken liver.

Sodium gradient-energized concentrative transport of adenosine in renal brush border vesicles

The uptake of adenosine in brush border vesicles of the proximal tubule of the rat kidney has been studied with a filtration technique. The initial rate of uptake was almost 6 times greater in the presence of NaCl than in the presence of KCl. The stimulatory effect of Na+ was strictly dependent on a gradient of Na+ (out greater than in). The time course of uptake showed an overshoot with a maximum at 20 s with a gradient of NaCl, but not with KCl. Inosine and 5'-AMP were produced from adenosine within the vesicles. In the presence of an inhibitor or adenosine deaminase adenosine was not significantly metabolized during the first 20 s of uptake. Thus, kinetic parameters of transport could be studied in the absence of interferences with metabolism. A Km of 1.1 microM and a Vmax of 232 pmol X min-1 X mg protein-1 were calculated for the Na+ gradient-dependent transport. The dependency on a Na+ gradient, the capacity for uphill transport and the high affinity for adenosine situate this transport system apart from the mechanisms of transport of nucleosides described so far. It may be relevant in regard to the role of adenosine in the regulation of glomerular filtration.