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Cox S. Studies on the Reversal of Azidothymidine Toxicity in Human Lymphocytes by Cytidine and Uridine. ACTA ACUST UNITED AC 2016. [DOI: 10.1177/095632029100200104] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
The toxicity of 3′-azidothymidine (AZT) in human lymphocytes has been shown previously to be reversed by co-incubation with the ribonucleosides cytidine or uridine. In the present paper, the effects of 3′-azidothymidine and cytidine/uridine, both alone and in combination, were studied upon key processes in lymphocytes in order to discover more about the mechanism of toxicity reversal. In these experiments 3′-azidothymidine had only minor effects on the ribonucleoside triphosphate pools. Cytidine increased the CTP pool, and uridine the UTP pool. Co-incubation with AZT caused similar changes to incubation with cytidine or uridine alone. Toxicity reversal was not linked to replacement of deficient ribonucleoside triphosphate pools. 3′-Azidothymidine caused the excretion of thymidine from lymphocytes. Incubation with cytidine and uridine increased the intracellular cytidine and uridine pools, respectively. Co-incubation with 3′-azidothymidine increased still further the intracellular cytidine and uridine pools. Cytidine and uridine did not affect the intracellular 3′-azidothymidine pool. The toxicity of 3′-azidothymidine was increased by co-incubation with the bases adenine, guanine, hypoxanthine, and uracil, but not by dihydrouracil, thymine, or xanthine.
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Affiliation(s)
- S. Cox
- Department of Virology, National Bacteriological Laboratory and Department of Virology, Karolinska Institute, S-10521 Stockholm, Sweden
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2
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Gonçalves P, Araújo JR, Martel F. Characterization of Butyrate Uptake by Nontransformed Intestinal Epithelial Cell Lines. J Membr Biol 2011; 240:35-46. [DOI: 10.1007/s00232-011-9340-3] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/22/2010] [Accepted: 01/02/2011] [Indexed: 10/18/2022]
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3
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Smith PL, Eddy EP, Lee CP, Wilson G. Exploitation of the Intestinal Oligopeptide Transporter to Enhance Drug Absorption. Drug Deliv 2008. [DOI: 10.3109/10717549309022763] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
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4
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Smith PL, Eddy EP, Lee CP, Wilson G. Exploitation of the intestinal oligopeptide transporter to enhance drug absorption. Drug Deliv 2008. [DOI: 10.3109/10717549609031181] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
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5
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Wu SK, Mathias NR, Kim KJ, Lee VHL. Functional and pharmacological mechanisms of nucleoside transport across the basolateral membrane of rabbit tracheal epithelial cells. Life Sci 2005; 78:310-20. [PMID: 16111717 DOI: 10.1016/j.lfs.2005.04.066] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/14/2005] [Accepted: 04/22/2005] [Indexed: 10/25/2022]
Abstract
The role of basolateral membrane nucleoside transport in primary cultured rabbit tracheal epithelial cells (RTEC) was studied. Primary cultured RTEC were grown on permeable support at an air-interface. Transport studies were conducted in the uptake, efflux, and transepithelial transport configurations using (3)H-uridine as a model substrate. Time, temperature and concentration dependency of (3)H-uridine transport were evaluated in parallel to the metabolism of this substrate using scintillation counting and thin layer chromatography. Inhibition of (3)H-uridine uptake from basolateral fluid was estimated in presence of all unlabeled natural nucleosides as well as analogs and nucleobases. Functional modulation pathways of (3)H-uridine uptake were studied after treatment of RTEC with pharmacological levels of A23187, forskolin, tamoxifen, H89 and colchicine. The basolateral aspect has a low-affinity and high-capacity transport system that exhibits characteristics of bi-directionality, temperature/concentration dependency, and broad specificity towards purines and pyrimidines without requiring Na(+). Basolateral equilibrative-sensitive/insensitive (es/ei) type transport machinery manifested as a biphasic dose response to nitro-benzyl-mercapto-purine-ribose (NBMPR) inhibition. In addition, a number of therapeutically relevant nucleoside analogs appeared to compete with the uptake of uridine from basolateral fluid. Short-term pre-incubation of primary cultured RTEC with the calcium ionophore A23187 inhibited basolateral uridine uptake without affecting the J(max) and K(m). The inhibitory effect was not reversible with a protein kinase C (PKC) antagonist, tamoxifen. In contrast, basolateral uridine uptake was increased by adenylyl cyclase activator forskolin (reversible with protein kinase A (PKA) inhibitor H89), resulting in a decreased K(m), but a lower J(max). Uridine exit across the basolateral membrane of primary cultured RTEC occurs via a facilitative diffusion carrier, which can be modulated by intracellular Ca(2+) levels and PKA. Information about these carriers will help improve the transportability of antitumor and antiviral nucleoside analogs in the pulmonary setting.
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Affiliation(s)
- Sharon K Wu
- Department of Pharmaceutical Sciences, University of Southern California, Los Angeles, CA 90089-9121, USA.
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6
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Aymerich I, Duflot S, Fernández-Veledo S, Guillén-Gómez E, Huber-Ruano I, Casado FJ, Pastor-Anglada M. The concentrative nucleoside transporter family (SLC28): new roles beyond salvage? Biochem Soc Trans 2005; 33:216-9. [PMID: 15667311 DOI: 10.1042/bst0330216] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
The concentrative nucleoside transporter (CNT) family (SLC28) has three members: SLC28A1 (CNT1), SLC28A2 (CNT2) and SLC28A3 (CNT3). The CNT1 and CNT2 transporters are co-expressed in liver parenchymal cells and macrophages, two suitable models in which to study cell cycle progression. Despite initial observations suggesting that these transporter proteins might contribute to nucleoside salvage during proliferation, their subcellular localization and regulatory properties suggest alternative roles in cell physiology. In particular, CNT2 is a suitable candidate for modulation of purinergic responses, since it is under the control of the adenosine 1 receptor. Increasing evidence also suggests a role for CNT2 in energy metabolism, since its activation relies on the opening of ATP-sensitive K+ channels. Animal and cell models genetically modified to alter nucleoside transporter expression levels may help to elucidate the particular roles of CNT proteins in cell physiology.
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Affiliation(s)
- I Aymerich
- Departament de Bioquímica i Biologia Molecular, Facultat de Biologia, Universitat de Barcelona, Diagonal 645, 08071 Barcelona, Spain
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7
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Gray JH, Owen RP, Giacomini KM. The concentrative nucleoside transporter family, SLC28. Pflugers Arch 2004; 447:728-34. [PMID: 12856181 DOI: 10.1007/s00424-003-1107-y] [Citation(s) in RCA: 306] [Impact Index Per Article: 15.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/24/2003] [Revised: 05/04/2003] [Accepted: 05/04/2003] [Indexed: 10/26/2022]
Abstract
The SLC28 family consists of three subtypes of sodium-dependent, concentrative nucleoside transporters, CNT1, CNT2, and CNT3 (SLC28A1, SLC28A2, and SLC28A3, respectively), that transport both naturally occurring nucleosides and synthetic nucleoside analogs used in the treatment of various diseases. These subtypes differ in their substrate specificities: CNT1 is pyrimidine-nucleoside preferring, CNT2 is purine-nucleoside preferring, and CNT3 transports both pyrimidine and purine nucleosides. Recent studies have identified key amino acid residues that are determinants of pyrimidine and purine specificity of CNT1 and CNT2. The tissue distributions of the CNTs vary: CNT1 is localized primarily in epithelia, whereas CNT2 and CNT3 have more generalized distributions. Nucleoside transporters in the SLC28 and SLC29 families play critical roles in nucleoside salvage pathways where they mediate the first step of nucleotide biosynthesis. In addition, these transporters work in concert to terminate adenosine signaling. SLC28 family members are crucial determinants of response to a variety of anticancer and antiviral nucleoside analogs, as they modulate the entry of these analogs into target tissues. Further, this family is involved in the absorption and disposition of many nucleoside analogs. Several CNT single nucleoside polymorphisms (SNPs) have been identified, but have yet to be characterized.
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Affiliation(s)
- Jennifer H Gray
- Department of Biopharmaceutical Sciences, University of California-San Francisco, 513 Parnassus Ave., S-926, San Francisco, CA 94143-0446, USA.
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8
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Toan SV, To KKW, Leung GPH, de Souza MO, Ward JL, Tse CM. Genomic organization and functional characterization of the human concentrative nucleoside transporter-3 isoform (hCNT3) expressed in mammalian cells. Pflugers Arch 2003; 447:195-204. [PMID: 14504928 DOI: 10.1007/s00424-003-1166-0] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/15/2003] [Accepted: 07/29/2003] [Indexed: 10/26/2022]
Abstract
Human CNT3 encodes the concentrative nucleoside transport N3 system. Previous expression studies in oocytes showed that the Km values for nucleosides of the cloned hCNT3 were 7- to 25-fold lower than the endogenous N3 transporter in HL60 cells. Therefore, in the present study we re-examined the kinetic properties of the cloned hCNT3 using mammalian cell expression systems by transient expression in Cos7L cells and stably expression in nucleoside transporter deficient PK15NTD cells. We demonstrated that hCNT3 is a Na-dependent, broadly-selective nucleoside transporter with affinities (<11 microM) for nucleosides closely resembling the endogenous N3 transporter. Pharmacological studies showed that phloridzin is a mixed-type inhibitor of hCNT3 (Ki=15 microM), and the dideoxyuridine analogs are poor substrates. By epitope-tagging, we further demonstrated that hCNT3 is N-glycosylated as PNGase F and Endo H deglycosylated hCNT3 from 67 kDa to 58 kDa. Searching the human genome database, we identified the genomic organization of hCNT3. This gene contains 19 exons and its exon-intron boundaries within the coding sequence exactly match with those of hCNT1 and hCNT2 with one additional exon in the N-terminus. Our data suggest that hCNT3 gene is evolutionarily conserved with hCNT1 and hCNT2. Physiologically, hCNT3 is a glycoprotein, which transports purine and pyrimidine nucleosides in a Na-dependent manner with high affinities.
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Affiliation(s)
- Shuy-Vang Toan
- Department of Medicine, Gastrointestinal Division, The Johns Hopkins University School of Medicine, Baltimore, MD 21205-2195, USA
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9
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Balimane PV, Sinko PJ. Involvement of multiple transporters in the oral absorption of nucleoside analogues. Adv Drug Deliv Rev 1999; 39:183-209. [PMID: 10837774 DOI: 10.1016/s0169-409x(99)00026-5] [Citation(s) in RCA: 56] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Many nucleoside analogues such as azt, ddI, ddC, d4T, 3TC, acv and vacv are currently being used in the treatment of patients infected with HIV, suffering from AIDS, or AIDS-related opportunistic infections. The transport of nucleoside analogues across the gastrointestinal tract is mediated by a number of transporters that fall into three broad categories, i.e., Na(+)-dependent concentrative transporters, Na(+)-independent equilibrative transporters and H(+)/peptide transporters. The first two transporter classes contain a large number of subtypes that are based on the substrate specificity. Recent studies have shown that most of the anti-HIV nucleoside analogues are transported by one or more of the nucleoside transporters. Furthermore, certain analogues, such as acv, appear to be absorbed by non-carrier-mediated diffusion, whereas vacv is apparently transported by non-nucleoside transporters (e.g., the oligopeptide transporter, PepT1 and possibly others). Thus, it is desirable to understand the precise nature of the absorption mechanism of these drugs to improve bioavailability and reduce the variability that is commonly observed in vivo in human patients. A complete understanding of the complex interactions of nucleoside analogues with the various transporters will help in designing better delivery systems and strategies to improve efficacy. In the current report, the mechanisms of nucleoside and nucleoside-analogue transport are reviewed. Also, methods of exploiting prodrugs to improve the bioavailability characteristics of drugs are highlighted.
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Affiliation(s)
- PV Balimane
- College of Pharmacy, Rutgers-The State University of New Jersey, 160 Frelinghuysen Road, Piscataway, NJ, USA
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10
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Liu MP, Beigelman L, Levy E, Handschumacher RE, Pizzorno G. Discrete roles of hepatocytes and nonparenchymal cells in uridine catabolism as a component of its homeostasis. THE AMERICAN JOURNAL OF PHYSIOLOGY 1998; 274:G1018-23. [PMID: 9696700 DOI: 10.1152/ajpgi.1998.274.6.g1018] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/08/2023]
Abstract
Previous studies indicated that uridine is essentially cleared in a single pass through a rat liver and replaced in a highly regulated manner by uridine formed presumably by de novo synthesis. We report a cellular basis for the catabolic component of this apparent paradox by dissociation of the liver with collagenase into two cell fractions, hepatocytes and a nonparenchymal cell population. Suspensions of the nonparenchymal cells rapidly cleave uridine to uracil, whereas in hepatocytes this activity was <5% of that in nonparenchymal cells. Conversely, hepatocytes cause extensive degradation of uracil to -alanine. These differences correlate with the uridine phosphorylase and dihydrouracil dehydrogenase activity in cell-free extracts of each cell type. We have documented the existence of a Na+-dependent, nitrobenzylthioinosine-insensitive transport system for uridine in the parenchymal cells (Michaelis constant 46 +/- 5 microM) that achieves a three- to fourfold concentration gradient in hepatocytes. A similar system is present in the nonparenchymal cell population. In addition, a highly specific and active Na+-dependent transport system for beta-alanine, the primary catabolic metabolite of uracil, has been demonstrated in hepatocytes.
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Affiliation(s)
- M P Liu
- Department of Internal Medicine, Yale University School of Medicine, New Haven, Connecticut 06520, USA
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11
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Mun EC, Tally KJ, Matthews JB. Characterization and regulation of adenosine transport in T84 intestinal epithelial cells. THE AMERICAN JOURNAL OF PHYSIOLOGY 1998; 274:G261-9. [PMID: 9486178 DOI: 10.1152/ajpgi.1998.274.2.g261] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
Adenosine release from mucosal sources during inflammation and ischemia activates intestinal epithelial Cl- secretion. Previous data suggest that A2b receptor-mediated Cl- secretory responses may be dampened by epithelial cell nucleoside scavenging. The present study utilizes isotopic flux analysis and nucleoside analog binding assays to directly characterize the nucleoside transport system of cultured T84 human intestinal epithelial cells and to explore whether adenosine transport is regulated by secretory agonists, metabolic inhibition, or phorbol ester. Uptake of adenosine across the apical membrane displayed characteristics of simple diffusion. Kinetic analysis of basolateral uptake revealed a Na(+)-independent, nitrobenzylthioinosine (NBTI)-sensitive facilitated-diffusion system with low affinity but high capacity for adenosine. NBTI binding studies indicated a single population of high-affinity binding sites basolaterally. Neither forskolin, 5'-(N-ethylcarboxamido)-adenosine, nor metabolic inhibition significantly altered adenosine transport. However, phorbol 12-myristate 13-acetate significantly reduced both adenosine transport and the number of specific NBTI binding sites, suggesting that transporter number may be decreased through activation of protein kinase C. This basolateral facilitated adenosine transporter may serve a conventional function in nucleoside salvage and a novel function as a regulator of adenosine-dependent Cl- secretory responses and hence diarrheal disorders.
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Affiliation(s)
- E C Mun
- Department of Surgery, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts 02215, USA
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12
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Yu H, Sinko PJ. Influence of the microporous substratum and hydrodynamics on resistances to drug transport in cell culture systems: calculation of intrinsic transport parameters. J Pharm Sci 1997; 86:1448-57. [PMID: 9423161 DOI: 10.1021/js960524q] [Citation(s) in RCA: 25] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
Although cell culture models are increasingly used to study drug transport and metabolism, the influence of the substratum on the transport properties of the cell monolayer has not been studied in great detail. Furthermore, the use of effective (or apparent) permeabilities (Peff) assumes that the contribution of the microporous filter substratum and the aqueous boundary layer (ABL) to transport are negligible or are at least constant for a series of drugs. In the present study, the permeabilities of the substratum, ABL, and monolayer were obtained for a series of compounds at variable flow rates in side-by-side diffusion chambers. Comparisons of transport properties were made between cell monolayers grown on substrata made of polycarbonate (PC) and polyester (PE). All paracellular markers demonstrated a reduction in permeability and a corresponding increase in transepithelial electrical resistance (TEER) through PE-grown monolayers. The permeabilities of two carrier-mediated compounds, phenylalanine and proline, were 55% higher and 48% lower through PE-grown monolayers than through the PC-grown monolayers, respectively. The resistance to progesterone transport attributed to the PE and PC filters was large (71% and 27% of total resistance, respectively) at a flow rate of 20 mL/min, indicating that the monolayer was not the rate-limiting transport barrier. Therefore, for highly permeable compounds, reporting Peff has limited value since it is an indicator of the transport properties of the substratum rather than of the monolayer. These results demonstrate that substratum properties (e.g., membrane composition, pore size, etc.) significantly affect the barrier properties of the Caco-2 cell monolayer. The most probable mechanism is by the modulation of the functional expression of nutrient and ion transporters resulting in variable transcellular and paracellular transport properties. These results further demonstrate the importance of calculating intrinsic membrane transport parameters if the monolayer is not maintained as the rate-determining barrier in the transport experiment. Using higher flow rates and higher porosity substrata supports may help maintain the monolayer as the rate-limiting transport barrier.
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Affiliation(s)
- H Yu
- College of Pharmacy, Rutgers University, Piscataway, NJ 08854, USA
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13
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Griffith DA, Jarvis SM. Nucleoside and nucleobase transport systems of mammalian cells. BIOCHIMICA ET BIOPHYSICA ACTA 1996; 1286:153-81. [PMID: 8982282 DOI: 10.1016/s0304-4157(96)00008-1] [Citation(s) in RCA: 377] [Impact Index Per Article: 13.5] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Affiliation(s)
- D A Griffith
- Research School of Biosciences, University of Kent, Canterbary, UK
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14
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Mercader J, Gomez-Angelats M, del Santo B, Casado FJ, Felipe A, Pastor-Anglada M. Nucleoside uptake in rat liver parenchymal cells. Biochem J 1996; 317 ( Pt 3):835-42. [PMID: 8760370 PMCID: PMC1217560 DOI: 10.1042/bj3170835] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
Rat liver parenchymal cells express Na(+)-dependent and Na(+)- independent nucleoside transport activity. The Na(+)-dependent component shows kinetic properties and substrate specificity similar to those reported for plasma membrane vesicles [Ruiz-Montasell, Casado, Felipe and Pastor-Anglada (1992) J. Membr. Biol. 128, 227-233]. This transport activity shows apparent K(m) values for uridine in the range 8-13 microM and a Vmax of 246 pmol of uridine per 3 min per 10(5) cells. Most nucleosides, including the analogue formycin B, cis-inhibit Na(+)-dependent uridine transport, although thymidine and cytidine are poor inhibitors. Inosine and adenosine inhibit Na(+)-dependent uridine uptake in a dose-dependent manner, reaching total inhibition. Guanosine also inhibits Na(+)-dependent uridine uptake, although there is some residual transport activity (35% of the control values) that is resistant to high concentrations of guanosine but may be inhibited by low concentrations of adenosine. The transport activity that is inhibited by high concentrations of thymidine is similar to the guanosine-resistant fraction. These observations are consistent with the presence of at least two Na(+)-dependent transport systems. Na(+)-dependent uridine uptake is sensitive to N-ethylmaleimide treatment, but Na(+)-independent transport is not. Nitrobenzylthioinosine (NBTI) stimulates Na(+)-dependent uridine uptake. The NBTI effect involves a change in Vmax, it is rapid, dose-dependent, does not need preincubation and can be abolished by depleting the Na+ transmembrane electrochemical gradient. Na(+)-independent uridine transport seems to be insensitive to NBTI. Under the same experimental conditions, NBTI effectively blocks most of the Na(+)-independent uridine uptake in hepatoma cells. Thus the stimulatory effect of NBTI on the concentrative nucleoside transporter of liver parenchymal cells cannot be explained by inhibition of nucleoside efflux.
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Affiliation(s)
- J Mercader
- Departament de Bioquímica i Biologia Molecular, Universitat de Barcelona, Spain
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15
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Waclawski AP, Sinko PJ. Oral absorption of anti-acquired immune deficiency syndrome nucleoside analogues. 2. Carrier-mediated intestinal transport of stavudine in rat and rabbit preparations. J Pharm Sci 1996; 85:478-85. [PMID: 8742938 DOI: 10.1021/js950363a] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
The intestinal transport and metabolism of stavudine (d4T), a nucleoside analogue of thymidine used in the treatment of AIDS, was studied using single-pass intestinal perfusion (SPIP), intestinal brush-border membrane vesicles (BBMV), and mucosal homogenates in rats and rabbits. In the SPIP, d4T demonstrated concentration-dependent mean wall permeability (P+/-w) at perfusate concentrations ranging from 0.001 to 25 mM. In coperfusion studies using 0.1 mM thymidine, 1 mM formycin B, or 5 microM NBTI as putative inhibitors of d4T transport, the P+/-w of 5 microM d4T was reduced to 48%, 62%, and 70% of the control value, respectively, suggesting the involvement of multiple nucleoside carriers in the intestinal uptake of d4T. d4T uptake in rat BBMV was significantly greater in the presence of a sodium ion gradient compared with a sodium-free (choline) gradient. The permeability of d4T, in the presence of a sodium gradient, was concentration-dependent and inhibited by 10 mM thymidine but not significantly reduced by 10 mM formycin B. In the presence of 10 microM NBTI, the permeability of d4T was not inhibited; however; the binding of d4T to rat and rabbit BBMV was significantly reduced. Formycin B did not significantly reduce the d4T uptake in rat or rabbit BBMV suggesting that d4T does not interact with the purine-selective N1 nucleoside carrier. However, because formycin B inhibited d4T uptake in the SPIP and since d4T inhibited formycin B uptake in rat but not rabbit BBMV, it appears to interact with the N3 carrier which has been demonstrated in rat but not rabbit intestine. Also, an interaction with the sodium-independent facilitative transporter at the basolateral membrane cannot be ruled out. The low hybrid K(m) and high passive permeability of d4T likely account for the lack of saturable absorption behavior observed in humans, whereas the brush-border and intracellular stability of d4T preserve the high bioavailability observed after oral dosing.
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Affiliation(s)
- A P Waclawski
- Department of Pharmaceutics, College of Pharmacy, Rutgers, State University of New Jersey, Piscataway 08855, USA
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Sinko PJ, Hu P, Waclawski AP, Patel NR. Oral absorption of anti-AIDS nucleoside analogues. 1. Intestinal transport of didanosine in rat and rabbit preparations. J Pharm Sci 1995; 84:959-65. [PMID: 7500281 DOI: 10.1002/jps.2600840811] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
The intestinal transport of didanosine (ddl), a nucleoside analog used in the treatment of human immunodeficiency virus (HIV) infection, was characterized using in situ and in vitro techniques. The zero-trans uptake of ddl in rat intestinal brush border membrane vesicles (BBMV) was linear over the range of 1 microM to 50 mM, ruling out a significant carrier-mediated absorption component. The lack of carrier-mediated transport was confirmed in a second species (rabbit). In order to quantitate the convective (Pconv) and diffusive (Pdiff) components of ddl intestinal permeability, the steady state wall permeability (P*w) was determined using an established perfusion technique in rats. Even though baseline P*w (pH 6.5, 290 mosm/kg, no modulator) and fluid absorption results were similar to those of furosemide, the ratios (ddl:furosemide) of Pdiff and phi, the sieving coefficient, were 0.31:1 and 1.70:1, respectively, demonstrating that ddl's Pdiff is low and Pconv is high relative to furosemide's, suggesting significant paracellular absorption of ddl. The apparent diffusive absorptive clearances (P'app) of ddl and furosemide were determined in BBMV, which lack functional tight junctions, and the ratios (ddl:furosemide) of P'app in rat and rabbit were 0.23:1 and 0.24:1, respectively. The BBMV results demonstrate that the majority of ddl intestinal transport does not occur by passive membrane diffusion, confirming the single pass intestinal perfusion (SPIP) findings. The results of these studies suggest that ddl is transported by nonfacilitated membrane and paracellular diffusion with paracellular transport being responsible for the majority of ddl absorption from the intestine.
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Affiliation(s)
- P J Sinko
- Department of Pharmaceutics, College of Pharmacy, Rutgers University, Piscataway, NJ 08855, USA
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18
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Hale CC, Rovetto MJ. A photoaffinity probe for the cardiac adenosine transporter. Life Sci 1995; 57:245-54. [PMID: 7596230 DOI: 10.1016/0024-3205(95)00268-b] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
We have developed and utilized a photoaffinity probe to identify the adenosine transporter in cardiac sarcolemmal (SL) vesicles. The probe is an azidosalicylate derivative of adenosine made by reacting adenosine with N-hydroxysuccinimidyl-4-azidosalicylic acid (NHS-ASA). Following synthesis and radiolabeling of the probe (ASA-adenosine), 125I-ASA-adenosine was purified by high pressure liquid chromatography. Iodine-125-ASA-adenosine, upon irradiation with uv light, covalently labeled a 65 kDa protein in bovine cardiac SL vesicles. Labeling of this protein was greatly diminished in the presence of nonradiolabeled adenosine, 5'-amino adenosine, or guanosine (inhibitors of purine nucleoside transport) but not by glucose (which does not inhibit transport). We conclude that the cardiac adenosine transporter is a protein with an apparent Mr of 65 kDa.
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20
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Wu X, Gutierrez MM, Giacomini KM. Further characterization of the sodium-dependent nucleoside transporter (N3) in choroid plexus from rabbit. BIOCHIMICA ET BIOPHYSICA ACTA 1994; 1191:190-6. [PMID: 8155674 DOI: 10.1016/0005-2736(94)90248-8] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Abstract
The Na+/nucleoside cotransporter in rabbit choroid plexus differs from Na+/nucleoside cotransporters in other tissues in terms of substrate selectivity and stoichiometry. The overall goal of this study was to further characterize the kinetics of this system (N3). Choroid plexus tissue slices obtained from rabbit brain were depleted of ATP and treated with valinomycin and K+. Na+/thymidine uptake at 30 s in the presence of an inside negative potential difference was significantly greater than in the absence of a potential difference. Na+/thymidine uptake was not significantly affected by replacing chloride with either thiocyanate or sulfate. The Km of Na+/guanosine uptake was 149, 85.2 and 30.5 microM in the presence of a 25, 50 and 100 mM Na+ gradient, respectively, whereas the Vmax was unaffected, suggesting that Na+ binds first to the cotransporter, then, the nucleoside. Therapeutically relevant base-modified nucleoside analogs, 5-fluorouridine, 2-chloroadenosine and 5-iododeoxyuridine, significantly inhibited Na+/thymidine uptake with IC50 values (mean +/- S.E.) of 12.0 +/- 2.3, 21.3 +/- 2.2 and 24.4 +/- 2.1 microM, respectively, whereas nucleoside analogs structurally modified on the ribose ring, 3'-azidothymidine, dideoxyinosine and dideoxycytidine (100 microM) did not. These studies suggest that Na+/nucleoside cotransport in the choroid plexus is electrogenic and is not dependent on chloride. This cotransporter, which is present in choroid plexus but not in renal brush-border membrane vesicles from rabbit, may play a role in the disposition of clinically relevant base-modified nucleoside analogs into and out of the brain.
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Affiliation(s)
- X Wu
- School of Pharmacy, University of California Medical Center, San Francisco 94143-0446
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21
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Huang Q, Harvey C, Paterson A, Cass C, Young J. Functional expression of Na(+)-dependent nucleoside transport systems of rat intestine in isolated oocytes of Xenopus laevis. Demonstration that rat jejunum expresses the purine-selective system N1 (cif) and a second, novel system N3 having broad specificity for purine and pyrimidine nucleosides. J Biol Chem 1993. [DOI: 10.1016/s0021-9258(20)80769-8] [Citation(s) in RCA: 54] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022] Open
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22
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Hu M. Comparison of uptake characteristics of thymidine and zidovudine in a human intestinal epithelial model system. J Pharm Sci 1993; 82:829-33. [PMID: 8377122 DOI: 10.1002/jps.2600820815] [Citation(s) in RCA: 25] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Abstract
The uptake of a natural nucleoside, thymidine (THY), and one of its deoxynucleoside analogues, zidovudine (AZT), was studied in a newly developed intestinal epithelial model system (CaCO-2). The results of the study indicated that the uptake rate of THY was saturable with a Km of 44.6 microM and maximum rate of 1.73 +/- 0.17 pmol/cm2/s. The uptake of AZT, on the other hand, was not saturable. When various naturally occurring nucleosides were used as inhibitors of THY uptake, it was shown that the uptake was inhibited by various pyrimidine nucleosides (e.g., uridine and cytidine), but not by purine nucleosides (e.g., guanosine). Neither group of nucleosides had any effect on the uptake of AZT. Finally, the uptake of THY was also inhibited by the metabolic inhibitor NaCN and a couple of specific nucleoside transporter inhibitors [i.e., dipyridamole and S-(p-nitrobenzyl-6-thioinosine], whereas none of these compounds had a significant effect on the uptake of AZT. The results showed, for the first time, the existence of a nucleoside carrier in the CaCO-2 cell culture model system. Also the results indicated that the uptake of AZT by the CaCO-2 cell monolayers is mainly via a passive diffusion process.
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Affiliation(s)
- M Hu
- Department of Pharmaceutical Sciences, College of Pharmacy, Washington State University, Pullman 99164-6510
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23
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Meckling-Gill KA, Guilbert L, Cass CE. CSF-1 stimulates nucleoside transport in S1 macrophages. J Cell Physiol 1993; 155:530-8. [PMID: 8491790 DOI: 10.1002/jcp.1041550311] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
We have examined nucleoside transport (NT) in a cell line derived from primary day 7 murine bone marrow macrophages (S1 macrophages) in response to the macrophage growth factor, colony-stimulating factor 1 (CSF-1). Adenosine and uridine transport in quiescent S1 macrophages occurred primarily by two facilitated diffusional routes, one that was sensitive and one that was relatively resistant to the inhibitor nitrobenzylthioinosine (NBMPR). Addition of CSF-1 to quiescent cultures resulted in increased adenosine and uridine transport with biphasic kinetics with respect to the cell cycle. Basal NT activity was elevated (about twofold) within 15 min of CSF-1 addition, returned to near basal levels by 1 h, and then increased again (three- to fourfold) 8-12 h later, returning again to basal levels by 48 h post CSF-1 stimulation. We propose that the large increase in NT activity at 8-12 h corresponded with the time when cultures synchronously began to enter the S phase of the cell cycle. In addition to these changes in the absolute rates, the proportions of NBMPR-sensitive and NBMPR-insensitive transport also change after CSF-1 addition. Quiescent cultures exhibited primarily NBMPR-insensitive transport while logrithmically growing cultures exhibited primarily NBMPR-sensitive nucleoside transport activity. The increase in the NBMPR-sensitive component of the transport process paralleled a similar increase in the number of high-affinity NBMPR binding sites, suggesting that the mechanism for upregulating NBMPR-sensitive NT activity involves increases in the number of NBMPR-sensitive transporter sites. Interestingly, we were unable to detect Na(+)-dependent concentrative uptake of adenosine, uridine, or formycin-B either in the S1 macrophage cell line or in primary (day 7) murine macrophages. Thus these bone marrow derived macrophages did not display the characteristically large Na(+)-dependent transport systems observed by others in peritoneal macrophages, implying that these two populations of macrophages are, indeed, functionally distinct.
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24
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Abstract
The goal of this study was to determine whether specific transport systems are involved in nucleoside elimination from the cerebrospinal fluid (CSF). First, in vitro studies were carried out in isolated choroid plexus tissue slices from rat to ascertain the mechanisms of transport of formycin B, a model nucleoside analogue. 3H-Formycin B accumulated against a concentration gradient in the presence of an Na+ gradient in the isolated ATP-depleted choroid plexus tissue slices. This accumulation was reduced by high concentrations of unlabeled formycin B. Nitrobenzylthioinosine (NBMPR), an equilibrative nucleoside transport inhibitor, inhibited the uptake of formycin B in the absence of an Na+ gradient. These data suggest that both equilibrative and secondary active Na(+)-nucleoside transport systems are present in rat choroid plexus. In vivo, formycin B, together with inulin as a bulk flow marker, was injected into the lateral ventricle of the anesthetized rat with the aid of a stereotaxic device, and CSF was sampled from the cisterna magna at various times after injection. Twelve rats were randomized and divided into a low- and a high-dose group. The CSF clearance (CLCSF) of formycin B was significantly higher than the CLCSF of inulin in both animal groups (P < 0.01), indicating that formycin B is cleared from CSF by a pathway(s) in addition to bulk flow. Formycin B CLCSF was significantly lower in the high-dose group than in the low-dose group (P < 0.05), suggesting a saturable CSF elimination. The CLCSF of formycin B was also significantly reduced in animals treated with NBMPR (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)
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Affiliation(s)
- X Wu
- School of Pharmacy, University of California, San Francisco 94143
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25
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Ruiz-Montasell B, Martinez-Mas JV, Enrich C, Casado FJ, Felipe A, Pastor-Anglada M. Early induction of Na(+)-dependent uridine uptake in the regenerating rat liver. FEBS Lett 1993; 316:85-8. [PMID: 8422942 DOI: 10.1016/0014-5793(93)81741-h] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Abstract
Na(+)-dependent uridine transport into liver plasma membrane vesicles from partially hepatectomized and sham-operated rats was studied. Preparations purified 6 h after 70% hepatectomy exhibited an increased Vmax of uridine uptake (3.7 vs. 1.4 pmol/mg prot/3 s) without any change in Km (6 microM). Incubation of the vesicles in the presence of monensin decreased uridine uptake although the differences between both experimental groups remained identical. It is concluded that uridine transport is induced early after partial hepatectomy by a mechanism which does not involve changes in the transmembrane Na+ gradient. This is the first evidence in favor of modulation of nucleoside transport into liver cells.
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Affiliation(s)
- B Ruiz-Montasell
- Departament de Bioquímica i Fisiologia, Universitat de Barcelona, Spain
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26
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Belt JA, Marina NM, Phelps DA, Crawford CR. Nucleoside transport in normal and neoplastic cells. ADVANCES IN ENZYME REGULATION 1993; 33:235-52. [PMID: 8356910 DOI: 10.1016/0065-2571(93)90021-5] [Citation(s) in RCA: 135] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Abstract
The permeation of nucleosides across the plasma membrane of mammalian cells is complex and mediated by at least five distinct transporters that differ in their sensitivity to inhibitors and in their specificity for nucleosides. The basic properties and permeant specificity of these transporters are summarized in Table 3. It appears that there may be differences in the distribution of these transporters in tumors and normal tissues that might be exploited for chemotherapeutic purposes. The human tumor cell lines examined express predominantly the NBMPR-sensitive equilibrative transporter es which can be blocked by low concentrations of NBMPR and dipyridamole. It is reasonable to expect that tumors with transport properties similar to the CCRF-CEM and Rh28 cell lines (Table 1) that have no detectable NBMPR-insensitive transport activity will be highly susceptible to the therapeutic approach of combining a transport inhibitor such as dipyridamole or NBMPR with an inhibitor of de novo pyrimidine biosynthesis. On the other hand, this approach to therapy is unlikely to succeed against tumors with transport phenotypes similar to the WI-L2 cell line that may permit the salvage nucleosides in the presence of these inhibitors. The majority of tumor cells examined, however, fall between these extremes, and it is not yet known what level of NBMPR-insensitive transport activity can be tolerated without seriously compromising this therapeutic approach. With respect to normal tissues, the mature absorptive cells of the intestine have predominantly Na(+)-dependent nucleoside transporters that are insensitive to NBMPR and dipyridamole. The proliferating crypt cells also appear to have Na(+)-dependent nucleoside transport, although they may also have an NBMPR-sensitive component of transport (Belt, unpublished data). Bone marrow granulocyte-macrophage progenitor cells also appear to have one or more concentrative nucleoside transporters. Thus these tissues, which are most vulnerable to the toxicity of antimetabolites, may be able to salvage nucleosides in the presence of inhibitors of equilibrative transport and be protected from the toxicity of de novo synthesis inhibitors. It is likely, however, that a successful application of this therapeutic approach will require the analysis of the nucleoside transport phenotype of individual tumors in order to identify those patients that may benefit from such therapy. Since the development of antibodies and cDNA probes for the various nucleoside transporters is currently underway in several laboratories, it is likely that analysis of the nucleoside transport phenotype of tumors from biopsy material will be feasible in the future.
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Affiliation(s)
- J A Belt
- Department of Biochemical and Clinical Pharmacology, St. Jude Children's Research Hospital, Memphis, TN 38101
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27
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Vijayalakshmi D, Dagnino L, Belt J, Gati W, Cass C, Paterson A. L1210/B23.1 cells express equilibrative, inhibitor-sensitive nucleoside transport activity and lack two parental nucleoside transport activities. J Biol Chem 1992. [DOI: 10.1016/s0021-9258(18)41877-7] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
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28
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Aran JM, Plagemann PG. Nucleoside transport-deficient mutants of PK-15 pig kidney cell line. BIOCHIMICA ET BIOPHYSICA ACTA 1992; 1110:51-8. [PMID: 1390836 DOI: 10.1016/0005-2736(92)90293-u] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
Previous studies indicated that PK-15 pig kidney cells express solely a nitrobenzylthioinosine-sensitive, equilibrative nucleoside transporter. In the present study, PK-15 cells were mutagenized by treatment with ICR-170 and nucleoside transport-deficient mutants selected in a single step in growth medium containing tubercidin and cytosine arabinoside at a frequency of about 2 x 10(6). The mutants were simultaneously at least 100-times more resistant to tubercidin, cytosine arabinoside and 5-fluorodeoxyuridine than the wild-type parent cells. The mutants failed to transport thymidine and uridine and had lost all high affinity nitrobenzylthioinosine binding sites. Residual low level uptake of thymidine by the mutants was shown to be due to nonmediated permeation (passive diffusion), which explains the sensitivity of the mutants to growth inhibition by high concentrations of the nucleoside drugs. Passive diffusion of thymidine at a concentration of 16 microM was not rapid enough to support the growth of nucleoside transport-deficient mutant cells that had been made thymidine-dependent by treatment with methotrexate, whereas wild-type cells grew normally under these conditions. The nucleoside transport-deficient mutants exhibited about the same growth rate and plating efficiency (60-80%) as wild-type cells, but formed larger colonies than wild-type cells because of a more extensive spread of the cells on the surface of culture dishes. PK-15 cells adhere very strongly to the surface of culture dishes and have been transformed with high efficiency with plasmid DNA either via lipofection or electroporation.
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Affiliation(s)
- J M Aran
- Department of Microbiology, University of Minnesota Medical School, Minneapolis 55455-0312
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29
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Aran JM, Plagemann PG. High-affinity, equilibrative nucleoside transporter of pig kidney cell line (PK-15). BIOCHIMICA ET BIOPHYSICA ACTA 1992; 1108:67-74. [PMID: 1379470 DOI: 10.1016/0005-2736(92)90115-3] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
Nucleoside transport was determined in the cloned porcine kidney cell line PK-15 which exhibits properties of tubular cells. The cells did not express any Na(+)-dependent, concentrative nucleoside transport. They exhibited only nitrobenzylthioinosine-sensitive equilibrative nucleoside transport. Their transport activity, however, was only about 10% of that observed in many other mammalian cell lines. This low transport activity correlated with a relatively low number of high-affinity nitrobenzylthioinosine binding sites (5 x 10(3)/cell). Furthermore, although the equilibrative transporter of PK-15 cells exhibited a similar broad substrate specificity as the equilibrative nucleoside transporters of other mammalian cells, it exhibited a much higher affinity for certain nucleosides, especially cytidine and uridine, than the latter. The Michaelis-Menten constants for zero-trans transport and equilibrium exchange of uridine in ATP-depleted cells were about the same (about 40 microM), indicating equal mobility of the nucleoside-loaded and empty carrier. Concentrative nucleoside transport was detected in one set of PK-15 cultures, but was found to be due to mycoplasma contamination.
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Affiliation(s)
- J M Aran
- Department of Microbiology, University of Minnesota Medical School, Minneapolis 55455
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30
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Wu X, Yuan G, Brett C, Hui A, Giacomini K. Sodium-dependent nucleoside transport in choroid plexus from rabbit. Evidence for a single transporter for purine and pyrimidine nucleosides. J Biol Chem 1992. [DOI: 10.1016/s0021-9258(19)50352-0] [Citation(s) in RCA: 55] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
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31
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Gutierrez MM, Brett CM, Ott RJ, Hui AC, Giacomini KM. Nucleoside transport in brush border membrane vesicles from human kidney. BIOCHIMICA ET BIOPHYSICA ACTA 1992; 1105:1-9. [PMID: 1567888 DOI: 10.1016/0005-2736(92)90156-g] [Citation(s) in RCA: 55] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
The goal of this study was to elucidate the mechanisms of nucleoside transport in the brush border membrane of the human kidney. [3H]Uridine was transported into brush border membrane vesicles (BBMV) from human kidney via Na(+)-independent and Na(+)-dependent processes. The Na(+)-dependent transport was saturable (Km = 4.76 +/- 0.39 microM; Vmax = 6.42 +/- 0.17 pmol/mg proteins per s) and was trans-stimulated by unlabeled uridine. Structural analogs of uridine (100 microM), 2'-deoxyuridine (2-dU) and dideoxyuridine (ddU), significantly inhibited Na(+)-uridine uptake into BBMV. Previous studies have suggested that Na(+)-nucleoside co-transport occurs via two major systems (Vijayalakshmi et al. (1988) J. Biol. Chem. 263, 19419-19423). One system (cit) is generally pyrimidine-selective; thymidine serves as a model substrate. The other system (cif) is generally purine-selective; formycin B serves as a model substrate. Uridine and adenosine are substrates of both systems. Thymidine and cytidine (100 microM), but not formycin B (100 microM) inhibited Na(+)-uridine uptake. In addition, [3H]thymidine exhibited an Na(+)-driven overshoot phenomenon whereas [3H]formycin B did not. Na(+)-thymidine uptake was inhibited by (100 microM) adenosine, uridine, guanosine, but not by formycin B and inosine. Further studies demonstrated that guanosine trans-stimulated thymidine uptake suggesting that guanosine and thymidine share a common transporter in the human renal BBMV. A different pattern was identified in BBMV from the rabbit kidney where both [3H]thymidine and [3H]formycin B as well as [3H]uridine exhibited a transient Na(+)-driven overshoot phenomenon. Collectively, these data suggest that in rabbit renal BBMV both cif and cit systems are present whereas in human renal BBMV, there appears to be a single concentrative Na(+)-nucleoside cotransport system that interacts with uridine, cytidine, thymidine, adenosine and guanosine but not with formycin B and inosine. The system is similar to the previously described cit system except that guanosine is also a substrate.
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Affiliation(s)
- M M Gutierrez
- School of Pharmacy, University of California, San Francisco 94143
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32
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Meckling-Gill KA, Cass CE. Effects of transformation by v-fps on nucleoside transport in Rat-2 fibroblasts. Biochem J 1992; 282 ( Pt 1):147-54. [PMID: 1540128 PMCID: PMC1130901 DOI: 10.1042/bj2820147] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
Important cellular nutrients, including nucleosides and hexose sugars, are rapidly taken up by cells, largely through mediated carrier systems. The present study examined nucleoside and hexose transport activity in normal Rat-2 fibroblasts and clonal derivatives that expressed either the wild-type (C10) or a temperature-sensitive mutant (NA9) form of v-fps, a transforming protein-tyrosine kinase. Initial uptake rates (transport) of adenosine, thymidine, 3-O-methylglucose and 2-deoxyglucose were greater in v-fps-transformed cells than in normal cells. Elevated transport rates were seen in cells that expressed the temperature-sensitive mutant v-fps only after growth at a temperature that was permissive for protein-tyrosine kinase activity. Nucleoside transport rates declined with increasing cell density in both normal and v-fps transformed cells. Analysis of the sensitivity of adenosine transport to inhibition by nitrobenzylthioinosine (NBMPR) indicated that Rat-2 fibroblasts, like many other rat cell types, possess at least two nucleoside transport systems, which can be distinguished by differences in sensitivity to NBMPR. Although both transport activities were elevated in v-fps-transformed cells, a greater increase was seen in the NBMPR-sensitive component than in the NBMPR-insensitive component. Mass law analysis of the binding of [3H]NBMPR indicated that transformed cells had either the same number (NA9) or a smaller number (C10) of NBMPR-binding sites than normal cells, and photolabelling of membrane proteins with [3H]NBMPR identified polypeptides with similar electrophoretic mobilities (55-75 kDa) in both normal and transformed cells. Thus transformation by v-fps resulted in an increase in NBMPR-sensitive transport activity which was not related to either the number of NBMPR-binding sites or the apparent molecular mass of NBMPR-binding polypeptides.
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33
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Fox M, Boyle JM, Kinsella AR. Nucleoside salvage and resistance to antimetabolite anticancer agents. Br J Cancer 1991; 64:428-36. [PMID: 1911182 PMCID: PMC1977642 DOI: 10.1038/bjc.1991.327] [Citation(s) in RCA: 27] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022] Open
Affiliation(s)
- M Fox
- CRC Department of Biochemical Genetics, Paterson Institute for Cancer Research, Manchester, UK
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34
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Geisbuhler TP, Rovetto MJ. Guanosine metabolism in adult rat cardiac myocytes: ribose-enhanced GTP synthesis from extracellular guanosine. Pflugers Arch 1991; 419:160-5. [PMID: 1961686 DOI: 10.1007/bf00373002] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
The metabolic fate of transported guanosine was examined in adult rat cardiac myocytes. Freshly isolated cells were incubated with 10 microM or 100 microM [3H]guanosine and the nucleotide products extracted and examined for radiolabel distribution. The data presented show significant incorporation of guanosine into the 5'-nucleotide pool, and a marked stimulation of that incorporation by ribose. An average of 233 pmol/mg cell protein extracellular guanosine was incorporated into the cellular 5'-nucleotides over 90 min at both 10 microM and 100 microM external nucleoside. This appeared primarily as GTP (approx. 204 pmol/mg cell protein in 90 min). Only guanine nucleotides contained radiolabel; adenine nucleotides and IMP remained unlabelled even after 90 min incubation of the cells with [3H]guanosine. Addition of 5 mM ribose to the medium stimulated guanosine incorporation into 5'-nucleotides 1.6-fold (380 pmol/mg protein vs 234 pmol/mg over 90 min at 10 microM guanosine), but did not enhance the amount of guanosine transported into the cells. Intracellular guanosine concentrations exceeded those of the incubation medium at both external guanosine concentrations studied. More [3H]guanosine was salvaged at 100 microM than at 10 microM external guanosine (562 vs 380 pmol/mg protein in 90 min), but only if ribose was present in the medium. We conclude from these studies that guanosine is salvaged by heart muscle, and that at high guanosine levels the rate of guanosine salvage appears dependent on the availability of phosphoribosylpyrophosphate within the cells. At lower guanosine levels in the presence of ribose, cell guanine concentrations limit the rate of guanosine incorporation into 5'-nucleotides.
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Affiliation(s)
- T P Geisbuhler
- Department of Physiology, University of Missouri, Columbia 65212
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35
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Plagemann PG. Na(+)-dependent, concentrative nucleoside transport in rat macrophages. Specificity for natural nucleosides and nucleoside analogs, including dideoxynucleosides, and comparison of nucleoside transport in rat, mouse and human macrophages. Biochem Pharmacol 1991; 42:247-52. [PMID: 1859446 DOI: 10.1016/0006-2952(91)90710-m] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
The effects of natural nucleosides and various analogs thereof on Na(+)-dependent, concentrative transport of formycin B by cultured rat macrophages were investigated. Concentrative transport is the sole nucleoside transport system of these cells. The results indicated that uridine, 5'-fluorouridine, all natural purine nucleosides, 2-chloroadenosine and 5'-deoxyadenosine are efficient substrates for the transporter. None of nine other pyrimidine nucleosides was transported. 3'-Deoxy-adenosine, 2',3'-dideoxyadenosine, 8-azidoadenosine, tubercidin, 5'-methylthioadenosine 6-mercaptopurine riboside and adenosine arabinoside were either poor substrates or not transported significantly. The substrate activity of some of the natural nucleosides and the lack of substrate activity of 3'-deoxyadenosine, 2',3'-dideoxyadenosine, 8-azidoadenosine and 2',3'-dideoxycytidine were confirmed by direct uptake measurements. No significant concentrative nucleoside transport was detected in cultured human monocytes/macrophages, whereas mouse macrophages possessed both concentrative and equilibrative nucleoside transporters.
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Affiliation(s)
- P G Plagemann
- Department of Microbiology, Medical School, University of Minnesota, Minneapolis 55455
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36
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Abstract
Cellular uptake of formycin B, a poorly metabolized analog of inosine, by the isolated epithelium of rabbit jejunum is three times higher in the presence of Na+ than without this cation. The Na(+)-dependent nucleoside transport system is located in the apical membrane of the enterocytes and is capable of uphill transport, as shown for formycin B and adenosine with brush border membrane vesicles. According to present and earlier evidence, nucleoside transport across the basolateral membrane appears to have the properties of facilitated diffusion. Na(+)-dependent formycin B transport activity in intestinal epithelium decreases from jejunum to ileum and is absent in descending colon. As with Na(+)-coupled cotransport systems for other organic solutes, apical entry of formycin B is driven by the electrochemical Na+ gradient into the cell. In contrast to the facilitated diffusion system for nucleosides, Na(+)-dependent formycin B transport is not inhibited by nitrobenzylthioinosine, but both carrier systems are sensitive to inhibitors of D-glucose transport. Natural purine nucleosides and uridine are strong inhibitors of Na(+)-dependent formycin B transport. Transepithelial flux measurements substantiated that the Na(+)-dependent transport mechanism for formycin B functions as an absorptive system.
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Affiliation(s)
- M Roden
- Pharmakologisches Institut, Universität Wien, Vienna, Austria
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37
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Plagemann PG. Na(+)-dependent, active nucleoside transport in S49 mouse lymphoma cells and loss in AE-1 mutant deficient in facilitated nucleoside transport. J Cell Biochem 1991; 46:54-9. [PMID: 1874800 DOI: 10.1002/jcb.240460109] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
S49 murine lymphoma cells were examined for expression of various nucleoside transport systems using a non-metabolized nucleoside, formycin B, as substrate. Nitrobenzylthioinosine (NBTI)-sensitive, facilitated transport was the primary nucleoside transport system of the cells. The cells also expressed very low levels of NBTI-resistant, facilitated nucleoside transport as well as of Na(+)-dependent, concentrative formycin B transport. Concentrative transport was specific for uridine and purine nucleosides, just as the concentrative nucleoside transporters of other mouse and rat cells. A nucleoside transport mutant of S49 cells, AE-1, lacked both the NBTI-sensitive, facilitated and Na(+)-dependent, concentrative formycin B transport activity, but Na(+)-dependent, concentrative transport of alpha-aminoisobutyrate was not affected.
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Affiliation(s)
- P G Plagemann
- Department of Microbiology, Medical School, University of Minnesota, Minneapolis 55455
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38
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Plagemann PG. Mycoplasma contamination greatly enhances the apparent transport and concentrative accumulation of formycin B by mammalian cell culture. BIOCHIMICA ET BIOPHYSICA ACTA 1991; 1064:162-4. [PMID: 1902747 DOI: 10.1016/0005-2736(91)90423-6] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
S49 mouse leukemia cells exhibit both equilibrative and Na(+)-dependent, concentrative formycin B transport. The latter represents only a minor nucleoside transport component and is detectable only when equilibrative nucleoside transport is inhibited by dipyridamole or another transport inhibitor. Thus in uncontaminated S49 cells formycin B accumulated only to slightly above the intracellular-extracellular equilibrium level. In contrast, in suspensions of S49 cells contaminated with mycoplasma, formycin B accumulated in the intracellular water space in unmodified form to 40-50-times the extracellular concentration in a dipyridamole-independent manner during 90 min of incubation at 37 degrees C. The mycoplasma active formycin B transport system was inhibited by all nucleosides tested, including thymidine and deoxycytidine, which are not substrates for the concentrative nucleoside transporter of S49 cells. Mycoplasma contamination was detected by the presence of cell-associated adenosine phosphorylase activity.
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Affiliation(s)
- P G Plagemann
- Department of Microbiology, Medical School, University of Minnesota, Minneapolis 55455
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39
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Dagnino L, Bennett LL, P. Paterson AR. Sodium-dependent nucleoside transport in mouse leukemia L1210 cells. J Biol Chem 1991. [DOI: 10.1016/s0021-9258(18)38118-3] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
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40
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Dagnino L, Bennett LL, P. Paterson AR. Substrate specificity, kinetics, and stoichiometry of sodium-dependent adenosine transport in L1210/AM mouse leukemia cells. J Biol Chem 1991. [DOI: 10.1016/s0021-9258(18)38119-5] [Citation(s) in RCA: 24] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
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41
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Crawford CR, Belt JA. Sodium-dependent, concentrative nucleoside transport in Walker 256 rat carcinosarcoma cells. Biochem Biophys Res Commun 1991; 175:846-51. [PMID: 2025258 DOI: 10.1016/0006-291x(91)91642-p] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
Nucleoside transport in Walker 256 cells was reexamined using formycin B, a nonmetabolized analog of inosine. In the presence of dipyridamole to inhibit the equilibrative (facilitated diffusion) transporter previously described in these cells, the initial rate of uptake of 1 microM formycin B was 10-fold greater in Na(+)-containing medium than in Na(+)-free medium. In the presence of Na+ and dipyridamole the intracellular concentration of formycin B exceeded that in the medium within one min and was 6-fold greater than that of the medium by 5 min. Na(+)-dependent transport of formycin B was inhibited by low concentrations of inosine, but not thymidine. Furthermore, Na(+)-dependent transport of uridine, but not thymidine, was apparent in the presence of dipyridamole. These data indicate that Walker 256 cells have, in addition to the previously described equilibrative transporter, a concentrative nucleoside transporter. The specificity of this transporter appears to correspond to one of the two Na(+)-dependent transporters previously described in mouse intestinal epithelial cells.
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Affiliation(s)
- C R Crawford
- Department of Biochemical and Clinical Pharmacology, St. Jude Children's Research Hospital, Memphis, TN 38101
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42
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Gross CJ, Savaiano DA. The effect of nutritional state and allopurinol on nucleotide formation in enterocytes from the guinea pig small intestine. BIOCHIMICA ET BIOPHYSICA ACTA 1991; 1073:260-7. [PMID: 2009279 DOI: 10.1016/0304-4165(91)90130-9] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
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.
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Affiliation(s)
- C J Gross
- Department of Food Science and Nutrition, University of Minnesota, St. Paul 55108
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43
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Multiple sodium-dependent nucleoside transport systems in bovine renal brush-border membrane vesicles. Biochem J 1991; 274 ( Pt 1):27-33. [PMID: 2001243 PMCID: PMC1149915 DOI: 10.1042/bj2740027] [Citation(s) in RCA: 81] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
Na(+)-dependent nucleoside transport was examined in bovine renal brush-border membrane vesicles. Two separate Na+/nucleoside cotransporters were shown to be present: (1) a system specific for purine nucleosides and uridine, designated as the N1 carrier, and (2) an Na(+)-dependent nucleoside transporter that accepts pyrimidine nucleosides, adenosine and analogues of adenosine, designated as the N2 system. Both systems exhibit a high affinity for nucleosides (apparent Km values approximately 10 microM), are insensitive to inhibition by facilitated-diffusion nucleoside transport inhibitors, are rheogenic and exhibit a high specificity for Na+. Na+ increases the affinity of the influx of guanosine and thymidine, nucleosides that serve as model permeants for the N1 and N2 nucleoside transporters respectively. The Na+/nucleoside coupling stoichiometry is consistent with 1:1 for both carriers.
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44
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Jakobs ES, Van Os-Corby DJ, Paterson AR. Expression of sodium-linked nucleoside transport activity in monolayer cultures of IEC-6 intestinal epithelial cells. J Biol Chem 1990. [DOI: 10.1016/s0021-9258(18)45691-8] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
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45
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Plagemann PG, Aran JM. Characterization of Na(+)-dependent, active nucleoside transport in rat and mouse peritoneal macrophages, a mouse macrophage cell line and normal rat kidney cells. BIOCHIMICA ET BIOPHYSICA ACTA 1990; 1028:289-98. [PMID: 2223800 DOI: 10.1016/0005-2736(90)90178-q] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
Peritoneal rat macrophages expressed solely an Na(+)-dependent, concentrative nucleoside transporter, which possesses a single Na(+)-binding site and transports purine nucleosides and uridine but not thymidine or deoxycytidine. The Michaelis-Menten constants for formycin B and Na+ were about 6 microns and 14 mM, respectively, and the estimated Na+:formycin B stoichiometry was 1:1. Rat macrophages accumulated 5 microM formycin B to a steady-state level exceeding that in the medium by about 500-fold during 60 min of incubation at 37 degrees C. Concentrative formycin B transport was resistant to inhibition by nitrobenzylthioinosine, lidoflazine, dilazep and nifedipine, but was slightly inhibited by high concentrations of dipyridamole (greater than 10 microM) and probenecid (greater than 100 microM). Mouse peritoneal macrophages and lines of mouse macrophages and normal rat kidney cells expressed Na(+)-dependent, active nucleoside transport but in addition significant Na(+)-independent, facilitated nucleoside transport. Facilitated nucleoside transport in these cells was sensitive to inhibition by nitrobenzylthioinosine, dilazep and dipyridamole. The presence of these inhibitors greatly enhanced the concentrative accumulation of formycin B by these cells by inhibiting the efflux via the facilitated transporter of the formycin B actively transported into the cells. Whereas rat macrophages lacked high-affinity nitrobenzylthioinosine-binding sites, mouse macrophages and normal rat kidney cells possessed about 10,000 such sites/cell. Rat and mouse erythrocytes, rat lymphocytes, and lines of Novikoff rat hepatoma cells, Chinese hamster ovary cells, Mus dunni cells and embryonic monkey kidney cells expressed only facilitated nucleoside transport.
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Affiliation(s)
- P G Plagemann
- Department of Microbiology, Medical School, University of Minnesota, Minneapolis 55455-0312
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Crawford C, Ng C, Belt J. Isolation and characterization of an L1210 cell line retaining the sodium-dependent carrier cif as its sole nucleoside transport activity. J Biol Chem 1990. [DOI: 10.1016/s0021-9258(18)77410-3] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022] Open
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47
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Baer HP, Moorji A. Sodium-dependent and inhibitor-insensitive uptake of adenosine by mouse peritoneal exudate cells. BIOCHIMICA ET BIOPHYSICA ACTA 1990; 1026:241-7. [PMID: 2378887 DOI: 10.1016/0005-2736(90)90070-5] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
[8-3H]Adenosine uptake in mouse peritoneal exudate cells, harvested following i.p. challenge with Complete Freund's Adjuvant from BALB/c mice, was found to be insensitive to common nucleoside transport inhibitors such as dilazep or 6-[(4-nitrobenzyl)mercapto]purine ribonucleoside and to require sodium ion, being inactive when sodium was replaced by lithium or potassium. These findings also applied to the adherent (macrophages) and nonadherent (polymorphonuclear cells) cell fractions prepared from the peritoneal cell mixture. Uptake was inhibited by several nucleosides including deoxyadenosine, inosine, uridine, thymidine and, to a lesser extent, by the adenosine analog tubercidin, while adenine, fructose, glucose and ribose were without effect. Uptake [8-3H]adenosine was fully matched by rapid intracellular phosphorylation to AMP, ADP and ATP. Inosine was a substrate for the transporter, but tubercidin was not. The system clearly is distinct from carrier-mediated, nonconcentrative transport and has similarities to concentrative, sodium-dependent nucleoside transporters described in other cell types.
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Affiliation(s)
- H P Baer
- King Faisal specialist Hospital and Research Centre, Department of Biological and Medical Research, Riyadh, Saudi Arabia
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Plagemann PG, Aran JM. Na(+)-dependent, active nucleoside transport in mouse spleen lymphocytes, leukemia cells, fibroblasts and macrophages, but not in equivalent human or pig cells; dipyridamole enhances nucleoside salvage by cells with both active and facilitated transport. BIOCHIMICA ET BIOPHYSICA ACTA 1990; 1025:32-42. [PMID: 2369575 DOI: 10.1016/0005-2736(90)90187-s] [Citation(s) in RCA: 32] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
Formycin B influx studies have shown that P388 and L1210 mouse leukemia cells, mouse L929 cells, mouse RAW 309 Cr.1 cells, LK35.2 mouse B-cell hybridoma cells and cultured mouse peritoneal macrophages express both Na(+)-dependent, active and nonconcentrative, facilitated nucleoside transport systems. In the mouse cell lines, active transport represented only a minor nucleoside transport component and was detected only by measuring formycin B uptake in the presence of dipyridamole or nitrobenzylthioinosine, strong inhibitors of facilitated, but not of active, nucleoside transport. Inhibition of facilitated transport resulted in the concentrative accumulation of formycin B in cells expressing active nucleoside transport. Concentrative formycin B accumulation was abolished by treatment of the cells with gramicidin or absence of Na+ in the extracellular medium and strongly inhibited by ATP depletion or ouabain treatment. Mouse macrophages accumulated formycin B to 70-times the extracellular concentration in the absence of dipyridamole during 90 min of incubation at 37 degrees C. Thus active transport represents a major nucleoside transport system of these cells, similarly as previously reported for mouse spleen lymphocytes. In contrast to the various types of mouse cells, active formycin B transport was not detected in human HeLa cells, human H9, Jurkat and CEM T lymphoidal cells and pig spleen lymphocytes. These cells expressed only facilitated nucleoside transport with kinetic properties similar to those of the facilitated transporters of other mammalian cells.
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Affiliation(s)
- P G Plagemann
- Department of Microbiology, University of Minnesota Medical School, Minneapolis
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Crawford CR, Ng CY, Ullman B, Belt JA. Identification and reconstitution of the nucleoside transporter of CEM human leukemia cells. BIOCHIMICA ET BIOPHYSICA ACTA 1990; 1024:289-97. [PMID: 2112949 DOI: 10.1016/0005-2736(90)90357-t] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
The major nucleoside transporter of the human T leukemia cell line CEM has been identified by photoaffinity labeling with the transport inhibitor nitrobenzylmercaptopurine riboside (NBMPR). The photolabeled protein migrates on SDS-PAGE gels as a broad band with a mean apparent molecular weight (75,000 +/- 3000) significantly higher than that reported for the nucleoside transporter in human erythrocytes (55,000) (Young et al. (1983) J. Biol. Chem. 258, 2202-2208). However, after treatment with endoglycosidase F to remove carbohydrate, the NBMPR-binding protein in CEM cells migrates as a sharp peak with an apparent molecular weight (47,000 +/- 3000) identical to that reported for the deglycosylated protein in human erythrocytes (Kwong et al. (1986) Biochem. J. 240, 349-356). It therefore appears that the difference in the apparent molecular weight of the NBMPR-sensitive nucleoside transporter between the CEM cell line and human erythrocytes is a result of differences in glycosylation. The NBMPR-binding protein from CEM cells has been solubilized with 1% octyl glucoside and reconstituted into phospholipid vesicles by a freeze-thaw sonication technique. Optimal reconstitution of uridine transport activity was achieved using a sonication interval of 5 to 10 s and lipid to protein ratios of 60:1 or greater. Under these conditions transport activity in the reconstituted vesicles was proportional to the protein concentration and was inhibited by NBMPR. Omission of lipid or protein, or substitution of a protein extract prepared from a nucleoside transport deficient mutant of the CEM cell line resulted in vesicles with no uridine transport activity. The initial rate of uridine transport, in the vesicles prepared with CEM protein, was saturable with a Km of 103 +/- 11 microM and was inhibited by adenosine, thymidine and cytidine. The Km for uridine and the potency of the other nucleosides as inhibitors of uridine transport (adenosine greater than thymidine greater than cytidine) were similar to intact cells. Thus, although the nucleoside transporter of CEM cells has a higher molecular weight than the human erythrocyte transporter, it exhibits typical NBMPR-sensitive nucleoside transport activity both in the intact cell and when reconstituted into phospholipid vesicles.
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Affiliation(s)
- C R Crawford
- Department of Biochemical and Clinical Pharmacology, St. Jude Children's Research Hospital, Memphis, TN 38101
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