The Concise Guide to PHARMACOLOGY 2023/24: Transporters

The Concise Guide to PHARMACOLOGY 2023/24 is the sixth in this series of biennial publications. The Concise Guide provides concise overviews, mostly in tabular format, of the key properties of approximately 1800 drug targets, and over 6000 interactions with about 3900 ligands. There is an emphasis on selective pharmacology (where available), plus links to the open access knowledgebase source of drug targets and their ligands (https://www.guidetopharmacology.org/), which provides more detailed views of target and ligand properties. Although the Concise Guide constitutes almost 500 pages, the material presented is substantially reduced compared to information and links presented on the website. It provides a permanent, citable, point-in-time record that will survive database updates. The full contents of this section can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.16182. Transporters are one of the six major pharmacological targets into which the Guide is divided, with the others being: G protein-coupled receptors, ion channels, nuclear hormone receptors, catalytic receptors and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. The landscape format of the Concise Guide is designed to facilitate comparison of related targets from material contemporary to mid-2023, and supersedes data presented in the 2021/22, 2019/20, 2017/18, 2015/16 and 2013/14 Concise Guides and previous Guides to Receptors and Channels. It is produced in close conjunction with the Nomenclature and Standards Committee of the International Union of Basic and Clinical Pharmacology (NC-IUPHAR), therefore, providing official IUPHAR classification and nomenclature for human drug targets, where appropriate.

THE CONCISE GUIDE TO PHARMACOLOGY 2021/22: Transporters

The Concise Guide to PHARMACOLOGY 2021/22 is the fifth in this series of biennial publications. The Concise Guide provides concise overviews, mostly in tabular format, of the key properties of nearly 1900 human drug targets with an emphasis on selective pharmacology (where available), plus links to the open access knowledgebase source of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. Although the Concise Guide constitutes over 500 pages, the material presented is substantially reduced compared to information and links presented on the website. It provides a permanent, citable, point-in-time record that will survive database updates. The full contents of this section can be found at http://onlinelibrary.wiley.com/doi/bph.15543. Transporters are one of the six major pharmacological targets into which the Guide is divided, with the others being: G protein-coupled receptors, ion channels, nuclear hormone receptors, catalytic receptors and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. The landscape format of the Concise Guide is designed to facilitate comparison of related targets from material contemporary to mid-2021, and supersedes data presented in the 2019/20, 2017/18, 2015/16 and 2013/14 Concise Guides and previous Guides to Receptors and Channels. It is produced in close conjunction with the Nomenclature and Standards Committee of the International Union of Basic and Clinical Pharmacology (NC-IUPHAR), therefore, providing official IUPHAR classification and nomenclature for human drug targets, where appropriate.

Deletion of murine slc29a4 modifies vascular responses to adenosine and 5-hydroxytryptamine in a sexually dimorphic manner

Equilibrative nucleoside transporter 4 (ENT4), encoded by SLC29A4, mediates the flux of both 5‐hydroxytryptamine (5‐HT) and adenosine across cell membranes. We hypothesized that loss of ENT4 function in mice would modify the effects of these established regulators of vascular function. Male and female wild‐type (WT) and slc29a4‐null (ENT4‐KO) mice were compared with respect to their hemodynamics and mesenteric vascular function. Male ENT4‐KO mice had a complete loss of myogenic tone in their mesenteric resistance arteries. This was accompanied by a decrease in blood flow in the superior mesenteric artery in the male ENT4‐KO mice, and a reduced responsiveness to 5‐HT. In contrast, endothelium‐dependent relaxations of mesenteric arteries from female ENT4‐KO mice were more sensitive to Ca²⁺‐activated K⁺ (K_Ca) channel blockade than WT mice. Female ENT4‐KO mice also demonstrated an enhanced vasodilatory response to adenosine in vivo that was not seen in males. Ketanserin (5‐HT_2A inhibitor) and GR55562 (5‐HT_1B/1D inhibitor) decreased 5‐HT‐induced tone, but only ketanserin inhibited the relaxant effect of 5‐HT in mesenteric arteries. 5‐HT‐evoked increases in tone were elevated in arteries from ENT4‐KO mice upon block of endothelial relaxant pathways, with arteries from female ENT4‐KO mice showing the greatest increase. Adenosine A_2b receptor expression was decreased, while other adenosine transporter subtypes, as well as adenosine deaminase and adenosine kinase were increased in mesenteric arteries from male, but not female, ENT4‐KO mice. These findings indicate that deletion of slc29a4 leads to sex‐specific changes in vascular function with significant consequences for regulation of blood flow and pressure by adenosine and 5‐HT.

Loss of ENT1 increases cell proliferation in the annulus fibrosus of the intervertebral disc

Mice lacking equilibrative nucleoside transporter 1 (ENT1 ^-/- ) demonstrate progressive calcification of spinal tissues including the annulus fibrosus (AF) of the intervertebral disc (IVD). We previously established ENT1 as the primary nucleoside transporter in the AF and demonstrated dysregulation of biomineralization pathways. To identify cellular pathways altered by loss of ENT1, we conducted microarray analysis of AF tissue from wild-type (WT) and ENT1 ^-/- mice before calcification (2 months of age) and associated with calcification (6 months of age). Bioinformatic analyses identified cell cycle dysregulation in ENT1 ^-/- AF tissues and implicated the E2f family of transcription factors as potential effectors. Quantitative polymerase chain reaction analysis confirmed increased expression of multiple E2f transcription factors and E2f interacting proteins ( Rb1 and Cdk2) in ENT1 ^-/- AF cells compared with WT at 6 months of age. At this time point, ENT1 ^-/- AF tissues showed increased JNK MAPK pathway activation, CDK1, minichromosome maintenance complex component 5 (Mcm5), and proliferating cell nuclear antigen (PCNA) protein expression, and PCNA-positive proliferating cells compared with WT controls. The current study demonstrates that loss of ENT1-mediated adenosine transport leads to increased cell proliferation in the AF of the IVD.

Changes in aortic reactivity associated with the loss of equilibrative nucleoside transporter 1 (ENT1) in mice

Slc29a1 encodes for equilibrative nucleoside transporter subtype 1 (ENT1), the primary mechanism of adenosine transfer across cell membranes. Previous studies showed that tissues isolated from Slc29a1-null mice are relatively resistant to injury caused by vascular ischemia-reperfusion. To determine if there are similar changes in the microvasculature, and investigate underlying mechanism, we examined aortas isolated from wildtype and Slc29a1-null mice. Aorta macrostructure and gene expression were examined histologically and by qPCR, respectively. Wire myography was used to assess the contractile properties of isolated thoracic aortic rings and their response to adenosine under both normoxic and hypoxic conditions. In vivo haemodynamic parameters were assessed using the tail-cuff method. Slc29a1-null mice had significantly (P<0.05) increased plasma adenosine (2.75-fold) and lower blood pressure (~15% ↓) than wild-type mice. Aortas from Slc29a1-null mice were stiffer with a smaller circumference (11% ↓), and had an enhanced contractile response to KCl and receptor-mediated stimuli. Blockade of ENT1 with nitrobenzylthioinosine significantly enhanced (by ~3.5-fold) the response of aorta from wild-type mice to phenylephrine, but had minimal effect on aortas from Slc29a1-null mice. Adenosine enhanced phenylephrine-mediated constriction in the wild-type tissue under both normoxic (11.7-fold) and hypoxic (3.6-fold) conditions, but had no effect on the Slc29a1-null aortic aorta. In conclusion, aortas from Slc29a1-null mice respond to hypoxic insult in a manner comparable to wild-type tissues that have been pharmacologically preconditioned with adenosine. These data also support a role for ENT1 in the regulation of the protective effects of adenosine on contractile function in elastic conduit arteries such as thoracic aorta.

Disruption of biomineralization pathways in spinal tissues of a mouse model of diffuse idiopathic skeletal hyperostosis

Equilibrative nucleoside transporter 1 (ENT1) mediates passage of adenosine across the plasma membrane. We reported previously that mice lacking ENT1 (ENT1(-/-)) exhibit progressive ectopic mineralization of spinal tissues resembling diffuse idiopathic skeletal hyperostosis (DISH) in humans. Here, we investigated mechanisms underlying aberrant mineralization in ENT1(-/-) mice. Micro-CT revealed ectopic mineralization of spinal tissues in both male and female ENT1(-/-) mice, involving the annulus fibrosus of the intervertebral discs (IVDs) of older mice. IVDs were isolated from wild-type and ENT1(-/-) mice at 2months of age (prior to disc mineralization), 4, and 6months of age (disc mineralization present) and processed for real-time PCR, cell isolation, or histology. Relative to the expression of ENTs in other tissues, ENT1 was the primary nucleoside transporter expressed in wild-type IVDs and mediated the functional uptake of [(3)H]2-chloroadenosine by annulus fibrosus cells. No differences in candidate gene expression were detected in IVDs from ENT1(-/-) and wild-type mice at 2 or 4months of age. However, at 6months of age, expression of genes that inhibit biomineralization Mgp, Enpp1, Ank, and Spp1 were reduced in IVDs from ENT1(-/-) mice. To assess whether changes detected in ENT1(-/-) mice were cell autonomous, annulus fibrosus cell cultures were established. Compared to wild-type cells, cells isolated from ENT1(-/-) IVDs at 2 or 6months of age demonstrated greater activity of alkaline phosphatase, a promoter of biomineralization. Cells from 2-month-old ENT1(-/-) mice also showed greater mineralization than wild-type. Interestingly, altered localization of alkaline phosphatase activity was detected in the inner annulus fibrosus of ENT1(-/-) mice in vivo. Alkaline phosphatase activity, together with the marked reduction in mineralization inhibitors, is consistent with the mineralization of IVDs seen in ENT1(-/-) mice at older ages. These findings establish that both cell-autonomous and systemic mechanisms contribute to ectopic mineralization in ENT1(-/-) mice.

Hypoxanthine uptake by skeletal muscle microvascular endothelial cells from equilibrative nucleoside transporter 1 (ENT1)-null mice: effect of oxidative stress

Adenosine is an endogenous regulator of vascular tone. This activity of adenosine is terminated by its uptake and metabolism by microvascular endothelial cells (MVEC). The predominant transporter involved is ENT1 (equilibrative nucleoside transporter subtype 1). MVEC also express the nucleobase transporter (ENBT1) which is involved in the cellular flux of adenosine metabolites such as hypoxanthine. Changes in either of these transport systems would impact the bioactivity of adenosine and its metabolism, including the formation of oxygen free radicals. MVEC isolated from skeletal muscle of ENT1(+/+) and ENT1(-/-) mice were subjected to oxidative stress induced by simulated ischemia/reperfusion or menadione. The functional activities of ENT1 and ENBT1 were assessed based on zero-trans influx kinetics of radiolabeled substrates. There was a reduction in the rate of ENBT1-mediated hypoxanthine uptake by ENT1(+/+) MVEC treated with menadione or after exposure to conditions that simulate ischemia/reperfusion. In both cases, the superoxide dismutase mimetic MnTMPyP attenuated the loss of ENBT1 activity, implicating superoxide radicals in the response. In contrast, MVEC isolated from ENT1(-/-) mice showed no reduction in ENBT1 activity upon treatment with menadione or simulated ischemia/reperfusion, but they did have a significantly higher level of catalase activity relative to ENT1(+/+) MVEC. These data suggest that ENBT1 activity is decreased in MVEC in response to the increased superoxide radical that is associated with ischemia/reperfusion injury. MVEC isolated from ENT1(-/-) mice do not show this reduction in ENBT1, possibly due to increased catalase activity.

The Concise Guide to PHARMACOLOGY 2013/14: overview

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties from the IUPHAR database. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. This compilation of the major pharmacological targets is divided into seven areas of focus: G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors & Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and GRAC and provides a permanent, citable, point-in-time record that will survive database updates.

Loss of equilibrative nucleoside transporter 1 in mice leads to progressive ectopic mineralization of spinal tissues resembling diffuse idiopathic skeletal hyperostosis in humans

Diffuse idiopathic skeletal hyperostosis (DISH) is a noninflammatory spondyloarthropathy, characterized by ectopic calcification of spinal tissues. Symptoms include spine pain and stiffness, and in severe cases dysphagia and spinal cord compression. The etiology of DISH is unknown and there are no specific treatments. Recent studies have suggested a role for purine metabolism in the regulation of biomineralization. Equilibrative nucleoside transporter 1 (ENT1) transfers hydrophilic nucleosides, such as adenosine, across the plasma membrane. In mice lacking ENT1, we observed the development of calcified lesions resembling DISH. By 12 months of age, ENT1(-/-) mice exhibited signs of spine stiffness, hind limb dysfunction, and paralysis. Micro-computed tomography (µCT) revealed ectopic mineralization of paraspinal tissues in the cervical-thoracic region at 2 months of age, which extended to the lumbar and caudal regions with advancing age. Energy-dispersive X-ray microanalysis of lesions revealed a high content of calcium and phosphorus with a ratio similar to that of cortical bone. At 12 months of age, histological examination of ENT1(-/-) mice revealed large, irregular accumulations of eosinophilic material in paraspinal ligaments and entheses, intervertebral discs, and sternocostal articulations. There was no evidence of mineralization in appendicular joints or blood vessels, indicating specificity for the axial skeleton. Plasma adenosine levels were significantly greater in ENT1(-/-) mice than in wild-type, consistent with loss of ENT1--a primary adenosine uptake pathway. There was a significant reduction in the expression of Enpp1, Ank, and Alpl in intervertebral discs from ENT1(-/-) mice compared to wild-type mice. Elevated plasma levels of inorganic pyrophosphate in ENT1(-/-) mice indicated generalized disruption of pyrophosphate homeostasis. This is the first report of a role for ENT1 in regulating the calcification of soft tissues. Moreover, ENT1(-/-) mice may be a useful model for investigating pathogenesis and evaluating therapeutics for the prevention of mineralization in DISH and related disorders.

Identification of cysteines involved in the effects of methanethiosulfonate reagents on human equilibrative nucleoside transporter 1

Inhibitor and substrate interactions with equilibrative nucleoside transporter 1 (ENT1; SLC29A1) are known to be affected by cysteine-modifying reagents. Given that selective ENT1 inhibitors, such as nitrobenzylmercaptopurine riboside (NBMPR), bind to the N-terminal half of the ENT1 protein, we hypothesized that one or more of the four cysteine residues in this region were contributing to the effects of the sulfhydryl modifiers. Recombinant human ENT1 (hENT1), and the four cysteine-serine ENT1 mutants, were expressed in nucleoside transport-deficient PK15 cells and probed with a series of methanethiosulfonate (MTS) sulfhydryl-modifying reagents. Transporter function was assessed by the binding of [(3)H]NBMPR and the cellular uptake of [(3)H]2-chloroadenosine. The membrane-permeable reagent methyl methanethiosulfonate (MMTS) enhanced [(3)H]NBMPR binding in a pH-dependent manner, but decreased [(3)H]2-chloroadenosine uptake. [2-(Trimethylammonium)ethyl] methane-thiosulfonate (MTSET) (positively charged, membrane-impermeable), but not sodium (2-sulfonatoethyl)-methanethiosulfonate (MTSES) (negatively charged), inhibited [(3)H]NBMPR binding and enhanced [(3)H]2-chloroadenosine uptake. Mutation of Cys222 in transmembrane (TM) 6 eliminated the effect of MMTS on NBMPR binding. Mutation of Cys193 in TM5 enhanced the ability of MMTS to increase [(3)H]NBMPR binding and attenuated the effects of MMTS and MTSET on [(3)H]2-chloroadenosine uptake. Taken together, these data suggest that Cys222 contributes to the effects of MTS reagents on [(3)H]NBMPR binding, and Cys193 is involved in the effects of these reagents on [(3)H]2-chloroadenosine transport. The results of this study also indicate that the hENT1-C193S mutant may be useful as a MTSET/MTSES-insensitive transporter for future cysteine substitution studies to define the extracellular domains contributing to the binding of substrates and inhibitors to this critical membrane transporter.

Nucleoside/nucleobase transport and metabolism by microvascular endothelial cells isolated from ENT1−/− mice

Nucleoside and nucleobase uptake is integral to mammalian cell function, and its disruption has significant effects on the cardiovasculature. The predominant transporters in this regard are the equilibrative nucleoside transporter subtypes 1 (ENT1) and 2 (ENT2). To examine the role of ENT1 in more detail, we have assessed the mechanisms by which microvascular endothelial cells (MVECs) from ENT1−/− mice transport and metabolize nucleosides and nucleobases. Wild-type murine MVECs express mainly the ENT1 subtype with only trace levels of ENT2. These cells also have a Na+-independent equilibrative nucleobase transport mechanism for hypoxanthine (ENBT1). In the ENT1−/− cells, there is no change in ENT2 or ENBT1, resulting in a very low level of nucleoside uptake in these cells, but a high capacity for nucleobase accumulation. Whereas there were no significant changes in nucleoside transporter subtype expression, there was a dramatic increase in adenosine deaminase and adenosine A2a receptors (both transcript and protein) in the ENT1−/− tissues compared with WT. These changes in adenosine deaminase and A2a receptors likely reflect adaptive cellular mechanisms in response to reduced adenosine flux across the membranes of ENT1−/− cells. Our study also revealed that mouse MVECs have a nucleoside/nucleobase transport profile that is more similar to human MVECs than to rat MVECs. Thus mouse MVECs from transgenic animals may prove to be a useful preclinical model for studies of the effects of purine metabolite modifiers on vascular function.

Equilibrative nucleoside transporter 1 plays an essential role in cardioprotection

To better understand the role of equilibrative nucleoside transporters (ENT) in purine nucleoside-dependent physiology of the cardiovascular system, we investigated whether the ENT1-null mouse heart was cardioprotected in response to ischemia (coronary occlusion for 30 min followed by reperfusion for 2 h). We observed that ENT1-null mouse hearts showed significantly less myocardial infarction compared with wild-type littermates. We confirmed that isolated wild-type adult mouse cardiomyocytes express predominantly ENT1, which is primarily responsible for purine nucleoside uptake in these cells. However, ENT1-null cardiomyocytes exhibit severely impaired nucleoside transport and lack ENT1 transcript and protein expression. Adenosine receptor expression profiles and expression levels of ENT2, ENT3, and ENT4 were similar in cardiomyocytes isolated from ENT1-null adult mice compared with cardiomyocytes isolated from wild-type littermates. Moreover, small interfering RNA knockdown of ENT1 in the cardiomyocyte cell line, HL-1, mimics findings in ENT1-null cardiomyocytes. Taken together, our data demonstrate that ENT1 plays an essential role in cardioprotection, most likely due to its effects in modulating purine nucleoside-dependent signaling and that the ENT1-null mouse is a powerful model system for the study of the role of ENTs in the physiology of the cardiomyocyte.

Hypoxanthine uptake and release by equilibrative nucleoside transporter 2 (ENT2) of rat microvascular endothelial cells

The cardioprotective actions of adenosine are terminated by its uptake into endothelial cells with subsequent metabolism through hypoxanthine to uric acid. This process involves xanthine oxidase-mediated generation of reactive oxygen species (ROS), which have been implicated in the vascular dysfunction observed in ischemia-reperfusion injury. The equilibrative nucleoside transporter, ENT2, mediates the transfer of hypoxanthine into cells. We hypothesize that ENT2 also mediates the cellular release of hypoxanthine, which would limit the amount of intracellular hypoxanthine available for xanthine oxidase-mediated ROS production. Rat microvascular endothelial cells (MVECs) were isolated from skeletal muscle by lectin-affinity purification. The transport of [(3)H]hypoxanthine was assessed using an oil-stop method, and hypoxanthine metabolites were identified by thin-layer chromatography. MVECs accumulated hypoxanthine with a K(m) of 300 microM and a V(max) of 2.8 pmol microl(-1) s(-1). ATP-depleted cells loaded with [(3)H]hypoxanthine released the radiolabel with kinetics similar to that obtained for [(3)H]hypoxanthine influx. The uptake and release of [(3)H]hypoxanthine were both blocked by ENT2 inhibitors with similar order of potency. Thus, ENT2 mediates both the influx and efflux of hypoxanthine. Inhibition of ENT2 in MVECs might be expected to increase the amount of intracellular hypoxanthine available for metabolism by xanthine oxidase and enhance the intracellular production of ROS.

Nucleoside and nucleobase transporters of primary human cardiac microvascular endothelial cells: characterization of a novel nucleobase transporter

Levels of cardiovascular active metabolites, like adenosine, are regulated by nucleoside transporters of endothelial cells. We characterized the nucleoside and nucleobase transport capabilities of primary human cardiac microvascular endothelial cells (hMVECs). hMVECs accumulated 2-[3H]chloroadenosine via the nitrobenzylmercaptopurine riboside-sensitive equilibrative nucleoside transporter 1 (ENT1) at a V(max) of 3.4 +/- 1 pmol.microl(-1).s(-1), with no contribution from the nitrobenzylmercaptopurine riboside-insensitive ENT2. Inhibition of 2-chloroadenosine uptake by ENT1 blockers produced monophasic inhibition curves, which are also compatible with minimal ENT2 expression. The nucleobase [3H]hypoxanthine was accumulated within hMVECs (K(m) = 96 +/- 37 microM; V(max) = 1.6 +/- 0.3 pmol.microl(-1).s(-1)) despite the lack of a known nucleobase transport system. This novel transporter was dipyridamole-insensitive but could be inhibited by adenine (K(i) = 19 +/- 7 microM) and other purine nucleobases, including chemotherapeutic analogs. A variety of other cell types also expressed the nucleobase transporter, including the nucleoside transporter-deficient PK(15) cell line (PK15NTD). Further characterization of [3H]hypoxanthine uptake in the PK15NTD cells showed no dependence on Na(+) or H(+). PK15NTD cells expressing human ENT2 accumulated 4.5-fold more [3H]hypoxanthine in the presence of the ENT2 inhibitor dipyridamole than did PK15NTD cells or hMVECs, suggesting trapping of ENT2-permeable metabolites. Understanding the nucleoside and nucleobase transporter profiles in the vasculature will allow for further study into their roles in pathophysiological conditions such as hypoxia or ischemia.

Differential regulation of mouse equilibrative nucleoside transporter 1 (mENT1) splice variants by protein kinase CK2

Nucleosides are accumulated by cells via a family of equilibrative transport proteins (ENTs). An alternative splice variant of the most common subtype of mouse ENT (ENT1) has been identified which is missing a protein kinase CK2 (casein kinase 2) consensus site (Ser(254)) in the central intracellular loop of the protein. We hypothesized that this variant (mENT1a) would be less susceptible to modulation by CK2-mediated phosphorylation compared to the variant containing the serine at position 254 (mENT1b). Each splice variant was transfected into nucleoside transporter deficient PK15 cells, and stable transfectants assessed for their ability to bind the ENT1-selective probe [(3)H]nitrobenzylthioinosine (NBMPR) and to mediate the cellular uptake of [(3)H]2-chloroadenosine, with or without treatment with the CK2 selective inhibitor, 4,5,6,7-tetrabromobenzotriazole (TBB). mENT1a had a higher affinity for NBMPR relative to mENT1b - measured both directly by the binding of [(3)H]NBMPR, and indirectly via inhibition of [(3)H]2-chloroadenosine influx by NBMPR. Furthermore, incubation of mENT1b-expressing cells with 10 microM TBB for 48 h decreased both the K(D) and B(max) of [(3)H]NBMPR binding, as well as the V(max) of 2-chloroadenosine uptake, whereas similar treatment of mENT1a-expressing cells with TBB had no effect. PK15 cells transfected with hENT1, which has Ser(254), was similar to mENT1b in its response to TBB. In conclusion, inhibition of CK2 activity, or deletion of Ser(254) from mENT1, enhances transporter affinity for the inhibitor, NBMPR, and reduces the number of ENT1 proteins functioning at the level of the plasma membrane.

Calculations of Molecular Properties in Hybrid Coupled-Cluster and Molecular Mechanics Approach

We report benchmark calculations obtained with our new coupled-cluster singles and doubles (CCSD) code for calculating the first- and second-order molecular properties. This code can be easily incorporated into combined [Valiev, M.; Kowalski, K. J. Chem. Phys. 2006, 125, 211101] classical molecular mechanics (MM) and ab initio coupled-cluster (CC) calculations using NWChem, enabling us to study molecular properties in a realistic environment. To test this methodology, we discuss the results of calculations of dipole moments and static polarizabilities for the Cl2O system in the CCl4 solution using the CCSD (CC with singles and doubles) linear response approach. We also discuss the application of the asymptotic extrapolation scheme (AES) [Kowalski, K.; Valiev, M. J. Phys. Chem. A 2006, 110, 13106] in reducing the numerical cost of CCSD calculations.

Pharmacological analysis and molecular cloning of the canine equilibrative nucleoside transporter 1

We studied the binding of [3H]nitrobenzylthioinosine (NBMPR) and the uptake of [3H]formycin B by the es (equilibrative inhibitor-sensitive) nucleoside transporter of Madin Darby Canine Kidney (MDCK) cells. NBMPR inhibited [3H]formycin B uptake with a Ki of 2.7+/-0.6 nM, and [3H]NBMPR had a KD of 1.3+/-0.3 nM for binding to these cells; these values are significantly higher than those obtained in human and mouse cell models. In contrast, other recognized es inhibitors, such as dipyridamole, were significantly more effective as inhibitors of [3H]NBMPR binding and [3H]formycin B uptake by MDCK cells relative to that seen for human cells. We isolated a cDNA encoding the canine es nucleoside transporter (designated cENT1), and assessed its function by stable expression in nucleoside transport deficient PK15NTD cells. The PK15-cENT1 cells displayed inhibitor sensitivities that were comparable to those obtained for the endogenous es nucleoside transporter in MDCK cells. These data indicate that the dog es/ENT1 transporter has distinctive inhibitor binding characteristics, and that these characteristics are a function of the protein structure as opposed to the environment in which it is expressed.

Nucleoside transporter subtype expression and function in rat skeletal muscle microvascular endothelial cells

1. Microvascular endothelial cells (MVECs) form a barrier between circulating metabolites, such as adenosine, and the surrounding tissue. We hypothesize that MVECs have a high capacity for the accumulation of nucleosides, such that inhibition of the endothelial nucleoside transporters (NT) would profoundly affect the actions of adenosine in the microvasculature. 2. We assessed the binding of [(3)H]nitrobenzylmercaptopurine riboside (NBMPR), a specific probe for the inhibitor-sensitive subtype of equilibrative NT (es), and the uptake of [(3)H]formycin B (FB), by MVECs isolated from rat skeletal muscle. The cellular expression of equilibrative (ENT1, ENT2, ENT3) and concentrative (CNT1, CNT2, CNT3) NT subtypes was also determined using both qualitative and quantitative polymerase chain reaction techniques. 3. In the absence of Na(+), MVECs accumulated [(3)H]FB with a V(max) of 21+/-1 pmol microl(-1) s(-1). This uptake was mediated equally by es (K(m) 260+/-70 microm) and ei (equilibrative inhibitor-insensitive; K(m) 130+/-20 microm) NTs. 4. A minor component of Na(+)-dependent cif (concentrative inhibitor-insensitive FB transporter)/CNT2-mediated [(3)H]FB uptake (V(i) 0.008+/-0.005 pmol microl(-1) s(-1) at 10 microm) was also observed at room temperature upon inhibition of ENTs with dipyridamole (2,6-bis(diethanolamino)-4,8-dipiperidinopyrimido-[5,4-d]pyrimidine)/NBMPR. 5. MVECs had 122,000 high-affinity (K(d) 0.10 nm) [(3)H]NBMPR binding sites (representing es transporters) per cell. A lower-affinity [(3)H]NBMPR binding component (K(d) 4.8 nm) was also observed that may be related to intracellular es-like proteins. 6. Rat skeletal muscle MVECs express es/ENT1, ei/ENT2, and cif/CNT2 transporters with characteristics typical of rat tissues. This primary cell culture model will enable future studies on factors influencing NT subtype expression, and the consequent effect on adenosine bioactivity, in the microvasculature.

Interaction of the Novel Adenosine Uptake Inhibitor 3-[1-(6,7-Diethoxy-2-morpholinoquinazolin-4-yl)piperidin-4-yl]-1,6-dimethyl-2,4(1H,3H)-quinazolinedione Hydrochloride (KF24345) with the es and ei Subtypes of Equilibrative Nucleoside Transporters

Nucleosides such as adenosine, as well as many nucleoside-based drugs, permeate cell membranes via a family of equilibrative nucleoside transporters (ENTs). We assessed the effects of (3-[1-(6,7-diethoxy-2-morpholino-quinazolin-4-yl)piperidin-4-yl]-1,6-dimethyl-2,4(1H,3H)-quinazolinedione hydrochloride (KF24345), a novel anti-inflammatory agent that potentiates the actions of adenosine, on the es (inhibitor-sensitive) and ei (inhibitor-resistant) subtypes of ENTs in human, mouse, and rat cells. KF24345 was similar to the prototypical high-affinity inhibitor nitrobenzylthioinosine (NBMPR) for blocking the human es transporter (K(I) of approximately 0.4 nM), but was 50-fold more effective than NBMPR at blocking the human ei transporter (K(I) of approximately 100 nM). KF24345 displayed significantly less species heterogeneity in its affinity for the es transporter than did dipyridamole, a widely used inhibitor of nucleoside transport; KF24345 may thus prove useful as an inhibitor for studies of nucleoside metabolism in a range of animal models. Furthermore, KF24345 seemed to act as a noncompetitive inhibitor of both [(3)H]NBMPR binding and [(3)H]nucleoside uptake by human es transporters, and these kinetics were consistent with an observed slow dissociation of KF24345 from the inhibitor binding site. KF24345 also exhibited unusual biphasic profiles for inhibition of [(3)H]NBMPR binding to membranes prepared from a recombinant human es transporter model (PK15-hENT1), suggesting the presence of multiple populations of NBMPR binding proteins in these membranes. The atypical tight binding interaction of KF24345 with the es transporter may prove useful for the molecular delineation of inhibitor binding domains and will facilitate its use as an in vivo inhibitor of nucleoside transport in studies focused on the biological effects of adenosine.

Complex effects of sulfhydryl reagents on ligand interactions with nucleoside transporters: evidence for multiple populations of ENT1 transporters with differential sensitivities to N-ethylmaleimide

Functional studies have implicated cysteines in the interaction of ligands with the ENT1 nucleoside transporter. To better define these interactions, N-ethylmaleimide (NEM) and p-chloromercuribenzylsulfonate (pCMBS) were tested for their effects on ligand interactions with the [(3)H] nitrobenzylthioinosine (NBMPR) binding site of the ENT1 transporters of mouse Ehrlich ascites cells and human erythrocytes. NEM had biphasic, concentration-dependent effects on NBMPR binding to intact Ehrlich cells, plasma membranes, and detergent-solubilized membranes, with about 35% of the binding activity being relatively insensitive to NEM inhibition. NBMPR binding to human erythrocyte membranes also displayed heterogeneity in that about 33% of the NBMPR binding sites remained, albeit with lower affinity for NBMPR, even after treatment with NEM at concentrations in excess of 1 mM. However, unlike that seen for Ehrlich cells, no "reversal" in NBMPR binding to human erythrocyte membranes was observed at the higher concentrations of NEM. pCMBS inhibited 100% of the NBMPR binding to both Ehrlich cell and human erythrocyte membranes, but had no effect on the binding of NBMPR to intact cells. The effects of NEM on NBMPR binding could be prevented by coincubation of membranes with nonradiolabeled NBMPR, adenosine, or uridine. Treatment with NEM and pCMBS also decreased the affinity of other nucleoside transport inhibitors for the NBMPR binding site, but enhanced the affinities of nucleoside substrates. These data support the existence of at least two populations of ENT1 in both erythrocyte and Ehrlich cell membranes with differential sensitivities to NEM. The interaction of NEM with the mouse ENT1 protein may also involve additional sulphydryl groups not present in the human ENT1.

Increased L-arginine uptake and inducible nitric oxide synthase activity in aortas of rats with heart failure

L-Arginine crosses the cell membrane primarily through the system y(+) transporter. The aim of this study was to investigate the role of L-arginine transport in nitric oxide (NO) production in aortas of rats with heart failure induced by myocardial infarction. Tumor necrosis factor-alpha levels in aortas of rats with heart failure were six times higher than in sham rats (P < 0.01). L-Arginine uptake was increased in aortas of rats with heart failure compared with sham rats (P < 0.01). Cationic amino acid transporter-2B and inducible (i) nitric oxide synthase (NOS) expression were increased in aortas of rats with heart failure compared with sham rats (P < 0.05). Aortic strips from rats with heart failure treated with L-arginine but not D-arginine increased NO production (P < 0.05). The effect of L-arginine on NO production was blocked by L-lysine, a basic amino acid that shares the same system y(+) transporter with L-arginine, and by the NOS inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME). Treatment with L-lysine and L-NAME in vivo decreased plasma nitrate and nitrite levels in rats with heart failure (P < 0.05). Our data demonstrate that NO production is dependent on iNOS activity and L-arginine uptake and suggest that L-arginine transport plays an important role in enhanced NO production in heart failure.

Molecular cloning and functional characterization of inhibitor-sensitive (mENT1) and inhibitor-resistant (mENT2) equilibrative nucleoside transporters from mouse brain

Mammalian cells express at least two subtypes of equilibrative nucleoside transporters, i.e. ENT1 and ENT2, which can be distinguished functionally by their sensitivity and resistance respectively to inhibition by nitrobenzylthioinosine. The ENT1 transporters exhibit distinctive species differences in their sensitivities to inhibition by dipyridamole, dilazep and draflazine (human>mouse>rat). A comparison of the ENT1 structures in the three species would facilitate the identification of the regions involved in the actions of these cardioprotective agents. We now report the molecular cloning and functional expression of the murine (m)ENT1 and mENT2 transporters. mENT1 and mENT2 encode proteins containing 458 and 456 residues respectively, with a predicted 11-transmembrane-domain topology. mENT1 has 88% and 78% amino acid identity with rat ENT1 and human ENT1 respectively; mENT2 is more highly conserved, with 94% and 88% identity with rat ENT2 and human ENT2 respectively. We have also isolated two additional distinct cDNAs that encode proteins similar to mENT1; these probably represent distinct mENT1 isoforms or alternative splicing products. One cDNA encodes a protein with two additional amino acids (designated mENT1b) that adds a potential protein kinase CK2 phosphorylation site in the central intracellular loop of the transporter, and is similar, in this regard, to the human and rat ENT1 orthologues. The other cDNA has a 5'-untranslated region sequence that is distinct from that of full-length mENT1. Microinjection of mENT1, mENT1b or mENT2 cRNA into Xenopus oocytes resulted in enhanced uptake of [(3)H]uridine by the oocytes relative to that seen in water-injected controls. mENT1-mediated, but not mENT2-mediated, [(3)H]uridine uptake was inhibited by nitrobenzylthioinosine and dilazep. Dipyridamole inhibited both mENT1 and mENT2, but was significantly more effective against mENT1. Adenosine inhibited both systems with a similar potency, as did a range of other purine and pyrimidine nucleosides. These results are compatible with the known characteristics of the native mENT1 and mENT2 transporters.

Interaction of a series of draflazine analogues with equilibrative nucleoside transporters: species differences and transporter subtype selectivity

The equilibrative nucleoside transporters of mammalian cells play an important role in the regulation of extracellular adenosine concentrations, and inhibition of these transporters potentiates the biological effects of adenosine. Two subtypes of equilibrative transporters have been defined by their differential sensitivities to inhibition by nitrobenzylthioinosine (NBMPR; es/ENT1, sensitive; ei/ENT2, insensitive). In addition, significant species differences have been noted in es/ENT1 transporter affinity for a subset of inhibitors including draflazine and dipyridamole. Draflazine and a series of 15 chemically related compounds were compared for their abilities to: (a) inhibit the binding of [3H]NBMPR to the es/ENT1 transporter in mouse Ehrlich cell and human erythrocyte membranes, and (b) inhibit the es/ENT1 and ei/ENT2 transporter-mediated uptake of [3H]uridine in Ehrlich cells. Compounds within this series represented over a 1000-fold range of affinities for the es/ENT1 and ei/ENT2 transporters with subtype selectivities (ENT1/ENT2) ranging from 370 for R70527 to 0.17 for soluflazine. Five other analogues were identified, in addition to soluflazine, that had significantly higher affinity for the ei/ENT2 transporter compared with es/ENT1. Structure activity analyses of these data identified the requirement of a hydrophobic group connected to a 2-aminocarbonyl piperazine by a 5-carbon chain for high-affinity interactions with es/ENT1. This hydrophobic moiety was not as important for ei/ENT2 affinity and, in contrast to es/ENT1, a shorter alkyl chain enhanced binding to ei/ENT2. These draflazine analogues also varied in their differential affinities for mouse vs. human es/ENT1 transporters, and the degree of species discrimination was strongly dependent on the position of the aminocarbonyl group on the piperazine ring. This information, combined with structural data derived from molecular studies with ENT1 and ENT2 recombinant proteins, should guide further development of subtype-selective inhibitors of the equilibrative nucleoside transporters.

[3H]gemcitabine uptake by nucleoside transporters in a human head and neck squamous carcinoma cell line

Cellular uptake of many chemotherapeutic nucleoside analogs is dependent on the activity of a family of nucleoside transport proteins located in the cell plasma membrane. In the present study, we examined the role of these transporters in the accumulation of gemcitabine by a human head and neck squamous carcinoma cell line. The uptake of [3H]gemcitibine was compared with that of [3H]uridine and [3H]formycin B in the parent cell line (HN-5a) and in a gemcitabine-resistant variant (GEM-8e). The HN-5a and GEM-8e cells were similar in their transport characteristics and expressed predominantly the es (equilibrative, inhibitor-sensitive) transporter subtype; less than 10% of the influx of [3H]formycin B or [3H]uridine was mediated by the ei (equilibrative inhibitor-resistant) system, and there was no evidence for Na+-dependent nucleoside transporters. [3H]Gemcitabine (10 microM) entered these cells via both the es and ei transporters with an initial rate of uptake similar to that seen with the use of [3H]formycin B or [3H]uridine. In addition, ATP-replete cells accumulated significantly less [3H]gemcitabine than did ATP-depleted cells, which is indicative of an active efflux mechanism for gemcitabine. These results show that gemcitabine is a substrate for both the es and ei nucleoside transporters of HN-5a and GEM-8e cells and that gemcitabine resistance of the GEM-8e cells cannot be attributed to changes in transporter activity. Further studies to define the characteristics of the putative efflux mechanism are clearly warranted because this system has the potential to significantly affect the clinical efficacy of gemcitabine.

Demonstration of equilibrative nucleoside transporters (hENT1 and hENT2) in nuclear envelopes of cultured human choriocarcinoma (BeWo) cells by functional reconstitution in proteoliposomes

The equilibrative nucleoside transporters (ENTs) are a newly recognized family of membrane proteins of which hENT1 is the nitrobenzylmercaptopurine ribonucleoside (NBMPR)-sensitive (es) and hENT2 the NBMPR-insensitive (ei) transporter of human cells. BeWo cells exhibit large numbers (>10(7)/cell) of NBMPR-binding sites and high es and ei nucleoside transport activities relative to other cell types. In this work, we have demonstrated that proliferating BeWo cells possess (i) mRNA encoding hENT1 and hENT2 and (ii) hENT1-specific immunoepitopes. We examined NBMPR binding and its inhibition of uridine transport in various BeWo membrane fractions and proteoliposomes derived therefrom to determine if NBMPR binding to intracellular membranes represented interaction with functional es transporters. Unfractionated membranes and fractions enriched 5-fold in plasma membranes relative to postnuclear supernatants exhibited high NBMPR binding activity. Intact nuclei and nuclear envelopes also exhibited abundant quantities of NBMPR-binding sites with affinities similar to those of enriched plasma membranes (Kd = 0.4-0.9 nM). When proteoliposomes were made from octyl glucoside-solubilized membranes, high affinity NBMPR-binding sites were not only observed in crude membrane preparations and plasma membrane-enriched fractions but also in nuclear envelope fractions. Proteoliposomes prepared from either unfractionated membranes or nuclear envelopes exhibited both hENT1-mediated (82-85%) and hENT2-mediated (15-18%) transport of [3H]uridine. These results provided evidence for the presence of functional es and ei transporters in nuclear membranes and endoplasmic reticulum, suggesting that hENT1 and hENT2 may function in the translocation of nucleosides between the cytosol and the luminal compartments of one or both of these membrane types.

Interaction of 2',2'-difluorodeoxycytidine (gemcitabine) and formycin B with the Na+-dependent and -independent nucleoside transporters of Ehrlich ascites tumor cells

The uptake of [3H]formycin B by Ehrlich ascites tumor cells was examined in both normal Na+ buffer (physiological) and nominally Na+-free buffer (iso-osmotic replacement with Li+). These studies were conducted to further characterize the equilibrative nucleoside transporter subtypes of Ehrlich cells and to assess the contribution of Na+-dependent concentrative transport mechanisms to the cellular accumulation of nucleoside analogues by these cells. Formycin B is poorly metabolized by mammalian cells and, hence, can be used as a substrate to measure transport kinetics in energetically competent cells. Initial studies established that formycin B inhibited [3H]uridine uptake by the ei (equilibrative inhibitor-insensitive) and es (equilibrative inhibitor-sensitive) transporters of Ehrlich cells with Ki values of 48 +/- 28 and 277 +/- 25 microM, respectively. Similarly, [3H]formycin B had Km values of 111 +/- 52 and 635 +/- 147 microM for uptake by the ei and es transporters, respectively. When assays were conducted in the presence of Na+, plus 100 nM nitrobenzylthioinosine to prevent efflux via the es transporters, the intracellular concentration of [3H]formycin B exceeded the initial medium concentration by more than 3-fold, indicating the activity of a Na+-dependent transporter. Interestingly, the initial rate of uptake of [3H]formycin B was significantly higher in the Li+ buffer (es-mediated Vmax = 65 +/- 10 pmol/microliter . sec) than in the Na+ buffer (Vmax = 8.4 +/- 0.9 pmol/microliter . sec); this may reflect trans-acceleration of [3H]formycin B uptake by elevated intracellular adenosine levels resulting from the low Na+ environment. This model was then used to assess the interaction of gemcitabine (2',2'-difluorodeoxycytidine) with the equilibrative and concentrative nucleoside transporters. Gemcitabine, which has shown considerable potential for the treatment of solid tumors, was a relatively poor inhibitor of [3H]formycin B uptake via the equilibrative transporters (IC50 approximately 400 microM). In contrast, gemcitabine was a potent inhibitor of the Na+-dependent nucleoside transporter of Ehrlich cells (IC50 = 17 +/- 5 nM). These results suggest that the cellular expression/activity of Na+-dependent nucleoside transporters may be an important determinant in gemcitabine cytotoxicity and clinical efficacy.

The construction and application of a lux-based nitrate biosensor

A plasmid-borne transcriptional fusion between the Escherichia coli nitrate reductase (narG) promoter and the Photorhabdus luminescens lux operon provides E. coli with a highly bioluminescent phenotype in the presence of nitrate. This E. coli biosensor can detect nitrate to a level of 5 x 10(-5) mol l-1 (0.3 ppm), levels relevant to those levels encountered in brewing water. Since induction of the narG promoter requires NarL, the plasmid-based sensor can also be used to interrogate enteric bacteria for the presence of functional homologues of this E. coli regulatory protein. Obesumbacterium proteus, an important bacterial brewery contaminant, failed to provide nitrate-dependent bioluminescence demonstrating divergence in this organism from E. coli in the mechanism of nitrate reductase regulation.

Enhancement of the functional stability of solubilized nucleoside transporters by substrates and inhibitors

Purification of functional nucleoside transporters has been hampered by the instability of detergent-solubilized proteins. The present study was undertaken to determine if the presence of specific transporter ligands in the solubilization medium could enhance the functional stability of the isolated proteins. Ehrlich cell plasma membranes were solubilized with 1% (w/v) octylglucoside (+/- transporter ligands) and reconstituted into liposomal membranes either immediately after solubilization or after storage for 48 hr at 6 degrees. Storage resulted in a parallel loss (approximately 60%) of [3H]nitrobenzylthioinosine (NBMPR) binding and reconstituted [3H]uridine uptake activities. furthermore, upon storage, the relative amount of NBMPR-resistant [3H]uridine uptake by the reconstituted system dropped from 19 +/- 2 to 8 +/- 1% of the total mediated influx. The inclusion of high concentrations (> 10 mM) of adenosine in the solubilization medium completely prevented the storage-induced loss of both [3H]NBMPR binding and [3H]uridine influx activity, and prevented the shift in NBMPR sensitivity. In addition, inclusion of adenosine in the solubilization procedure increased the relative amount of NBMPR-resistant [3H]uridine uptake to 33 +/- 2% of the total influx in proteoliposomes prepared immediately after the proteins were extracted from the plasma membrane (i.e. no storage). A partial protection of [3H]NBMPR binding activity was also obtained using 2'-deoxyadenosine, 2-chloroadenosine, uridine, and non-radiolabelled NBMPR, but not with cytidine, inosine, diazepam, dipyridamole, or dilazep. These results suggest that both NBMPR sensitivity and transporter stability are dependent upon the conformational state of the protein. The protective effects of adenosine analogues and other nucleosides are likely due to their binding to the substrate translocation site, thereby effectively "locking" the transporter in a stable conformation.

Effect of detergents on ligand binding and translocation activities of solubilized/reconstituted nucleoside transporters

Purification of equilibrative nucleoside transporters has been hampered by the functional instability of the detergent-solubilized proteins. A variety of nonionic detergents were compared with octylglucoside (the most commonly used detergent in this regard) for their abilities to solubilize functionally stable nucleoside transporter proteins from Ehrlich cell plasma membranes. Transporter stability was assessed through the binding of the specific probe [3H]nitrobenzylthioinosine to freshly solubilized and stored (48 h/6 degrees C) preparations. The most promising detergents were decylmaltoside and cyclohexylbutylmaltoside, both of which, like octylglucoside, solubilized over 70% of the transporters from the membrane. Decylmaltoside- and cyclohexylbutylmaltoside-solubilized transport proteins retained 61 and 83%, respectively, of their [3H]-nitrobenzylthioinosine binding activity upon storage, compared to about 30% using octylglucoside. Decylmaltoside was also superior to octylglucoside in its capacity to solubilize the transporter in a state that retained its high affinity for the transport inhibitors dilazep (Ki = 11 nM, vs 75 nM in octylglucoside) and dipyridamole (Ki = 260 nM, vs 12 microM in octylglucoside). Reconstitution studies indicated that both the decylmaltoside- and cyclohexylbutylmaltoside-solubilized transporters were capable of mediating the uptake of [3H]uridine. Decylmaltoside was superior to cyclohexylbutylmaltoside, however, in both the enhanced transport activity of the resulting proteoliposomes (Vi = 21 pmol/mg/s vs 13 pmol/mg/s, respectively) and the lower nonmediated uptake observed in the decylmaltoside-derived vesicles (27% of total uptake at 4 min incubation). Nevertheless, cyclohexylbutylmaltoside may be useful in initial solubilization procedures due to its ability to selectively solubilize the nucleoside transporter from the plasma membrane. The rational use of these detergents will enable a more extensive purification of functional nucleoside transporters.

Characterization of nucleoside transport activity in rabbit cortical synaptosomes

Rabbit central nervous system (CNS) preparations have been used to study the central effects of adenosine, but little is known about the specific uptake mechanisms in rabbit brain involved in the regulation of extracellular adenosine concentrations. The present study assessed the kinetic and pharmacological characteristics of the uptake of [3H]uridine (a poorly metabolized substrate for adenosine transporters) by rabbit cortical synaptosomes, to define the transporter subtypes involved and to evaluate species variability in transporter characteristics. [3H]Uridine transport into rabbit cortical synaptosomes was mediated by two saturable, facilitated diffusion systems with characteristics compatible with the es and ei transporter subtypes identified in other mammalian species. About 65% of the total transport was mediated by the es system, and Km estimates of 320 and 94 microM were determined for [3H]uridine uptake by the es and ei transporter, respectively. These results differ significantly from the subtype ratio and kinetic characteristics reported for rat and guinea pig cortical synaptosomes, where most of the transport was mediated by an ei subtype. Dipyridamole, dilazep, nitrobenzylthioinosine, R75231, soluflazine, and mioflazine were relatively more effective as inhibitors of es-mediated uptake (compared with ei), while the substrates adenosine, cytidine, and guanosine did not distinguish between the es and ei transporters in rabbit cortical synaptosomes. These results highlight the significant species-tissue variability in nucleoside transporter characteristics and subtype expression, and emphasize the need to characterize the transporters in human CNS tissue to allow the rational development of CNS-active therapeutics based on inhibition of nucleoside transport.

Genetically-modified brewing yeasts for the 21st century. Progress to date

Academic studies and traditional breeding of yeasts depend upon their sporulation lifestyle. The strains used have been specially selected to sporulate readily and to mate producing new yeast types. Unfortunately brewing yeast strains do not behave in this way. They sporulate poorly, any spores which are formed are usually non-viable and any haploid strains produced are invariably non-maters. Only in recent years, with the development of recombinant-DNA techniques, has the specific breeding of new brewing yeast strains become widespread. Strains have been produced with the ability to ferment a wider range of carbohydrates, with altered flocculation properties and which produce beers with modified flavours. Many have been tested on the pilot scale and one, an amylolytic brewing yeast, has received approval for commercial use.

Functional reconstitution of pharmacologically distinct subtypes of nucleoside transporters in liposomal membranes

We examined the functional and pharmacological characteristics of liposome-reconstituted nucleoside transporter proteins obtained by detergent (octylglucoside) extraction from Ehrlich ascites tumor cell plasma membranes. Optimal reconstitution was achieved using a lipid composition of phosphatidylcholine, cholesterol, phosphatidylethanolamine and phosphatidylserine in a molar ratio of 33:33:26:8 and with a lipid-to-protein ratio of 30. This preparation had Km and Vmax values of 280 microM and 570 pmol/mg/sec, respectively, for the transporter-mediated uptake of [3H]uridine and bound 8.3 pmol of [3H]nitrobenzyl-thioinosine per milligram of protein. In general, the reconstituted system had kinetic and pharmacological characteristics comparable to those of the native membrane-located system, including an 80:20 ratio of nitrobenzylthioinosine-sensitive to -resistant [3H]uridine influx. The uridine translocation capacity of the optimally reconstituted system was 56 molecules per transporter per second compared with 104 molecules per transporter per second in intact cells, indicating that more than half of the inserted proteins were capable of mediating the influx of [3H]uridine (assuming that each functioning transporter was operating at optimal efficiency). Differences between the native and reconstituted transporters included the appearance of a [3H]uridine influx component (inhibited by adenosine) that was resistant to inhibition by R75231 (18%) and dipyridamole (10%). Dilazep was also significantly less effective in inhibiting the nitrobenzylthioinosine-sensitive transporter in the reconstituted preparations (IC50 = 41 nM) relative to that seen in intact Ehrlich cells (IC50 = 1.4 nM). These results suggest that inhibitor sensitivity may be a factor of the lipid microenvironment of the transporter or may involve other cellular components that could dissociate from the complex on detergent solubilization.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of cellular differentiation on nucleoside transport in human neuroblastoma cells

The nucleoside transport characteristics of undifferentiated and differentiated LA-N-2 human neuroblastoma cells were compared through measurement of the cellular accumulation of [3H]formycin B in the absence and presence of specific nucleoside transport blockers such as dipyridamole and nitrobenzylthioinosine (NBMPR). [3H]NBMPR was also used as a high affinity probe to obtain an estimate of the number of NBMPR-sensitive nucleoside transport proteins. Undifferentiated LA-N-2 cells accumulated [3H]formycin B (25 microM) via a NBMPR/dipyridamole sensitive, Na(+)-independent, nucleoside transport system (Vi = 1.52 pmol/microliters/s; maximum intracellular concentration = 45 pmol/microliters cell water). The undifferentiated cells also had a high density of site-specific [3H]NBMPR binding sites (135,000 sites/cell; KD = 0.4 nM). When cell differentiation was induced by exposure to a serum-free defined medium, the initial rate of transporter-mediated [3H]formycin B uptake increased to 1.92 pmol/microliters/s, and the steady-state intracellular concentration of [3H]formycin B also increased significantly to 73 pmol/microliters. However, there was no concomitant change in the number of [3H]NBMPR binding sites, and the additional uptake was not Na(+)-dependent. This enhanced uptake in the differentiated cells appeared to be due, in part, to an increased functional expression of a NBMPR-resistant form of facilitated nucleoside transporter. Approximately 18% of the transporter-mediated uptake in the differentiated cells was resistant to inhibition by NBMPR at concentrations that blocked transport completely in the undifferentiated cells. This cell model may prove useful for basic studies on regulation of nucleoside transporter subtype expression in neural tissues, and for evaluation of the efficacy and potential host toxicity of cytotoxic nucleoside analogues (+/- specific transport blockers) in the treatment of neuroblastoma.

Biochemical and Molecular Characterization of Obesumbacterium proteus , a Common Contaminant of Brewing Yeasts

We have evaluated the effectiveness of API 20E, Biolog testing, plasmid profiling, ribotyping, and enteric repetitive intergenic consensus (ERIC)-PCR to characterize, classify, and differentiate nine bacterial isolates of the common brewery contaminant Obesumbacterium proteus. Of the five typing techniques, Biolog testing, plasmid profiling, and ERIC-PCR provided the most differentiation, and API 20E testing and ribotyping were relatively indiscriminate. The molecular biology approach of ERIC-PCR offered the ideal combination of speed, simplicity, and discrimination in this study. Overall, the results are supportive of the view that O. proteus can be subdivided into two biogroups, biogroup 1, which has considerable biochemical and genetic homology to Hafnia alvei , and biogroup 2, which is relatively heterogeneous.

Interaction of the mioflazine derivative R75231 with the nucleoside transporter: evidence for positive cooperativity

This study investigated the interaction of the mioflazine derivative R75231 with the nucleoside transport system of rabbit cortical synaptosomes, and assessed the binding of [3H]R75231 to human erythrocyte ghost membranes. R75231 was a potent inhibitor of [3H]nitrobenzylthioinosine binding and [3H]uridine uptake in synaptosomes (Ki < 10 nM). This inhibition was evident even after extensive washing of the synaptosomes, subsequent to exposure to R75231. In addition to its tight binding characteristics, R75231 was shown to be a 'mixed' type inhibitor of [3H]nitrobenzylthioinosine binding (increased KD, decreased Bmax). [3H]R75231 bound with high affinity (KD = 0.4 nM) to erythrocyte membranes with a Bmax of 44 pmol/mg protein, which is comparable to the number of [3H]nitrobenzylthioinosine binding sites in this preparation. Binding of [3H]R75231 to these membranes was reversible, but the rate of dissociation was dependent upon the displacer used. Nitrobenzylthioinosine and dipyridamole each induced a complete dissociation of site-bound [3H]R75231 at rates not significantly different from those observed using a protocol involving a 100-fold dilution with buffer (no displacer). However, R75231 and mioflazine slowed the rate of dissociation of [3H]R75231 and actually caused an initial increase in the amount of site-bound [3H]R75231. Adenosine, on the other hand, enhanced the rate of [3H]R75231 dissociation. These results indicate that R75231 binding to the nucleoside transporter is a complex reaction, which may involve multiple interacting sites demonstrating positive cooperativity.

Differential uptake of [3H]guanosine by nucleoside transporter subtypes in Ehrlich ascites tumour cells

Intracellular metabolism of [3H]guanosine was minimal (< 15%) during the first 22 s of incubation, and hence reasonable estimates of initial-rate influx kinetics could be derived by using metabolically active cells. Na(+)-dependent concentrative [3H]guanosine uptake was not observed. Data suggest that [3H]guanosine was accumulated primarily via the nitrobenzylthioguanosine (NBTGR)-sensitive subtype of facilitated nucleoside transporter. Incubation of cells with 100 nM-NBTGR significantly decreased the potency of guanosine as an inhibitor of [3H]uridine influx. The Vmax. for [3H]guanosine influx (9.2 pmol/s per microliters) was significantly lower than that for [3H]uridine influx (16 pmol/s per microliters). The Km for transporter-mediated [3H]guanosine influx determined in the presence of 100 nM-NBTGR was 16-fold higher (1780 microM) than that determined in its absence, whereas the Km for [3H]uridine influx was shifted by only 2-fold. In other respects, the cellular accumulations of [3H]guanosine and [3H]uridine were similar; both had Km values of approx. 140 microM for total mediated influx, and both were inhibited similarly by other nucleosides and transport inhibitors. These characteristics, and the fact that guanosine is an endogenous nucleoside, suggest that [3H]guanosine may prove useful as a poorly metabolized, relatively selective, substrate for study of the NBTGR-sensitive nucleoside transport systems of mammalian cells.

Heterogeneity of [3H]dipyridamole binding to CNS membranes: correlation with [3H]nitrobenzylthioinosine binding and [3H]uridine influx studies

The relationship between the nucleoside transport system and the nitrobenzylthioinosine-sensitive and -resistant [3H]dipyridamole binding sites was examined by comparing the characteristics of [3H]dipyridamole binding with those of [3H]nitrobenzylthioinosine binding and [3H]-uridine influx in rabbit and guinea pig cerebral cortical synaptosomes. Two distinct high-affinity synaptosomal membrane-associated [3H]dipyridamole binding sites, with different sensitivities to inhibition by nitrobenzylthioinosine, were characterized in the presence of 3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulfonate (CHAPS, 0.01%) to prevent [3H]dipyridamole binding to glass tubes and filters. The nitrobenzylthioinosine-resistant [3H]-dipyridamole binding sites represented a greater proportion of the total membrane sites in guinea pig than in rabbit (40 vs. 10% based on inhibition studies). In rabbit, nitrobenzylthioinosine-sensitive [3H]dipyridamole binding (KD = 1.4 +/- 0.2 nM) and [3H]nitrobenzylthioinosine binding (KD = 0.30 +/- 0.01 nM) appeared to involve the same membrane site associated with the nitrobenzylthioinosine-sensitive nucleoside transporter. By mass law analysis, [3H]-dipyridamole binding in guinea pig could be resolved into two components based on sensitivity to inhibition by 1 microM nitrobenzylthioinosine. The nitrobenzylthioinosine-resistant [3H]dipyridamole binding sites were relatively insensitive to inhibition by all of the nucleoside transport substrates and inhibitors tested, with the exception of dipyridamole itself. In guinea pig synaptosomes, 100 microM dilazep blocked nitrobenzylthioinosine-resistant [3H]uridine transport completely but inhibited the nitrobenzylthioinosine-resistant [3H]dipyridamole binding component by only 20%. Furthermore, a greater percentage of the [3H]dipyridamole binding was nitrobenzylthioinosine resistant in guinea pig compared with rabbit, yet both species had a similar percentage of nitrobenzylthioinosine-resistant [3H]uridine transport.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative pharmacology of the nitrobenzylthioguanosine-sensitive and -resistant nucleoside transport mechanisms of Ehrlich ascites tumor cells

A variety of nucleoside transport inhibitors and substrates were compared for their capacities to inhibit the zero-trans influx of [3H]uridine in Ehrlich ascites tumor cells. ATP-depleted cells accumulated [3H]uridine primarily by facilitated diffusion (Vmax = 16 pmol/sec/microliter cell water) via both nitrobenzylthioguanosine (NBTGR)-sensitive (IC50 = 0.53 nM, 100 microM [3H]uridine) and NBTGR-resistant (IC50 = 71 microM, 100 microM [3H]uridine) mechanisms with uridine Km estimates of 99 and 284 microM, respectively. Dilazep also distinguished between the transporter subtypes with IC50 values of 1.4 nM and 1.8 microM, respectively, for inhibiting 100 microM [3H]uridine influx. Incubation of cells with 50 nM NBTGR allowed the selective study of inhibitor effects on NBTGR-resistant [3H]uridine influx. Dipyridamole, cyclopentyladenosine, 2-phenylaminoadenosine, etoposide, teniposide, diazepam, chlordiazepoxide, triazolam and the lidoflazine derivative 2-(aminocarbonyl)-N-(4-amino-2,6-dichlorophenyl)-4-[5,5-bis-(4- fluorophenyl)pentyl]-1-piperazineacetamide (R75231), were significantly less potent as inhibitors of NBTGR-resistant influx, when compared with their capacities to inhibit the total mediated influx of [3H]uridine. In contrast, 2-fluoroadenosine, 2-chloroadenosine, 5'-N-ethylcarboxamidoadenosine and soluflazine were relatively more effective as inhibitors of the NBTGR-resistant component. Mioflazine, a compound related to both soluflazine and R75231, did not distinguish between transporter subtypes. The NBTGR-resistant transporter also had a distinctive substrate specificity; guanosine, 2'-deoxyguanosine, cytidine and 2'-deoxycytidine were significantly less effective as inhibitors of NBTGR-resistant [3H]uridine influx.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetic analysis of ligand binding to the Ehrlich cell nucleoside transporter: pharmacological characterization of allosteric interactions with the [3H]nitrobenzylthioinosine binding site

Kinetic analysis of the binding of [3H]nitrobenzylthioinosine ([3H] NBMPR) to Ehrlich ascites tumor cell plasma membranes was conducted in the presence and absence of a variety of nucleoside transport inhibitors and substrates. The association of [3H] NBMPR with Ehrlich cell membranes occurred in two distinct phases, possibly reflecting functional conformation changes in the [3H]NBMPR binding site/nucleoside transporter complex. Inhibitors of the equilibrium binding of [3H]NBMPR, tested at submaximal inhibitory concentrations, generally decreased the rate of association of [3H]NBMPR, but the magnitude of this effect varied significantly with the agent tested. Adenosine and diazepam had relatively minor effects on the association rate, whereas dipyridamole and mioflazine slowed the rate dramatically. Inhibitors of nucleoside transport also decreased the rate of dissociation of [3H]NBMPR, with an order of potency significantly different from their relative potencies as inhibitors of the equilibrium binding of [3H]NBMPR. Dilazep, dipyridamole, and mioflazine were effective inhibitors of both [3H]NBMPR dissociation and equilibrium binding. The lidoflazine analogue R75231, on the other hand, had no effect on the rate of dissociation of [3H]NBMPR at concentrations below 300 microM, even though it was one of the most potent inhibitors of [3H]NBMPR binding tested (Ki less than 100 nM). In contrast, a series of natural substrates for the nucleoside transport system enhanced the rate of dissociation of [3H]NBMPR with an order of effectiveness that paralleled their relative affinities for the permeant site of the transporter. The most effective enhancers of [3H]NBMPR dissociation, however, were the benzodiazepines diazepam, chlordiazepoxide, and triazolam. Comparable effects of adenosine and dipyridamole on [3H]NBMPR dissociation rate were obtained upon solubilization of the membranes with octylglucoside, suggesting that this phenomenon was not due to changes in membrane fluidity. These results are compatible with the existence of specific ligand recognition sites on the nucleoside transport complex of Ehrlich cells that are pharmacologically distinct from, but allosterically linked to, the high affinity binding sites for [3H]NBMPR. The marked effects on [3H]NBMPR binding kinetics that result from ligand interactions with these sites must be considered in the design and analysis of all studies involving the use of [3H]NBMPR as a high affinity probe for the nucleoside transport system.

The effect of detergents on firefly luciferase reactions

The reaction rate of ATP-limited firefly luciferase-catalysed reactions, is affected by the presence of detergents. Anionic detergents inhibit luciferase activity without causing significant enzyme inactivation during the reaction. Cationic detergents increase reaction rate several-fold with a sharply defined optimum concentration of detergent for the effect. However, cationic detergents inactivate firefly luciferase during the reaction, resulting in a continuously decreasing reaction rate. Under such conditions, peak light intensity must be used as an indication of initial reaction rate. The inactivation rate increases with increasing detergent concentration. Non-ionic and zwitterionic detergents increase reaction rate over a broad range of detergent concentrations. Enzyme stability during the reaction is not affected by non-ionic detergents and only affected by zwitterionic detergents at high detergent concentration. Cyclodextrins, which can increase reaction rates of some chemiluminescent reactions, have little effect on firefly luciferase activity. Assays for ATP using firefly luciferase must be internally standardized by the constant addition technique in which a known amount of ATP is added to the test sample, since external calibration of such assays, by reference to a previously prepared standard curve, can lead to imprecision when detergents are present.