The Experts below are selected from a list of 2511 Experts worldwide ranked by ideXlab platform

Simon M. Jarvis - One of the best experts on this subject based on the ideXlab platform.

  • A highly selective, high-affinity transporter for uracil in Trypanosoma brucei brucei: evidence for proton-dependent transport
    Biochemistry and Cell Biology, 1998
    Co-Authors: Harry P De Koning, Simon M. Jarvis
    Abstract:

    The presence of an uptake mechanism for uracil in procyclic forms of the protozoan parasite Trypanosoma brucei brucei was investigated. Uptake of [3H]uracil at 22°C was rapid and saturable and appeared to be mediated by a single high-affinity transporter, designated U1, with an apparent Km of 0.46 ± 0.09 µM and a Vmax of 0.65 ± 0.08 pmol·(107 cells)-1·s-1. [3H]Uracil uptake was not inhibited by a broad range of purine and pyrimidine nucleosides and nucleobases (concentrations up to 1 mM), with the exception of uridine, which acted as an apparent weak inhibitor (Ki value of 48 ± 15 µM). Similarly, most chemical analogues of uracil, such as 5-chlorouracil, 3-deazauracil, and 2-thiouracil, had little or no affinity for the U1 carrier. Only 5-fluorouracil was found to be a relatively potent inhibitor of uracil uptake (Ki = 3.2 ± 0.4 µM). Transport of uracil was independent of extracellular sodium and potassium gradients, as replacement of NaCl in the assay buffer by N-methyl-D-glucamine, KCl, LiCl, CsCl, or RbCl did not affect initial rates of transport. However, the proton ionophore Carbonyl Cyanide Chlorophenylhydrazone inhibited up to 70% of [3H]uracil flux. These data show that uracil uptake in T. b. brucei procyclics is mediated by a single high-affinity transporter with high substrate selectivity and are consistent with a nucleobase-H+-symporter model for this carrier.Key words: uracil, trypanosome, proton-nucleobase cotransport, nucleobase transport.

  • A highly selective, high-affinity transporter for uracil in Trypanosoma brucei brucei: evidence for proton-dependent transport
    Biochemistry and Cell Biology, 1998
    Co-Authors: Harry P De Koning, Simon M. Jarvis
    Abstract:

    The presence of an uptake mechanism for uracil in procyclic forms of the protozoan parasite Trypanosoma brucei brucei was investigated. Uptake of [3H]uracil at 22 degrees C was rapid and saturable and appeared to be mediated by a single high-affinity transporter, designated U1, with an apparent Km of 0.46 +/- 0.09 microM and a Vmax of 0.65 +/- 0.08 pmol x (10(7) cells)(-1) x s(-1). [3H]Uracil uptake was not inhibited by a broad range of purine and pyrimidine nucleosides and nucleobases (concentrations up to 1 mM), with the exception of uridine, which acted as an apparent weak inhibitor (Ki value of 48 +/- 15 microM). Similarly, most chemical analogues of uracil, such as 5-chlorouracil, 3-deazauracil, and 2-thiouracil, had little or no affinity for the U1 carrier. Only 5-fluorouracil was found to be a relatively potent inhibitor of uracil uptake (Ki = 3.2 +/- 0.4 microM). Transport of uracil was independent of extracellular sodium and potassium gradients, as replacement of NaCl in the assay buffer by N-methyl-D-glucamine, KCl, LiCl, CsCl, or RbCl did not affect initial rates of transport. However, the proton ionophore Carbonyl Cyanide Chlorophenylhydrazone inhibited up to 70% of [3H]uracil flux. These data show that uracil uptake in T. b. brucei procyclics is mediated by a single high-affinity transporter with high substrate selectivity and are consistent with a nucleobase-H+-symporter model for this carrier.

  • hypoxanthine uptake through a purine selective nucleobase transporter in trypanosoma brucei brucei procyclic cells is driven by protonmotive force
    FEBS Journal, 1997
    Co-Authors: Harry P De Koning, Simon M. Jarvis
    Abstract:

    The mechanism of purine nucleobase transport in procyclic cells of the protozoan parasite Trypanosoma brucei brucei was investigated. Hypoxanthine uptake at 22°C was rapid and saturable, exhibiting an apparent Km, of 9.3±2.0 μM and a Vmax of 4.5±0.8 pmol · (107cells)−1· s−1. All the natural purine nucleobases tested (Km 1.8–7.2 μM), as well as the purine analogues oxypurinol and allopurinol, inhibited hypoxanthine influx in a manner consistent with the presence of a single high-affinity carrier. Nucleosides and pyrimidine nucleobases had little or no effect on hypoxanthine influx. The uptake process was independent of extracellular sodium, but inhibited by ionophores inducing cytosolic acidification (Carbonyl Cyanide Chlorophenylhydrazone, nigericin, valinomycin) or membrane depolarisation (gramicidin) as well as by the adenosine triphosphatase inhibitors N-ethylmaleimide and N, N′-dicyclohexylcarbodiimide. Using the fluorescent dyes bisoxonol and 2′, 7′-bis-(carboxyethyl)-5, 6-carboxy-fluorescein to determine membrane potential and intracellular pH (pHi, the rate of hypoxanthine uptake was shown to be directly proportional to the protonmotive force. Similarly, under alkaline extracellular conditions hypoxanthine uptake was reversibly inhibited alongside a reduction in protonmotive force. In addition, hypoxanthine accelerated the rate of pH, recovery to pH7 after base-loading with NH4Cl, indicative of a proton influx concurrent with hypoxanthine transport. Finally, after pretreatment of cells with N-ethylmaleimide, hypoxanthine induced a slow membrane depolarisation, demonstrating that hypoxanthine transport is electro-genic. These data show that hypoxanthine uptake in T b. brucei procyclic cells is dependent on the protonmotive force, and are consistent with a nucleobase/H+-symporter model for this transporter.

Harry P De Koning - One of the best experts on this subject based on the ideXlab platform.

  • A highly selective, high-affinity transporter for uracil in Trypanosoma brucei brucei: evidence for proton-dependent transport
    Biochemistry and Cell Biology, 1998
    Co-Authors: Harry P De Koning, Simon M. Jarvis
    Abstract:

    The presence of an uptake mechanism for uracil in procyclic forms of the protozoan parasite Trypanosoma brucei brucei was investigated. Uptake of [3H]uracil at 22°C was rapid and saturable and appeared to be mediated by a single high-affinity transporter, designated U1, with an apparent Km of 0.46 ± 0.09 µM and a Vmax of 0.65 ± 0.08 pmol·(107 cells)-1·s-1. [3H]Uracil uptake was not inhibited by a broad range of purine and pyrimidine nucleosides and nucleobases (concentrations up to 1 mM), with the exception of uridine, which acted as an apparent weak inhibitor (Ki value of 48 ± 15 µM). Similarly, most chemical analogues of uracil, such as 5-chlorouracil, 3-deazauracil, and 2-thiouracil, had little or no affinity for the U1 carrier. Only 5-fluorouracil was found to be a relatively potent inhibitor of uracil uptake (Ki = 3.2 ± 0.4 µM). Transport of uracil was independent of extracellular sodium and potassium gradients, as replacement of NaCl in the assay buffer by N-methyl-D-glucamine, KCl, LiCl, CsCl, or RbCl did not affect initial rates of transport. However, the proton ionophore Carbonyl Cyanide Chlorophenylhydrazone inhibited up to 70% of [3H]uracil flux. These data show that uracil uptake in T. b. brucei procyclics is mediated by a single high-affinity transporter with high substrate selectivity and are consistent with a nucleobase-H+-symporter model for this carrier.Key words: uracil, trypanosome, proton-nucleobase cotransport, nucleobase transport.

  • A highly selective, high-affinity transporter for uracil in Trypanosoma brucei brucei: evidence for proton-dependent transport
    Biochemistry and Cell Biology, 1998
    Co-Authors: Harry P De Koning, Simon M. Jarvis
    Abstract:

    The presence of an uptake mechanism for uracil in procyclic forms of the protozoan parasite Trypanosoma brucei brucei was investigated. Uptake of [3H]uracil at 22 degrees C was rapid and saturable and appeared to be mediated by a single high-affinity transporter, designated U1, with an apparent Km of 0.46 +/- 0.09 microM and a Vmax of 0.65 +/- 0.08 pmol x (10(7) cells)(-1) x s(-1). [3H]Uracil uptake was not inhibited by a broad range of purine and pyrimidine nucleosides and nucleobases (concentrations up to 1 mM), with the exception of uridine, which acted as an apparent weak inhibitor (Ki value of 48 +/- 15 microM). Similarly, most chemical analogues of uracil, such as 5-chlorouracil, 3-deazauracil, and 2-thiouracil, had little or no affinity for the U1 carrier. Only 5-fluorouracil was found to be a relatively potent inhibitor of uracil uptake (Ki = 3.2 +/- 0.4 microM). Transport of uracil was independent of extracellular sodium and potassium gradients, as replacement of NaCl in the assay buffer by N-methyl-D-glucamine, KCl, LiCl, CsCl, or RbCl did not affect initial rates of transport. However, the proton ionophore Carbonyl Cyanide Chlorophenylhydrazone inhibited up to 70% of [3H]uracil flux. These data show that uracil uptake in T. b. brucei procyclics is mediated by a single high-affinity transporter with high substrate selectivity and are consistent with a nucleobase-H+-symporter model for this carrier.

  • hypoxanthine uptake through a purine selective nucleobase transporter in trypanosoma brucei brucei procyclic cells is driven by protonmotive force
    FEBS Journal, 1997
    Co-Authors: Harry P De Koning, Simon M. Jarvis
    Abstract:

    The mechanism of purine nucleobase transport in procyclic cells of the protozoan parasite Trypanosoma brucei brucei was investigated. Hypoxanthine uptake at 22°C was rapid and saturable, exhibiting an apparent Km, of 9.3±2.0 μM and a Vmax of 4.5±0.8 pmol · (107cells)−1· s−1. All the natural purine nucleobases tested (Km 1.8–7.2 μM), as well as the purine analogues oxypurinol and allopurinol, inhibited hypoxanthine influx in a manner consistent with the presence of a single high-affinity carrier. Nucleosides and pyrimidine nucleobases had little or no effect on hypoxanthine influx. The uptake process was independent of extracellular sodium, but inhibited by ionophores inducing cytosolic acidification (Carbonyl Cyanide Chlorophenylhydrazone, nigericin, valinomycin) or membrane depolarisation (gramicidin) as well as by the adenosine triphosphatase inhibitors N-ethylmaleimide and N, N′-dicyclohexylcarbodiimide. Using the fluorescent dyes bisoxonol and 2′, 7′-bis-(carboxyethyl)-5, 6-carboxy-fluorescein to determine membrane potential and intracellular pH (pHi, the rate of hypoxanthine uptake was shown to be directly proportional to the protonmotive force. Similarly, under alkaline extracellular conditions hypoxanthine uptake was reversibly inhibited alongside a reduction in protonmotive force. In addition, hypoxanthine accelerated the rate of pH, recovery to pH7 after base-loading with NH4Cl, indicative of a proton influx concurrent with hypoxanthine transport. Finally, after pretreatment of cells with N-ethylmaleimide, hypoxanthine induced a slow membrane depolarisation, demonstrating that hypoxanthine transport is electro-genic. These data show that hypoxanthine uptake in T b. brucei procyclic cells is dependent on the protonmotive force, and are consistent with a nucleobase/H+-symporter model for this transporter.

Haijian Zhou - One of the best experts on this subject based on the ideXlab platform.

Xue Zhong - One of the best experts on this subject based on the ideXlab platform.

Gang Cheng - One of the best experts on this subject based on the ideXlab platform.

  • First emergence of acrAB and oqxAB mediated tigecycline resistance in clinical isolates of Klebsiella pneumoniae pre-dating the use of tigecycline in a Chinese hospital.
    PLoS ONE, 2014
    Co-Authors: Xue Zhong, Dongke Chen, Haijian Zhou, Gang Cheng
    Abstract:

    Tigecycline is one of the few therapeutic options for treating infections caused by some multi-drug resistant pathogens, such as Klebsiella pneumoniae. However, tigecycline-resistant K. pneumoniae has been discovered recently in China. From 2009 to 2013, nine tigecycline-resistant K. pneumoniae isolates were identified in our hospital. Six of nine strains were identified before using tigecycline. To investigate the efflux-mediated resistance mechanisms of K. pneumoniae, the expression of efflux pump genes (acrA, acrB, tolC, oqxA and oqxB) and pump regulators (acrR, marA, soxS, rarA, rob and ramA) were examined by real-time RT-PCR. Molecular typing of the tigecycline resistant strains was performed. ST11 was the predominant clone of K. pneumoniae strains, while ST1414 and ST1415 were novel STs. Efflux pump inhibitor (EPI)-Carbonyl Cyanide Chlorophenylhydrazone (CCCP) was able to reverse the resistance patterns of 5 resistant K. pneumoniae strains. In comparison with strain A111, a tigecycline-susceptible strain (negative control), we found that the expression levels of efflux pump genes and pump regulators were higher in a majority of resistant strains. Higher expression levels of regulators rarA (2.41-fold, 9.55-fold, 28.44-fold and 18.31-fold, respectively) and pump gene oqxB (3.87-fold, 31.96-fold, 50.61-fold and 29.45-fold, respectively) were observed in four tigecycline resistant strains (A363, A361, A368, A373, respectively). Increased expression of acrB was associated with ramA and marA expression. To our knowledge, studies on tigecycline resistance mechanism in K. pneumoniae are limited especially in China. In our study, we found that both efflux pump AcrAB-TolC and OqxAB contributed to tigecycline resistance in K. pneumoniae isolates.