Ionophore Function Research Articles

New nanostructured extracellular potassium ion probe for assay of cellular K+ transport

The concentration of potassium ion is an important indicator for human health, and its abnormality is often accompanied by various diseases. However, most tools currently used to study potassium ion transport are low throughput. Herein, we reported a new K+ fluorescent nanoprobe CP1-KS with high selectivity and sensitivity to K+ (fluorescence enhanced factor was up to 9.91 at 20 mM K+). The polymeric fluorescent probe CP1-KS was composed of the small-molecular K+ indicator KS and amphiphilic copolymer CP1. This sensor can be easily and uniformly dispersed in cell culture medium and is suitable for high throughput analysis. To assess the utility of the probe CP1-KS in biological field, this probe was employed as an extracellular fluorescent probe to monitor the efflux of K+ from cells (E coli, B. Subtilis 168, Hela and MCF-7 cells) under various stimulation including lysozyme, nigericin, digitonin, and ATP. Results demonstrated that CP1-KS is an effective analysis tool for extracellular K+ concentration. We believe that the nanoprobe has great potential in antibacterial drug screening, K+ ionophore function, K+ channel activity, cell membrane permeability analysis or other K+ related field in the future.

Expanding the antibacterial selectivity of polyether ionophore antibiotics through diversity-focused semisynthesis.

Polyether ionophores are complex natural products capable of transporting cations across biological membranes. Many polyether ionophores possess potent antimicrobial activity and a few selected compounds have ability to target aggressive cancer cells. Nevertheless, ionophore function is believed to be associated with idiosyncratic cellu-lar toxicity and, consequently, human clinical development has not been pursued. Here, we demonstrate that structurally novel polyether ionophores can be efficiently constructed by recycling components of highly abundant polyethers to afford analogues with enhanced anti-bacterial selectivity compared to a panel of natural polyether ionophores. We used classic degradation reactions of the natural polyethers lasalocid and monensin and combined the resulting fragments with building blocks provided by total synthesis, including halogen-functionalized tetronic acids as cation-binding groups. Our results suggest that structural optimization of polyether ionophores is possible and that this area represents a potential opportunity for future methodological innovation.

Isolation of Microcystin-LR and Its Potential Function of Ionophore

Isolation of Microcystin-LR and Its Potential Function of Ionophore

Spectroscopic Insights into the Structural Dynamics and Mechanism of Ionophore Function of Valinomycin in Lipid Membranes

The ionophore valinomycin has been well studied as a model carrier depsipeptide, whose proposed mechanism involves the selective complexation of a monovalent cation in solution which is carried across the membrane and released. However, the proposed mechanisms of transport rely primarily on studies done in non-polar organic solvent. Here we present the first spectroscopic studies of valinomycin aimed at structural information done in the presence of a model lipid membrane without the need for sample or solvent modification. Deep-UV resonance Raman spectroscopy with a 197-206 nm excitation range is ideally suited to probe the secondary structure without interference from the lipid moiety. Two lipid environments were investigated, micelles and single unilamellar vesicles, which induce different conformations of valinomycin. Complexation of valinomycin with potassium was also investigated, both with and without competing sodium ions present. Implications for the mechanism of ion transport are discussed.View Large Image | View Hi-Res Image | Download PowerPoint Slide

ChemInform Abstract: Chiral Environment Specifically Induced by Metal Ion: Asymmetric α-Alkylation of α-Amino Esters Using Pyridoxal Derivatives Having a Chiral Ionophore Function.

Abstract Stereoselective alkylation of aldimines, prepared from α-amino esters and pyridoxal models having an ionophoric side-chain composed of a chiral glycerol structure, proceeded in the presence of Li+ or Na+ to afford α,α-dialkyl amino esters after acidic hydrolysis. Both the structure of the side chain and the metal ion were found to be in relation with the stereoselectivity, affording the highest stereoselectivity when the side-chain having a 2-naphthylmethoxy group and a methoxy group at the respective 3′- and 2′-positions was employed in the presence of Na+.

Synthesis of heteroaromatic tropeines and heterogeneous binding to glycine receptors

Heteroaromatic carboxylic esters of (nor)tropine were synthesized. Tropine esters displaced [ 3H]strychnine binding to glycine receptors of rat spinal cord with low Hill slopes. Two-site displacement resulted in nanomolar IC 50,1 and micromolar IC 50,2 values, and IC 50,2/IC 50,1 ratios up to 615 depending on the heteroaromatic rings and N-methyl substitution. Nortropeines displayed high affinity and low heterogeneity. IC 50,1 and IC 50,2 values of tropeines did not correlate suggesting different binding modes/sites. Glycine potentiated only the nanomolar displacement reflecting positive allosteric interactions and potentiation of ionophore function. Affinities of three (nor)tropeines were different for glycine receptors but identical for 5-HT 3 receptors.

A 17β‐derivative of allopregnanolone is a neurosteroid antagonist at a cerebellar subpopulation of GABAA receptors with nanomolar affinity

High-affinity, subtype-selective antagonists of the neurosteroid binding sites of GABA(A) receptors are not available. We have characterized an allopregnanolone derivative as an antagonist of cerebellar GABA(A) receptors with nanomolar affinity. Receptor binding and electrophysiological methods were used for the allosteric modulation of cerebellar GABA(A) receptors by an allopregnanolone derivative, (20R)-17beta-(1-hydroxy-2,3-butadienyl)-5alpha-androstane-3alpha-ol (HBAO). GABA(A) receptors of rat cerebellar membranes were labelled with the chloride channel blocker [(3)H]ethynylbicycloorthobenzoate (EBOB). The ionophore function of GABA(A) receptors was studied by whole-cell patch clamp electrophysiology in cultured rat cerebellar granule and cortical cells. Partial displacement of cerebellar [(3)H]EBOB binding by nanomolar HBAO was attenuated by 0.1 mM furosemide, an antagonist of alpha(6) and beta(2-3) subunit-containing GABA(A) receptors. Displacement curves of HBAO were reshaped by 30 nM GABA and shifted to the right. However, the micromolar potency of full displacement by allopregnanolone was not affected by 0.1 mM furosemide or 30 nM GABA. The nanomolar, but not the micromolar phase of displacement of [(3)H]EBOB binding by GABA was attenuated by 100 nM HBAO. Submicromolar HBAO did not affect [(3)H]EBOB binding to cortical and hippocampal GABA(A) receptors. HBAO up to 1 microM did not affect chloride currents elicited by 0.3-10 microM GABA, while it abolished potentiation by 1 microM allopregnanolone with nanomolar potency in cerebellar but not in cortical cells. Furosemide attenuated cerebellar inhibition by 100 nM HBAO. HBAO is a selective antagonist of allopregnanolone, a major endogenous positive modulator via neurosteroid sites of cerebellar (probably alpha(6)beta(2-3)delta) GABA(A) receptors.

Pyridoxal Derivatives Having an Ionophore Function: Application to the Synthesis of α,α‐Dialkyl Amino Acid Derivatives

AbstractFor Abstract see ChemInform Abstract in Full Text.

Nanomolar allopregnanolone potentiates rat cerebellar GABA A receptors

The ionophore function of γ-aminobutyric acid A (GABA A) receptors was studied by whole-cell patch clamp electrophysiology in primary cultures of rat cerebellar cortex. Chloride currents elicited by 1 μM GABA were potentiated by allopregnanolone with a plateau of high affinity (EC 50 = 14 nM) and a peak of potentiation around 1 μM allopregnanolone. Furosemide (0.1 mM) eliminated the high affinity phase and increased the EC 50 to 685 nM. GABA A receptors of rat cerebellar synaptosomal membranes were labelled with [ 3H]ethynylbicycloorthobenzoate (EBOB). Allopregnanolone displaced [ 3H]EBOB binding with IC 50 = 320 nM. The displacing potency of allopregnanolone was strongly enhanced (IC 50 = 39 nM) in the presence of 400 nM GABA and 60 nM SR 95531. Nanomolar potentiation by allopregnanolone can be associated with cerebellar GABA A receptors containing α 6, β 2–3 and δ subunits. This might be suitable for physiological modulation of tonic inhibitory neurotransmission via extrasynaptic GABA A receptors in cerebellar granule cells by neurosteroids.

Activation of ionotropic receptors and thermodynamics of binding

The structure, thermodynamics and activation mechanism of Cys-loop ionotropic receptors such as glycine, nicotinic acetylcholine, 5-HT 3-type serotonin and A-type γ-aminobutyric acid receptors are discussed. Based on the interrelationship of receptor binding and ionophore function, a ternary displacement mechanism of binding including the activation of ionophores is outlined. This displacement model can explain the enigmatic thermodynamic discrimination of agonists versus antagonists of Cys-loop ionotropic receptors. Binding of both agonists and antagonists is exothermic while activation is endothermic driven by large increases in entropy. Closure of the binding cavities around agonists in concert with subunit rotations and/or removal of water-filled crevices between transmembrane (TM) regions can account for entropy increases. Recombinant glycine and γ-aminobutyric acid A receptors and their point mutations support the predominant role of entropy in receptor activation.

Artificial model for cystathionine β-synthase: construction of a catalytic cycle with a pyridoxal model compound having an ionophore function

Catalytic transformation of serine- O-carbonate to S-aryl cysteine derivatives was successfully achieved in the presence of Li + by the use of a pyridoxal model compound having an ionophore function, which is the first example mimicking cystathionine β-synthase, artificially.

Artificial model for cystathionine β-synthase: efficient β-replacement reaction with thiols employing a novel pyridoxal model compound having an imidazole function

As a second-generation pyridoxal model compound for cystathionine β-synthase, we designed a novel model compound having an ionophore function and an imidazole function, application of which to the β-replacement reaction with various thiols smoothly took place to give S-substituted cysteines. Peptides having a serine- O-carbonate residue at the N-terminal position were also converted to the corresponding peptides having an S-substituted cysteine residue under the catalytic conditions of the novel pyridoxal model compound.

β‐Replacement Reaction of Serine‐O‐carbonate Derivatives with Thiols Catalyzed by a Pyridoxal Model Having an Ionophore Side‐Chain.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Beta-replacement reaction of serine-O-carbonate derivatives with thiols catalyzed by a pyridoxal model having an ionophore side-chain.

Serine-O-carbonate derivatives, including peptides having a serine-O-carbonate residue at the N-terminal position, are catalytically transformed into S-substituted cysteine derivatives employing the pyridoxal model having an ionophore function in the presence of Li+; this is the first artificial model mimicking cystathionine Beta-synthase.

Bicarbonate and thiocyanate ions affect the gating of γ-aminobutyric acidA receptors in cultured rat cortical cells

The ionophore function of γ-aminobutyric acidA (GABAA) receptors was studied by whole-cell patch clamp electrophysiology in primary cultures of rat cerebral cortex. Chloride ions were replaced for SCN− (thiocyanate) and HCO3− (bicarbonate) ions. The EC50 values of the GABAA agonist muscimol (HCO3−>Cl−>SCN−) varied parallel with the free energies of dehydration of the anions, while the inhibition constants of the GABAA antagonist bicuculline methiodide were not affected. These findings might be relevant in the interpretation of the contribution of HCO3− versus Cl− currents to the pharmacological differences between depolarizing and hyperpolarizing GABA responses.

ChemInform Abstract: Asymmetric α-Alkylation of α-Amino Esters Using Pyridoxal Derivatives Having a Chiral Ansa-Structure and a Chiral Ionophore Function: A Novel Example of Double Asymmetric Induction.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Asymmetric α-Alkylation of α-Amino Esters Using Pyridoxal Derivatives Having a Chiral Ansa-Structure and a Chiral Ionophore Function: a Novel Example of Double Asymmetric Induction

Stereoselective alkylation of aldimines, prepared from α-amino esters and a pyridoxal model having a chiral ansa-structure and an ethoxyethoxy group at C-3, proceeded in the presence of Li + to give α,α-dialkyl amino esters after acidic hydrolysis. Double asymmetric induction effect was also observed in the alkylation reaction by combination of the chiral ansa-structure and a chiral ionophore side chain at C-3.

Chiral environment specifically induced by metal ion: Asymmetric α-alkylation of α-amino esters using pyridoxal derivatives having a chiral ionophore function

Stereoselective alkylation of aldimines, prepared from α-amino esters and pyridoxal models having an ionophoric side-chain composed of a chiral glycerol structure, proceeded in the presence of Li + or Na + to afford α,α-dialkyl amino esters after acidic hydrolysis. Both the structure of the side chain and the metal ion were found to be in relation with the stereoselectivity, affording the highest stereoselectivity when the side-chain having a 2-naphthylmethoxy group and a methoxy group at the respective 3′- and 2′-positions was employed in the presence of Na +.

Selective blocking effects of tropisetron and atropine on recombinant glycine receptors.

Some serotonin 5-HT3 receptor ligands of tropeine structure have been recently shown to modulate ionophore function and binding of glycine receptors. This led us to study the effects of the tropeines tropisetron and atropine on recombinant human glycine receptors transiently expressed in Xenopus oocytes by using whole-cell voltage-clamp electrophysiology. Glycine currents were inhibited by atropine in an apparently competitive manner and with considerable selectivity of the tropeines for alpha2 versus alpha1 subunits. Coexpression of beta with alpha subunits and replacement of the N-terminal region of the alpha1 subunits by the corresponding beta segment resulted in similar increases in the inhibitory potencies. Our data suggest common sites of the tropeines for inhibition on the N-terminal region of glycine receptors. The point mutations R271K and R271L of the alpha1 subunit decreased, whereas a T112A substitution increased, the inhibition constants (Ki) of the tropeines. These changes in the Ki values of the tropeines were associated with opposite changes in the EC50 of glycine. Selectivities for the tropeines versus glycine (EC50/Ki) varied within three orders of magnitude. These results, when expressed in terms of free energy changes, can be interpreted according to a two-state receptor model.

Bidirectional allosteric modulation of strychnine-sensitive glycine receptors by tropeines and 5-HT 3 serotonin receptor ligands

Specific binding of [ 3H]strychnine was studied on membranes prepared from rat spinal cord. Several antagonists and agonists of 5-HT 3 receptors and tropane derivatives displaced [ 3H]strychnine binding with micromolar potencies. In the presence of 10 μM glycine a high affinity (nanomolar) component of displacement was also observed for the tropeines zatosetron, bemesetron and tropisetron. The displacing potency of glycine was also enhanced by these agents which are therefore termed glycine-positive. In contrast, atropine, SR 57227A, m-chlorophenylbiguanide, metoclopramide and granisetron are termed glycine-negative, because they decreased the displacing potency of glycine while glycine decreased the displacing potencies of atropine and metoclopramide. The dissociation of [ 3H]strychnine binding was accelerated in the presence of m-chlorophenylbiguanide, SR 57227A, atropine and zatosetron with a concentration dependence (EC 50 values and Hill slopes) similar to their displacing effects. This demonstrates that the displacement of strychnine binding is associated with allosteric interactions between different binding sites. Structure-activity analysis revealed that the tropeine structure is essential for high affinity binding, and its substitutions (in scopolamine and cocaine) or its replacement (in ondansetron and metoclopramide) strongly decrease the potency and/or efficacy of allosteric modulation. High affinity modulatory sites for tropeines appear to be associated with the potentiation of ionophore function, but distinct from the low affinity channel blocking sites as well as from the binding sites of strychnine and glycine.