The detection of ionophorous antibiotic-cation complexes in water with fluorescent probes

From Molecular and Emulsified Ion Sensors to Membrane Electrodes: Molecular and Mechanistic Sensor Design

Selective molecular ion probes are often insoluble in water and require a hydrophobic solvent environment for strong and selective binding, which runs counter to the desire of utilizing them in a homogeneous solution. This Account aims to guide the reader on how such molecules, often coined ionophores, can be harnessed to design exceptionally useful optical and electrochemical sensors. We start here with some historical context on the design of such ionophores and continue with the explanation of the response mechanism of optical and potentiometric sensors and the role of combined components to build a robust ion sensor. This Account is addressed to nonspecialist readers and for this reason avoids extensive use of equations or theoretical considerations. The interested reader should turn to the original literature for further reading. Emulsified optical sensors are introduced as an initial example. Here, multiple reagents are confined in an attoliter sensing nanodroplet of the organic phase, immiscible with the aqueous sample phase. In this case, the ionophore molecules may retain their high affinity and selectivity to the target ion and the aqueous sample phase does not have to be modified. Emulsified optical sensors allow one to achieve the selective chemical sensing of ions, even with optically silent ionophores. Such ionophore-based nanodroplets are also discussed as a useful novel class of complexometric titration reagents and optical end point indicators with unique selectivities. We then turn our attention to potentiometric sensing probes and briefly discuss the unique opportunity of a direct characterization of ion-ionophore complexation properties offered by membrane electrodes. A carbonate-selective membrane electrode containing a highly selective tweezer-type ionophore with trifluoroacetophenone functional groups is then used as an example for the construction of a robust all-solid-state sensor. This potentiometric probe, in combination with a pH electrode, can directly measure PCO₂ in freshwater lakes, demonstrating a dramatically improved response time relative to traditional sensors equipped with a gas-permeable membrane. In recent years, new sensing modes and electrode designs have been introduced to expand the application scope of ionophore-based potentiometric sensors. Membrane electrodes containing ionophores are placed under dynamic electrochemistry control to give important progress in the field. We specifically highlight our recent works by membranes that are controlled by chronopotentiometry (controlled current) for speciation analysis, by ion transfer voltammetry on thin sensing films for multianalyte detection, by exhaustive coulometry for potentially calibration-free sensors and with coulometric membrane pumps for the selective delivery of reagents.

Pyroptosis is driven by non-selective gasdermin-D pore and its morphology is different from MLKL channel-mediated necroptosis

Necroptosis and pyroptosis are two forms of programmed cell death with a common feature of plasma membrane rupture. Here we studied the morphology and mechanism of pyroptosis in comparison with necroptosis. Different from necroptosis, pyroptosis undergoes membrane blebbing and produces apoptotic body-like cell protrusions (termed pyroptotic bodies) prior to plasma membrane rupture. The rupture in necroptosis is explosion-like, whereas in pyroptosis it leads to flattening of cells. It is known that the execution of necroptosis is mediated by mixed lineage kinase domain-like (MLKL) oligomers in the plasma membrane, whereas gasdermin-D (GSDMD) mediates pyroptosis after its cleavage by caspase-1 or caspase-11. We show that N-terminal fragment of GSDMD (GSDMD-N) generated by caspase cleavage also forms oligomer and migrates to the plasma membrane to kill cells. Both MLKL and GSDMD-N are lipophilic and the N-terminal sequences of both proteins are important for their oligomerization and plasma membrane translocation. Unlike MLKL which forms channels on the plasma membrane that induces influx of selected ions which osmotically swell the cells to burst, GSDMD-N forms non-selective pores and does not rely on increased osmolarity to disrupt cells. Our study reveals the pore-forming activity of GSDMD and channel-forming activity of MLKL determine different ways of plasma membrane rupture in pyroptosis and necroptosis.

K+/Na+ selectivity in K channels and valinomycin: over-coordination versus cavity-size constraints

Potassium channels and valinomycin molecules share the exquisite ability to select K(+) over Na(+). Highly selective K channels maintain a special local environment around their binding sites devoid of competing hydrogen bond donor groups, which enables spontaneous transfer of K(+) from states of low coordinations in water into states of over-coordination by eight carbonyl ligands. In such a phase-activated state, electrostatic interactions from these 8-fold binding sites, constrained to maintain high coordinations, result in K(+)/Na(+) selectivity with no need for a specific cavity size. Under such conditions, however, direct coordination from five or six carbonyl ligands does not result in selectivity. Yet, valinomycin molecules achieve selectivity by providing only six carbonyl ligands. Does valinomycin use additional coordinating ligands from the solvent or does it have special structural features not present in K channels? Quantum chemical investigations undertaken here demonstrate that valinomycin selectivity is due to cavity size constraints that physically prevent it from collapsing onto the smaller sodium ion. Valinomycin enforces these constraints by using a combination of intramolecular hydrogen bonds and other structural features, including its specific ring size and the spacing between its connected ligands. Results of these investigations provide a consistent explanation for the experimental data available for the ion-complexation properties of valinomycin in solvents of varying polarity. Together, investigations of these two systems reveal how nature, despite being popular for its parsimony in recycling functional motifs, can use different combinations of phase, coordination number, cavity size, and rigidity (constraints) to achieve K(+)/Na(+) selectivity.

Determination of complex formation constants of lipophilic neutral ionophores in solvent polymeric membranes with segmented sandwich membranes

A potentiometric method to determine ionophore complex formation constants in solvent polymeric membrane phases, proposed originally by Russian researchers, is critically evaluated and compared to other established methods. It requires membrane potential measurements on two-layer sandwich membranes, where only one side contains the ionophore. The resulting initial membrane potential reflects the ion activity ratio at both aqueous phase--membrane interfaces and can be conveniently used to calculate complex formation constants in situ. This method is potentially useful, since it does not require the use of a reference ion or second ionophore in the measurement. In this paper, the five ionophores valinomycin, BME-44, ETH 2120, tert-butylcalix[4]arene tetraethyl ester, and S,S'-methylenebis(diisobutyldithiocarbamate) are characterized in poly(vinyl chloride) (PVC) plasticized with dioctyl sebacate (DOS) and compared with other established methods. The resulting formation constants correspond well to literature values. The influence of varying membrane concentrations and different anionic site additives is studied and found to be relatively small. Experiments are also performed with and without lipophilic inert electrolytes and with ionophore-free sandwich membranes to illustrate the effect of ion pairing and the membrane internal diffusion potential on the response of such sandwich membranes. These experiments suggest that ions are completely associated in PVC-DOS membranes, but that such ion pairs are rather nonspecific. Diffusion potentials seem to play a minor role with these systems. The results are explained with theory. This work indicates that the characterization of electrically charged ionophores, anion-selective ionophores, and ionophores in membrane matrixes other than PVC plasticized with DOS may now be experimentally accessible.

Movement of fatty acids, fatty acid analogues, and bile acids across phospholipid bilayers

How lipophilic acids move across membranes, either model or biological, is the subject of controversy. We describe experiments which better define the mechanism and rates in protein-free phospholipid bilayers. The transbilayer movement of lipophilic acids [fatty acids (FA), covalently-labeled FA, bile acids, and retinoic acid] was monitored by entrapping pyranin, a water-soluble, pH-sensitive fluorescent molecule to measure pH inside unilamellar vesicles [Kamp, F., & Hamilton, J.A. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 11367-11370]. Equations for the pseudo-unimolecular rate constants for transbilayer movement of un-ionized (kappa FAH) and ionized (kappa FA-) acids are derived. All FA studied (octanoic, lauric, myristic, palmitic, stearic, oleic, elaidic, linoleic, linolelaidic, and arachidonic) and retinoic acid exhibited rapid transbilayer movement (t 1/2 < 1 s) via the un-ionized form across small unilamellar egg phosphatidylcholine (PC) vesicles. FA produced by phospholipase A2 in the outer leaflet of PC vesicles equilibrated rapidly to the inner leaflet. Ionized FA showed enhanced transbilayer movement (kappa FA- = 0.029 s-1) in the presence of equimolar valinomycin. The three FA analogues [12-(9-anthroyloxy)stearic acid, 5-doxylstearic acid, and 1-pyrenenonanoic acid] moved across PC bilayers via the un-ionized form; except for the anthroyloxy FA (kappa FAH = 4.8 x 10(-3) s-1), the rates were too fast to measure (t 1/2 < 1 s). The rate for cholic acid (CA) transbilayer movement was slow (kappa CAH = 0.056 s-1) compared to that of the more hydrophobic bile acids, deoxy- and chenodeoxycholic acid (t 1/2 < 1 s). The taurine conjugates of the three bile acids did not cross the bilayer (t 1/2 > 1 h). A further application of the pyranin method was to measure the partitioning of FA and bile acids among water, albumin, and PC vesicles. Our results show that the ability of lipophilic acids to permeate a PC bilayer rapidly is dependent on the presence of the un-ionized acid in the membrane interface. Considering the fast unfacilitated movement of FA across protein-free phospholipid bilayers, it is unlikely that there is a universal need for a transport protein to enhance movement of FA across membrane bilayers. Physiological implications of proton movement accompanying fast movement of un-ionized lipophilic acids (and the consequent generation of a pH gradient) are discussed.

Model ion channels: gramicidin and alamethicin

We have discussed in some detail a variety of experimental studies which were designed to elucidate the conformational and dynamic properties of gramicidin and alamethicin. Although the behavior of these peptides is by no means fully characterized, these studies have already permitted aspects of ion channel activity to be understood in molecular terms. Studies with gramicidin in a variety of organic solutions have revealed conformational heterogeneity of this peptide; at least five major isomers exist, several of which have been characterized in detail using NMR spectroscopy and X-ray crystallography. When added to lipid membranes gramicidin undergoes a further conformational conversion. Although the conformation of gramicidin in membranes is not as well characterized as the solution conformation(s) and an X-ray structure is not yet available, detailed data, particularly from solid-state NMR studies, continue to become available and a right-handed beta 6.3 helical conformation of the peptide backbone is now generally accepted. Two of these beta 6.3 helices joined at their N-termini are believed to form the conducting channel. The conformational behavior of the side-chains of gramicidin in the membrane-bound form is not well established and several NMR, CD, fluorescence and theoretical studies are now focussed on this. Although the side-chains do not directly contact the permeating ions, they can have distinct effects on conductance and selectivity by altering the electrostatic environment sensed by the ion. The dynamics of both side-chain and backbone conformations of gramicidin appear critical to a detailed understanding of the ion transport process in this channel. As the description of the membrane-bound conformation of gramicidin becomes more detailed, simulations of ion transport using computational methods are likely to improve and will further our understanding of the processes of ion transport. As well as internal motion of the backbone and side-chains, gramicidin undergoes rotational and translational motion in the plane of the membrane. These motions do not appear to be essential for the process of ion transport but can affect channel lifetime since lifetime is determined by the rate of association and dissociation of gramicidin monomers. Gramicidin-membrane interactions are also likely to be involved in the frequency of occurrence of channel subconductance states, the frequency of channel flickering and fundamentally in the stability of the membrane-bound gramicidin conformation. Alamethicin forms channels in membranes which are strongly voltage-dependent. The molecular origin of voltage-dependent conductances has been a fundamental problem in biophysics for many years.(ABSTRACT TRUNCATED AT 400 WORDS)

1H NMR study of valinomycin conformation in a phospholipid bilayer

Our understanding of how ions pass across biological membranes has been greatly advanced by the study of small molecules which are capable of enhancing ion transport. The concepts of ion movement through channels or via mobile ion carriers have arisen from studies of model systems. However, direct probing at the molecular level of the process of ion movement in a membrane system has proved difficult. The electrical properties of black lipid membrane model systems do not provide information about the details of ionophore location or conformation. Spectroscopic methods which are suited for probing the details of ionophore conformation and the stoichometry of ion binding have been confined largely to organic solvent systems which are limited as models for biological membranes. We report here proton magnetic resonance (1H NMR) spectroscopic studies which investigate valinomycin conformation and ion binding in small bilayer vesicles.

Ionomycin stimulates mast cell histamine secretion by forming a lipid-soluble calcium complex

The activation of cellular function following the direct introduction of Ca2+ into the cytosol by the use of a Ca2+-transporting ionophore has served to confirm the widely held idea that Ca2+ has the status of a second messenger in many cell types. However, this evidence has been obtained largely from the use of a single ionophore, the antibiotic A23187; experiments with X537A, which is another ionophorous antibiotic capable of transporting Ca2+ (ref. 3), have failed to show the expected characteristics. For example, histamine release from rat mast cells mediated by X537A is neither dependent on extracellular Ca2+ nor prevented by metabolic inhibitors. Ionomycin is a recently described polyether antibiotic produced by Streptomyces conglobatus ATCC 31005, and is active against Gram-positive bacteria. The antibiotic action is presumably due to its ionophorous properties, as it extracts Ca2+ from an aqueous phase into an organic phase with a stoichiometry of 1:1 (ref. 6). The ionophore is also capable of transporting 45Ca2+ across biological membranes (our unpublished results). Here we report the application of ionomycin to rat mast cells. We show that ionomycin stimulates mast cell secretion solely through its ability to form a lipid-soluble calcium complex, and thus to convey Ca2+ across the hydrocarbon region of the cell membrane.

Resolution of cytochrome oxidase into two component complexes

Cytochrome c oxidase (ferrocytochrome c: oxygen oxidoreductase, EC 1.9.3.1) has been resolved into a pair of complexes of unequal molecular weight. The larger complex (electron transfer complex) contains exclusively the oxidation-reduction proteins characteristic of cytochrome oxidase; the smaller complex (ion transfer complex) shows exclusively the capability for cation-dependent induction of the fluorescence of 8-anilino-1-naphthalenesulfonic acid--a capability demonstrable in preparations of cytochrome oxidase. The duplex nature of cytochrome oxidase has important implications for the mechanism of energy coupling.

Resolution of complex III of the mitochondrial electron transfer chain into two component complexes

Complex III can be resolved into an electron transfer complex and an ion transfer complex. The electron transfer complex contains exclusively the oxidation-reduction proteins of complex III, and the ion transfer complex the capability for augmenting cation-induced fluorescence of 8-anilino-1-naphthalenesulfonic acid. This finding has important implications for the molecular mechanism of mitochondrial energy coupling.

Isolation of an electrogenic K+/Ca2+ ionophore from an ionophoroprotein of beef heart mitochondria

A K+/Ca2+ electrogenic ionophore has been isolated from an ionophoroprotein of beef heart mitochondria and identified as a neutral peptide of molecular weight 1600. The amino acid composition and cationic specificity of the ionophore have been determined. The free ionophore was released from the ionophoroprotein as a consequence of tryptic digestion. The ionophoroprotein can be converted to an ionophoro peptide (molecular weight 5,100) by proteolysis without release of the free ionophore. The isolation of a K+/Ca2+ ionophore thus provides an introduction to the general technology of extracting ionophoro proteins and ofreleasing ionophores from these proteins by proteolytic digestion.

Coupling in cytochrome c oxidase

Cytochrome c oxidase (ferrocytochrome c: oxygen oxidoreductase; EC 1.9.3.1) can be resolved into an electron transfer complex (ETC) and an ionophore transfer complex (ITC). Coupling requires an interaction between the moving electron in the ETC and a moving, positively charged ionophore-cation adduct in the ITC. The duplex character of cytochrome oxidase facilitates this interaction. The ITC mediates cyclical cation transport. It can be replaced as the coupling partner by the combination of valinomycin and nigericin in the presence of K(+) when cytochrome oxidase is incorporated into liposomes containing acidic phospholipids or by the combination of lipid cytochrome c and bile acids in an ITC-resolved preparation of the ETC. Respiratory control can be induced by incorporating cytochrome oxidase into vesicles of unfractionated whole mitochondrial lipid. The activity of the ITC is suppressed by such incorporation and this suppression leads to the emergence of respiratory control. The ionophoroproteins of the ITC can be extracted into organic solvents; some 50% of the total protein of cytochrome oxidase is extractable. The release of free ionophore is achieved by tryptic digestion of the ionophoroprotein. Preliminary to this release the ionophoroprotein is degraded to an ionophoropeptide. Electrogenic ionophores, as well as uncoupler, are liberated by such proteolysis. The ITC contains a set of ionophoroproteins imbedded in a matrix of phospholipid.

Use of 1-anilino-8-naphthalene-sulfonate as a probe of gastric vesicle transport

The interaction of 1-anilino-8-naphthalene-sulfonate (ANS) with vesicles derived from hog fundic mucosa was studied in the presence of valinomycin and with the addition of ATP. Evidence was found for two classes of sites, those rapidly accessible to ANS with a KD of 7.5 micronM and those slowly accessible, but rapidly accessed in the presence of valinomycin with a KD of 2.5 micronM. ATP transiently increases the quantum yield of the latter ANS binding sites only in the presence of valinomycin, but does not alter the number of KD of those sites. The time course of this increase correlates with H+ uptake and Rb+ extrusion by those vesicles and H+ carries such as tetrachlorsalicylanilide or nigericin abolish the ATP response. With ATP addition in the presence of SC14N and valinomycin there is transient uptake of SCN-. It is concluded that ANS is acting as a probe of a structural change dependent on a potential and H+ gradient.

Role of ionophores in energy coupling

If, as we deem inevitable, the principles of energy coupling are universal, then the necessity for charge-separating devices will apply across the board to all bioenergetic systems. Since, apart from the elctron transfer chain, ionophores are the only charge-separating devices available in bioenergetic systems, model of energy coupling that does not feature this central role of ionophores can be taken seriously. The ionophore approach thus opens the royal highway to the ultimate solution of all bioenergetic problems.

Isolation, properties, and structural features of divalent cation ionophores derived from beef heart mitochondria

The notion of small molecular weight ion carriers in biological systems is herein documented by a description of the isolation and ionophoretic properties of a family of oxyoctadecadienoate congeners derived from beef-heart mitochondria. Although certain members of this family of compounds have been shown to possess unique ionophoretic properties, one should not lose sight of the fact that the compounds that we have described represented only a portion of the total picture. Other chemically unrelated, yet structurally unknown species have been isolated from beef-heart mitochondria, and compounds similar in both chromatographic and spectroscopic properties to the oxyoctadecadienoate family, as well as other unique structures, have been isolated in our laboratory from sarcoplasmic reticulum and chloroplasts. The important points to be derived from these findings are that there is an apparent abundance of natural ionophores and we should no longer concern should address ourselves to the more relevant task of digging them out and ourselves with the question "are there ionophores in biological systems?" but describing their chemical and physical properties. In view of the apparent abundance of natural ionophores, this is an enormous task, especially when one considers that it only represents half of the problem. The isolation and description of the ionophoroprotein or channel-forming complexes share equally in the overall significance and level of understanding attributable to this area of inquiry and it would appear that many fruitful collaborative ventures are, or should be, on the horizon.

Transmembrane electrophoresis of 8-anilino-1-naphthalenesulfonate through egg lecithin liposome membranes

Valinomycin has been shown to increase the amount of 8-anilino-1-naphthalenesulfonate (ANS) bound to egg lecithin liposomes and also to increase the maximum fluorescence value, as derived from double reciprocal plots. The assay conditions were such that addition of valinomycin would not produce a transmembrane potential. The formation of a valinomycin potassium ANS complex in the micelle membrane is proposed. This could account for the increase in the maximum fluorescence value and, by acting as an ANS transporter, could also account for the increase in ANS bound. Tributylamine was also shown to increase the binding and maximum fluorescence of ANS. In assay conditions where the addition of valinomycin would produce a transmembrane potential negative inside, the tributylamine-induced fluorescence was reversed. The fluorescense decrease is interpreted as transmembrane electrophoresis of ANS in response to a transmembrane potential.

Effects of unstirred layers on the steady-state zero-current conductance of bilayer membranes mediated by neutral carriers of ions

Some effects of diffusion polarization and chemical reactions on the steady-state zero-current conductance of lipid bilayers mediated by neutral carriers of ions have been studied theoretically and experimentally. Assuming that ion permeation across the interfaces occurs via a heterogeneous reaction between ions in the solution and carriers in the membrane, the relationship between the conductance and the aqueous concentration of carriers is shown to be linear only in a limited range of sufficiently low concentrations. At higher carrier concentrations, which for the most strongly bound cations are within the range of the experimentally accessible values, the conductance is expected to become limited by diffusion of the carried ion in the unstirred layers and therefore reach an upper limiting value independent of the membrane properties. This expectation has been successfully verified for glyceryl-monooleate membranes in the presence of the ions K+, Rb+ and NH+4 and carriers such as valinomycin and trinactin. The experimental results support, at least for the present system, the generally accepted view that complexation between ions and the macrocyclic antibiotics occurs at the membrane surface; it is shown, in fact, that for a different mechanism, such as that by which the complexes would form in the aqueous solutions and cross the interfaces as lipid-soluble ions, the same type of saturation would be expected to be observable only for unrealistically high values of the rate constants of the ion-carrier association. A previously proposed criterion to distinguish between these two mechanisms, based on the dependence of the conductance on the ion concentration, is discussed from the viewpoint of this more comprehensive model.

Conformational changes of alamethicin induced by solvent and temperature. A 13C-NMR and circular-dichroism study

13C nuclear magnetic resonance (NMR) and circular dichroism (CD) have been used for studies on the conformation of alamethicin. The 13C NMR spectrum is assigned with the aid of signals of synthetic partial sequences and selective proton decoupling. The solvent and temperature-dependence of the 13C NMR spectra, T1 measurements and the use of lanthanide-shift reagents allow the differentiation between the amino acids belonging to a rigid alpha-helical portion of the alamethicin sequence and those belonging to a more flexible part. The 13C NMR results are in agreement with results obtained from extended solvent and temperature-dependent CD studies which indicate a highly stabilized nonpolar and intrachenar alpha-helical part. The concentration-dependence of the CD spectrum of alamethicin in a nematic phase revealed aggregation phenomena which might simulate those observed in natural and synthetic membranes. After dissolving alamethicin in aqueous alcohol there is a time-dependence of the ellipticity of the Cotton effects showing a sort of memory effect on the mode of dissolution. Four different conformations can be characterized by CD spectra depending on the solvent and concentration. A model illustrating the dynamic conformations and aggregation phenomena within a membrane is proposed.

Paired moving charge model of energy coupling. III. Intrinsic ionophores in energy coupling systems

The experimental basis for the postulated role of intrinsic ionophores in mitochondrial ion transport and energy coupling is summarized. Intrinsic ionophores appear to be linked to, or contained within, specific ionophoroproteins localized in the inner membrane, and the isolation of these ionophores requires their release from the ionophoroproteins. At least ten different species of ionophores have been isolated from the mitochondrion, five of which have been wholly or in part chemically identified. Intrinsic ionophores have been implicated in the activation of inorganic phosphate in ATP synthesis and hydrolysis, and in the contol of the coupling modes. The presence of ionophores in soluble proteins such as troponin and in ATP-energized kinases has been demonstrated.