The action of certain antibiotics on mitochondrial, erythrocyte and artificial phospholipid membranes. The role of induced proton permeability

Selectivity of cation transport across lipid membranes by the antibiotic salinomycin

The ionophoric antibiotic salinomycin is in the phase of preclinical tests against several types of malignant tumors including breast cancer. Notwithstanding, the data on its ion selectivity, although being critical for its therapeutic activity, are rather scarce. In the present work, we studied the ability of salinomycin to exert cation/H+-exchange across artificial bilayer lipid membranes (BLM) by measuring electrical potential on planar BLM in the presence of a protonophore and fluorescence responses of the pH-sensitive dye pyranine entrapped in liposomes. The following order of ion selectivity was obtained by these two methods: K+ > Na+ > Rb+ > Cs+ > Li+. Measurements of the monovalent cation-induced quenching of fluorescence of thallium ions in methanol showed that salinomycin effectively binds potassium and calcium but poorly binds sodium and lithium ions. At high concentrations, salinomycin transports Ca2+ through membranes of liposomes and mitochondria, as measured by using the calcium-sensitive dye Fluo-5 N. The data obtained can be used in the mechanistic studies of the anti-tumor activity of salinomycin and its selective cytotoxicity towards cancer stem cells.

Nigericin Boosts Anti-Tumor Immune Response via Inducing Pyroptosis in Triple-Negative Breast Cancer

Although immune checkpoint inhibitors improved the clinical outcomes of advanced triple negative breast cancer (TBNC) patients, the response rate remains relatively low. Nigericin is an antibiotic derived from Streptomyces hydrophobicus. We found that nigericin caused cell death in TNBC cell lines MDA-MB-231 and 4T1 by inducing concurrent pyroptosis and apoptosis. As nigericin facilitated cellular potassium efflux, we discovered that it caused mitochondrial dysfunction, leading to mitochondrial ROS production, as well as activation of Caspase-1/GSDMD-mediated pyroptosis and Caspase-3-mediated apoptosis in TNBC cells. Notably, nigericin-induced pyroptosis could amplify the anti-tumor immune response by enhancing the infiltration and anti-tumor effect of CD4+ and CD8+ T cells. Moreover, nigericin showed a synergistic therapeutic effect when combined with anti-PD-1 antibody in TNBC treatment. Our study reveals that nigericin may be a promising anti-tumor agent, especially in combination with immune checkpoint inhibitors for advanced TNBC treatment.

Boosting intracellular sodium selectively kills hepatocarcinoma cells and induces hepatocellular carcinoma tumor shrinkage in mice

Pharmacological treatments for advanced hepatocellular carcinoma (HCC) have a partial efficacy. Augmented Na⁺ content and water retention are observed in human cancers and offer unexplored targets for anticancer therapies. Na⁺ levels are evaluated upon treatments with the antibiotic cation ionophore Monensin by fluorimetry, ICP-MS, ²³Na-MRI, NMR relaxometry, confocal or time-lapse analysis related to energy production, water fluxes and cell death, employing both murine and human HCC cell lines, primary murine hepatocytes, or HCC allografts in NSG mice. Na⁺ levels of HCC cells and tissue are 8-10 times higher than that of healthy hepatocytes and livers. Monensin further increases Na⁺ levels in HCC cells and in HCC allografts but not in primary hepatocytes and in normal hepatic and extrahepatic tissue. The Na⁺ increase is associated with energy depletion, mitochondrial Na⁺ load and inhibition of O₂ consumption. The Na⁺ increase causes an enhancement of the intracellular water lifetime and death of HCC cells, and a regression and necrosis of allograft tumors, without affecting the proliferating activity of either HCCs or healthy tissues. These observations indicate that HCC cells are, unlike healthy cells, energetically incapable of compensating and surviving a pharmacologically induced Na⁺ load, highlighting Na⁺ homeostasis as druggable target for HCC therapy.

Ester-stabilized phosphorus ylides as protonophores on bilayer lipid membranes, mitochondria and chloroplasts

Triphenylphosphonium ylides are commonly used as key intermediates in the Wittig reaction. Based on the known acidities of stabilized ylide precursors, we proposed that a methylene group adjacent to phosphorus in these compounds can ensure proton shuttling across lipid membranes. Here, we synthesized (decyloxycarbonylmethyl)triphenylphosphonium bromide (CMTPP-C₁₀) by reaction of triphenylphosphine with decyl bromoacetate. This phosphonium salt precursor of the ester-stabilized phosphorus ylide along with its octyl (CMTPP-C₈) and dodecyl (CMTPP-C₁₂) analogues was found to be a carrier of protons in mitochondrial, chloroplast and artificial lipid membranes, suggesting that it can reversibly release hydrogen ions and diffuse through the membranes in both zwitterionic (ylide) and cationic forms. The CMTPP-C₁₀-mediated electrical current across planar bilayer lipid membranes exhibited pronounced proton selectivity. Similar to conventional protonophores, known to uncouple electron transport and ATP synthesis, CMTPP-C_n (n = 8, 10, 12) stimulated mitochondrial respiration, while decreasing membrane potential, at micromolar concentrations, thereby showing the classical uncoupling activity in mitochondria. CMTPP-C₁₂ also caused dissipation of transmembrane pH gradient on chloroplast membranes. Importantly, CMTPP-C₁₀ exhibited substantially lower toxicity in cell culture, than C₁₂TPP. Thus, we report the finding of a new class of ylide-type protonophores, which is of substantial interest due to promising therapeutic properties of uncouplers.

Alkyl esters of 7-hydroxycoumarin-3-carboxylic acid as potent tissue-specific uncouplers of oxidative phosphorylation: Involvement of ATP/ADP translocase in mitochondrial uncoupling

An impressive body of evidence has been accumulated now on sound beneficial effects of mitochondrial uncouplers in struggling with the most dangerous pathologies such as cancer, infective diseases, neurodegeneration and obesity. To increase their efficacy while gaining further insight in the mechanism of the uncoupling action has been remaining a challenge. Encouraged by our previous promising results on lipophilic derivatives of 7-hydroxycoumarin-4-acetic acid (UB-4 esters), here, we use a 7-hydroxycoumarin-3-carboxylic acid scaffold to synthesize a new series of 7-hydroxycoumarin (umbelliferone, UB)-derived uncouplers of oxidative phosphorylation - alkyl esters of umbelliferone-3-carboxylic acid (UB-3 esters) with varying carbon chain length. Compared to the UB-4 derivatives, UB-3 esters proved to be stronger uncouplers: the most effective of them caused a pronounced increase in the respiration rate of isolated rat heart mitochondria (RHM) at submicromolar concentrations. Both of these series of UB derivatives exhibited a striking difference between their uncoupling patterns in mitochondria isolated from liver and heart or kidney, namely: a pronounced but transient decrease in membrane potential, followed by its recovery, was observed after the addition of these compounds to isolated rat liver mitochondria (RLM), while the depolarization of RHM and rat kidney mitochondria (RKM) was rather stable under the same conditions. Interestingly, partial reversal of this depolarization in RHM and RKM was caused by carboxyatractyloside, an inhibitor of ATP/ADP translocase, thereby pointing to the involvement of this mitochondrial membrane protein in the uncoupling activity of both UB-3 and UB-4 esters. The fast membrane potential recovery in RLM uncoupled by the addition of the UB esters was apparently associated with hydrolysis of these compounds, catalyzed by mitochondrial aldehyde dehydrogenase (ALDH2), being in high abundance in liver compared to other tissues. Protonophoric properties of the UB derivatives in isolated mitochondria were confirmed by measurements of RHM swelling in the presence of potassium acetate. In model bilayer lipid membranes (BLM), proton-carrying activity of UB-3 esters was demonstrated by measuring fluorescence response of the pH-dependent dye pyranine in liposomes. Electrophysiological experiments on identified neurons from Lymnaea stagnalis demonstrated low neurotoxicity of UB-3 esters. Resazurin-based cell viability assay showed low toxicity of UB-3 esters to HEK293 cells and primary human fibroblasts. Thus, the present results enable us to consider UB-3 esters as effective tissue-specific protonophoric mitochondrial uncouplers.

Mitochondrial Side Effects of Surgical Prophylactic Antibiotics Ceftriaxone and Rifaximin Lead to Bowel Mucosal Damage

Despite their clinical effectiveness, a growing body of evidence has shown that many classes of antibiotics lead to mitochondrial dysfunction. Ceftriaxone and Rifaximin are first choice perioperative antibiotics in gastrointestinal surgery targeting fundamental processes of intestinal bacteria; however, may also have negative consequences for the host cells. In this study, we investigated their direct effect on mitochondrial functions in vitro, together with their impact on ileum, colon and liver tissue. Additionally, their impact on the gastrointestinal microbiome was studied in vivo, in a rat model. Rifaximin significantly impaired the oxidative phosphorylation capacity (OxPhos) and leak respiration in the ileal mucosa, in line with increased oxidative tissue damage and histological changes following treatment. Ceftriaxone prophylaxis led to similar changes in the colon mucosa. The composition and diversity of bacterial communities differed extensively in response to antibiotic pre-treatment. However, the relative abundances of the toxin producing species were not increased. We have confirmed the harmful effects of prophylactic doses of Rifaximin and Ceftriaxone on the intestinal mucosa and that these effects were related to the mitochondrial dysfunction. These experiments raise awareness of mitochondrial side effects of these antibiotics that may be of clinical importance when evaluating their adverse effects on bowel mucosa.

Trialkyl(vinyl)phosphonium Chlorophenol Derivatives as Potent Mitochondrial Uncouplers and Antibacterial Agents

Trialkyl phosphonium derivatives of vinyl-substituted p-chlorophenol were synthesized here by a recently developed method of preparing quaternary phosphonium salts from phosphine oxides using Grignard reagents. All the derivatives with a number (n) of carbon atoms in phosphonium alkyl substituents varying from 4 to 7 showed pronounced uncoupling activity in isolated rat liver mitochondria at micromolar concentrations, with a tripentyl derivative being the most effective both in accelerating respiration and causing membrane potential collapse, as well as in provoking mitochondrial swelling in a potassium-acetate medium. Remarkably, the trialkyl phosphonium derivatives with n from 4 to 7 also proved to be rather potent antibacterial agents. Methylation of the chlorophenol hydroxyl group suppressed the effects of P₅₅₅ and P₄₄₄ on the respiration and membrane potential of mitochondria but not those of P₆₆₆, thereby suggesting a mechanistic difference in the mitochondrial uncoupling by these derivatives, which was predominantly protonophoric (carrier-like) in the case of P₅₅₅ and P₄₄₄ but detergent-like with P₆₆₆. The latter was confirmed by the carboxyfluorescein leakage assay on model liposomal membranes.

Transmembrane Transport of Bicarbonate Unravelled*

Anion receptors can be used to transport ions across lipid bilayers, which has potential for therapeutic applications. Synthetic bicarbonate transporters are of particular interest, as defects in transmembrane transport of bicarbonate are associated with various diseases. However, no convenient method exists to directly observe bicarbonate transport and study the mechanisms involved. Here, an assay is presented that allows the kinetics of bicarbonate transport into liposomes to be monitored directly and with great sensitivity. The assay utilises an encapsulated europium(III) complex, which exhibits a large increase in emission intensity upon binding bicarbonate. Mechanisms involving CO2 diffusion and the dissipation of a pH gradient are shown to be able to lead to an increase in bicarbonate concentration within liposomes, without transport of the anion occurring at all. By distinguishing these alternative mechanisms from actual bicarbonate transport, this assay will inform the future development of bicarbonate transporters.

Anomalous potentials on bilayer lipid membranes in the presence of usnic acid: Markin-Sokolov versus Nernst-Donnan equilibrium

To gain insight into the mechanisms of ionophoric activity of usnic acid (UA), we examined the UA-induced generation of potentials on a planar bilayer lipid membrane (BLM) in the presence of concentration gradients of hydrogen and magnesium or calcium ions under open-circuit conditions. Remarkably, the BLM potential generated by UA at the proton concentration gradient of 1 pH unit was approximately twice the Nernst equilibrium level. With a concentration gradient of magnesium or calcium ions, the BLM potential generated by UA had the opposite sign. The observed anomalies in the membrane potentials were consistent with a theory developed by Markin and Sokolov (Bioelectrochem. Bioenerg. 1990) for the case of ionophore-mediated coupled fluxes of several ions across a membrane.

Monitoring the heterogeneity in single cell responses to drugs using electrochemical impedance and electrochemical noise

Impedance spectroscopy is a widely used technique for monitoring cell-surface interactions and morphological changes, typically based on averaged signals from thousands of cells. However, acquiring impedance data at the single cell level, can potentially reveal cell-to-cell heterogeneity for example in response to chemotherapeutic agents such as doxorubicin. Here, we present a generic platform where light is used to define and localize the electroactive area, thus enabling the impedance measurements for selected single cells. We firstly tested the platform to assess phenotypic changes in breast cancer cells, at the single cell level, using the change in the cell impedance. We next show that changes in electrochemical noise reflects instantaneous responses of the cells to drugs, prior to any phenotypical changes. We used doxorubicin and monensin as model drugs and found that both drug influx and efflux events affect the impedance noise signals. Finally, we show how the electrochemical noise signal can be combined with fluorescence microscopy, to show that the noise provides information on cell susceptibility and resistance to drugs at the single cell level. Together the combination of electrochemical impedance and electrochemical noise with fluorescence microscopy provides a unique approach to understanding the heterogeneity in the response of single cells to stimuli where there is not phenotypic change.

Factors governing the competition between group IA and IB cations for monensin A: a DFT/PCM study

The results obtained suggest that the metal selectivity of monensin can be modulated by changing the solvents used.

Chemiosmotic energy for primitive cellular life: Proton gradients are generated across lipid membranes by redox reactions coupled to meteoritic quinones

Transmembrane proton gradients coupled to, and maintained by, electron transport are ubiquitous sources of chemiosmotic energy in all life today, but how this system first emerged is uncertain. Here we report a model liposome system in which internal ferricyanide serves as an oxidant and external ascorbate or dithionite provide a source of electrons to electron carriers embedded in liposome membranes. Quinones linked the donor to the acceptor in a coupled redox reaction that released protons into the vesicle internal volume as electrons were transported across the membranes, thereby producing substantial pH gradients. Using this system, we found that one or more quinones in extracts from carbonaceous meteorites could serve as coupling agents and that substantial pH gradients developed in the acidic interior of liposomes. If amphiphilic compounds present on the prebiotic Earth assembled into membranous compartments that separate reduced solutes in the external medium from an encapsulated acceptor, quinones can mediate electron and proton transport across the membranes, thereby providing a source of chemiosmotic energy for primitive metabolic reactions.

Short-chain diamines are the physiological substrates of PACE family efflux pumps

Acinetobacter baumannii has rapidly emerged as a major cause of gram-negative hospital infections worldwide. A. baumannii encodes for the transport protein AceI, which confers resistance to chlorhexidine, a widely used antiseptic. AceI is also the prototype for the recently discovered proteobacterial antimicrobial compound efflux (PACE) family of transport proteins that confer resistance to a range of antibiotics and antiseptics in many gram-negative bacteria, including pathogens. The gene encoding AceI is conserved in the core genome of A. baumannii, suggesting that it has an important primordial function. This is incongruous with the sole characterized substrate of AceI, chlorhexidine, an entirely synthetic biocide produced only during the last century. Here we investigated a potential primordial function of AceI and other members of the PACE family in the transport of naturally occurring polyamines. Polyamines are abundant in living cells, where they have physiologically important functions and play multifaceted roles in bacterial infection. Gene expression studies revealed that the aceI gene is induced in A. baumannii by the short-chain diamines cadaverine and putrescine. Membrane transport experiments conducted in whole cells of A. baumannii and Escherichia coli and also in proteoliposomes showed that AceI mediates the efflux of these short-chain diamines when energized by an electrochemical gradient. Assays conducted using 8 additional diverse PACE family proteins identified 3 that also catalyze cadaverine transport. Taken together, these results demonstrate that short-chain diamines are common substrates for the PACE family of transport proteins, adding to their broad significance as a novel family of efflux pumps.

Membrane potential and delta pH dependency of reverse electron transport-associated hydrogen peroxide production in brain and heart mitochondria

Succinate-driven reverse electron transport (RET) is one of the main sources of mitochondrial reactive oxygen species (mtROS) in ischemia-reperfusion injury. RET is dependent on mitochondrial membrane potential (Δψ_m) and transmembrane pH difference (ΔpH), components of the proton motive force (pmf); a decrease in Δψ_m and/or ΔpH inhibits RET. In this study we aimed to determine which component of the pmf displays the more dominant effect on RET-provoked ROS generation in isolated guinea pig brain and heart mitochondria respiring on succinate or α-glycerophosphate (α-GP). Δψ_m was detected via safranin fluorescence and a TPP⁺ electrode, the rate of H₂O₂ formation was measured by Amplex UltraRed, the intramitochondrial pH (pH_in) was assessed via BCECF fluorescence. Ionophores were used to dissect the effects of the two components of pmf. The K⁺/H⁺ exchanger, nigericin lowered pH_in and ΔpH, followed by a compensatory increase in Δψ_m that led to an augmented H₂O₂ production. Valinomycin, a K⁺ ionophore, at low [K⁺] increased ΔpH and pH_in, decreased Δψ_m, which resulted in a decline in H₂O₂ formation. It was concluded that Δψ_m is dominant over ∆pH in modulating the succinate- and α-GP-evoked RET. The elevation of extramitochondrial pH was accompanied by an enhanced H₂O₂ release and a decreased ∆pH. This phenomenon reveals that from the pH component not ∆pH, but rather absolute value of pH has higher impact on the rate of mtROS formation. Minor decrease of Δψ_m might be applied as a therapeutic strategy to attenuate RET-driven ROS generation in ischemia-reperfusion injury.

Antibiotics May Trigger Mitochondrial Dysfunction Inducing Psychiatric Disorders

Clinical usage of several classes of antibiotics is associated with moderate to severe side effects due to the promotion of mitochondrial dysfunction. We contend that this may be due to perturbation of unique evolutionary relationships that link selective biochemical and molecular aspects of mitochondrial biology to conserved enzymatic processes derived from bacterial progenitors. Operationally, stereo-selective conformational matching between mitochondrial respiratory complexes, cytosolic and nuclear signaling complexes appears to support the conservation of a critically important set of chemical messengers required for existential regulation of homeostatic cellular processes. Accordingly, perturbation of normative mitochondrial function by select classes of antibiotics is certainly reflective of the high degree of evolutionary pressure designed to maintain ongoing bidirectional signaling processes between cellular compartments. These issues are of critical importance in evaluating potentially severe side effects of antibiotics on complex behavioral functions mediated by CNS neuronal groups. The CNS is extremely dependent on delivery of molecular oxygen for maintaining a required level of metabolic activity, as reflected by the high concentration of neuronal mitochondria. Thus, it is not surprising to find several distinct behavioral abnormalities conforming to established psychiatric criteria that are associated with antibiotic usage in humans. The manifestation of acute and/or chronic psychiatric conditions following antibiotic usage may provide unique insights into key etiological factors of major psychiatric syndromes that involve rundown of cellular bioenergetics via mitochondrial dysfunction. Thus, a potential window of opportunity exists for development of novel therapeutic agents targeting diminished mitochondrial function as a factor in severe behavioral disorders.

The antimicrobial activity of gramicidin A is associated with hydroxyl radical formation

Gramicidin A is an antimicrobial peptide that destroys gram-positive bacteria. The bactericidal mechanism of antimicrobial peptides has been linked to membrane permeation and metabolism disruption as well as interruption of DNA and protein functions. However, the exact bacterial killing mechanism of gramicidin A is not clearly understood. In the present study, we examined the antimicrobial activity of gramicidin A on Staphylococcus aureus using biochemical and biophysical methods, including hydroxyl radical and NAD+/NADH cycling assays, atomic force microscopy, and Fourier transform infrared spectroscopy. Gramicidin A induced membrane permeabilization and changed the composition of the membrane. The morphology of Staphylococcus aureus during gramicidin A destruction was divided into four stages: pore formation, water permeability, bacterial flattening, and lysis. Changes in membrane composition included the destruction of membrane lipids, proteins, and carbohydrates. Most interestingly, we demonstrated that gramicidin A not only caused membrane permeabilization but also induced the formation of hydroxyl radicals, which are a possible end product of the transient depletion of NADH from the tricarboxylic acid cycle. The latter may be the main cause of complete Staphylococcus aureus killing. This new finding may provide insight into the underlying bactericidal mechanism of gA.

Real-time attack of LL-37 on single Bacillus subtilis cells

Time-lapse fluorescence microscopy of single, growing Bacillus subtilis cells with 2-12s time resolution reveals the mechanisms of antimicrobial peptide (AMP) action on a Gram-positive species with unprecedented detail. For the human cathelicidin LL-37 attacking B. subtilis, the symptoms of antimicrobial stress differ dramatically depending on the bulk AMP concentration. At 2μM LL-37, the mean single-cell growth rate decreases, but membrane permeabilization does not occur. At 4μM LL-37, cells abruptly shrink in size at the same time that Sytox Green enters the cytoplasm and stains the nucleoids. We interpret the shrinkage event as loss of turgor pressure (and presumably the membrane potential) due to permeabilization of the membrane. Movies of Sytox Green staining at 0.5frame/s show that nucleoid staining is initially local, more consistent with pore formation than with global permeabilization models. In a novel "growth recovery" assay, cells are incubated with LL-37 for a variable period and then rinsed with fresh growth medium lacking LL-37. The growth rate attenuation observed at 2μM LL-37 is a recoverable symptom, while the abrupt cell shrinkage observed at 4μM LL-37 is not.

Poly(aniline) nanowires in sol-gel coated ITO: a pH-responsive substrate for planar supported lipid bilayers

Facilitated ion transport across an artificial lipid bilayer coupled to a solid substrate is a function common to several types of bioelectronic devices based on supported membranes, including biomimetic fuel cells and ion channel biosensors. Described here is fabrication of a pH-sensitive transducer composed of a porous sol-gel layer derivatized with poly(aniline) (PANI) nanowires grown from an underlying planar indium-tin oxide (ITO) electrode. The upper sol-gel surface is hydrophilic, smooth, and compatible with deposition of a planar supported lipid bilayer (PSLB) formed via vesicle fusion. Conducting tip AFM was used to show that the PANI wires are connected to the ITO, which convert this electrode into a potentiometric pH sensor. The response to changes in the pH of the buffer contacting the PANI nanowire/sol-gel/ITO electrode is blocked by the very low ion permeability of the overlying fluid PSLB. The feasibility of using this assembly to monitor facilitated proton transport across the PSLB was demonstrated by doping the membrane with lipophilic ionophores that respond to a transmembrane pH gradient, which produced an apparent proton permeability several orders of magnitude greater than values measured for undoped lipid bilayers.

Ethambutol-induced optic neuropathy linked to OPA1 mutation and mitochondrial toxicity

Ethambutol (EMB), widely used in the treatment of tuberculosis, has been reported to cause Leber's hereditary optic neuropathy in patients carrying mitochondrial DNA mutations. We study the effect of EMB on mitochondrial metabolism in fibroblasts from controls and from a man carrying an OPA1 mutation, in whom the drug induced the development of autosomal dominant optic atrophy (ADOA). EMB produced a mitochondrial coupling defect together with a 25% reduction in complex IV activity. EMB induced the formation of vacuoles associated with decreased mitochondrial membrane potential and increased fragmentation of the mitochondrial network. Mitochondrial genetic variations may therefore be predisposing factors in EMB-induced ocular injury.

Differences in nitric oxide steady states between arginine, hypoxanthine, uracil auxotrophs (AHU) and non-AHU strains of Neisseria gonorrhoeae during anaerobic respiration in the presence of nitrite

Neisseria gonorrhoeae can grow by anaerobic respiration using nitrite as an alternative electron acceptor. Under these growth conditions, N. gonorrhoeae produces and degrades nitric oxide (NO), an important host defense molecule. Laboratory strain F62 has been shown to establish and maintain a NO steady-state level that is a function of the nitrite reductase/NO reductase ratio and is independent of cell number. The nitrite reductase activities (122-197 nmol NO2 reduced x min(-1) x OD600(-1)) and NO reductase activities (88-155 nmol NO reduced x min(-1) x OD600(-1)) in a variety of gonococcal clinical isolates were similar to the specific activities seen in F62 (241 nmol NO2 reduced x min(-1) x OD600(-1) and 88 nmol NO reduced x min(-1) x OD600(-1), respectively). In seven gonococcal strains, the NO steady-state levels established in the presence of nitrite were similar to that of F62 (801-2121 nmol x L-1 NO), while six of the strains, identified as arginine, hypoxanthine, and uracil auxotrophs (AHU), that cause asymptomatic infection in men had either two- to threefold (373-579 nmol x L-1 NO) or about 100-fold (13-24 nmol x L-1 NO) lower NO steady-state concentrations. All tested strains in the presence of a NO donor, 2,2'-(hydroxynitrosohydrazono)bis-ethanimine/NO, quickly lowered and maintained NO levels in the noninflammatory range of NO (<300 nmol x L-1). The generation of a NO steady-state concentration was directly affected by alterations in respiratory control in both F62 and an AHU strain, although differences in membrane function are suspected to be responsible for NO steady-state level differences in AHU strains.

Toxoplasma gondii: induction of egress by the potassium ionophore nigericin

The obligate intracellular parasite Toxoplasma gondii is an important pathogen of humans and animals. Some of the devastating consequences of toxoplasmosis are in part due to the lysis of the host cell during parasite egress. The process of egress is poorly understood and since it is asynchronous in tissue culture its study has been limited to those conditions that induce it, such as artificial permeabilisation of the host cell and induction of calcium fluxes with ionophores. Given that permeabilisation leads to egress by the activation of motility upon a drop in host cell potassium concentration, we investigated whether the ionophore nigericin, which selectively causes efflux of potassium from the cell without the need for permeabilisation, would cause egress. Nigericin effectively causes intracellular parasites to exit their host cell within 30 min of treatment with the drug. Our results show that nigericin-induced egress depends on an efflux of potassium from the cell and requires phospholipase C function and parasite motility. This novel method of inducing and synchronising egress mimics the effect of artificial permeabilisation in all respects. Nevertheless, since the membrane remains intact during the treatment, in our nigericin-induced egress we are able to detect parasite-dependent permeabilisation of the host cell, a known step in induced egress. In addition, consistent with the model that loss of host cell potassium leads to egress through the activation of intraparasitic calcium fluxes, a previously isolated Toxoplasma mutant lacking a sodium hydrogen exchanger and defective in responding to calcium fluxes does not undergo nigericin-induced egress. Thus, the discovery that nigericin induces egress presents a novel assay that allows for the genetic and biochemical analysis of the signalling mechanisms that lead to the induction of motility and egress.

Monensin improves the effectiveness of meso-dimercaptosuccinate when used to treat lead intoxication in rats

Among divalent cations, the ionophore monensin shows high activity and selectivity for the transport of lead ions (Pb2+) across phospholipid membranes. When coadministered to rats that were receiving meso-dimercaptosuccinate for treatment of Pb intoxication, monensin significantly increased the amount of Pb removed from femur, brain, and heart. It showed a tendency to increase Pb removal from liver and kidney but had no effect of this type in skeletal muscle. Tissue levels of several physiologic (calcium, cobalt, copper, iron, magnesium, manganese, molybdenum, zinc) and nonphysiologic (arsenic, cadmium, chromium, nickel, strontium) elements were also determined after the application of these compounds. Among the physiologic elements, a number of significant changes were seen, including both rising and falling values. The size of these changes was typically around 20% compared with control values, with the largest examples seen in femur. These changes often tended to reverse those of similar size that had occurred during Pb administration. Among the nonphysiologic elements, which were present in trace amounts, the changes were smaller in number but larger in size. None of these changes appears likely to be significant in terms of toxicity, and there were no signs of overt toxicity under any of the conditions employed. Monensin may act by cotransporting Pb2+ and OH- ions out of cells, in exchange for external sodium ions. The net effect would be to shuttle intracellular Pb2+ to extracellular dimercaptosuccinic acid thereby enhancing its effectiveness. Thus, monensin may be useful for the treatment of Pb intoxication when applied in combination with hydrophilic Pb2+ chelators.

Effect of phloretin on ionophore mediated electroneutral transmembrane translocations of H(+), K(+) and Na(+) in phospholipid vesicles

Rates of M(+)/H(+) exchange (M(+)=K(+), Na(+)) across phospholipid membranes by ionophore mediated electroneutral translocations and transports through channels could either increase or decrease or change negligibly on adding the polar molecule phloretin to the membrane. The changes depend on pH, the concentration and choice of M(+) and choice of ionophore/channel. Such diverse behaviours have been inferred from studies on the decay of the pH difference across soybean phospholipid vesicular membrane (=Delta pH). The transporters used in this study are (a) the exchange ionophores: nigericin, monensin; (b) combinations of alkali metal ion carriers, valinomycin or nonactin with weak acids carbonyl cyanide m-chlorophenylhydrazone or 2,4-dinitrophenol and (c) channels formed by gramicidin A. All the diverse results can be rationally explained if we take note of the following. (i) The rate limiting steps are associated with the transmembrane translocations involving the rate limiting species identified in the literature. (ii) Phloretin in the membrane decreases the apparent M(+) dissociation constant, K(M), of the M(+) bound ionophores/channels which has the effect of increasing the concentration of these species. (iii) The concentrations of H(+) bound ionophores/channels decrease on adding phloretin. (iv) Phloretin inhibits ternary complex formation (involving valinomycin or nonactin, M(+) and an anion) by forming 1:2 complexes with valinomycin-M(+) or nonactin-M(+). (v) On adding 6-ketocholestanol to the membrane (instead of phloretin) K(M) increases. The decreases/increases in K(M) mentioned above are consistent with the consequences of a hypothesis in which phloretin decreases and 6-ketocholestanol increases the positive internal membrane dipole potential.

Hydrogen ion gradients across the mitochondrial, endosomal and plasma membranes in bloodstream forms of trypanosoma brucei solving the three-compartment problem

Conditions for the use of both [14C]methylamine and 5, 5-dimethyl[14C]oxa-azolidine-2,4-dione (DMO) to measure the H+ concentration of intracellular compartments of monomorphic long thin bloodstream forms of Trypanosoma brucei were established. Neither probe was actively transported or bound to internal components of the cell and both probes equilibrated passively with a t1/2 close to 8 min. DMO was excluded from cells, while methylamine was accumulated but not metabolized. Solution of the three-compartment problem revealed that, when cells were respiring aerobically on glucose at an external pH of 7.5, the cytoplasmic pH was in the range 6.99-7.03, the pH of the mitochondrial matrix was 7.71-7.73, and the algebraic average pH of the various endosomal compartments was 5.19-5.50. Similar values were found when cells were respiring aerobically on glycerol. However, bloodstream forms of T. brucei could not maintain a constant internal H+ concentration outside the external pH range 7.0-7.5, and no evidence for the presence of an H+/Na+ exchanger was found. Full motility and levels of pyruvate production were maintained as the external pH was raised as high as 9.5, suggesting that these cells tolerate significant internal alkalinisation. However, both motility and pyruvate production were severely inhibited under acidic conditions, and the cells deteriorated rapidly below an external pH of 6.5. Physiologically, the plasma membrane of T. brucei had low permeability to H+ and the internal pH was unaffected by changes in Deltapsip, which is dominated by the potassium diffusion potential. However, in the presence of FCCP, the internal pH fell rapidly about 0.5 pH unit and came into equilibrium with Deltapsip. Oligomycin abolished the mitochondrial pH gradient (DeltapHm) selectively, whereas chloroquine abolished only the endosomal pH gradient (DeltapHe). The pH gradient across the plasma membrane (DeltapHp) alone could be abolished by careful osmotic swelling of cells. The plasma membrane had an inwardly directed proton-motive force (DeltaPp) of -52 mV and an inwardly directed sodium-motive force (DeltaNp) of -149 mV, whereas the mitochondrial inner membrane had only an inwardly directed DeltaPm of -195 mV. The pH gradient across the endosomal membranes was not accompanied by an electrical gradient. Consequently, endosomal membranes had an algebraically average outwardly directed DeltaPl within the range + 89 to +110 mV, depending on the measurement method.

Theoretical calculations of the permeability of monensin-cation complexes in model bio-membranes

Monensin is one of the best-characterized ionophores; it functions in the electroneutral exchange of cations between the extracellular and cytoplasmic sides of cell membranes. The X-ray crystal structures of monensin in free acid form and in complex with Na(+), K(+) and Ag(+) are known and we have recently measured the diffusion rates of monensin in free acid form (Mo-H) and in complex with Na(+) (Mo-Na) and with K(+) (Mo-K) using laser pulse techniques. The results have shown that Mo-H diffuses across the membrane one order of magnitude faster than Mo-Na and two orders of magnitude faster than Mo-K. Here, we report calculations of the translocation free energy of these complexes across the membrane along the most favorable path, i.e. the lowest free energy path. The calculations show that the most favorable orientation of monensin is with its hydrophobic furanyl and pyranyl moieties in the hydrocarbon region of the membrane and the carboxyl group and the cation at the water-membrane interface. Further, the calculations show that Mo-H is likely to be inserted deeper than Mo-Na into the bilayer, and that the free energy barrier for transfer of Mo-H across the membrane is approximately 1 kcal/mol lower than for Mo-Na, in good agreement with our measurements. Our results show that the Mo-K complex is unlikely to diffuse across lipid bilayers in its X-ray crystal structure, in contrast to the Mo-H and Mo-Na complexes. Apparently, when diffusing across the membrane, the Mo-K complex assumes a different conformation and/or thinning defects in the bilayer lower significantly the free energy barrier for the process. The suitability of the model for treating the membrane association of small molecules is discussed in view of the successes and failures observed for the monensin system.

Effects of neutral ionophores on membrane electrical characteristics of NG108-15 cells

The effects of several K(+)-selective neutral ionophores on membrane electrical characteristics of differentiated NG108-15 (neuroblastoma X glioma hybrid) cells were examined. Specifically, alterations in membrane resting potential (V(m)), input resistance (R(in)) and electrically-induced action potential generation were determined upon bath application of enniatin (0.1-10 microg/ml), nonactin (0. 1-10 microM) and valinomycin (0.1-10 microM). Although some cells exhibited a slight hyperpolarization and/or reduced R(in), i.e. membrane electrical correlates of enhanced K(+) loss, neither V(m) nor R(in) were significantly altered by any of the ionophores. However, valinomycin and especially nonactin affected action potentials induced by electrical stimulation. This was apparent in the ablation of action potentials in some cells and in the occurrence of degenerative changes in action potential shape in others. The simultaneous administration of the neutral ionophores and the protonophore CCCP or the superfusion of enniatin, nonactin or valinomycin in high (50 mM) glucose-containing physiological solution did not yield more extensive alterations in V(m) or R(in). These data suggest that the neutral ionophores are unable to materially enhance K(+) flux above the relatively high resting level in NG108-15 cells. Thus, alterations in action potentials appear to be unrelated to K(+) transport activity.

The mechanism of channel formation by alamethicin as viewed by molecular dynamics simulations

Alamethicin is a 20-residue channel-forming peptide that forms a stable amphipathic alpha-helix in membrane and membrane-mimetic environments. This helix contains a kink induced by a central Gly-X-X-Pro sequence motif. Alamethicin channels are activated by a cis positive transbilayer voltage. Channel activation is suggested to correspond to voltage-induced insertion of alamethicin helices in the bilayer. Alamethicin forms multi-conductance channels in lipid bilayers. These channels are formed by parallel bundles of transmembrane helices surrounding a central pore. A change in the number of helices per bundle switches the single channel conductance level. Molecular dynamics simulations of alamethicin in a number of different environments have been used to explore its channel-forming properties. These simulations include: (i) alamethicin in solution in water and in methanol; (ii) a single alamethicin helix at the surface of a phosphatidylcholine bilayer; (iii) single alamethicin helices spanning a phosphatidylcholine bilayer; and (iv) channels formed by bundles of 5, 6, 7 or 8 alamethicin helices spanning a phosphatidylcholine bilayer. The total simulation time is c. 30 ns. Thus, these simulations provide a set of dynamic snapshots of a possible mechanism of channel formation by this peptide.

Evidence for dimer participation and evidence against channel mechanism in A23187-mediated monovalent metal ion transport across phospholipid vesicular membrane

The decay of the pH difference (DeltapH) across soybean phospholipid vesicular membrane by ionophore A23187 (CAL)-mediated H+/M+ exchange (M+ = Li+, Na+, K+, and Cs+) has been studied in the pH range 6-7.6. The DeltapH in these experiments were created by temperature jump. The observed dependence of DeltapH relaxation rate 1/tau on the concentration of CAL, pH, and the choice of M+ in vesicle solutions lead to the following conclusions. 1) The concentrations of dimers and other oligomers of A23187 in the membrane are small compared to the total concentration of A23187 in the membrane, similar to that in chloroform solutions reported in the literature. 2) In the H+ transport cycle leading to DeltapH decay, the A23187-mediated H+ translocation across the membrane is a fast step, and the rate-limiting step is the A23187-mediated M+ translocation. 3) Even though the monomeric Cal-H is the dominant species translocating H+, Cal-M is not the dominant species translocating M+ (even at concentrations higher than [Cal-H]), presumably because its dissociation rate is much higher than its translocation rate. 4) The pH dependence of 1/tau shows that the dimeric species Cal2LiLi, Cal2NaNa, Cal2KH, and Cal2CsH are the dominant species translocating M+. The rate constant associated with their translocation has been estimated to be approximately 5 x 10(3) s-1. With this magnitude for the rate constants, the dimer dissociation constants of these species in the membrane have been estimated to be approximately 4, 1, 0.05, and 0.04 M, respectively. 5) Contrary to the claims made in the literature, the data obtained in the DeltapH decay studies do not favor the channel mechanism for the ion transport in this system. 6) However, they support the hypothesis that the dissociation of the divalent metal ion-A23187 complex is the rate limiting step of A23187-mediated divalent metal ion transport.

Relative magnitudes of the rate constants associated with monensin-mediated H+, Na+ and K+ translocations across phospholipid vesicular membranes

Monensin (Mon)-mediated decay of the pH difference (DeltapH) across soyabean phospholipid vesicular membrane has been studied as a function of K+ and Na+ ion concentrations. In these experiments, the DeltapH was created using temperature jump, and ionic strength was regulated at 0.3 using CsCl. Rate constants associated with the translocation of Mon-H, Mon-K and Mon-Na have been estimated (without making any assumptions) from an analysis of the DeltapH decay data. These estimates contradict the claim made in the literature (E. Nachliel, Y. Finkelstein, M. Gutman, Biochim. Biophys. Acta, 1285 (1996) 131-145) that the translocation rate constants of the three above-mentioned species are significantly different. Our observations on the changes in DeltapH decay rate on adding carbonyl cyanide m-chlorophenylhydrazone (CCCP) also suggest that the dominant barrier to the DeltapH decay process is not the 'polar region' of the membrane. Therefore, the differences in the electric dipole moments of Mon-H, Mon-K and Mon-Na are unlikely to cause large differences in their translocation rate constants.

Role of metal ion free valinomycin-carbonyl cyanide m-chlorophenylhydrazone complex in the enhancement of the rates of gramicidin facilitated net H+, Li+ and Na+ transport across phospholipid vesicular membrane

The studies on the decay of the pH difference, delta pH, across soyabean phospholipid vesicular membrane have shown that the rates of net proton transport and the associated Li+ and Na+ ion transport across the membrane can be enhanced by the combined action of gramicidin, valinomycin and carbonyl cyanide m-chlorophenylhydrazone (CCCP) in K(+)-free vesicle solutions. The data obtained under different experimental conditions suggest that this enhancement is a consequence of facilitation of CCCP- transport (1) by complexing CCCP- with the highly membrane permeant valinomycin without the metal ion bound to it and (2) by the associated Li+ or Na+ transport through the gramicidin channel such that no net charge is transported across the membrane. The dissociation constant of the weak valinomycin-CCCP- complex has been estimated to be > 200 mM in the membrane. The delta pH in these experiments were created by temperature jump.

Two mechanisms of H+/OH- transport across phospholipid vesicular membrane facilitated by gramicidin A

Two rate-limiting mechanisms have been proposed to explain the gramicidin channel facilitated decay of the pH difference across vesicular membrane (delta pH) in the pH region 6-8 and salt (MCI, M+ = K+, Na+) concentration range 50-300 mM. 1) At low pH conditions (approximately 6), H+ transport through the gramicidin channel predominantly limits the delta pH decay rate. 2) At higher pH conditions (approximately 7.5), transport of a deprotonated species (but not through the channel) predominantly limits the rate. The second mechanism has been suggested to be the hydroxyl ion propogation through water chains across the bilayer by hydrogen bond exchange. In both mechanisms alkali metal ion transport providing the compensating flux takes place through the gramicidin channels. Such an identification has been made from a detailed study of the delta pH decay rate as a function of 1) gramicidin concentration, 2) alkali metal ion concentration, 3) pH, 4) temperature, and 5) changes in the membrane order (by adding small amounts of chloroform to vesicle solutions). The apparent activation energy associated with the second mechanism (approximately 3.2 kcal/mol) is smaller than that associated with the first mechanism (approximately 12 kcal/mol). In these experiments, delta pH was created by temperature jump, and vesicles were prepared using soybean phospholipid or a mixture of 94% egg phosphatidylcholine and 6% phosphatidic acid.

Increased inhibitory effect of phorbol ester on cytosolic Ca2+ level and contraction in rat myometrium after gestation

Activation of voltage-dependent Ca2+ channels by high K+ (40 mM) increased the cytosolic Ca2+ level ([Ca2+]i) (estimated by fura-PE3 fluorescence ratio) and force in myometrium isolated from pregnant (21 days after gestation) and non-pregnant (estrus) rats. 12-Deoxyphorbol 13-isobutyrate (DPB, 1 mM) decreased the high (K+)-stimulated [Ca2+]i and force in a concentration-dependent manner. The inhibitory effect was stronger in the pregnant myometrium than in the non-pregnant myometrium. In the pregnant myometrium, the increase in Ca2+ permeability by ionomycin (1 microM) greatly increased [Ca2+]i and force, which were only partially inhibited by verapamil (10 microM). DPB (1 microM) inhibited the verapamil-insensitive component of the increases in [Ca2+]i and muscle tension. Oxytocin (100 nM) and thapsigargin (1 microM) also induced a verapamil-insensitive increase in [Ca2+]i and force, and DPB (1 microM) inhibited these increments. Ca2+ sensitivity of contractile elements, estimated from the relationships between Ca2+ and muscle force in intact and alpha-toxin permeabilized muscle, was not significantly changed by DPB (1 microM). In summary, DPB inhibits the increase in [Ca2+]i more strongly in myometrium isolated from pregnant rats than that from non-pregnant rats without any change in the [Ca2+]i/tension relationship. Since DPB decreased [Ca2+]i-rise induced by three different mechanisms, DPB may activate Ca2+ extrusion, rather than to inhibit a specific influx pathway, to decrease [Ca2+]i.

Enhancement of ricin toxin A chain immunotoxin activity: synthesis, ionophoretic ability, and in vitro activity of monensin derivatives

Site-selective toxin delivery was achieved by coupling monoclonal antibody to the A chain subunit of ricin (RTA-IT). The cell-killing potency of RTA-IT can be drastically increased in vitro by using ionophores such as monensin. To reduce the intrinsic toxicity of monensin and to enhance its in vitro and in vivo activity, we synthesized 7 derivatives characterized by different lipophilicities. These derivatives were also analyzed for ionophoretic activity on intact cells, toxicity, and RTA-IT-enhancing activity. Two different RTA-IT were assayed on a human leukemia cell line. A correlation between lipophilicity, ionophoretic activity, and RTA-IT enhancement was observed. The compounds with the highest polar charge showed low intrinsic toxicity, revealed moderate ionophoretic activity, and were able to enhance RTA-IT only at high concentrations, whereas more lipophilic compounds (with a C28 tail or a phenyl group) showed significant ionophoretic activity and good enhancing properties.

Surrogate cells or Trojan horses. The discovery of liposomes

An autobiographical account of the liposome, from the perplexities of a blood smear to the growth of a multi-million pound business.

Enhancement of rates of H+, Na+ and K+ transport across phospholipid vesicular membrane by the combined action of carbonyl cyanide m-chlorophenylhydrazone and valinomycin: temperature-jump studies

Enhancement of delta pH relaxation rate by the combined action of valinomycin (VAL) and carbonyl cyanide m-chlorophenylhydrazone (CCCP) has been studied under a variety of concentration conditions in soyabean phospholipid (SBPL) vesicles after creating a pH gradient across the vesicular membrane delta pH by temperature jump. After taking note of the changes by VAL and CCCP induced membrane disorder (using nigericin and monensin mediated delta pH decay as probes) the following could be inferred about the mechanism of enhancement of delta pH decay rate: (i) in solutions containing KCl, the rate limiting species have been identified to be (a) Val-K(+)-CCCP-, at low [Val]0 and [CCCP]0 (with translocation rate constant k2 approximatley 3.2 x 10(3) s-1); (b) CCCPH, at high [Val]0 (with translocation rate constant k1 approximately 2 x 10(5) s-1); (c) the neutral valinomycin species Val, at high [CCCP]0. (ii) In solutions containing NaCl, in our concentration range, the rate limiting species are Val-Na(+)-CCCP-. (iii) The apparent dissociation constant K*M of Val-M+ decreases with pH in SBPL vesicles but is independent of pH in vesicles prepared from PC + 6% PA. (iv) The differences in the ionic strength dependencies of kinetic data shows that the environments of Na+ and K+ binding sites on VAL are different. (v) In vesicle solutions containing 100 mM MCl, the cation selectivity of VAL (towards K+ in preference to Na+) is reduced when CCCP- is already bound to it in the membrane. The CCCP- dissociation constant of Val-M(+)-CCCP- is smaller with M+ = Na+ (approximatley 0.22 mM at 100 mM NaCl) when compared to that with M+ = K+ (approximately 2 mM at 100 mM KCl). Attributing these differences to the differences in electrostatic interaction between CCCP- and M+ in Val-M(+)-CCCP-, we can say that CCCP- binds closer to the Na+ binding site than to the K+ binding site on VAL.

Effect of Na+ reduction and monensin on ion content and contractile response in normoxic and ischaemic reperfused rat hearts

The possibility was explored whether the functional properties of Na+/Ca2+ exchange are altered after ischaemia, thereby contributing to the elevated intracellular (i) Ca2+ levels in ischaemic reperfused hearts. The intracellular Na+, K+ and Ca2+ contents in rat Langendorff heart preparations were determined by atomic absorption spectrometry under normoxic conditions, after ischaemia (30 min) and after ischaemia (30 min) plus reperfusion (30 min). In addition, the influence of modulating the Na+ gradient (Na+o/Na+i) across the sarcolemma was studied with respect to cardiac contractility and intracellular ion content. This was done by either decreasing extracellular (o) Na+ or by increasing Na+i with monensin, both in normoxic and reperfused hearts. Both Na+o reduction and monensin led to an increase in contractility and coronary flow, an effect which was nearly abolished in reperfused hearts. Under normoxic conditions the intracellular ion contents amounted to Na+ = 12.4 +/- 0.4, K+ = 99.0 +/- 3.1 and Ca2+ = 0.64 +/- 0.02 mmol/kg cell (means +/- SEM, n = 7). In normoxic hearts, lowering Na+o reduced and monensin increased Na+i, thereby both leading to a decrease in Na+ gradient; no effect on total Ca2+i content was observed. Na+i increased twofold after ischaemia as compared to the normoxic situation, an effect which was aggravated (4 fold increase) in reperfused hearts. The opposite effects were observed for K+i with a 25% decrease after ischaemia and a 40% decrease in reperfused hearts. Only after ischaemia plus reperfusion was Ca2+i increased (6 fold).(ABSTRACT TRUNCATED AT 250 WORDS)

Na/K competitive transport selectivity of (221) C10-cryptand: effect of temperature

The kinetics of the competitive transport of Na+ and K+ ions across the membrane of large unilamellar vesicles (LUV) were determined when transport was induced by (221)C10-cryptand at various temperatures in order to quantify the temperature-dependence of the Na/K competitive transport selectivity of this ionizable mobile carrier. At any given temperature, the apparent affinity of (221)C10 for Na+ was higher and less dependent on the concentration of the other competing ion than that for K+. Its enthalpy for Na+ (delta H(KmNa) = 50.6 kJ/mol) was not significantly different from that for K+ (delta H(KmK) = 52.7 kJ/mol). The Na/K competitive transport selectivity (SC(Na/K)) of (221)C10 increased linearly with the Na+ concentrations and decreased hyperbolically with increasing those of K+. When the cation concentrations were equal, this competitive selectivity amounted to about 2 at any given temperature. Equations were established to describe the variations of the competitive transport selectivity (SC) of cryptands, and for comparison of their noncompetitive selectivity (SNC), with the ionic concentrations and the Michaelis parameters of the cations. It is theoretically demonstrated that the ratio between the competitive and noncompetitive transport selectivities, i.e., SC/SNC, of mobile carriers does not depend on the Jmax of the competing ions and that its value amounts to 1 when the specific concentrations (C'S/Km) of the ions are equal. Under these conditions, the transport selectivity of any given mobile carrier has the same value whether determined from competition or separated experiments. The reaction order in Na+ (n(Na)) increased significantly as the temperature rose and decreased significantly as the K+ concentration increased. The results are discussed in terms of the structural, physicochemical and electrical characteristics of carriers and complexes.