Effects of parathion on membrane organization and its implications for the mechanisms of toxicity

Chlorpyrifos enrichment enhances tolerance of Anabaena sp. PCC 7119 to dimethoate

Organophosphorus (OP) insecticides are widely used for on-field pest control, constituting about 38% of global pesticide consumption. Insecticide tolerance has been recorded in microorganisms isolated from the contaminated soil. However, the cross-tolerance of laboratory-enriched cultures remains poorly understood. A chlorpyrifos tolerant (T) strain of Anabaena sp. PCC 7119 was developed through continuous enrichment of the wild strain (W). The cross-tolerance of the T strain to the OP insecticide dimethoate was assessed by measuring photosynthetic performance, key enzyme activities and degradation potential. The presence of dimethoate led to a significant reduction in the growth and pigment content of the W strain. In contrast, the T strain demonstrated improved growth and metabolic performance. Chl a and carotenoids were degraded faster than phycobiliproteins in both strains. The T strain exhibited superior photosynthetic performance, metabolic efficiency and photosystem functions, than of W strain, at both the tested dimethoate concentrations (100 and 200 μM). The treated T strain had more or less a normal OJIP fluorescence transient and bioenergetic functions, while the W strain showed a greater fluorescence rise at ≤ 300 μs indicating the inhibition of electron donation to PS II, and at 2 ms due to reduced electron release beyond QA. The T strain had significantly higher levels of esterase and phosphatases, further enhanced by insecticide treatment. Dimethoate degradation efficiency of the T strain was significantly higher than of the W strain. T strain also removed chlorpyrifos more efficiently than W strain at both the tested concentrations. The BCFs of both chlorpyrifos and dimethoate were lower in the T strain compared to the W strain. These findings suggest that the enriched strain exhibits promising results in withstanding dimethoate toxicity and could be explored for its potential as a bioremediating organism for OP degradation.

Study the effect of insecticide dimethoate on photosynthetic pigments and photosynthetic activity of pigeon pea: Laser-induced chlorophyll fluorescence spectroscopy

Pigeon pea is one of the most important legume crops in India and dimethoate is a widely used insecticide in various crop plants. We studied the effect of dimethoate on growth and photosynthetic activity of pigeon pea plants over a short and long term exposure. Plant growth parameters, photosynthetic pigment content and chlorophyll fluorescence response of pigeon pea (Cajanus cajan L.) plants treated with various concentrations of the insecticide dimethoate (10, 20, 40 and 80 ppm) have been compared for 30 days at regular intervals of 10 days each. Laser induced chlorophyll fluorescence spectra and fluorescence-induction kinetics (FIK) curve of dimethoate treated pigeon pea plants were recorded after 10, 20 and 30 days of treatment. Fluorescence intensity ratio at the two fluorescence maxima (F685/F730) was calculated by evaluating curve-fitted parameters. The variable chlorophyll fluorescence decrease ratio (Rfd) was determined from the FIK curves. Our study revealed that after 10 days of treatment, 10 ppm of dimethoate showed stimulatory response whereas 20, 40 and 80 ppm of dimethoate showed inhibitory response for growth and photosynthetic activity of pigeon pea plants, but after 20 and 30 days of treatment all the tested concentrations of dimethoate became inhibitory. This study clearly shows that dimethoate is highly toxic to the pigeon pea plant, even at very low concentration (10 ppm), if used for a prolonged duration. Our study may thus be helpful in determining the optimal dose of dimethoate in agricultural practices.

Amelioration of prallethrin-induced oxidative stress and hepatotoxicity in rat by the administration of Origanum majorana essential oil

This study was carried out to evaluate the adverse effects of exposure to prallethrin on oxidant/antioxidant status and liver dysfunction biomarkers and the protective role of Origanum majorana essential oil (EO) in rat. Male rats were divided into 4 groups: (i) received only olive oil (ii) treated with 64.0 mg/kg body weight prallethrin (1/10 LD₅₀) in olive oil via oral route daily for 28 days, (iii) treated with 64.0 mg/kg body weight prallethrin (1/10 LD₅₀) and EO (160 μL/kg b.wt.) in olive oil and (iv) received EO (160 μL/kg b.wt.) in olive oil via oral route twice daily for 28 days. Prallethrin treatment caused decrease in body weight gain and increase in relative liver weight. There was a significant increase in the activity of serum marker enzymes, aspartate transaminase, alanine transaminase, and alkaline phosphatase. It caused increase in thiobarbituric acid reactive substances and reduction in the activities of superoxide dismutase, catalase, and glutathione-S-transferase in liver. Consistent histological changes were found in the liver of prallethrin treatment. EO showed significant protection with the depletion of serum marker enzymes and replenishment of antioxidant status and brought all the values to near normal, indicating the protective effect of EO. We can conclude that prallethrin caused oxidative damage and liver injury in male rat and co-administration of EO attenuated the toxic effect of prallethrin. These results demonstrate that administration of EO may be useful, easy, and economical to protect human against pyrethroids toxic effects.

Cytotoxicity and membrane interaction of tamoxifen as affected by ca(2+) and mg(2+): use of a bacterial model system

A strain of Bacillus stearothermophilus was used as a model to study the interaction of tamoxifen (TAM) with the membrane and the cytostatic antiproliferative effects not related to estrogen binding. TAM inhibits the growth of B. stearothermophilus as a function of concentration. The supplementation of the growth medium with Ca(2+) or Mg(2+) partially relieves the growth inhibition by TAM, allowing growth at TAM concentrations that fully impair growth in the basal medium. Fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH) and of its propionic acid derivative (DPH-PA) reveals opposite effects induced by TAM and Ca(2+). The addition of Ca(2+) to liposomes of bacterial lipids promoted physical ordering as opposed to disordering induced by TAM. Thus, it is predictable that growth impairment induced by TAM is mediated through perturbations at the membrane level.

Non-cholinergic intervention of sarin nerve agent poisoning

The protective effects of selected anesthetic regimens on sarin (GB) were investigated in domestic swine. At 30% oxygen, the toxicity of this agent in isoflurane anesthetized animals (LD(50)=10.1μg/kg) was similar to literature sited values in awake swine (LD(50)=11.8μg/kg) and slightly higher than that of both ketamine (LD(50)=15.6μg/kg) and propofol (LD(50)=15.3μg/kg) anesthetized swine. Use of 100% oxygen in ketamine anesthetized animals resulted in three-fold protective effects compared to 30% oxygen. Use of 100% oxygen in both isoflurane and propofol anesthetized animals, compared to 30% resulted in profound protection against GB poisoning (>33×). There were no differences in the severity of the poisoning or recovery time in animals treated over dose ranges of 10-350μg/kg (isoflurane) or 15-500μg/kg GB (propofol). Survivors of high GB challenges that were revived from propofol anesthetic exhibited no signs of cognitive impairment seven days later. Protective treatments did not attenuate cholinesterase (ChE) inhibition; survivors of otherwise supralethal GB concentrations exhibited very low blood ChE activities. This work indicates that propofol has protective effects against GB, and that oxygen tension may have an important role in treating nerve agent casualties. More importantly, it demonstrates that non-cholinergic protective mechanisms exist that may be exploited in the future development of medical countermeasures against organophosphorous nerve agents.

Cytotoxic effect of fenitrothion and lambda-cyhalothrin mixture on lipid peroxidation and antioxidant defense system in rat kidney

A mixture of pyrethroids plus organophosphates was assessed for their potential effects on lipid peroxidation, the antioxidant defense system and lactate dehydrogenase (LDH) in rat kidney in vitro. Various insecticide concentrations were incubated with kidney homogenate at 37°C for different incubation times. Treatment with fenitothion (FNT) plus lambda-cyhalothrin (LC) caused a significant induction (P < 0.05) in thiobarbituric acid reactive substances (TBARS), which might be associated to decreased levels of reduced glutathione (GSH), superoxide dismutase (SOD), catalase (CAT), glutathione S-transferase (GST) activities and protein content in rat kidney. However, a significant induction of lactate dehydrogenase (LDH) activity was observed. The effect was concentration and time dependent. It can be concluded that depletion of GSH might indicate that reactive oxygen species (ROS) could be involved in the toxic effects of FNT plus LC which lead to marked perturbations in antioxidant defense system.

Thionate versus Oxon: comparison of stability, uptake, and cell toxicity of ((14)CH(3)O)(2)-labeled methyl parathion and methyl paraoxon with SH-SY5Y cells

The stability, hydrolysis, and uptake of the organophosphates methyl parathion and methyl paraoxon were investigated in SH-SY5Y cells. The stabilities of ((14)CH(3)O)(2)-methyl parathion ((14)C-MPS) and ((14)CH(3)O)(2)-methyl paraoxon ((14)C-MPO) at 1 microM in culture media had similar half-lives of 91.7 and 101.9 h, respectively. However, 100 microM MPO caused >95% cytotoxicity at 24 h, whereas 100 microM MPS caused 4-5% cytotoxicity at 24 h ( approximately 60% cytotoxicity at 48 h). Greater radioactivity was detected inside cells treated with MPO as compared to MPS, although >80% of the total MPO uptake was primarily dimethyl phosphate (DMP). Maximum uptake was reached after 48 h of (14)C-MPS or (14)C-MPO exposure with total uptakes of 1.19 and 1.76 nM/10(6) cells for MPS and MPO, respectively. The amounts of MPS and MPO detected in the cytosol after 48 h of exposure time were 0.54 and 0.37 nM/10(6) cells, respectively.

Oxidative stress induced by lambda-cyhalothrin (LTC) in rat erythrocytes and brain: Attenuation by vitamin C

The objective of this study was to investigate the propensity of lambda-cyhalothrin (LTC) to induce oxidative stress in blood and brain of male Wistar rats and its possible attenuation by vitamin C. Rats were randomly divided into four groups: group I served as control rats. group II was treated daily with 200mgvit C/(kgbw) administered by intraperitoneal way. Rats of group III have received orally 668ppm LTC. Animals of group IV were treated with LTC and vitamin C. A decrease of some hematologic parameters (RBC, Hb, Ht: p<0.01) and a significant increase of MDA levels (p<0.05) in erythrocytes and brain were observed in LTC group compared to controls. Antioxidant enzyme activities in both tissues were modified in LTC group compared to controls. Administration of vitamin C ameliorated these parameters. Our results indicated the potential effects of LTC to induce oxidative damage in tissues and the ability of vitamin C to attenuate LTC-induced oxidative damage.

Membrane fluidity changes in goat sperm induced by cholesterol depletion using beta-cyclodextrin

Cholesterol efflux from membranes promotes acrosome reaction in goat spermatozoa. In 1 h of incubation of sperm in the presence of beta-cyclodextrin (beta CD), all the interchangeable cholesterol is desorbed from sperm membranes, although acrosome reaction is fully accomplished only after 3-4 h of incubation, as previously published. In the present paper we investigate the effect of cholesterol removal from mature goat spermatozoa on the overall membrane "fluidity" of live cell membranes and of liposomes from sperm lipid extracts. Using steady state fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH), we studied the average thermotropic behaviour of membrane lipids, after incubation of live sperm for 1 h in BSA-free medium with the presence/absence of 8 mM beta-cyclodextrin, as a cholesterol acceptor. Unimodal and bimodal theoretical sigmoids fitted best to the experimental thermotropic profiles of liposomes and whole cells, respectively. In the case of whole sperm, two phase transitions, attributable to different lipid domains, were clearly separated by using the fitting parameters. After cholesterol removal, important changes in the relative anisotropy range of the two transitions were found, indicating an increase in the "fluidity" of some of the lipid microdomains of sperm membranes. These changes in sperm lipid dynamics are produced before the onset of sperm acrosome reaction.

Arachnid lipoproteins: comparative aspects

Findings on hemolymph lipoproteins in the class Arachnida are reviewed in relation to their lipid and protein compositions, hydrated densities, the capacity of apoproteins to bind lipids, and the influence of xenobiotics on their structures and functionality. The occurrence of hemolymphatic lipoproteins in arachnids has been reported in species belonging to the orders Araneida, Scorpionida, Solpugida and Acarina. However, lipoproteins were properly characterized in only three species, Eurypelma californicum, Polybetes pythagoricus and Latrodectus mirabilis. Like insect and crustaceans the arachnids examined contain high density lipoproteins (HDLs) as predominant circulating lipoproteins. Although in most arachnids these particles resemble those of insect HDLs called "lipophorins", in two arachnid species they differ from lipophorins in their apoproteins, total mass and lipid composition. The hemolymph of P. pythagoricus and L. mirabilis contains another HDL of higher density, while P. pythagoricus and E. californicum hemolymph contain a third lipoprotein of very high density (VHDL). Composition of arachnid lipoproteins regarding apoprotein classes as well as lipid classes differ among species. Hemocyanin, in addition to the classical role of this protein as respiratory pigment, is presented here performing the function of apolipoprotein in some arachnid species. Reports on experiments demonstrating the capacity of hemocyanin to bind neutral and polar lipid classes, including ecdysteroids, are commented. Recent works about the changes evoked by a phosphorous pesticide on the structures and functionality of spider lipoproteins are also reviewed.

Fluorescence polarization in studies of bacterial cytoplasmic membrane fluidity under environmental stress

The integrity of the bacterial cytoplasmic membrane is critical in maintaining the viability of cells and their metabolic functions, particularly under stress. Bacteria actively adjust membrane fluidity through changes in lipid composition in response to variations in temperature, pressure, ion concentrations, pH, nutrient availability, and xenobiotics. Fluorescence polarization methods are valuable for measuring bacterial cytoplasmic membrane fluidity. In this review we discuss the mechanisms of bacterial membrane adaptations and present data from research using 1,6-diphenyl-1,3,5-hexatirene (DPH) as a measure of membrane fluidity and phase transitions. We illustrate the range of fluidity in viable cells, extracted membranes, and liposomes under optimal and stressed physiological conditions.

Parathion effects on protein synthesis in the seminiferous tubules of mice

Agropesticides impart high risks of exposure for animals including humans. This work analysed the effect of parathion upon protein synthesis in seminiferous tubules cultured in vitro from adult male CF1 mice. They were incubated with parathion solutions (0.8, 0.4, 0.04, 0.004, 0.0004 mM) for 5 h. One hour after ending incubation tritiated leucine was added to each sample. In addition, groups treated with 0.8 mM parathion were incubated for 5 h, and thereafter transferred to a parathion-free medium; tritiated leucine incorporation was measured 1, 2, or 3 h afterwards. Results of protein synthesis denote an inhibitory dose-dependent effect of parathion. This is reversed after pesticide-free incubation. Testicular protein synthesis inhibition induced by parathion may compromise male fertility in view of the fact that, in spermatogenesis, proliferation (mitosis, meiosis) and cell differentiation occurs.

Comparative study of tributyltin toxicity on two bacteria of the genus Bacillus

Tributyltin is a potent biocide mainly used in marine anti-fouling paints. Owing to its widespread distribution in coast areas and its high toxicity to aquatic organisms, the use of this compound is generally restricted and under government regulation. Despite of that, it persists in the aquatic environment. Organotins used in industry have also been detected in terrestrial environments. The persistence and high lipophilicity explain bioaccumulation. The role of bacteria in recycling organic matter prompted us to study the interaction of tributyltin with two ubiquitous bacilli, B. stearothermophilus and B. subtilis, proposed as biological indicators of pollutants with ecological impact. These bacteria have been used as suitable models for the study of toxicity mechanisms of unselective lipophilic compounds (e.g., DDT and endosulfan). Drug effects on growth parameters, oxygen consumption and membrane organization were assessed. Bacteria growth in a liquid complex medium was disturbed by concentrations of TBT as low as 25 nM (8 microgL(-1)), close to the concentration in polluted environments. The respiratory activity is affected by TBT in both microorganisms. Membrane organization, assessed by fluorescence polarization of two fluidity probes, 1,6-diphenyl-1,3,5-hexatriene (DPH) and a propionic acid derivative (DPH-PA), was also perturbed by the xenobiotic. Alterations on growth, oxygen consumption and physical properties of membrane lipids are stronger in B. stearothermophilus as compared to B. subtilis. A putative relationship between growth inhibition and respiratory activity impairment induced by TBT and its effects on the physical behaviour of bacterial membrane lipids is suggested.

Fluorescent methods to study DNA, RNA, proteins and cytoplasmic membrane polarization in the pentachlorophenol-mineralizing bacterium Sphingomonas sp. UG30 during nutrient starvation in water

The effect of sodium pentachlorophenolate (NaPCP) exposure on the nutrient-starved pentachlorophenol (PCP)-mineralizing bacterium Sphingomonas sp. UG30 was assessed using fluorescent methods to measure DNA, RNA, total cellular protein, and cytoplasmic membrane proteins. UG30 cells were inoculated into sterilized Speed River (Guelph, ON, Canada) water samples in the presence of 50, 100, and 250 ppm NaPCP. No marked changes were observed in the total cellular DNA, RNA or protein levels over 90 d, indicating the macromolecular composition of UG30 was not affected by both nutrient limitation and NaPCP. Total cell counts as determined by DAPI staining also did not change over 90 d. Over the same period, viable counts decreased with increasing concentrations of NaPCP. At 250 ppm NaPCP, viable cell counts decreased over 6 orders of magnitude after 1 hr exposure. Cell numbers partially recovered once NaPCP was degraded. The UG30 cytoplasmic membrane polarization ratio also decreased after NaPCP was depleted. The decreased polarization value at the end of the study period suggested the UG30 membrane was more fluid and that this increase in fluidity was due to nutrient starvation effects rather than exposure to NaPCP. These results indicated that UG30 is a robust organism that is able to degrade NaPCP even under adverse conditions and fluorescent methods are useful for determining macromolecular concentrations and cytoplasmic membrane polarization values.

Morphological and biochemical perturbations in rat erythrocytes following in vitro exposure to Fenvalerate and its metabolite

Erythrocytes are a convenient model to understand the membrane oxidative damage induced by various xenobiotic-prooxidants. In this investigation, we have examined the potency of Fenvalerate (FEN) and its metabolite, p-chlorophenyl isovaleric acid (p-CPIA) to induce oxidative stress response in rat erythrocytes in vitro in terms of lipid peroxidation and effects on selected antioxidant enzymes. Susceptibility of erythrocytes to FEN exposure was further investigated in terms of morphological alterations by scanning electron microscopy and protein damage by gel electrophoresis of erythrocyte ghosts. Following in vitro exposure, FEN caused a significant induction of oxidative damage in erythrocytes at concentrations beyond 0.1 mM as evidenced by increased thiobarbituric acid reactive substances (TBARS) levels. The response was both concentration and time dependent. At higher concentrations, significant decreases in the activities of vital antioxidant enzymes viz., catalase, superoxide dismutase, glutathione transferase and glutathione reductase were also discernible clearly suggesting the potency of both, parent compound and its metabolite to induce oxidative stress in erythrocytes. Scanning electron micrographs of erythrocytes following FEN exposure at higher concentrations revealed various degrees of distortion in shape and ruptured membranes. Furthermore, gel electrophoresis studies revealed consistent and significant aggregation of only band 3 protein in erythrocyte membranes exposed to either FEN or p-CPIA at higher concentrations. These in vitro findings show that FEN and its metabolite have the propensity to cause significant oxidative damage in rat erythrocytes, which is associated with marked damage to membrane proteins. These data suggest that both structural and functional perturbations may ensue in erythrocytes following exposure to FEN at higher concentrations under in vivo situations.

Effects of deltamethrin on functions of rat liver mitochondria and on native and synthetic model membranes

Deltamethrin (DTM) is a synthetic pyrethroid insecticide used wideworld in agriculture, home pest control, protection of foodstuff, and disease vector control. It has widespread applications in Brazilian agriculture. The effects of DTM on mitochondrial respiratory parameters and on the organization of artificial and native membranes are described. DTM (200 nmol mg(-1) protein) on isolated liver mitochondria decreased oxygen consumption of both, state III and state IV, as well as the inner mitochondrial membrane potential (Deltapsi). Analysis of segments of the respiratory chain suggested that the DTM inhibition site is located between complex II and complex III. Mitochondrial swelling, energized or driven by the K+ diffusion potential using valinomycin, were partially inhibited by DTM (200 nmol mg(-1) protein). Fluorescence polarization of DPH and DPH-PA, probing the core and outer regions, respectively, of dimyristoylphosphatidylcholine (DMPC) and native mitochondrial membranes, indicated that DTM shifts the midpoint phase transition to lower values, besides broadening the phase transition. DTM decreased the lipid order of DMPC bilayers, at temperatures lower than the transition temperature and also caused a disordering effect on native membranes. However at temperatures above the transition temperature, the pesticide increased the rigidity of the membrane. These results suggest that DTM causes perturbations in lipid-lipid and lipid-protein interactions, interferes in transport mechanisms operating at the membrane level, and causes alterations of membrane permeability and mitochondrial enzyme activities. These effects could be associated with the toxicity of deltamethrin.

Impacts of carbamate pesticides on olfactory neurophysiology and cholinesterase activity in coho salmon (Oncorhynchus kisutch)

Many freshwater aquatic environments in the Pacific Northwest of North America contain neurotoxic pesticides, an issue of concern given the use of many of these habitats by Pacific salmon (Oncorhynchus sp.). Pesticides such as carbamates are known to affect fundamental physiological systems (such as the enzyme acetylcholinesterase (AChE)), and have been shown to affect salmonid olfactory-mediated behaviors. A neurophysiological measure of olfactory function, the electro-olfactogram (EOG), was used in this study to examine the impacts of acute localized exposure to three carbamates (the insecticide carbofuran, the antisapstain IPBC, and the fungicide mancozeb) on olfactory function in the coho salmon (Oncorhynchus kisutch). We also examine the potential for these pesticides to alter AChE levels in the primary olfactory system and brain with brief exposures (30 min to only the olfactory rosette (OR)). In results, we find that the EOG in coho salmon is highly sensitive to brief localized exposures of two of these three carbamate pesticides. The effective nominal concentration required to cause a 50% reduction in EOG amplitude (EC50) for carbofuran was 10.4 microg/l and for IPBC was 1.28 microg/l. For mancozeb, the EC50 was higher at 2.05 mg/l. All three carbamates also affected AChE activity levels in the OR and brain (BR): carbofuran exposure at 200 microg/l significantly inhibited AChE activity in the OR, and both IPBC and mancozeb significantly increased AChE activity in BR at multiple concentrations with acute localized exposure. These carbamate effects highlight the sensitivity of salmon olfactory neurophysiology to pesticides acting not only potentially via AChE-inhibition, but also by other currently unknown modes of action.

Fenitrothion-induced structural and functional perturbations in the yolk lipoproteins of the shrimp Macrobrachium borellii

Two lipovitellin (LV) forms containing the same apoproteins but differing in their lipid composition were isolated from Macrobrachium borelii eggs at early (LVe) and late (LVI) embryogenic stages and characterized. These two forms of LV, as well as liposomes prepared with lipids extracted from them, were used as simpler models to study the effect of the pesticide fenitrothion (FS) on their structures and functions. Rotational diffusion and fluorescence lifetime of two fluorescent probes [1,6-diphenyl-1,3,5-hexatriene (DPH) and 3-(p-(6-phenyl)-1,3,5-hexatrienal)phenylpropionic acid (DPH-PA)] were used to obtain information on structural changes induced by FS in the inner and outer regions of the LV, respectively. Comparison of the rotational behavior of these probes in native LV and liposomes (LP) from extracted LV lipids suggests that apoprotein-lipid interactions result in an ordered neutral lipid core. FS increased the lipid phase polarity of both LV and LP forms. The rotation of these probes in LP was not affected, suggesting a dependence of FS action on lipid-protein interactions. DPH-PA steady-state anisotropy showed that, unlike the LVe form, the LVI form was sensitive to extremely low FS concentrations. The ability of both LV to transfer palmitic acid to albumin was increased, but in a dissimilar manner, by the presence of FS. Such differences in the sensitivity of the LV at different steps of embryogenesis to FS influence the toxic action of this insecticide.

Morphological effects of neuropathy-inducing organophosphorus compounds in primary dorsal root ganglia cell cultures

Chick embryo dorsal root ganglia (DRG) cultures were used to explore early pathological events associated with exposure to neuropathy-inducing organophosphorus (OP) compounds. This approach used an in vitro neuronal system from the species that provides the animal model for OP-induced delayed neuropathy (OPIDN). DRG were obtained from 9-day-old chick embryos, and grown for 14 days in minimal essential medium (MEM) supplemented with bovine and human placental sera and growth factors. Cultures were then exposed to 1 microM of the OP compounds phenyl saligenin phosphate (PSP) or mipafox, which readily elicit OPIDN in hens, paraoxon, which does not cause OPIDN, or the DMSO vehicle. The medium containing these toxicants was removed after 12 h, and cultures maintained for 4-7 days post-exposure. Morphometric analysis of neurites was performed by inverted microscopy, which demonstrated that neurites of cells treated with mipafox or PSP but not with paraoxon had decreased length-to-diameter ratios at day 4 post-exposure. Ultrastructural alterations of neurons treated with PSP and mipafox included dissolution of microtubules and neurofilaments and degrading mitochondria. Paraoxon-treated and DMSO control neuronal cell cultures did not show such evident ultrastructural changes. This study demonstrates that chick DRG show pathological changes following exposure to neuropathy-inducing OP compounds.

Molecular mechanisms of the metabolite 4-hydroxytamoxifen of the anticancer drug tamoxifen: use of a model microorganism

A strain of the thermophilic eubacterium Bacillus stearothermophilus was used as a model system to identify membrane mediated cytotoxic effects of 4-hydroxytamoxifen, following previous studies with tamoxifen. With this experimental approach we attempted to further clarify tamoxifen and 4-hydroxytamoxifen membrane interactions often evoked as responsible for their multiple cellular effects. Bacterial growth and the oxygen consumption rate provided quantitative data of the cytotoxic action of hydroxytamoxifen. The effects of hydroxytamoxifen on the physical properties of bacterial lipid membrane preparations were also evaluated by fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene. Cultures of B. stearothermophilus grown in a complex medium containing hydroxytamoxifen in the concentration range of 1 to 7 microM exhibited progressively longer lag adapting periods, decreased specific growth rates and lower growth yields, as compared to control cultures. Hydroxytamoxifen also affected the electron redox flow of B. stearothermophilus protoplasts and induced significant perturbation of the structural order of bacterial lipid dispersions. We concluded that the bacterial model provides useful information about the nature and repercussion of membrane physical interactions of this lipophilic drug, on the basis of an easy and economic methodology.

Use of the microorganism Bacillus stearothermophilus as a model to evaluate toxicity of the lipophilic environmental pollutant endosulfan

Microorganisms are very powerful tools for the supply of information about the toxic effects of lipophilic compounds, since an impairment of cell growth usually occurs as a result of perturbations related, in most cases, with the partition of toxicants in membranes. The thermophilic eubacterium Bacillus stearothermophilus has been used as a model system to identify alpha- and beta-endosulfan interactions with the membrane possibly related with the insecticide toxicity. Two approaches have been pursued: (a) bacterial growth is followed and the effects of endosulfan isomers determined; (b) biophysical studies with the fluorescent fluidity probe 1,6-diphenyl-1,3,5-hexatriene (DPH) were performed to assess the effects of alpha- and beta-endosulfan on the organization of the membrane lipid bilayer. The effects on growth were quantitatively evaluated by determination of growth parameters, namely the lag phase, the specific growth rate and the cell density reached by cultures in the stationary phase. Growth inhibition by alpha and beta-endosulfan dependent on the concentration is diminished or removed by the addition of 2.5 mM Ca2+ to bacterial cultures. Fluorescence DPH polarization consistently showed opposite effects of Ca2+ and alpha- and beta-endosulfan on the physical state of bacterial polar lipid dispersions.

Effect of selected environmental and physico-chemical factors on bacterial cytoplasmic membranes

Membranes lipids are one of the most adaptable molecules in response to perturbations. Even subtle changes of the composition of acyl chains or head groups can alter the packing arrangements of lipids within the bilayer. This changes the balance between bilayer and nonbilayer lipids, serving to affect bilayer stability and fluidity, as well as altering lipid-protein interactions. External factors can also change membrane fluidity and lipid composition; including temperature, chemicals, ions, pressure, nutrients and the growth phase of the microbial culture. Various biophysical techniques have been used to monitor fluidity changes within the bacterial membrane. In this review, bacterial cytoplasmic membrane changes and related functional effects will be examined as well as the use of fluorescence polarization methods and examples of data obtained from research with bacteria.

Effect of fenitrothion on the physical properties of crustacean lipoproteins

The effect of the liposoluble organophosphorus insecticide fenitrothion (FS) on lipid packing and rotation of two crustacean plasma HDL was investigated. These lipoproteins, HDL-1 and HDL-2, differed in their lipid composition, but their lipid/protein ratios were similar. The rotational behavior of the fluorescent probes 1,6-diphenyl-1,3,5-hexatriene (DPH) and 3-(p-(6-phenyl)-1,3,5-hexatrienyl) phenylpropionic acid (DPH-PA) was used to obtain information about the lipid dynamics in the outer and inner regions, respectively, of the lipid phase of the lipoproteins. Fluorescent steady-state anisotropy (r(s)), lifetime (tau), rotational correlation time (tau(r)), and the limiting anisotropy (r(infinity)) of these probes were measured in the lipoproteins exposed to different concentrations of FS in vitro. The results showed the penetration of FS into both plasma lipoproteins, altering the lipid dynamics of the inner as well as the outer regions. The overall effect of the insecticide was to induce an increase in the lipid order in a concentration-dependent fashion. DPH and DPH-PA fluorescence-lifetime shortening indicated that FS increased the polarity of the probe environment, suggesting an enhanced water penetration into the lipoprotein lipid phase, may be due to the induction of failures in the lipid packing. Even in the absence of FS, a higher ordering of the lipid phase was found in HDL-2 compared to HDL-1, a fact that might be attributed to a higher percentage of sphingomyelin in HDL-2.

Cypermethrin-induced plasma membrane perturbation on erythrocytes from rats: reduction of fluidity in the hydrophobic core and in glutathione peroxidase activity

The effects of treatment with the synthetic insecticide cypermethrin on plasma membrane fluidity, lipid peroxidation and antioxidant status in rat erythrocytes were investigated. Rats were treated by gavage with a low dose (12.5 mg/kg body weight per day) of cypermethrin in corn oil for 60 days. DPH and TMA-DPH fluorescence anisotropy experiments show that cypermethrin treatment, compared with controls, induced a significant decrease in erythrocyte membrane fluidity measured by DPH, while no changes were observed using TMA-DPH. Cypermethrin treatment also induced a significant increase in the lipid peroxidation, measured by the formation of conjugated dienes. The increased oxidative stress resulted in a significant decrease in the activity of glutathione peroxidase. The results are discussed in terms of preferential localization of cypermethrin in the hydrophobic core of the membrane, where it increases lipid packing and consequently decreases membrane fluidity.

Ethylazinphos interaction with membrane lipid organization induces increase of proton permeability and impairment of mitochondrial bioenergetic functions

Ethylazinphos increases the passive proton permeability of lipid bilayers reconstituted with dipalmitoylphosphatidylcholine (DPPC) and mitochondrial lipids. A sharp increase of proton permeability is detected at insecticide/lipid molar ratios identical to those inducing phase separation in the plane of DPPC bilayers, as revealed by differential scanning calorimetry (DSC). Ethylazinphos progressively depresses the transmembrane potential (DeltaPsi) of mitochondria supported by piruvate/malate, succinate, or ascorbate/TMPD. Additionally, a decreased depolarization induced by ADP depends on ethylazinphos concentration, reflecting a phosphorylation depression. This loss of phosphorylation is a consequence of a decreased DeltaPsi. A decreased respiratory control ratio is also observed, since ethylazinphos stimulates state 4 respiration and inhibits ADP-stimulated respiration (state 3). Ethylazinphos concentrations up to 100 nmol/mg mitochondrial protein increase the rate of state 4 together with a decrease in DeltaPsi, without significant perturbation of state 3 and carbonyl cyanide-p-trifluoromethoxyphenylhydrazone (FCCP)-uncoupled respiration. For increased insecticide concentrations, the state 3 and FCCP-uncoupled respiration are inhibited to approximately the same extent. The perturbations are more pronounced when the energization is supported by pyruvate/malate and less effective when succinate is used as substrate. The present data, in association with previous DSC studies, indicate that ethylazinphos, at concentrations up to 100 nmol/mg mitochondrial protein, interacts with the lipid bilayer of mitochondrial membrane, changing the lipid organization and increasing the proton permeability of the inner membrane. The increased proton permeability explains the decreased oxidative phosphorylation coupling. Resulting disturbed ATP synthesis may significantly underlie the mechanisms of ethylazinphos toxicity, since most of cell energy in eukaryotes is provided by mitochondria.

Changes induced by malathion, methylparathion and parathion on membrane lipid physicochemical properties correlate with their toxicity

Perturbations induced by malathion, methylparathion and parathion on the physicochemical properties of dipalmitoylphosphatidylcholine (DPPC) were studied by fluorescence anisotropy of DPH and DPH-PA and by differential scanning calorimetry (DSC). Methylparathion and parathion (50 microM) increased the fluorescence anisotropy evaluated by DPH-PA and DPH, either in gel or in the fluid phase of DPPC bilayers, but mainly in the fluid phase. Parathion is more effective than methylparathion. On the other hand, malathion had almost no effect. All the three xenobiotics displaced the phase transition midpoint to lower temperature values and broadened the phase transition profile of DPPC, the effectiveness following the sequence: parathion>methylparathion>>malathion. A shifting and broadening of the phase transition was also observed by DSC. Furthermore, at methylparathion/lipid molar ratio of 1/2 and at parathion/lipid molar ratio of 1/7, the DSC thermograms displayed a shoulder in the main peak, in the low temperature side, suggesting coexistence of phases. For higher ratios, the phase transition profile becomes sharp as the control transition, but the midpoint is shifted to the previous shoulder position. Conversely to methylparathion and parathion, malathion did not promote phase separation. The overall data from fluorescence anisotropy and calorimetry indicate that the degree of effect of the insecticides on the physicochemical membrane properties correlates with toxicity to mammals. Therefore, the in vivo effects of organophosphorus compounds may be in part related with their ability to perturb the phospholipid bilayer structure, whose integrity is essential for normal cell function.

Effect of fenitrothion on dipalmitoyl and 1-palmitoyl-2-oleoylphosphatidylcholine bilayers

The effects of the organophosphorous insecticide fenitrothion (phosphorothioic acid, O,O-dimethyl O-(3-methyl-4-nitrophenyl) ester; FS) on the physical state of pure dipalmitoyl (DPPC) and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) membranes were investigated. FS lowers the phase transition temperature of DPPC. It has no large effects on the DPPC gel phase, but it increases the order of the liquid-crystalline state of DPPC and POPC. FS also decreases 1,6-diphenyl-1,3,5-hexatriene (DPH) lifetime (tau) in the DPPC and POPC liquid-crystalline states. Since a direct quenching of DPH emission by FS was ruled out, tau shortening is assigned to an increased water penetration in the bilayer. The effect of FS is different from most perturbing agents for which an increased order is accompanied by a higher tau. Furthermore, quenching of DPH by KI was increased by FS in POPC liposomes indicating an increased accessibility of the quencher to the hydrophobic core where DPH distributes. The effect of FS on dipole relaxation at the hydrophilic-hydrophobic interface of POPC bilayers was studied with 2-dimethylamino-6-lauroylnaphthalene (Laurdan). FS produces a decrease in Laurdan tau and a narrowing of its emission band. FS significantly increases the generalized polarization values at both emission band ends. These results indicate that FS may allow the coexistence of microdomains that have different physical properties.

Organophosphorus compound-induced modification of SH-SY5Y human neuroblastoma mitochondrial transmembrane potential

Organophosphorus (OP) compounds inhibit mitochondrial enzymes, respiration, and ATP generation, in addition to inducing structural changes such as matrix swelling. This implicates mitochondria as primary subcellular targets for these compounds. In this study, the health and function of cellular mitochondria following OP compound exposure were assessed by evaluating the mitochondrial transmembrane potential (DeltaPsi(m)). This was done by measuring the changes in DeltaPsi(m) in SH-SY5Y human neuroblastoma cells incubated with the cationic fluorochrome, rhodamine 123 (5 microg/ml), and the OP compounds tri-ortho-tolyl phosphate (TOTP), triphenyl phosphite (TPPi), or parathion for 7.5 to 960 minutes. OP compounds (100 microM to 1 mM) induced significant concentration-dependent mitochondrial hyperpolarization with peak maxima occurring at 60 (TOTP, TPPi) or 120 (parathion) min. Following this, the mitochondrial membranes gradually depolarized. Pretreatment with cyclosporin A (500 nM, 30 h), a mitochondrial permeability transition pore (PTP) inhibitor, decreased the hyperpolarization. In contrast, 30-h pretreatment with the muscarinic receptor agonist carbachol (1 mM) significantly increased DeltaPsi(m) and delayed subsequent depolarization. Hyperpolarization and subsequent depolarization of mitochondrial membranes occurred 16 to 24 h prior to a loss of substrate adhesion or an increase in DNA fragmentation, indicating that mitochondria were a primary target in OP compound-initiated cytotoxicity.

Biophysical perturbations induced by ethylazinphos in lipid membranes

Perturbations induced by ethylazinphos on the physical organization of dipalmitoylphosphatidylcholine (DPPC) and DPPC/cholesterol membranes were studied by differential scanning calorimetry (DSC) and fluorescence polarization of 2-, 6-, 12-(9-anthroyloxy) stearic acids and 16-(9-anthroyloxy) palmitic acid. Ethylazinphos (50 and 100 microM) increases the fluorescence polarization of the probes, either in the gel or in the fluid phase of DPPC bilayers, and this concentration dependent effect decreases from the surface to the bilayer core. Additionally, the insecticide displaces the phase transition to a lower temperature range and broadens the transition profile of DPPC. A shifting and broadening of the phase transition is also observed by DSC. Furthermore at insecticide/lipid molar ratios higher than 1/7, DSC thermograms, in addition to the normal transition centered at 41 degrees C, also display a new phase transition centered at 45.5 degrees C. The enthalpy of this new transition increases with insecticide concentration, with a corresponding decrease of the main transition enthalpy. Ethylazinphos in DPPC bilayers with low cholesterol (< or = 20 mol%) perturbs the membrane organization as described above for pure DPPC. However, cholesterol concentrations higher than 20 mol% prevent insecticide interaction, as revealed by fluorescence polarization and DSC data. Apparently, cholesterol significantly modulates insecticide interaction by competition for similar distribution domains in the membrane. The present results strongly support our previous hypothesis that ethylazinphos locates in the cooperativity region, i.e. the region of C1-C9 atoms of the acyl chains, and extends to the lipid-water interface, where it increases lipid packing order sensed across all the thickness of the bilayer. Additionally, and, on the basis of DSC data, a lateral regionalization of ethylazinphos is here tentatively suggested.

The role of phosphotriesterases in the detoxication of organophosphorus compounds

The enzymes that hydrolyze organophosphorus compounds are called phosphotriesterases. The presence of phosphotriesterases has been described in a variety of tissues. The physiological role of these enzymes is not known, although a clear correlation exists between the levels of phosphotriesterases and susceptibility of the species to the toxic effects of organophosphorus compounds. Thus, mammals that possess high levels of phosphotriesterases in serum and liver are more tolerant to the toxic effects of these compounds than birds and insects - these being species considered lacking of phosphotriesterases. Because most of these enzymes are not well characterized, they are usually differentiated according to their different patterns of response to activators and/ or inhibitors. Phosphotriesterases usually depend of divalent cations and therefore EDTA usually inhibits them. A peculiar EDTA-resistant phosphotriesterase has been described in serum albumin. The biotechnological and therapeutical applications of phosphotriesterases are currently subject to study.

Effect of dimethoate and chlorfenvinphos on plasma membrane integrity of Synechocystis sp. PCC 6803

The organophosphorus (OP) insecticides dimethoate and chlorfenvinphos, at all selected concentrations (0-500 micromol liter-1), reduced the rate of accumulation of uranine and enhanced fluorescence quenching of 9-amino-6-chloro-2-methoxyacridine (ACMA). There was a significant nonlinear negative correlation between insecticide concentrations and uranine accumulation. The reduction in amount of uranine trapped inside the cells was greater in cultures treated with chlorfenvinphos, whereas quenching of ACMA fluorescence was found to be greater with dimethoate treatment. A marked difference between the effects of dimethoate and chlorfenvinphos on permeability changes in the plasma membrane of Synechocystis was also noted. The release of cellular organic carbon was observed at each concentration of chlorfenvinphos, whereas with dimethoate such an effect was noted at >/=200 micromol liter-1. The rate of uptake of substrates like 2-deoxyglucose and 2-aminoisobutyric acid (AIBA) was significantly (negatively) correlated with concentrations of both the insecticides. Chlorfenvinphos was more effective than dimethoate in reducing the active uptake of nonmetabolizable sugar and amino acid analogues.

Lipid composition changes induced by tamoxifen in a bacterial model system

A putative relationship between growth impairment of Bacillus stearothermophilus by tamoxifen (TAM) and TAM-induced perturbation of the physical properties of bacterial membrane lipids has been observed. The supplementation of the growth medium with Ca2+ (a membrane stabilizer) partially relieves growth inhibition by TAM, allowing growth at TAM concentrations that fully impair growth in the basal medium. B. stearothermophilus modifies the membrane lipid composition in response to the addition of TAM to the growth medium and the response is sensitive to Ca2+. Changes in lipid composition are observed in the acyl chains and in the polar head groups of phospholipids. The physical effects of alteration in these lipids was studied by fluorescence polarization of DPH and DPH-PA. Polar lipid dispersions from TAM-adapted cells grown in a Ca2+ medium show a shift of Tm to higher temperatures and a significant increase of the structural order as compared to lipids from control cells, suggesting that TAM-induced lipid composition changes compensate for the destabilizing effects of the cytostatic on membrane organization. The polar lipids from cells grown in the basal medium containing tamoxifen are also altered, but these alterations do not promote order increase of the bilayer in spite of a deviation of Tm to higher temperatures as detected by DPH. Data indicate that B. stearothermophilus controls the membrane lipid composition in response to tamoxifen, to compensate for TAM-promoted disordering in membranes and to provide an appropriate packing of phospholipid molecules in a stable bilayer, putatively disturbed by TAM incorporation.

The organochlorine pesticide heptachlor disrupts the structure of model and cell membranes

Heptachlor is an organochlorine pesticide which is particularly toxic for aquatic life. A significant source of this pesticide for infants is breast milk, where its concentration is considerably higher than in dairy milk. Given the lipophilic character of heptachlor, lipid-rich cell membranes are a very plausible target for its interaction with living organisms. In order to evaluate its toxicity towards cell membranes, heptachlor was made to interact with human erythrocytes and molecular models of the red cell membrane. These consisted of multilayers of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylethanolamine (DMPE), which are types of phospholipids that are respectively located in the outer and inner monolayers of the erythrocyte membrane, and large unilamellar vesicles (LUV) of DMPC. Observations by scanning electron microscopy showed that 10 mM heptachlor produced various degrees of shape alterations to erythrocytes, which ranged from a few blebs in some cells to a great number of protuberances in others. On the other hand, experiments performed by X-ray diffraction on DMPC and DMPE indicated that the bilayer structure of DMPC was much more affected by heptachlor than that of DMPE. Measurements by fluorescence spectroscopy on DMPC LUV confirmed the X-ray diffraction results in that both the hydrocarbon chain and polar head regions of DMPC were structurally perturbed by heptachlor. The results obtained from the model studies could explain the shape changes induced to red cells by heptachlor. According to the bilayer hypothesis, they were due to the preferential interaction of heptachlor with the phosphatidylcholine-rich external moiety of the erythrocyte membrane. It is therefore concluded that toxic effects of this pesticide can be related to its capacity to perturb the phospholipid bilayer structure, whose integrity is essential for cell membrane functions.

Interaction of 2,4-dichlorophenoxyacetic acid (2,4-D) with cell and model membranes

2,4-dichlorophenoxyacetic acid (2,4-D), a widely used herbicide, is a component of the "agent orange' whose toxicity has been extensively studied without definite conclusions. In order to evaluate its perturbing effect upon cell membranes, 2,4-D was made to interact with human erythrocytes and molecular models. These studies were performed by scanning electron microscopy on red cells, fluorescence spectroscopy on dimyristoylphosphatidylcholine (DMPC) large unilamellar vesicles and X-ray diffraction on multilayers of DMPC and dimyristoylphosphatidylethanolamine (DMPE). It was observed that 2,4-D induced a pronounced shape change to the erythrocytes. This effect is explained by the herbicide interaction with the outer monolayer of the red cell membrane.

Interaction of ethylazinphos with the physical organization of model and native membranes

The interaction of ethylazinphos with the physical organization of model and native membranes was investigated by means of fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH) and of its propionic acid derivative (DPH-PA). Ethylazinphos shifts the phase transition midpoint to lower temperature values and broadens the phase transition profile of bilayers reconstituted with dimyristoyl-, dipalmitoyl- and distearoylphosphatidylcholines (DMPC, DPPC, DSPC), as detected by DPH and DPH-PA. Additionally, both probes detect significant effects of ethylazinphos in the fluid phase of the above lipid bilayers. The insecticide perturbations are more pronounced in bilayers of short-chain lipids, e.g., DMPC, in correlation with the higher partition in these membranes. On the other hand, the insecticide increases to some extent the ordering promoted by cholesterol in the fluid phase of DMPC, but high cholesterol concentrations (> or = 30 mol%) almost prevent insecticide interaction, as revealed by DPH and DPH-PA. In agreement with the results in models of synthetic lipids, the increase of intrinsic cholesterol in fluid native membranes depresses the partition values of ethylazinphos and consequently its effects.

Effects of the pyrethroid insecticide permethrin on membrane fluidity

The interaction of permethrin with dimyristoyl- (DMPC), dipalmitoyl- (DPPC) and distearoyl- (DSPC) bilayers has been investigated by differential scanning calorimetry (DSC) and DPH and TMA-DPH fluorescence anisotropy. In experiments performed by DSC, we show that the addition of permethrin to liposomes, in a 5:1 phospholipid/pyrethroid ratio, decreases the phase transition temperature (Tm) of DMPC, DPPC and DSPC by 3.2, 2.3 and 1.1 degrees C, respectively. Furthermore, DSC profiles reveal that permethrin decreases the cooperativity for the phase transition of DMPC, DPPC and DSPC membranes. DPH and TMA-DPH fluorescence anisotropy experiments show that permethrin increases membrane fluidity at temperatures below the Tm. The results are discussed in terms of a preferential localization of permethrin in the hydrophobic core of the membrane, where it diminishes the lipid packing in the gel phase and has no effect in the liquid-crystalline phase.

Partition of DDE in synthetic and native membranes determined by ultraviolet derivative spectroscopy

Partition coefficients of DDE (2,2-bis(p-chlorophenyl)-1,1-dichloroethylene) were determined, in model and native membranes, as a function of temperature, lipid chain length, cholesterol content and DDE concentration, by means of second derivative ultraviolet spectrophotometry. DDE incorporation increases with the temperature, since the partition values in dimyristoylphosphatidylcholine (DMPC), at 24, 30 and 37 degrees C, are 5722 +/- 138, 10356 +/- 763 and 14006 +/- 740, respectively. The insecticide incorporates better into bilayers of DMPC as compared with DPPC (dipalmitoylphosphatidylcholine). The partition decreases from 10355 +/- 763 in DMPC to 6432 +/- 613 in DPPC, at temperatures 5-7 degrees C above the midpoint of their transitions. The addition of cholesterol to fluid membranes of DMPC depresses the partition of DDE. In agreement with the results in models of synthetic lipids, the partition of DDE into native membranes increases with the temperature and decreases with the intrinsic cholesterol. It is concluded that a fluid membrane favors the accumulation of DDE.

Calorimetric and theoretical studies of the effects of lindane on lipid bilayers of different acyl chain length

The effects of the insecticide lindane on the phase transition in multilamellar bilayers of saturated diacylphosphatidylcholines of different acyl chain length (DC14PC, DC16PC, and DC18PC) have been studied by means of differential scanning calorimetry (DSC), as well as computer-simulation calculations on a molecular interaction model. The calorimetric data show that increasing concentrations of lindane lower the transition temperature and lead to a broadening of the specific heat in a systematic way depending on the lipid acyl chain length. Kinetic effects in the observed calorimetric traces indicate that the incorporation of lindane into multilamellar lipid bilayers is slow, but faster for the shorter lipid species. Large unilamellar vesicles do not show such kinetic effects. The transition enthalpy is for all three lipid species found to be independent of the lindane concentration which implies that the entropy of mixing is vanishingly small. This lends support to a microscopic molecular interaction model which assigns the absorbed lindane molecules to interstitial sites in the bilayer. Computer-simulation calculations on this model, which assumes a specific interaction between lindane and certain excited acyl chain configurations, lead to predictions of the lipid-water partition coefficient in qualitative agreement with experimental measurements (Antunes-Madeira and Madeira (1985) Biochim. Biophys. Acta 820, 165-172). The partition coefficient has a peak near the phase transition which is a consequence of enhanced interfacial adsorption of lindane at lipid-domain interfaces.