Effects of DDE on the fluidity of model and native membranes: implications for the mechanisms of toxicity

Phase structure of liposome in lipid mixtures

Gas microbubbles present in ultrasound imaging contrast agents are stabilized by lipid aggregates that typically contain a mixture of lipids. In this study, the phase structure of the lipid mixtures that contained two or three lipids was investigated using three different methods: dynamic light scattering, (1)H NMR, and microfluidity measurements with fluorescence probes. Three lipids that are commonly present in imaging agents (DPPC, DPPE-PEG, and DPPA) were used. Two types of systems, two-lipid model systems and simulated imaging systems were investigated. The results show that liposomes were the dominant aggregates in all the samples studied. The polar PEG side chains from the PEGylated lipid lead to the formation of micelles and micellar aggregates in small sizes. In the ternary lipid systems, almost all the lipids were present in bilayers with micelles absent and free lipids at very low concentration. These results suggest that liposomes, not micelles, contribute to the stabilization of microbubbles in an ultrasound imaging contrast agent.

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.

Differential effects induced by alpha- and beta-endosulfan in lipid bilayer organization are reflected in proton permeability

The effects of two insecticides isomers, alpha- and beta-endosulfan, on the passive proton permeability of large unilamellar vesicles (LUV) reconstituted with dipalmitoylphosphatidylcholine (DPPC) or mitochondrial lipids were reported. In DPPC (LUV) gel phase, at 30 degrees C, the global kinetic constant (K) of proton permeability (proportional to the proton permeability) initially increased slightly with the increase of alpha-endosulfan/lipid molar ratio up to 0.143. In the range from 0.143 to 0.286, a discontinuity in the increment occurred and, above this range, the proton permeability increased substantially. In DPPC fluid phase, at 48 degrees C, the proton permeability showed a behavior identical to that observed in gel DPPC, with a sharp increase for alpha-endosulfan/lipid molar ratios ranging from 0.143 to 0.286. At these and higher concentrations, alpha-endosulfan induced phase separation in the plane of DPPC membranes, as revealed by differential scanning calorimetry (DSC). Conversely to alpha-endosulfan, beta-endosulfan induced only a slight increase in the proton permeability, either in the fluid or the gel phase of DPPC, for all beta-endosulfan/lipid molar ratios tested. Additionally, the effects of the endosulfan isomers on the proton permeability of mitochondrial fluid lipid dispersions, at 37 degrees C, are similar to those described for DPPC. The beta-isomer induced a very small effect, and alpha-endosulfan, at low concentrations, increased slightly the proton permeability, but for insecticide/lipid molar ratios above 0.143 the permeability increased substantially. Consequently, the membrane physical state of synthetic and native lipid dispersions, as affected by the structural features of alpha- and beta-endosulfan, influenced the proton permeability. The effects here observed in vitro suggest that the formation of lateral membrane domains may underlay the biological activity of alpha-endosulfan in vivo, contributing to its higher degree of toxicity as compared with beta-endosulfan.

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.

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.

HgCl2 disrupts the structure of the human erythrocyte membrane and model phospholipid bilayers

The structural effects of Hg(II) ions on the erythrocyte membrane were studied through the interactions of HgCl2 with human erythrocytes and their isolated resealed membranes. Studies were carried out by scanning electron microscopy and fluorescence spectroscopy, respectively. Hg(II) induced shape changes in erythrocytes, which took the form of echinocytes and stomatocytes. This finding means that Hg(II) locates in both the outer and inner monolayers of the erythrocyte membrane. Fluorescence spectroscopy results indicate strong interactions of Hg(II) ions with phospholipid amino groups, which also affected the packing of the lipid acyl chains at the deep hydrophobic core of the membrane. HgCl2 also interacted with bilayers of dimyristoylphosphatidylcholine and dimyristoylphosphatidylethanolamine, representative of phospholipid classes located in the outer and inner monolayers of the erythrocyte membrane, respectively. X-ray diffraction indicated that Hg(II) ions induced molecular disorder to both phospholipid bilayers, while fluorescence spectroscopy of dimyristoylphosphatidylcholine large unilamellar vesicles confirmed the interaction of Hg(II) ions with the lipid polar head groups. All these findings point to the important role of the phospholipid bilayers in the interaction of Hg(II) on cell membranes.

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.

Perturbations induced by alpha- and beta-endosulfan in lipid membranes: a DSC and fluorescence polarization study

The interaction of alpha- and beta-endosulfan isomers with lipid bilayers was searched by differential scanning calorimetry (DSC) and fluorescence polarization of 2-, 6- and 12-(9-anthroyloxy) stearic acids (2-AS, 6-AS and 12-AS) and 16-(9-anthroyloxy) palmitic acid (16-AP). Both endosulfan isomers, at insecticide/lipid molar ratios ranging from 1/40 to 1/1, shift the phase transition midpoint to lower temperature values and broaden the transition profile of dipalmitoylphosphatidylcholine (DPPC) bilayers. At insecticide/lipid molar ratios of 1/40, the isomers fully abolish the bilayer pretransition. Conversely to beta-endosulfan, alpha-endosulfan promotes a new phase transition, centered at 35.4 degrees C, in addition to the main phase transition of DPPC. Therefore, the alpha-isomer may undergo a heterogeneous distribution in separate domains in the plane of the membrane, whereas the beta-isomer may undergo a homogeneous distribution. Fluorescence polarization data indicate that alpha-endosulfan increases the lipid structural order in the regions probed by 2-AS and decreases it in the regions probed by 6-AS, 12-AS and 16-AP. On the other hand, the beta-isomer produces disordering effects in the upper regions of the bilayers, probed by 2-AS, and ordering in deeper regions, probed by 6-AS, 12-AS and 16-AP, mainly in the gel phase. The incorporation of cholesterol into DPPC bilayers progressively decreases the effects of beta-isomer which are vanished at 20 mol% cholesterol. However, this and higher cholesterol concentrations did not prevent alpha-endosulfan membrane interaction, as revealed by DSC and fluorescence polarization. The distinct effects promoted by alpha- and beta-endosulfan are discussed in terms of molecular orientation and positioning within the bilayer. Apparently, the alpha-isomer preferentially locates closer to the phospholipid headgroups whereas the beta-isomer distributes in deeper domains of the bilayer.

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.

Interactions of 2,2-bis(p-chlorophenyl)-1,1-dichloroethylene with mitochondrial oxidative phosphorylation

The effects of DDE (2,2-bis(p-chlorophenyl)-1,1-dichloroethylene), the major metabolite of DDT (2,2-bis(p-chlorophenyl)-1,1,1-trichloroethane), on rat liver mitochondrial bioenergetic activities were examined. The approach developed by M. D. Brand (Biochim Biophys Acta 1018: 128-133, 1990) was used to assess the effects of DDE because it is possible to discriminate the sites of action of compounds having pleiotypic effects on oxidative phosphorylation. Data were further confirmed using a "classical" approach, including measurements of transmembrane potential, respiratory indexes, enzymatic activities and membrane permeability to protons. DDE up to 40 nmol/mg protein affected the proton motive force generating system. In fact, DDE interacted with succinate dehydrogenase (complex II), decreasing respiration and membrane potential. In this concentration range, the permeability of the inner membrane to protons remained intact. Only higher concentrations (> or = 80 nmol/mg) increased permeability to protons, uncoupling oxidation from phosphorylation. The phosphorylative system was not affected because the rate of ATP synthesis was unchanged. In addition, data from carbonyl cyanide m-chlorophenylhydrazone-uncoupled rotenone-inhibited preparations or submitochondrial particles indicated that F0F1 ATPase activity is not affected by DDE. Therefore, DDE inhibition of complex II and putative inhibition of succinate translocation explain the depression of mitochondrial respiration. The use of appropriate substrates and assay conditions indicates that complexes I, III and IV were not affected by DDE. The uncoupling of oxidative phosphorylation at high concentrations (> 80 nmol DDE/mg protein) was probably related to deleterious effects on the integrity of the mitochondrial membrane. We confirmed that the technique originally proposed by Brand is useful for characterizing the effects of xenobiotics on oxidative phosphorylation. In addition, data provided by this technique closely agree with data from classical studies.

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.

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.

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

The effects of the organophosphorus insecticide parathion (O,O-diethyl O-(p-nitrophenyl)phosphorothioate) on the physical state of synthetic and native membranes was investigated by fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH), probing the bilayer core, and by its anionic propionic acid derivative (DPH-PA), probing the outer regions of the bilayer. Parathion disorders the gel phase of liposomes reconstituted with dimyristoylphosphatidylcholine (DMPC), broadening the transition profile and shifting the temperature midpoint of the phase transition, as detected by both probes. The insecticide strongly orders the fluid phase either in the hydrophobic core or in the outer regions of the membrane, as evaluated by DPH and DPH-PA, respectively. These ordering effects of parathion were further confirmed in fluid models of egg-yolk phosphatidylcholine. Parathion increases to some extent the ordering promoted by cholesterol in DMPC bilayers, but high cholesterol concentrations (> or = 30 mol%) prevent parathion interaction. The results in native membranes correlate reasonably with those obtained in models of synthetic lipids. Thus, parathion does not exert detectable effects in cholesterol-rich membranes, namely, erythrocytes, but moderate ordering effects of parathion are detected by both probes in brain microsomes, i.e., membranes with a lower content of cholesterol. Again, in agreement with the models of synthetic lipids, pronounced ordering effects of parathion are detected in cholesterol-poor membranes, e.g., sarcoplasmic reticulum and mitochondria.