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.
Centro de Neurociências, Departamento de Zoologia, 3000, Coimbra, Portugal.
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D015060:1,2-Dipalmitoylphosphatidylcholine / Q000737:chemistry; D002152:Calorimetry, Differential Scanning; D002784:Cholesterol / Q000737:chemistry; D004726:Endosulfan / Q000737:chemistry*; D005454:Fluorescence Polarization; D006843:Hydrocarbons, Chlorinated; D007306:Insecticides / Q000737:chemistry*; D007536:Isomerism; D008051:Lipid Bilayers / Q000737:chemistry*; D013816:Thermodynamics
D006843:Hydrocarbons, Chlorinated; D007306:Insecticides; D008051:Lipid Bilayers; D015060:1,2-Dipalmitoylphosphatidylcholine; D002784:Cholesterol; D004726:Endosulfan