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.