Membrane permeabilization induced by Triton X-100: The role of membrane phase state and edge tension

Detergents are widely used to solubilize and separate biomembrane components. It is therefore relevant to study and understand the mechanistic details underlying detergent-lipid interactions using biomimetic systems. Here, we have investigated in detail the process of membrane permeabilization and the nature of pores induced by sub-solubilizing concentrations of the detergent Triton X-100 (TX-100) in bilayers composed of palmitoyl oleoyl phosphatidylcholine (POPC), sphingomyelin (SM) and binary mixtures of these phospholipids with 30 mol% cholesterol (chol). A fluorescence quenching assay was used to evaluate the permeability of large unilamellar vesicles (LUVs) in the presence of increasing concentrations of TX-100. Confocal microscopy was employed to visualize and quantify the permeability of giant unilamellar vesicles (GUVs) to two fluorescent dyes of different sizes in the presence of TX-100. Both methods showed that POPC, POPC/chol and SM membranes become fully permeable at a specific TX-100 concentration, followed by complete (POPC and SM) and partial (POPC/chol) solubilization at a higher detergent concentration. The confocal microscopy experiments revealed that opening of pores occurs as a well-defined event and that for POPC and POPC/chol the pores were initially selective to the small probe and then grew and allowed passage of the larger dye as well. On the other hand, the insoluble SM/chol membranes exhibited only a mild TX-100-induced permeabilization. The membrane edge tension of the liquid phases was measured from the closure rate of macropores induced by electric pulses in GUVs. Membrane edge tension was shown to be sensitive to membrane composition and to decrease in the presence of TX-100. We propose that extensive permeabilization occurs below a critical membrane edge tension, which is eventually reached in the partially and fully soluble compositions, but not in the insoluble mixture.