Effects Caused by Triton X-100 on Lipid Bilayers of Different Composition

Abstract

Detergents, such as Triton X-100, are widely used as solubilizing agents of biological membranes. Due to their amphiphilic character, detergents are incorporated in lipid bilayers at low concentrations, but are able to solubilize the membrane at higher concentrations. Here we study the incorporation of Triton X-100 into model membranes of different lipid composition and the subsequent membrane solubilization. The membranes were composed of pure POPC (palmitoyl oleoyl phoshatidylcholine), pure SM (sphingomyelin) and binary mixtures of these lipids with 30 mol% cholesterol. The process of incorporation/solubilization was followed at room temperature by optical microscopy of giant unilamellar vesicles (GUVs), isothermal titration calorimetry (ITC) and turbidity measurements. Optical microscopy revealed different responses of GUVs to Triton X-100, depending on membrane composition. Incorporation of Triton X-100 in the bilayer caused an increase in the surface area of GUVs composed of POPC, changes in the spontaneous curvature of GUVs of POPC/chol, and no morphological effects in GUVs made of SM and SM/chol. These observations were discussed in terms of a modulation of the flip-flop rate of bilayer-incorporated Triton X-100 by membrane phase and packing. Eventually, Triton X-100 was able to solubilize GUVs composed of POPC, POPC/chol and SM. During the solubilization process several holes opened and the bilayer gradually vanished. On the other hand, GUVs composed of SM/chol were insoluble in Triton X-100. ITC and turbidity measurements were used to determine the incorporation and solubility thresholds of Triton X-100 in the different membrane compositions. Financial support: INCT-FCx and FAPESP.