Monitoring of cholesterol oxidation in a lipid bilayer membrane using streptolysin O as a sensing and signal transduction element.

Streptolysin O (SLO), which recognizes sterols and forms nanopores in lipid membranes, is proposed as a sensing element for monitoring cholesterol oxidation in a lipid bilayer. The structural requirements of eight sterols for forming nanopores by SLO confirmed that a free 3-OH group in the β-configuration of sterols is required for recognition by SLO in a lipid bilayer. The extent of nanopore formation by SLO in lipid bilayers increased in the order of cholestanol<cholesterol<25-OH cholesterol and in a sterol concentration-dependent manner. The immobilization of liposomes consisting of PC, cholesterol and 4-cholesten-3-one exhibited a linear relationship between calcein permeability and the molar ratio of cholesterol and 4-cholesten-3-one. The SLO-based method was successfully applied for monitoring of cholesterol oxidase-mediated oxidation of cholesterol in a lipid bilayer. The potential of the SLO nanopore-based method for monitoring cholesterol oxidation in a lipid bilayer by other oxidative enzymes is also discussed.