Oxidation of Cholesterol Changes the Physical Properties of Lipid Membranes

Oxysterols are oxidative derivatives of cholesterol playing a crucial role in many regulatory processes in the human body. They are produced naturally from cholesterol during an enzymatic side chain hydroxylation (catalysed by cytochrome P450) or non-enzymatic oxidation (due to direct exposure of cholesterol to reactive oxygen species). Using several experimental techniques, including dynamic light scattering, fluorescence spectroscopy, and NMR, together with atomistic molecular dynamics simulations and a quantitative description of molecular tilt, we characterized the behavior of a family of oxysterols in phospholipid membranes and compared the resulting data to that of cholesterol. We found that the two distinct groups of oxysterols, i.e., ring-oxidized sterols (mostly produced by free radicals) and tail-oxidized sterols (mostly produced enzymatically) behave differently in lipid membranes, influencing them in different ways. Unlike tail-oxidized sterols or cholesterol, ring-oxidized sterols can efficiently acquire tilted orientations in the bilayer, leading to a stronger disruption of the membrane structure. These changes in physical properties of lipid bilayers may have implications in membrane biochemistry: tail-oxidized sterols behave similarly to cholesterol in terms of membrane stiffening, indicating that their effect on biochemical processes in membranes is similar to that of cholesterol, while ring-oxidized sterols exhibit behavior distinct from cholesterol, implying potential disruptions in membrane functionality such as in signalling processes.