Topography of alphaxalone and delta 16-alphaxalone in membrane bilayers containing cholesterol

We have used small-angle X-ray diffraction and differential scanning calorimetry (DSC) to study the topographies of alphaxalone and its biologically inactive analog delta 16-alphaxalone in dimyristoylphosphatidylcholine (DMPC) and DMPC/cholesterol model membranes. Diffraction patterns were obtained and analyzed for preparations of bilayers without and with the steroids. Temperature dependence of the total period repeat distance (d-spacing) allowed us to identify equivalent temperatures at which the preparations had similar d-spacing and were in the same mesomorphic state. The combination of X-ray and DSC data showed that the anesthetic steroid alphaxalone broadens the membrane phase transition and increases the ratio of gauche: trans conformers in the membranes in contrast to the inactive steroid delta 16-alphaxalone which affects the membranes only marginally. In model DMPC membranes alphaxalone and delta 16-alphaxalone are located near the bilayer interface. This location is maintained by alphaxalone when cholesterol is incorporated in the bilayer as evidenced by the X-ray measurements. However, when delta 16-alphaxalone is incorporated in cholesterol containing bilayers, a decrease in the electron density profile of the preparation is observed. This can be explained by invoking the formation of a delta 16-alphaxalone-cholesterol complex. The delta 16-alphaxalone complex shows no periodicity and is therefore, not detected in the X-ray diffraction experiment. Presumably, this complex forms aggregates either on the surface or inside the bilayer. This explanation corroborates DSC results which show that delta 16-alphaxalone sharpens the phase transition of DMPC/cholesterol preparations, an indication that some cholesterol is excluded from the bilayer preparation after the addition of the biologically inactive steroid.