Relative activity of cholesterol in OPPC/cholesterol/sphingomyelin mixtures measured with an acoustic sensor

Abstract

Acoustic devices are sensitive to the mole fraction of cholesterol present in liposomes adsorbed to the device surface as a result of the different mechanical properties of the liposomes. This fact was exploited to develop an acoustic assay to determine the relative affinity of cholesterol for different lipid mixtures. In the assay described here, the initial rate of β-cyclodextrin-induced removal of cholesterol was measured for liposomes having a range of compositions. The initial rate of cholesterol removal was found to be directly proportional to the concentration of β-cyclodextrin (βCD) present over the range of 0–7.5 mg/ml (0–6.6 mM), consistent with other assays measuring the βCD-accelerated transfer of cholesterol between liposomes. The affinity of cholesterol for 1-oleoyl-2-palmitoyl-sn-glycero-3-phosphocholine (OPPC) liposomes with a sphingomyelin mole fraction, χ_SPM, of 0.2 was found to be 1.4× higher than that for pure OPPC liposomes. For liposomes composed only of OPPC and cholesterol in varying ratios, the initial rate of cholesterol removal was determined as a function of cholesterol mole fraction (χ_C). The initial rate of removal showed an increase at χ_C = 0.13, consistent with phase diagrams showing the start of liquid ordered domain formation, but no such increase at χ_C = 0.25, in contrast to the predictions of the umbrella model for OPPC/cholesterol interactions.