INFLUENCE OF OXYGENATED STEROL COMPOUNDS ON PHASE-TRANSITIONS IN MODEL MEMBRANES - A STUDY BY DIFFERENTIAL SCANNING CALORIMETRY

A marked influence of oxygenated sterol compounds (OSC: 7.alpha.-, 7.beta.- and 25-hydroxycholesterol and 7-ketocholestanol) on the reversible gel to liquid-crystalline phase transition behavior of cholesterol-free and cholesterol-containing model membranes is evidenced by high-sensitivity differential scanning calorimetry. Liposomes of dipalmitoylphosphatidylcholine (DPPC) were chosen as model membranes. Each of the investigated OSC exerts an individual influence on the phase transition of DPPC liposomes, which expresses itself in the temperature range, the corresponding enthalpy and the peak shape of calorimetric curves. The onset temperature of the phase transition is lowered in the following range of effectiveness: 7.beta.-hydroxycholesterol > 7.alpha.-hydroxycholesterol > 7-ketocholestanol > cholesterol. The mutual presence of cholesterol and of OSC leads to the following order: 7.alpha.-hydroxycholesterol .apprxeq. 7.beta.-hydroxycholesterol > 7-ketocholestanol > cholesterol (without OSC) > 25-hydroxycholesterol. The enthalpy of the phase transition is decreased with increasing content of cholesterol, 7.alpha.- or 7.beta.-hydroxycholesterol, or 7-ketocholestanol. At a concentration of about 10 mol% of the latter 3 OSC, the corresponding enthalpy value of the transition is lowered from 9.1 kcal/mol for pure DPPC to about 7.5 kcal/mol, whereas 10 mol% cholesterol lowers the enthalpy value to 7.0 kcal/mol. Incorporation of both cholesterol and one of the OSC into the DPPC liposomes leads to enthalpy values depending on the total sterol content only, except in the case of 7.beta.-hydroxycholesterol at relatively high concentration (about 10 mol%); here, a higher value was found. Cholesterol-free DPPC liposomes containing 25-hydroxycholesterol display a thermotropic behavior very similar to that of pure DPPC liposomes. This effect is attributed to the sparing solubility of 25-hydroxycholesterol in DPPC bilayers. The mutual presence of cholesterol and 25-hydroxycholesterol in DPPC liposomes, however, leads to a thermotropic behavior resembling that of model membranes containing cholesterol and either 7.alpha.- and 7.beta.-hydroxycholesterol or 7-ketocholestanol.