Correlation of side chain mobility with cholesterol retention by phospholipid vesicles

Development of tea tree oil-loaded liposomal formulation using response surface methodology

The aim of this study is to prepare tea tree oil liposome (TTOL) and optimize the preparation condition by single factor experiment and statistical design. TTOL was prepared using a thin-film hydration with the combination of sonication method and the preparation conditions of TTOL were optimized with response surface methodology (RSM). The optimal preparation conditions for TTOL by response surface methodology were as follows: the mass ratio of PC and Cho 5.51, TTO concentration 1.21% (v/v) and Tween 80 concentration 0.79% (v/v). The response surface analysis showed that the significant (p < 0.05) second-order polynomial regression equations successfully fitted for all dependent variables with no significant (p > 0.05) lack of fit for the reduced models. Furthermore, the interaction of the mass ratio of PC/Cho and TTO concentration had a significant effect. The amounts of Tween 80 required were also reduced with RSM. Under these conditions, the experimental encapsulation efficiency of TTOL was 97.81 ± 0.33%, which was close with the predicted value. Therefore, the optimized preparation condition was very reliable. The increased entrapment efficiency would significantly improve the TTO stability and bioavailability.

Cholesterol limits estrogen uptake by liposomes and erythrocyte membranes

Multilamellar vesicles (MLV) were prepared from phospholipids with and without cholesterol in equimolar amounts and [4-14C]estradiol. Unincorporated estrogen was removed by petroleum ether extraction or by aqueous buffer washes. In either case, cholesterol-containing vesicles incorporated one-half the estradiol as vesicles without sterol. Addition of estradiol to preformed vesicles followed by buffer washes showed that vesicles without cholesterol invariably retained more estradiol than those with the sterol. Reduction of the cholesterol content to one-half increased estradiol incorporation. The pattern of estradiol removal from MLV with successive buffer washes indicated that much of the steroid associated with cholesterol-containing vesicles was superficially bound to the membrane but vesicles without cholesterol incorporated the estrogen into the bilayer structure. To test the role of cholesterol in limiting the uptake of an estrogen by cells, right-side out resealed ghosts of ox erythrocytes were prepared. They were partially depleted of cholesterol by exposure to small unilamellar vesicles of dioleoylphosphatidyl choline. A decrease in cholesterol content correlates with an increase in estradiol uptake by red cell ghosts. The experiments described point to a central role of cholesterol in limiting the uptake of steroids. The loss of cholesterol of steroid producing cells caused by tropic hormones may be key to their mode of action in promoting secretion of steroid hormones. Likewise, the long-term genomic responses of steroid target cells may depend upon their cholesterol content and the ease by which the steroid can penetrate the membrane barrier.

Cell membranes and multilamellar vesicles: influence of pH on solvent induced damage

Pigment leakage from sheep and horse erythrocytes and from red beet tissue induced by non-polar solvents was determined as a function of pH. The results were compared to disruption of multilamellar vesicles (MLV) composed of phospholipids with equimolar cholesterol under identical conditions of solvent exposure and pH. Solvent access to cholesterol was used to measure vesicle disruption. MLV were made from 1,2-dioleoyl phosphatidylethanolamine, sphingomyelin (SP) and various phosphatidylcholines to simulate the major lipid components of membranes. Pigment leakage from erythrocytes caused by petroleum hydrocarbon (b.p. 60-80 degrees C) was maximal at pH 2-4 and at pH 10, but minimal at pH 6.8; alcohols caused less pigment leakage than petroleum hydrocarbon. Beta-cyanin leakage from beet tissue induced by petroleum hydrocarbon was maximal at pH 2, with very little leakage at pH 4, 6.6 and pH 10. Alcohols caused minimal damage to beet tissue above pH 2. Cholesterol removal by petroleum hydrocarbon from MLV of mixed lipid composition was maximal at pH 2-4, reduced at pH 6.8 and minimal at pH 10. Lipid mixtures in which fatty acyl side chains of one phospholipid were of a different length than the other lost more sterol than mixtures in which the acyl side chains were of identical chain length. MLV with more than 25% SP lost more sterol than those with less or no SP. Results show that in mixtures of phospholipids, SP exposes the hydrocarbon phase of a bilayer to solvent extraction, a property that was also observed in native membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Unusual product ratios resulting from the gamma-irradiation of cholesterol in liposomes

Cholesterol in aqueous suspensions of multilamellar vesicles (MLV) was exposed to gamma-irradiation (0.5-10 kGy) at 0-4 degrees C. Cholesterol oxidation products resulting from the irradiation were isolated by dry column extraction followed by preparative thin-layer chromatography (TLC) and were quantitated by on-column gas chromatography (GC). The ratio of 7-ketocholesterol/cholesterol 5,6-epoxides generated by irradiation was less than one, much lower than the ratio of ten commonly produced by autoxidation. Irradiation also produced relatively higher amounts of 7-hydroxycholesterol than did autoxidation. These unique product ratios may be suitable indicators of past exposure to irradiation.

Effect of pH on the affinity of phospholipids for cholesterol

The ability of multilamellar vesicles of phosphatidylcholine and phosphatidylethanolamine in aqueous phase to prevent access to cholesterol by a nonpolar solvent was examined. Phosphatidylethanolamine vesicles. In mixed vesicles, cholesterol was retained in proportion to the amount of phosphatidylcholine. To alter the charge and hydration of head groups, pH was adjusted from 1.2 to 12.5. Above pH 8, both phosphatidylethanolamine and phosphatidylcholine retained sterol in a 1:1 molar ratio of phospholipid to cholesterol, regardless of acyl side chain composition. Between pH 2.0 and pH 8.0, sterol retention varied with type of head group and side chain. Lipids with 16-carbon saturated side chains retained more sterol than 18-carbon unsaturated or 12-carbon saturated side chains. Between pH 1.1 and 2.0, none of the phosphatidylethanolamines retained sterol, but long chain phosphatidylcholines, saturated or unsaturated, retained sterol in a 1:1 molar ratio of phospholipid to sterol. Short chain phosphatidylethanolamines and phosphatidylcholines retained 0 to 20% at the low- to mid-pH range. Size of multilamellar vesicles, measured by Doppler effect light scattering analysis, had no bearing on sterol retention. Sonication of vesicles, which increases surface curvature, increases the retention of sterol. Fluorescence polarization indicated that cholesterol does not interact with DPPC or DLPC side chains. The observations can be interpreted in terms of space requirements of head groups, including charge repulsion and hydration. Other factors, such as monovalent cation replacement by protons, juxtaposition of charged groups on vesicle surfaces and length and unsaturation of acyl side chains affect the affinity of phospholipids for cholesterol.