Partitioning of selected estrogenic compounds between synthetic membrane vesicles and water: Effects of lipid components

Partition coefficients of the steroid estrogens 17beta-estradiol, estriol, estrone, and 17alpha-ethynylestradiol and the industrial estrogenic compounds p-nonylphenol, p-(tert)octylphenol, bisphenol A, butylbenzylphthalate, and dibutylphthalate between liposome membrane vesicles and water (K-lipw values) were determined using equilibrium dialysis. A moderate linear correlation with R-2 values of as low as 0.679 were found for the relationship between log K-lipw and log K-ow for these compounds. Effects of lipid components used to prepare the membrane vesicles on the partitioning of 17beta-estradiol and p-nonylphenol were also evaluated. For both, K-lipw values were larger for the vesicles prepared from phospholipids composed of shorter acyl chains such as dilauroyl-phosphatidylcholine than those composed of longer acyl chains such as distearoylphosphatidylcholine. Partition coefficients were higher for the vesicles prepared from phospholipids including unsaturated acyl chains such as dioleoyl-phosphatidylcholine than those solely composed of saturated acyl chains such as distearoyl-phosphatidylcholine. Both shorter acyl chains and double bonds resulted in a more fluid conformation of the lipid bilayer with the liquid crystalline phase rather than the gel phase and greater partitioning. In contrast, higher cholesterol contents reduced the partitioning coefficient. The presence of cholesterol in the void of the lipid bilayer possibly led to the stabilization of the bilayer and the decrease in the partitioning of 17beta-estradiol or p-nonylphenol molecules. These results suggest that phase transition is of critical importance in selecting lipid components to evaluate the bioconcentration for these compounds.