Partitioning of moderately hydrophobic endocrine disruptors between water and synthetic membrane vesicles

The partition coefficient between water and lipid membrane vesicles (K _lipw) has been used as an alternative to the 1-octanol-water partition coefficient (K_ow) between water and organic solvent, because it more closely represents actual biological membranes. Despite theoretical differences, log K_lipw correlates well with log K _ow for conventional nonpolar organic pollutants. In the present study, K_lipw values of 11 structurally diverse endocrine-disrupting chemicals (EDCs) were measured for three different types of lipid membrane vesicles from dipalmitoylphosphatidylcholine (DPPC), DPPC/cholesterol, and palmitoyloleoylphosphatidylcholine. Correlation analyses were conducted to evaluate the effects of hydrophobicity, molar liquid volume (MLV), and polar surface area (PSA) for 20 EDCs, including nine from a previous study. Correlations that include MLV and PSA reduce the predicted value of log K _lipw, suggesting that lipid membranes are less favorable than 1-octanol for a hydrophobic solute because of the higher molar volume and higher hydrogen-bonding potential. These results suggested that K_ow alone has limited potential for estimating K_lipw and that additional descriptors are required. In addition, K_lipw values vary by as much as two orders of magnitude because of the changes in membrane fluidity and the amount of cholesterol in the lipid bilayer. Therefore, lipid components should be chosen carefully to evaluate the bioconcentration of these compounds.