Calorimetric approach of the interaction and absorption of polycyclic aromatic hydrocarbons with model membranes

The ability of polycyclic aromatic hydrocarbons (PAHs) to interact with cell membranes outer lipid layer and subsequently to penetrate inside cells can be a prerequisite for exhibiting a mutagenic and carcinogenic activity. The effect exerted by pyrene, benzo[a]pyrene, and anthracene, three structurally similar polycyclic aromatic hydrocarbons possessing mutagenic and carcinogenic activity on the thermotropic behavior of model membranes represented by dimyristoylphosphatidyicholine (DMPC) vesicles, was investigated by differential scanning calorimetry (DSC). The examined compounds, when dispersed in liposomes during their preparation, exerted a different action on the gel-to-liquid crystal phase transition of DMPC multilamellar vesicles. Pyrene and benzo[a]pyrene affected the transition temperature (T-m), shifting it toward lower values with a concomitant decrease of the associated enthalpy changes (DeltaH). Anthracene does not significantly affect the thermotropic behavior of lipid vesicles for all tested concentrations. The interaction between PAHs and model membranes was also studied by considering the ability of such compounds as a finely powdered solid or adsorbed on soil surrogate (constituted by silica gel) to migrate through an aqueous medium. This transfer process was compared with the PAHs intermembrane transfer from PAH loaded liposomes to empty membranes. These processes can mimic absorption kinetics mediated by hydrophilic or lipophilic media. No interaction occurred between model membranes and solid PAHs. A very small effect was also observed for PAHs released by silica gel, suggesting that the migration and absorption are hindered by the aqueous layer and that their low hydrophilic character inhibits migration through the aqueous layer surrounding the multilamellar vesicles (MLV). Different behavior was observed by considering the time-dependent studies carried out by contacting, for increasing times, equivalent amounts of empty DMPC vesicles with PAH loaded ones; all compounds were able to migrate between the two different kinds of model membranes. Thus, PAHs are unable to reach and penetrate biological membranes migrating through an aqueous layer but, when dispersed in a lipophilic medium, are able to penetrate and diffuse inside a membrane. The obtained experimental results seem to validate the employment of the DSC technique in order to study the ability of bioactive compounds, not only to interact with biological membranes, but also to be adsorbed inside a cell when dispersed in a lipophilic medium.