A calorimetric evidence of the interaction and transport of environmentally carcinogenic compounds through biomembranes

The effect exerted by pyrene and nitro-pyrene (N-pyrene), two structurally similar polycyclic aromatic hydrocarbons (PAH,), possessing mutagenic and c arcinogenic activity, on the thermotropic behaviour of model membranes cons tituted by dimyristoylphosphatidylcholine (DMPC) vesicles. was investigated by differential scanning calorimetry (DSC). Attention was directed to eval uate modifications in mutagen-lipid interaction induced by compound structu re and lipophily and evidences in their membrane penetration. The two exami ned compounds, when dispersed in liposomes during their preparation, were f ound to exert a very different action on the L-beta to L-alpha gel-to-liqui d crystal phase transition of DMPC multilamellar Vesicles (MLV). Pyrene cau sed a detectable effect on the transition temperature (T-m) shifting it tow ards lower values with a concomitant decrease of the associated enthalpy (D eltaH) changes, while N-pyrene was able to modify the lipid vesicles thermo tropic behaviour only for low molar fractions, without deep changes in the DeltaH. Modifications induced by pyrene were a function of mutagen concentr ation while the different behaviour of N-pyrene can be due to different pol arity induced by the presence of the nitro-group, which attribute an higher hydrophilic character. Solid pyrene and N-pyrene and MLV aqueous dispersio ns (0.12 molar fraction) were left in touch for long incubation times at te mperature higher than the transitional temperature of DMPC to detect their spontaneous transfer through the medium. By following this procedure, no in teraction was detected for both pyrene and N-pyrene with Lipid vesicles, su ggesting that their low hydrophilic character avoid their migration through the aqueous layer surrounding the MLV vesicles. Carrying out a kinetic mea surement leaving for increasing incubation times charged MLV (0.12 molar fr action) with empty DMPC vesicles the uptake of the PAHs by the empty vesicl es and their successive interaction, after several periods of incubations, was monitored. Pyrene and N-pyrene showed a different kinetic behaviour: th e N-pyrene rate transfer was faster than that of pyrene but both final tran sfer and interaction were on the same order that detected by preparation of MLV charged with a 0.06 molar fraction of PAH. carried out in organic solv ent. The obtained results suggest that the PAH,. even if unable to reach an d penetrate the biological membranes migrating through an aqueous layer, wh en dispersed in a lipophilic medium are able to penetrate and diffuse insid e a model membrane. The different effects observed could be explained in te rms of compound hydrophobicity and a relation between compound structure an d membrane interaction can be suggested. This allows the membrane interacti on with pyrene for all the tested molar fractions, but the structure of N-p yrene seems to suggest the formation of aggregate on the membrane surface f or molar fractions higher than 0.09. (C) 2001 Elsevier Science B.V. All rig hts reserved.