Liposomes as models to study the distribution of porphyrins in cell membranes.

Unilamellar liposomes of dipalmitoylphosphatidylcholine (DPPC) have been chosen as suitable models of cell membranes in studies aimed at defining the influence of specific parameters on the distribution properties of selected hydrophobic photosensitizers, namely hematoporphyrin (HP) and protoporphyrin (PP), in normal and tumour tissues. To better mimic in vivo situations, DPPC liposomes were sometimes mixed with cardiolipin (Card) or cholesterol (Chol). Two techniques were mainly used: the quenching of porphyrin fluorescence by methyl viologen, which can discriminate different dye populations inside the vesicles as well as their degree of accessibility to the external medium, and the polarization of porphyrin fluorescence, which gives information on the dye microenvironment through its degree of rotational freedom. The nature of the porphyrin binding sites in each phospholipid monolayer is found to be a function of the degree of hydrophobicity and the concentration of the dye as well as the chemical composition of the liposomes. In DPPC and DPPC-Chol liposomes, all PP molecules are deeply embedded into very rigid, hydrophobic domains of the inner lipid monolayer. Only in the presence of cardiolipin, for [PP] greater than 2.5 microM, a partial shift of the dye molecules towards the outer lipid monolayer is observed. HP mostly localizes at the inner lipid/water interface in all liposomes: at very low concentrations ([HP] approximately equal to 0.5 microM) the dye is bound to the polar heads of the lipids through its carboxylate groups, leaving the rest of the molecule dissolved in the inner aqueous pool. At higher concentrations, HP molecules change their orientation: the ionized propionic chains still interact with the polar heads while the hydrophobic core lies in the lipid phase in DPPC and DPPC-Card vesicles. HP incorporated into DPPC-Chol mixed liposomes projects from the inner lipid phase into the aqueous compartment in all the concentration range studied by us. A very small fraction of HP population (corresponding to 5-10% of the overall fluorescence) is localized at the water/lipid external interface in DPPC and DPPC-Chol liposomes. This fraction increases in the presence of cardiolipin (up to 30% of the overall fluorescence). The possible implications of these findings for the nature of the targets of photosensitization in cell membranes are discussed.