Effect of haematoporphyrin-induced photosensitization on lipid membranes

In vitro cultured mouse myeloma (Sp-2/0-Ag14) cells and phosphatidylcholine liposomes were used to study the membrane effects of photosensitization with an He-Ne laser activated haematoporphyrin (HP). Lipophilic HP molecules, intercalated between the membrane lipid molecules, caused morphological changes of cell membranes on light activation. Steady state and time-resolved fluorescence spectroscopic studies of membrane-bound HP molecules provide information about the change in membrane lipid dynamics (fluidity). Increased HP fluorescence anisotropy was found after laser irradiation in the case of cell membrane. This finding can be related either to the increased rotation correlation time of the rotating fluorophore (HP) (decreased membrane fluidity) or to the decrease in the angular range of molecular rotation, which corresponds to an increased lipid order after photosensitization. Changes in the ratio of saturated:unsaturated fatty acid content of membrane lipids or other chemical events such as cross-linking of membrane components during the photosensitization process can also account for the observed effects.