TEMPERATURE-INDUCED CHANGES IN FLUORESCENCE PROPERTIES AS A PROBE OF PORPHYRIN MICROENVIRONMENT IN LIPID-MEMBRANES .2. THE PARTITION OF HEMATOPORPHYRIN AND PROTOPORPHYRIN IN MITOCHONDRIA

The temperature dependence of hematoporphyrin and protoporphyrin fluor escence quantum yields (Phi(F)) was studied after delivery to whole mi tochondria or isolated inner (IMM) and outer (OMM) mitochondrial membr anes, obtained from liver of Wistar rats. These studies are very sensi tive to variations of the porphyrin lipid environment. Before incorpor ation, the polphyrins were dissolved in 0.01 M sodium phosphate, 0.15 M NaCl, pH 7.4) NaCl/P-i (only hematoporphyrin) or dispersed into lipo somes of dipalmitoylphosphoglycerocholine (Pam(2)GroPCho), sometimes e nriched with cholesterol or cardiolipin. Whole mitochondria show highe r incorporation capacity of hematoporphyrin and protoporphyrin than is olated IMM and OMM, probably because additional, energy-sensitive tran sport mechanisms for the porphyrin uptake occur in intact organelles. A small decrease in protoporphyrin uptake is observed in OMM in compar ison with IMM; in contrast, the decrease in hematoporphyrin uptake by OMM is rather significant. A comparison between the results obtained w ith IMM, OMM and whole mitochondria show that both porphyrins, when re leased to the intact organelles, preferentially localize in the IMM, i rrespective of the lipid carrier used. NaCl/P-i-dissolved hematoporphy rin probably interacts with some membrane proteins, due to the similar ity of the Arrhenius plots with those obtained fbr liposome-entrapped human serum albumin/hematoporphyrin complexes which were used as model s to mimic hematoporphyrin-membrane protein binding sites. Liposomal h ematoporphyrin and protoporphyrin bind to lipid domains. Hematoporphyr in accumulates in specific, localized lipid regions, perhaps in the bo undary lipids area surrounding some inner-mitochondrial carriers; prot oporphyrin accomodates in more rigid, lipid areas. On these bases, the higher photoactivity of hematoporphyrin, previously observed in mitoc hondria, in comparison with protoporphyrin, can be easily explained. F ormation of linear dimers/aggregates, endowed with higher Phi(F) than that of the monomers, are postulated to occur for both porphyrins only in the inner mitochondrial membrane.