INTERACTIONS OF DICARBOXYLIC PORPHYRINS WITH UNILAMELLAR LIPIDIC VESICLES - DRASTIC EFFECTS OF PH AND CHOLESTEROL ON KINETICS

The effect of pH on the transfer of deuteroporphyrin from dimyristoylp hosphatidylcholine (DMPC) unilamellar vesicles to human serum albumin is investigated using a stopped-flow with fluorescence detection. The kinetics of this process allows for the determination of the rate cons tants for the porphyrin exit from the outer vesicle layer to the bulk aqueous medium (k(off)), and for its movement from the inner to the ou ter vesicle layer (k(to)). Both k(off) and k(to) are found to strongly depend on the pH. The observed behavior can be described by classical titration curves and is most likely due to protonation equilibria inv olving the two carboxylic groups of the porphyrin. A pH increase accel erates the exit of the porphyrin. The reverse effect is observed for i ts movement through the bilayer. The presence of cholesterol in the DM PC bilayer also strongly affects the interactions of the porphyrin wit h the vesicles. The rate constant k(to) is dramatically reduced by inc reasing the cholesterol content. An irregularity is noted around 10-20 mol % cholesterol. The results are discussed in relation to the prefe rential uptake of porphyrins by tumors, a basis of photodynamic therap y, and to possible pH-mediated relocalization of porphyrins among subc ellular structures.