PROTEIN LOCATION IN LIPOSOMES, A DRUG CARRIER - A PREDICTION BY DIFFERENTIAL SCANNING CALORIMETRY

Location of protein drugs in lipid carriers often determines the stabi lity, loading efficiency, and release rate of these drugs from the car riers following administration. On the basis of conventional different ial scanning calorimetry (DSC) measurements, Papahadjopoulos ct al. (B iochim. Biphys. Acta 1975, 401, 317-335) proposed that proteins can be classified into three categories depending on their effects on the th ermotropic behavior of the lipids, e.g., transition temperature and en thalpy. Interactions are usually electrostatic, hydrophobic, or their combination. The nature of these interactions are reflected by changes in various thermotropic parameters. Our study aims to test the validi ty of Papahadjopoulos' classification. Hydrophilic ribonuclease A, cyt ochrome c, and superoxide dismutase (SOD), as well as hydrophobic cycl osporin A, are used as model proteins. Neutral lipids, e.g., dipalmito ylphosphatidylcholine, and/or negatively charged lipids, e.g., dipalmi toylphosphatidylglycerol (DPPG), are used to prepare liposomes. Result s from conventional and high-sensitivity DSC are compared. High-sensit ivity DSC gives significant, more reproducible results. We find that t he classification of Papahadjopoulos et al. needs to be modified. No h ydrophilic proteins bind to liposomes exclusively on the surface by el ectrostatic interactions, and some degree of penetration is observed i n most cases. An unexpected binding between SOD and DPPG liposomes is observed. The binding of SOD to negatively charged lipids may account, at least in part, for its ability to protect lipid membranes against oxygen-mediated injury.