Protein location in liposomes, a drug carrier: a prediction by differential scanning calorimetry

Location of protein drugs in lipid carriers often determines the stability, loading efficiency, and release rate of these drugs from the carriers following administration. On the basis of conventional differential scanning calorimetry (DSC) measurements, Papahadjopoulos et al. (Biochim. Biphys. Acta 1975, 401, 317-335) proposed that proteins can be classified into three categories depending on their effects on the thermotropic behavior of the lipids, e.g., transition temperature and enthalpy. 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 validity of Papahadjopoulos' classification. Hydrophilic ribonuclease A, cytochrome c, and superoxide dismutase (SOD), as well as hydrophobic cyclosporin A, are used as model proteins. Neutral lipids, e.g., dipalmitoylphosphatidylcholine, and/or negatively charged lipids, e.g., dipalmitoylphosphatidylglycerol (DPPG), are used to prepare liposomes. Results from conventional and high-sensitivity DSC are compared. High-sensitivity DSC gives significant, more reproducible results. We find that the classification of Papahadjopoulos et al. needs to be modified. No hydrophilic proteins bind to liposomes exclusively on the surface by electrostatic interactions, and some degree of penetration is observed in 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.