(200c) Effects of Unsaturated Phospholipid Dilinoleoylphosphatidylcholine on Degradation of Phospholipid Vesicles Catalyzed By a Model Phospholipase A2

Secreted phospholipase A2 (sPLA₂) is a lipolytic enzyme that catalyzes the degradation of phospholipids and is widespread in the venoms of insects and reptiles and extracellular spaces of mammalian tissues. Research has shown that dysregulation of sPLA₂ is related to a variety of inflammatory diseases. The intrinsic relationship of in vivo sPLA₂ levels to certain diseases can be exploited for medical diagnosis and treatment. For example, overexpression of sPLA₂ in cancerous regions is a potential trigger for increasing local release of drugs from lipid-based drug carriers. Understanding the interactions of phospholipid membranes with sPLA₂ is necessary to reveal the roles it plays in related physiological processes and to design lipid vesicles for drug delivery. To date, the research on sPLA₂ activity on membranes has mainly focused on the saturated phospholipids. The interactions of unsaturated phospholipids with sPLA₂ are also essential in the human body, yet there is a lack of systematic in situ investigations of these interactions.

In this study, dilinoleoylphosphatidylcholin (DLPC) was used as a model unsaturated phospholipid. Its acidic degradation product, linoleic acid (LA), is one of the two essential fatty acids and is important to our inflammatory defenses. Through inclusion of DLPC in the dipalmitoylphosphatidylcholin (DPPC) saturated membranes, the effects of unsaturation on the degradation of giant unilamellar vesicles (GUV) were investigated by recording the vesicles’ deformation using fluorescence microscopy. The mechanisms of the observed differences in the degradation of the DPPC and DPPC-DLPC GUVs were analyzed by comparing the phase behaviors of a series of DPPC-DLPC monolayers (x_DLPC =0, 0.2, 0.4, 0.63, and 1) using a Langmuir trough-coupled fluorescence microscope. The Angstrom-length scale analysis of the interfacial packing structures of these films before and after degradation was conducted using X-ray surface scattering techniques.

We found that inclusion of unsaturated phospholipids in the membranes not only enhanced the sPLA₂-induced degradation of vesicles, but also changed the vesicle deformation processes and the products’ configurations. The higher enzyme activity on DPPC-DLPC GUVs compared to DPPC GUVs resulted from the differing phase behaviors and molecular packing structures of DPPC and DLPC moleculse at the interface. On the other hand, the differing deformation processes and final configurations of DPPC and DPPC-DLPC GUVs were related to the distinct interfacial organization of the acid degradation products of DPPC and DLPC. These study findings advance the understanding of the influence of unsaturated phospholipids on the interaction of phospholipid membranes with sPLA₂ at the molecular level. Thus the study sheds light on the roles of sPLA₂ in related physiological processes and supports development of a systematic mechanism-based approach for designing lipid-based drug delivery systems.