Lipid Interfacial Organization and Interaction with a Model Enzyme

The intrinsic overexpressed enzymes have potential to be exploited to reduce the non-specific drug release of lipid-based drug delivery systems (LDDS) and enhance their target delivery efficacy. To explore this potential, this study investigated lipid molecular packing structures and interactions of lipids with a model enzyme, secretory phospholipase A2 (sPLA2). By employing a Langmuir trough integrated with X-ray reflectivity and grazing incidence X-ray diffraction techniques and using 1,2-palmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayers as model lipid substrates, the interactions of lipids with sPLA2 were quantified through the changes in lipid interfacial organization during interaction. The results showed that the interactions exhibited Ca2+ dependence. Without Ca2+ in the subphase, sPLA2 was adsorbed to the interface and tightened the packing of DPPC molecules. In contrast, with Ca2+ present, sPLA2 catalyzed DPPC degradation, leading to disruption of the ordered monolayer structure and generated a mixture film contained multilayer domains of palmitic acid (PA) – Ca2+ complexes and a loosely packed monolayer phase. To better understand the mechanisms, the spontaneous reorganization of PA molecules on buffers was investigated and molecular simulations were conducted. In addition, the effects of polyunsaturation and polyethylene glycol coating (PEGylation) on lipid organization and sPLA2-catalyzed degradation were investigated. The results showed that inclusion of a high molar fraction of polyunsaturated phospholipids in membranes enhanced the degradation and prevented crystal formation. On the other hand, PEGylation impeded the reorganization of degraded products, suggesting a hindering effect on the degradation of LDDSs. In summary, our results showed that the degradation kinetics of LDDSs catalyzed by sPLA2 highly depended on the interfacial organization of lipid substrates and degradation products. A combination of saturated phospholipids, polyunsaturated phospholipids, and PEGylated phospholipids shows promise for achieving local drug release from LDDSs at target sites where sPLA2 is overexpressed. The results obtained in this study provide a foundation for mechanism-based design and optimization methods for sPLA2-triggered LDDSs and shed light on the roles of sPLA2 in related biological processes.