Quantification of Phospholipid Vesicle Degradation Kinetics Catalyzed By a Model Phospholipase

In this study, kinetics of giant unilamellar vesicles (GUVs) degradation upon addition of secretory phospholipase A2 (sPLA₂) were monitored under inverted microscopy. The effects of PEGylation and different combinations of saturated and unsaturated phospholipids were quantified. Secretory phospholipase A2 (sPLA₂) is an enzyme present in the human body and can be found in excess in many pro-inflammatory conditions. The rationale directing the design of switchable liposomal drug carriers is based on the hypothesis that liposomes remain stable at low concentrations of sPLA₂ and degrade spontaneously at a concentration higher than the threshold concentration of sPLA₂.

The morphologies of the GUVs were observed before and after adding enzyme and degradation kinetics were quantified by calculating the change in surface area of the GUVs. A threshold concentration of sPLA₂ was discovered, beyond which the GUVs shrink significantly fast. More specifically, combinations of 1, 2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1, 2-dilinoleoyl-sn-glycero-3-phosphocholine (DLPC, 18:2) were used in this study.

It was found that above 1μg/ml of sPLA₂, DPPC GUV degradation became obvious within the observation time (30 min). With the presence of the unsaturated DLPC, the threshold concentration of sPLA₂ is reduced. The structures formed after degradation were also different for saturated and unsaturated lipids. While saturated lipids formed crystal-like structures, unsaturated lipids were seen momentarily shrinking and then relaxing back to a spherical shape. These results will help design a system for targeted drug delivery.