Vesicles of one monoglyceride and two phospholipids: phase behavior and susceptibility to hydrolysis by glyceride lipase and phospholipase A(2)

Mono-disperse, unilamellar vesicles of mixtures of three lipids: monomyristoylglycerol (MMG), dimyristoylphosphatidylcholine (DMPC), and dimyristoylphosphatidylglycerol (DMPG) were produced. The lipid suspensions were characterized by multi-angle dynamic light scattering (DLS); phase behavior was investigated by differential, scanning calorimetry (DSC).

The zwitterionic DMPC and the anionic DMPG display almost ideal mixing as binary mixtures under our experimental conditions. DSC further showed that MMG induces a gel-fluid co-existence and an increase in the main phase transition temperature for binary mixtures of 0-50 mol% MMG in DMPC and DMPG, respectively.

Similar non-ideality was observed for ternary mixtures of 0-50 mol% MMG in lipid suspensions where the ratio of DMPC and DMPG was fixed at a molar ratio of 70:30. Thermodynamic calculations on the phase behavior of the lipid mixtures were made by means of a two-dimensional regular solution model. The theoretical results are in fine accordance with the experimentally observed non-ideal mixing behavior of the MMG-phospholipid systems.

Repulsive interaction parameters between MMG and the two phospholipids are found, in both the gel and the fluid phase. Strongest destabilizing effect of the monoglyceride is seen in the fluid phase. The susceptibility of the vesicles suspension to hydrolytic activity by a glyceride lipase and a phospholipase A(2) (PLA(2)) was investigated.

As a common platform to study the enzymatic activities, a ternary mixture of 23 mol% MMG in bilayers of 70:30 mol% DMPC/DMPG was chosen. Enzymatic reactions were analyzed by spectrofluorometric measurements in combination with quantitative HPLC. The phase behavior of the lipid substrates strongly affected the enzymatic activities when probing different regions in the phase diagram.

In this system PLA(2)-hydrolysis proceeded with no latency phase, and varied expectedly with the bilayer phase state with general preference for DMPG over DMPC. Vesicle stability was compromized by the PLA(2) activity. Hydrolysis of MMG, catalyzed by the glyceride lipase, shows a different scenario than established for the PLA(2) reactions; vesicles remained stable over the course of reaction, and a latency phase was found in experiments where a gel phase was present.