Monolayer-bilayer equilibrium of phospholipid: stabilization of neutral lipid droplets in aqueous medium and catabolism of plasma lipoproteins

Abstract

Phospholipid spreads as monolayer at air/water and oil/water interfaces. Interfacial pressure of the monolayer in equilibrium with a definite bulk phase of the lipid, (equilibrium) spreading pressure, depends on lyotropic and thermotropic polymorphic state of the lipid bulk phase. Phosphatidylcholine (PC) in hydrated liquid crystalline state (bilayers in L alpha state) gives an appreciably large value of spreading pressure, 45-46 mN/m. Monolayer-bilayer equilibrium of a neutral lipid-phospholipid mixture is determined by miscibilities of the lipids in mixed monolayer, PC bilayer and bulk (liquid or solid) phase of the neutral lipid. A neutral lipid of limited solubility in PC bilayer forms a separate phase in aqueous medium. The phase is stabilized as small particles in the medium by PC monolayer at the particle surface. The monolayer is in equilibrium with the bilayer. This sort of equilibrium plays important roles in formation and catabolism of triglyceride- and cholesteryl ester-rich lipoprotein particles in animal plasma. The equilibrium is a critical factor also in stabilization of aqueous dispersion of lipophilic vitamin (neutral lipid). Coexistence of emulsion particles (neutral lipid core covered with PC monolayer) and vesicles made of PC bilayer are observed in a stable dispersion.