Fluidity and osmotic sensitivity changes of phospholipase A 2 -treated liposomes

Membrane fluidity and osmotic sensitivity were examined in DPPC liposomes treated with phospholipase A₂ (PL.A₂) in the presence of Ca²⁺ or Mg²⁺. The amount of liposome phospholipid hydrolyzed differed with the two ions. Embedded DPH, a rod-like fluorescent probe, was employed in the determination of membrane fluidity. Membrane fluidity decreased according to the degree of phospholipid hydrolization in liposomes by PL.A₂. The reciprocal value of absorption at 450 nm was measured as the index of osmotic sensitivity of liposomes. Intact sonicated liposomes showed osmotic insensitivity. PL.A₂-treated liposomes in which about 40 % of total phospholipid was hydrolyzed showed osmotic sensitivity. No change in the membrane fluidity was obtained when PL.A₂-treated liposomes were exposed to hypertonic or hypotonic solution. These results suggested that the motion of the acyl-chain of phospholipids and free fatty acids was resisted in PL.A₂-treated liposomes. The resistance may be due to a phase separation between phospholipids and free fatty acids. The pore for water permeation might be induced in the border between phase-separated domains in PL.A₂-treated liposomes.