Association of lysozyme with phospholipid vesicles is accompanied by membrane surface dehydration

Lysozyme is a globular protein which is known to bind to negatively charged phospholipid vesicles. In order to study the relationship between binding of the protein and the subsequent destabilization of the phospholipid vesic les a set of experiments was performed using phospholipid monolayers and ve sicles. Using microelectrophoresis the binding of lysozyme to phospholipid vesicles made of PS was determined. At low ionic strength and mild acidic p H of the solution lysozyme reduced the magnitude of the negative zeta poten tial of PS vesicles at lower concentrations compared to neutral pH and high ionic strength. In contrast, the bound fraction of lysozyme to PS vesicles was nearly constant at acidic and neutral pH. At low pH, the binding of ly sozyme was accompanied by a strong aggregation of the vesicles. Lysozyme bi nding to PS vesicles is accompanied by its penetration into the PL monolaye r. This was measured by surface tension and film balance measurements at lo w pH and low ionic strength. The interaction of lysozyme with negatively ch arged vesicles lead to a decrease of the vesicle surface hydration as measu red by the shift of the emission peak of the fluorescent probe DPE. The bin ding of bis-ANS increased at low pH after addition of lysozyme to the vesic les. This indicates that more hydrophobic patches of the lysozyme-PS comple x are exposed at low pH. At low pH the binding process of lysozyme to PS ve sicles was followed by an extensive intermixing of phospholipids between th e aggregated vesicles, accompanied by a massive leakage of the aqueous cont ent of vesicles.