Adsorption of detergent-solubilized and phospholipase C-solubilized alkaline phosphatase at air/liquid interfaces

To investigate the influence of a hydrophobic anchor on protein adsorption, equilibrium and dynamic aspects of the adsorption of two different solubilized forms of rat osseous plate alkaline phosphatase on Langmuir monolayers of dimyristoylphosphatidic acid (DMPA) were studied. Surface pressure and surface potential measurements at air/liquid interfaces were carried out using the detergent-solubilized form (DSAP) of alkaline phosphatase, which holds a glycosylphosphatidylinositol (GPI) hydrophobic anchor, and the glycosylphosphatidylinositol-specific phospholipase C-solubilized form (PLSAP), lacking the GPI anchor. Similar surface transitions observed for both DMPA and DMPA/PLSAP mixed monolayers indicate that the presence of PLSAP does not promote significant changes in surface packing of the DMPA monolayer. However, PLSAP interacts with the polar portion of the phospholipid even at high lateral compression. The presence of the GPI anchor increases the adsorption of DSAP at a plain air/liquid interface and also enables the penetration of the protein into the DMPA monolayers. The penetration is dependent on both time and surface pressure. Up to 20 mN/m, the surface pressure increases smoothly indicating a diffusion followed by an adsorption process. Above 20 mN/m, after a fast increase, the surface pressure slowly decays to equilibrium values quite close to the initial surface pressures. The results indicate that the molecular packing of the lipid layer drives the enzyme adsorption to the interface either through the GPI anchor or by the polypeptide moiety.