Influence of the glycosylphosphatidylinositol anchor in the morphology and roughness of Langmuir-Blodgett films of phospholipids containing alkaline phosphatases

The morphology of phospholipid Langmuir-Blodgett films containing a glycosylphosphatidylinositol-anchored rat osseous plate alkaline phosphatase was studied by atomic force microscopy. Two forms of the enzyme were used in this study: an anchor-containing detergent-solubilized form, and a phospholipase-solubilized form, without the hydrophobic moiety. Direct measurements of catalytic activity in enzyme/phospholipid mixed films, and nanogravimetric estimations by quartz crystal microbalance confirmed the adsorption of both forms of alkaline phosphatase. Atomic force microscopy analysis showed that both enzyme forms originated aggregates when adsorbed on phospholipid Langmuir-Blodgett films. However, the phospholipase-solubilized form showed a higher roughness when compared to the detergent solubilized alkaline phosphatase. A model suggesting different alignments of the major axis of the ellipsoid polypeptide moiety relative to the film surface is proposed. This model, supported by surface density and catalytic activity data, might explain the difference of roughness observed for films containing each enzyme form adsorbed onto a phospholipid-modified solid support.