Lipid-coated particles--a new approach to fix membrane-bound enzymes onto carrier surfaces

A new method to covalently link phosphatidylethanolamine via the headgroup at the surface of cell-size spherical polymer particles is described. Because the density of the reactive groups linked to the polymer beads is extremely high, a dense, tightly bonded lipid monolayer is formed. When a solubilized lipid is added to the suspension of monolayer-coated polymer beads, the spontaneous formation of a bilayer-like structure is observed. The upper layer of lipid can be removed by washing with detergent solution or organic solvents, ethanol or butanol, and can be replaced in a relipidation step by any other phospholipid; thus, an asymmetric lipid bilayer structure can be formed. Membrane-bound enzymes such as alanine aminopeptidase or dipeptidyl peptidase IV may be inserted with their hydrophobic anchor segments in a stable and enzymatically active form in this artificial system. Incorporation of integral membrane enzymes such as bacteriorhodopsin with membrane-spanning domains and bulky segments at both sides of the membrane succeeded only when a hydrophilic spacer of appropriate length (e.g., pentaalanine) was introduced between the carrier surface and the lipid headgroups.