Interactions of the drug amphotericin B with phospholipid membranes containing or not ergosterol: new insight into the role of ergosterol.

Amphotericin B (AmB) is an amphipathic polyene antibiotic which permeabilizes ergosterol-containing membranes, supposedly by formation of pores. In water, AmB forms chiral aggregates, modelled as stacks of planar dimers in which the joined polyene chains in each dimer turn round, from one dimer to the following in these stacks, by forming a helical array. Studies of the binding of AmB with L-dipalmitoylphosphatidylcholine (L-DPPC) and L-dilauroylphosphatidylcholine (L-DLPC) bilayers disclose the main following results. (1) An inversion of the helicity of the L-DPPC-bound AmB aggregates, when the L-DPPC bilayers are in the gel phase, is inferred from the evolution of the circular dichroism spectra of AmB+L-DPPC mixtures. (2) An AmB-induced gel-to-subgel transformation of L-DPPC bilayers, in the previous mixtures, is revealed by a differential scanning calorimetry study. (3) The role played by ergosterol in the location of phospholipid-bound AmB aggregates with respect to a phospholipid bilayer is directly demonstrated from atomic force microscopy observations of mica-supported AmB+L-DLPC mixtures, in the presence or absence of ergosterol. While in the absence of ergosterol AmB aggregates remained at the surface of the bilayer, in the presence of ergosterol they appeared embedded within this bilayer and became hollow-centered. As such an embedding in the hydrophobic core of a bilayer requires a rearrangement of the aggregates with respect to their architecture in water, this rearrangement is held responsible for the hollowing of aggregates. The hollow-centered sublayer-embedded AmB aggregates are thought to be the precursors of the formation of AmB pores.