Partitioning of triphenylalkylphosphonium homologues in gel bead-immobilized liposomes: chromatographic measurement of their membrane partition coefficients

Unilamellar liposomes of small or large size, SUVs and LUVs, respectively, were stably immobilized in the highly hydrophilic Sepharose 4B or Sephacryl S-1000 gel beads as a membrane stationary phase for immobilized liposome c hromatography (ILC). Lipophilic cations of triphenylmethylphosphonium and t etraphenylphosphonium (TPP+) have been used as probes of the membrane poten tial of cells, interaction of TPP+ and triphenylalkylphosphonium homologues with the immobilized liposomal membranes was shown by their elution profil es on both zonal and frontal ILC. Retardation of the lipophilic cations on the liposome gel bed was increased as the hydrophobicity of the cations inc reased, indicating the partitioning of lipophilic cations into the hydrocar bon region of the membranes. The cations did not retard on the Sepharose or Sephacryl gel bed without liposomes, confirming that the cations only inte ract with the immobilized liposomes. Effects of the solute concentration, f low rate, and gel-matrix substance on the ILC were studied. The stationary phase volume of the liposomal membranes was calculated from the volume of a phospholipid molecule and the amount of the immobilized phospholipid, whic h allowed us to determine the membrane partition coefficient (K-LM) for the lipophilic cations distributed between the aqueous mobile and membrane sta tionary phases. The values of K-LM were generally increased with the hydrop hobicity of the solutes increased, and were higher for the SUVs than for th e LUVs. The ILC method described here can be applied to measure membrane pa rtition coefficients for other lipophilic solutes (e.g., drugs). (C) 1999 E lsevier Science B.V. All rights reserved.