Partial specific volumes of liposome systems

Abstract

The apparent partial specific volumes ( V ) of liposomes of various compositions and in the presence of solutes have been determined from accurate density measurements. Both acetylsalicylic acid (ASA) and cortisone acetate (CA) caused a decrease in the V of dimyristoylphosphatidylcholine (DMPC) liposomes but by different mechanisms, ASA having the most pronounced effect. An increase in pH of the medium was more effective at reducing the V of ASA/DMPC liposomes at a higher ionic strength because of greater binding of anionic ASA to the phospholipid molecules. Mole ratios of cholesterol (CHOL) or dicetylphosphate (DCP) with DMPC, or CHOL with dipalmitoylphosphatidylcholine (DPPC) increased the V to a maximum at 0.4 and 0.17, respectively, by about 4.5% compared to 6% in DPPC liposomes at a 1.0 DCP/DPPC mole ratio. Δ V at the maximum in liposomes of stearylamine/DMPC was 15-fold greater even though these preparations were clear and viscous. Sphingomyelin had no significant effect on the V of DMPC liposomes up to a 1.0 mole ratio. In contrast, dipalmitoylphosphatidylethanol amine (DPPE) in DPPE/DMPC liposomes decreased the V to a minimum at 0.7 mole ratio by 2.3%. It is concluded that changes in the composition of multilamellar liposomes may result in substantive volume changes which could exert pronounced effects on membrane stability and permeability.