Low concentrations of bile salts increase the rate of spontaneous phospholipid transfer between vesicles

The rate of 1-palmitoyl-2-[12-[(7-nitro-2,1,3-benzoxadiazol-4-yl)amino] dodecanoyl] phosphatidylcholine (P-C12-NBD-PC) transfer between dioleoylphosphatidylcholine vesicles was measured by a technique based on resonance energy transfer between P-C12-NBD-PC and N-(lissamine rhodamine B sulfonyl)dioleoylphosphatidylethanolamine [Nichols, J. W., & Pagano, R. E. (1982) Biochemistry 21, 1720-1726]. Addition of bile salts at concentrations below their critical micelle concentrations increased the rate of spontaneous P-C12-NBD-PC transfer without disrupting the vesicles. The effectiveness in increasing the transfer rate was dependent on the structure of the bile salt. In general, conjugated bile salts were more effective than unconjugated, and mono- and dihydroxy bile salts were more effective than trihydroxy. The kinetics of intervesicular P-C12-NBD-PC transfer in the presence of cholate were found to be consistent with a mass action kinetic model based on the premise that bile salts bind to the vesicles, alter the dissociation and/or association rate constants for phospholipid monomer-vesicle interaction, and increase the rate of phospholipid transfer via the diffusion of soluble monomers through the aqueous phase. Temperature dependence studies indicated that cholate binding to vesicles is an entropy-driven process and that cholate binding lowers the free energy of activation for phospholipid monomer-vesicle dissociation by producing compensatory decreases in both the enthalpy and entropy of activation.