Kinetic analysis of phospholipid exchange between phosphatidylcholine/taurocholate mixed micelles: effect of the acyl chain moiety of the micellar phosphatidylcholine.

A fluorescent assay based on concentration-dependent self-quenching of the fluorescent phospholipid N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)phosphatidylethanolamine was used to measure the rate of phospholipid exchange between taurocholate/phosphatidylcholine mixed micelles. Two NBD-labeled phosphatidylethanolamine probes (dilauryl and dimyristoyl) were tested in taurocholate/phosphatidylcholine mixed micelles prepared from phosphatidylcholine molecules varying in saturated chain length from 12 to 18. All combinations of probes and micellar phosphatidylcholines gave kinetic results that were best described by a transfer model in which phospholipids exchange predominantly through the water phase at low micellar concentrations and through transient micelle fusions at higher concentrations. Increasing the chain length of the micellar-saturated diacylphosphatidylcholine from 12 to 18 carbons resulted in a decrease in the overall rate of exchange by a factor of 127 for NBD-labeled dilaurylphosphatidylethanolamine and a factor of 2490 for NBD-labeled dipalmitoylphosphatidylethanolamine. The reduction in the overall rate resulted from decreases in both mechanisms of transfer. These results argue that the hydrophobicity of the lipophilic core of bile salt/phospholipid mixed micelles is the predominant determinant of the rate of formation of transfer-competent, transient micelle fusions and a major determinant of the rate of micelle to water phospholipid dissociation.