Doug A. Fullington, David G. Shoemaker, and J. Wylie Nichols* Department of Physiology, Emory University School of Medicine, Atlanta, Georgia 30322 Received June 28,¡ 989; Revised Manuscript Received September 21, 1989 abstract: Concentration-dependent self-quenching of the fluorescent phospholipid Ar-(7-nitro-2, l, 3-benzoxadiazol-4-yl) phosphatidylethanolamine (7V-NBD-PE) was used to measure the rate of 7V-NBD-PE transfer between phosphatidylcholine-bile salt mixed micelles. In a previous study using the same technique, the rate of N-NBD-PE transfer between phosphatidylcholine-taurocholate mixed micelles was found to be several orders of magnitude faster than its transfer between phosphatidylcholine vesicles as a result of an increased rate of transfer through the water at low micelle concentrations and an increased rate of transfer during transient micelle collisionsat higher micelle concentrations [Nichols, J. W.(1988) Biochemistry 27, 3925-3931], In this study we have determined the influence of bile salt structure, incorporation of cholesterol, and temperature on the rate and mechanism of phospholipid transfer between mixed micelles. We found that both transfer pathways were a common property of mixed micelles prepared from a series of different bile salts and that the rates of transfer by both pathways increased as a function of the degree of bile salt hydrophobicity. Cholesterol incorporation into phosphatidylcholine-taurocholate mixed micelles displaced taurocholatefrom the micelles and resulted in an increased rate of transfer through the water and a decreased rate of transfer during micelle collisions. The temperature dependence of the transfer rates was used to calculate the activation free energy, enthalpy, and entropy for both mechanisms. The activation enthalpy was the majorbarrier to transfer by both mechanisms. However, the observed increase in the rate of phospholipid transfer through the water between mixed micelles relative to vesicles, and the increased rate of collision-dependent transfer between mixed micelles prepared with the more hydrophobic bile salts, are both primarily the result of increased activation entropy.

Fluorescent-labeled NBD1 phospholipids have recently been shown to transferrapidly between phospholipid-taurocholate mixed micelles when compared to their transfer between vesicles (Nichols, 1988). This increased rate of transfer was shown to result from an increase in the rate of phospholipid dissociation into the water phase (aqueous diffusion mecha-nism), which is the rate-limiting step at low concentrations of micelles, and to result predominantly from collision-de-