Lipid exchange

Kawasaki, K., J.-J. Yin, W. K. Subczynski, J. S. Hyde, and A. Kusumi. 2001. Pulse EPR detection of lipid exchange between protein-rich raft and bulk domains in the membrane Methodology development and its application to studies of influenza viral membrane. Biophys. J. 80 738-748. 

Figure 5.27 Lipoprotein metabolism. Lipid exchange between lipoprotein particles and cells.

Lipoproteins are classified into five groups. In order of decreasing size and increasing density, these are chylomicrons, VLDLs (very-low-density lipoproteins), IDLs (inter-mediate-density lipoproteins), LDLs (low-density lipoproteins), and HDLs (high-density lipoproteins). The proportions of apoproteins range from 1 % in chylomicrons to over 50% in HDLs. These proteins serve less for solubility purposes, but rather function as recognition molecules for the membrane receptors and enzymes that are involved in lipid exchange. 

While reconstitution of the calcium-dependent ATPase from the lipid deprived enzyme can easily be achieved, attempts to reconstitute simultaneously the abolished accumulation of calcium had no success55,70. Yet, in a number of reports the reconstitution of calcium transport from the enzyme after purification and/or after lipid exchange has been described160,, 70 172) jn these experiments it was attempted to reconstitute vesicles which could retain calcium ions which were transported into the vesicular space by the transport protein across the lipid bilayer. Different lipid pro-... 

FIGURE 14.1 Scheme of the proposed interaction mechanisms of liposomes with cells (a) adsorption, (b) endocytosis, (c) lipid exchange, and (d) fusion. (From Ostro, M. J. (1S8J.)Am. 257, 102-111. With permission.)... 

DE Epps, KA Greenlee, JS Harris, EW Thomas, CK Castle, JF Fisher, RR Hozak, CK Marschke, GW Melchior, FL Kezdy. Kinetics and inhibition of lipid exchange catalyzed by plasma cholesteryl ester transfer protein (lipid transfer protein). Biochemistry 34 12560-12569, 1995. 

The hydrophobic lipid anchors can be either fatty chains (e.g. derived from oleic or myristic acid) or a cholesterol group. Lipid anchors help in forming liposomes (or micellar structures) and determine the physical properties of a lipid bilayer, such as membrane rigidity and rate of lipid exchange between lipid... 

Rapid and selective aggregation (in less than 30 s) occurs between the complementary liposomes, which is followed by lipid exchange (within 30 min after mixing). The lipid exchange, which takes place when the membranes are in contact, results either in fusion or, if fusion does not occur, to a redispersion of the liposomes within 17 h. 

Molecular dynamics, diffusion, trans-bilayer translocation, and lipid exchange in bilayers... 

PHYSICAL PROPERTIES MOLECULAR DYNAMICS, DIFFUSION, TRANS-BILAYER TRANSLOCATION, AND LIPID EXCHANGE IN BILAYERS... 

Theoretically, the fluorescence of the mock fusion control should be the same as fluorescent plus nonfluorescent liposomes in a 1 10 ratio that are 100% fused (and thus this value could be used as Tmax or factored into the equation provided in Subheading 3.2.2., step 5). However, in practical application, the efficiency of fluorescent lipid incorporation tends to be variable, and the distribution of the fluorescent lipid in the two faces of the liposomal bilayer may not be equivalent. An additional concern is that lipid exchange may occur. These factors are difficult to control. 

To quantitate the lipid transfer activity of a protein, one measures the movement of labeled lipids from one membrane, the donor, to a second membrane, the acceptor. Typically, the donor and acceptor membranes are incubated in the presence and absence of transfer protein. After the incubation, the particles are separated and either the loss of radiolabeled lipids from the donor particles or the appearance of radiolabeled lipids in the acceptor particles is quantitated. The rate of lipid transfer in the presence of protein minus the transfer that occurs in the absence of protein is a measure of the lipid transfer activity of the protein. The transfer activity is expressed as a percent of the donor lipid transferred or the number of nmols lipid transferred per unit of time. To determine if the rate of lipid transfer also represents the rate of exchange, it must first be established that lipid exchange occurs between donors and acceptors. Exchange occurs when the rate of lipid transfer from donor to acceptor equals the rate of transfer from acceptor to donor or when the chemical composition of the donor and acceptor membranes does not change during the transfer reaction. 

Morgan, C.G., Yianni, Y.P., Sandhu, S.S., and Mitchell, A.C. (1995a) Liposome fusion and lipid exchange on ultraviolet irradiation of liposomes containing a photochromic phospholipid, Photochem. Photobiol., 62, 24—29. 

---