Lipid exchange proteins

The non-random distribution of the responsive (18 0,20 4)phosphatidyl inositol and its derivatives implies that these responsive molecules are not freely transported from membrane to membrane by lipid exchange protein and that they do not have free lateral... 

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. 

Lipids are transported between membranes. As indicated above, lipids are often biosynthesized in one intracellular membrane and must be transported to other intracellular compartments for membrane biogenesis. Because lipids are insoluble in water, special mechanisms must exist for the inter- and intracellular transport of membrane lipids. Vesicular trafficking, cytoplasmic transfer-exchange proteins and direct transfer across membrane contacts can transport lipids from one membrane to another. The best understood of such mechanisms is vesicular transport, wherein the lipid molecules are sorted into membrane vesicles that bud out from the donor membrane and travel to and then fuse with the recipient membrane. The well characterized transport of plasma cholesterol into cells via receptor-mediated endocytosis is a useful model of this type of lipid transport. [9, 20]. A brain specific transporter for cholesterol has been identified (see Chapter 5). It is believed that transport of cholesterol from the endoplasmic reticulum to other membranes and of glycolipids from the Golgi bodies to the plasma membrane is mediated by similar mechanisms. The transport of phosphoglycerides is less clearly understood. Recent evidence suggests that net phospholipid movement between subcellular membranes may occur via specialized zones of apposition, as characterized for transfer of PtdSer between mitochondria and the endoplasmic reticulum [21]. 

The method of introduction of the fluorophore into the membrane is also important. Many probes are introduced into preexisting vesicles, natural membranes, or whole cells by the injection of a small volume of organic solvent containing the fluorophore. For DPH, tetrahydrofuran is commonly used, while methanol is often employed for other probes. The amount of solvent used should be the absolute minimum possible to avoid perturbation of the lipids, since the solvent will also partition into the membrane. With lipid vesicles this potential problem can be avoided by mixing the lipids and fluorophore followed by evaporation of the solvent and codispersing in buffer. For fluorophores attached to phospholipids, this is the only way to get the fluorophore into the bilayer with natural membranes, phospholipid exchange proteins or other techniques may have to be employed. 

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-... 

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. 

Human lipoproteins exist in several sizes and densities with differing lipid to protein ratios. These various lipoproteins have different origins in the body, different destinations and different functions (10). Thus, chylomicrons are extremely large low density particles formed in the intestine and designed to deliver dietary fat to adipose tissue. Very low density lipoproteins (VLDL), on the other hand, are smaller, more dense particles designed to deliver lipids from the liver to adipose and other tissues. Low density lipoproteins (LDL), formed from VLDL or produced in the liver or intestine deliver cholesterol to peripheral tissue, while high density lipoproteins (HDL) function to return cholesterol from peripheral tissues to the liver for catabolism. There is a complex exchange of lipids and apoproteins between the lipoprotein classes. 

Diffusion is the random movement of a particle because of an exchange of thermal energy with its environment. Membrane lipids and proteins participate in highly anisotropic translational and rotational diffusion motion. Translational diffusion in the plane of the membrane is described by the mean square lateral displacement after a time At (r ) = TD At. Lipid lateral diffusion coefficients in fluid phase bilayers are typically in the range Dj 10 to 10 cm /s (3). 

Plasma lipoproteins are in a dynamic state. Their continuous synthesis and degradation are accompanied by rapid exchanges of lipid and protein components between... 

The bioconcentration factor is the concentration of a chemical in a tissue per concentration of the chemical in water (generally adimensional) [Pavan, Netzeva et al, 2008]. This physical property characterizes the uptake of pollutants due to chemical partitioning from environmental phase (e.g., air or water) into an organic phase (e. g., lipids or proteins) through an exchange surface (e.g., gills of fish). 

In the late 1960s, Wirtz and Zilversmit (1968) found a soluble factor in rat liver cytosol that accelerated the exchange of phospholipids between biological membranes. Since this discovery, many lipid transfer proteins have been purified. A list of these and their physical properties is found in Table I. Although there has been considerable speculation concerning the physiological role of lipid transfer proteins, their precise function remains in doubt (Wirtz, 1982). 

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