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Studies on the roles of lipids in membrane structure and function

University of British Columbia

Lipids in membranes satisfy two basic roles. First they provide a matrix with which membrane proteins are associated and second, they provide a structural bilayer for maintaining a permeability barrier. This thesis investigates certain aspects of the structural and permeability barrier properties of lipids employing simple protein free model membrane systems. It is demonstrated, employing ³¹P and ²H NMR techniques, that cholesterol engendered formation of non-bilayer structures in multilamellar vesicles (MLVs) composed of phosphatidylethanolamine (PE) and either phosphatidylserine (PS) or phosphatidylcholine (PC). Further the presence of cholesterol, in conjunction with Mg²⁺, facilitated Ca²⁺triggered formation of non-bilayer organizations in the PS containing systems. It is indicated that in these complex multicomponent systems where multiple structural phases (ie. bilayer, hexagonal, and "isotropic") coexist, that the phospholipids exhibit ideal mixing behaviour. A basic consequence of the barrier properties of a lipid bilayer is an ability to maintain a membrane potential (Δψ), which is required for a variety of membrane mediated transport processes. To investigate the role of lipids in maintaining Δψ, and the direct effect of Δψ on transport functions, a large unilamellar vesicle (LUV) preparation free of impurities is required. This thesis describes a novel procedure for generating LUVs, by extrusion of MLVs through polycarbonate filters (pore size 100 nm). LUVs can thus be obtained from a wide variety of lipid species and mixtures in the absence of lipid solubilizing agents. Vesicles exhibiting Δψ in response to a Na⁺/K⁺ ion gradient (K⁺ inside) are characterized. It is shown that such a K⁺ diffusion potential can drive the uptake of a variety of biologically active molecules (eg. local anaesthetics, antineoplastic agents, biogenic amines (dopamine)) which have cationic and lipophilic characteristics. The transport process appears to proceed by a antiport cation/lipophilic cation exchange process that is driven by the transmembrane potential.

Text

2010-05-31

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http://hdl.handle.net/2429/25247

Biochemistry and Molecular Biology

University of British Columbia

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