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Laxman Mainali, Marija Raguz, William J. O'Brien, Witold K. Subczynski; Comparison of Properties of Lens Lipid Membranes and Intact Plasma Membranes Derived from Porcine Eye-Lens Cortical and Nuclear Fiber Cells. Invest. Ophthalmol. Vis. Sci. 2011;52(14):1535.
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© ARVO (1962-2015); The Authors (2016-present)
Fiber-cell plasma membranes of the eye lens are saturated with cholesterol and are dense with membrane proteins. Cholesterol affects the organization of lipids in the lipid bilayer portion of the intact plasma membrane, inducing formation of immiscible cholesterol crystalline domains (CCDs). These effects can be investigated in lens lipid membranes. The structure and dynamics of the lipid bilayer portion of the intact membrane are also affected by the presence of membrane proteins. To understand the effects of proteins, we have compared the properties of intact membranes to those obtained for lens lipid membranes.
Conventional and saturation-recovery (SR) electron paramagnetic resonance (EPR) techniques with phospholipid and cholesterol analogue spin labels were used to assess physical properties and to discriminate membrane domains in lens lipid and intact membranes derived from the cortex and the nucleus of two-year-old pig eye lenses.
Three lipid environments were discriminated in the cortical and nuclear intact membranes compared to the one, and two lipid domains, respectively, in the cortical and nuclear lens lipid membranes. In intact membranes, the three domains are assigned to bulk, boundary, and trapped lipids (in protein aggregates). The amount of boundary and trapped lipids is greater in nuclear intact membranes than in cortical membranes, making them more rigid and resistant to oxygen permeation. In cortical lens lipid membranes, only homogeneous bulk lipids are present. While in nuclear lens lipid membranes, bulk lipids coexist with the CCD. CCDs cannot be discriminated in intact membranes using SR EPR because the oxygen transport parameter measured in the CCD is the same as in the domain formed by trapped lipids. The hydrophobic barrier to permeation of polar molecules is high in all membranes.
High cholesterol and protein contents in fiber-cell plasma membranes are responsible for the rigidity of the lipid bilayer portion of cortical and nuclear membranes. High amounts of cholesterol induce formation of the CCD, while contact with proteins induces formation of the lipid boundary layer and the domain formed by lipids trapped between proteins.
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