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M. Raguz, W. J. O'Brien, W. K. Subczynski; Characterization of the Porcine Eye-Lens Cortical and Nuclear Fiber-Cell Plasma Membranes. Invest. Ophthalmol. Vis. Sci. 2010;51(13):6356.
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One of the most unique features of the eye lens fiber-cell plasma membrane is its extremely high cholesterol content. Cholesterol not only saturates the bulk phospholipid bilayer, but also induces formation of the immiscible cholesterol crystalline domains (CCD). The fiber-cell membranes are dense with membrane proteins. It has been estimated that more than 50% of lipid molecules in native lens membranes are in contact with intrinsic proteins. We hypothesize that the association of the membrane proteins with membrane lipids should significantly affect the physical properties of the lipid-bilayer portion of the membrane.
Lipid n-doxylstearic acid spin labels and conventional as well as saturation recovery electron paramagnetic resonance (EPR) spectroscopy were used to assess physical properties of the lipid-bilayer portion of the plasma membranes-including profiles of the order parameter, hydrophobicity, and the oxygen diffusion-concentration product (oxygen transport parameter).
The physical properties, obtained for membranes containing membrane proteins, were compared with the properties of membranes made from the total lipid extract of porcine eye lenses, which did not contain proteins. We found that the lipid-bilayer portion of the intact membranes was significantly less fluid and better ordered than the lipid bilayer made of extracted lipids. Additionally, the movement of small hydrophobic molecules, such as molecular oxygen, was drastically restricted. However, hydrophobicity profiles obtained for biological and model membranes were nearly the same, showing the existence of a high hydrophobic barrier to movement of polar molecules across the fiber-cell plasma membrane. It was also evident that the rigidity of nuclear fiber-cell plasma membranes was greater than that of cortical membranes. The permeability of oxygen across the lipid portion of the fiber-cell plasma membrane (4.4 - 20.6 cm s-1) was significantly lower than that across the lipid bilayer made of the lipid extract (57.2 - 65.1 cm s-1).
The fiber-cell plasma membrane forms a significant barrier to oxygen transport, which should help maintain a low oxygen concentration in the eye-lens interior. A great deal of evidence suggests that an increase in oxygen concentration in the lens interior can lead to the development of cataracts. Our data provide the basis for a mechanism by which the transport of oxygen within the eye lens is controlled.
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