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Marija Raguz, Ivana Krpan, William J. O'Brien, Laxman Mainali, Witold K. Subczynski; Physical Properties of the Cortical and Nuclear Fiber-Cell Plasma Membranes from the Human Eye Lens. Invest. Ophthalmol. Vis. Sci. 2011;52(14):3413.
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The cholesterol content of human lenses is significantly higher than in lenses from other animals. Cholesterol-to-phospholipid molar ratios in fiber cell membranes range from 1 to 2 in the cortex to 3 to 4 in the nucleus. At highest content, cholesterol saturates the phospholipid bilayer and forms cholesterol crystalline domains. It is estimated that in human lens membranes more than 50% of the lipid molecules are in contact with intrinsic proteins forming boundary and/or trapped lipid domains. We hypothesize that such composition should strongly affect structure and dynamics of the lipid-bilayer portion of the membrane.
Profiles of the alkyl chain order, hydrophobicity, and oxygen transport parameter (oxygen diffusion-concentration product), were obtained using conventional and saturation-recovery electron paramagnetic resonance spectroscopy and lipid spin labels. These methods provide a unique opportunity to assess physical properties and to discriminate coexisting membrane domains of cells.
Profiles of physical properties were obtained for membranes of cortical and nuclear fiber cells isolated from 20 human lenses from 61 to 80 year-old donors. We found that the lipid-bilayer portions of membranes of cells from both regions were significantly less fluid and better ordered than lipid bilayers with similar lipid composition without proteins. Hydrophobicity profiles for cortical and nuclear membranes were almost identical, showing the existence of a high hydrophobic barrier to permeation of polar molecules. Two different lipid domains, bulk-boundary and slow oxygen transport domains (formed by trapped lipids), were discriminated in membranes of cells from both regions. It is evident that the rigidity of nuclear fiber-cell membranes was greater than that of membranes from cortical cells. Nuclear fiber-cell membranes were less permeable to oxygen than membranes of cortical cells.
The fiber-cell plasma membrane forms a significant barrier for permeation of oxygen. This barrier is greater for membranes of nuclear fiber cells than membranes of cortical cells, a property that helps maintain a low oxygen concentration in the eye-lens center. This physical property likely contributes to the prevention of cataracts by reducing oxygen concentrations in the interior of the lens.
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