Structural characterization of lipid membranes from clear and cataractous human lenses.

Lipid composition related structural changes in human cataractous lenses was explored by characterizing the hydrophobic hydrocarbon chains in lipid membranes corresponding to twelve Indian cataractous lenses and eight American clear lenses of similar age. The nuclear-lipid phase transitions corresponding to the clear lenses exhibited significantly higher average transition temperatures (nucleus 33 degrees C, cortex 26.3 degrees C) and cooperativities, 38.1, as compared to the value of 24.1 for the cortical-lipid phase transitions. At 36 degrees C, the phase transitions corresponding to cortical and nuclear lipids indicate a similar degree of disorder, 63%, in the hydrocarbon chains, i.e., similar relative amounts of gauche and trans rotomers. The twelve cataractous lenses investigated all had nuclear opacities, four were brunescent and four had cortical opacities. No significant differences were observed in the phase transition parameters (temperature, cooperativity, magnitude, enthalpy) evaluated for the nuclear-lipid membranes corresponding to the different types of cataracts. Furthermore, for the cataractous membranes, the phase transition parameters obtained for the nuclear lipids were comparable to those evaluated for the cortical lipid membranes. However, the cortical lipids exhibited the highest order in membranes from nuclear cataracts without cortical opacity. The cortical lipids from clear, non-cataractous lenses had the lowest level of order. At 36 degrees C, the degree of order in the cortical lipid from clear lenses was comparable to that from nuclear cataractous lenses without cortical opacity. The transition temperature, and cooperativity were significantly higher for cortical lipids from cataractous lenses as compared to those from clear lenses. At 36 degrees C, the degree of order in the cortical lipid membranes was lower for all cataract types vs. clear lens fractions. Our results suggest the possibility that lipid-lipid interactions could be different in cataractous lens membranes. Lipid compositional and chemical differences must account for these altered lipid interactions. These studies will provide a basis for studying lipid-protein interactions and structure-function relationships in the lens membrane.