ESR spin label and ultrastructural monitoring of protein-lipid interactions in the lens fiber-cell plasma membranes in relation to human ageing and cataractogenesis.

The Electron Spin Resonance (ESR) technique and the protein spin labels 2,2,4,4,9-pentamethyl-1,2,3,4-tetrahydro-gamma-carboline-3-oxyl and 4-(N-maleimido)-2,2,6,6-tetramethylpiperidine-1-oxyl were used in this study to probe the fluidity and binding ability to protein functional groups in human lens membranes. The image and the stage of cataract as well as the ultrastructural characteristics of the lens fiber cell membranes were evaluated in parallel studies. ESR measurements in membrane structures of the transparent lenses of different ages have shown that the sorbtion parameter of the carboline label on the surface of protein-lipid components was usually weakly expressed but increased with aging and the extent of immobilization of the bound label was not significant. At different stages of the lens opacification the carboline analogue spin label increasingly bound to the lens membranous structures. A spin label signal can be enhanced by addition to the samples of the paramagnetic probe K3Fe(CN)6. The maleimide spin label bound to the protein SH-groups in the cataractous lens membranes at a slow rate, however in transparent lenses a gradual increase of the rapidly binding phase of this label could be detected. The results show an appearance of at least two types of the reactive SH-groups in membranes of human transparent lenses. The electron microscopic studies suggested that the age-related and cataractogenic changes in the lens matter are accompanied by deterioration of the lenticular fiber plasma membranes and formation of the coalescing globules with a diameter of 220-500 nm. At the stage of cataract with advanced opacities, which is biochemically characterized by the increase in number of the carboline label binding sites with the surface proteins and annular lipids of membranes, a mass of amorphous aggregates filled with the electron-grey debris is formed contributing to significant scatter of light. It appears, therefore, that the suggested ESR spin label technique can be effectively used to monitor the aggregation process of protein membrane components which takes place during human cataractogenesis.