Calcium ATPase activity and membrane structure in clear and cataractous human lenses

Purpose. Maintenance of calcium homeostasis is imperative for the clarity of the lens. Ca²⁺-ATPase is essential for the removal of cytosolic calcium, either across the plasma membrane or through intracellular organelles such as the endoplasmic reticulum. In this study, membranes prepared from clear lens epithelium were compared to membranes prepared from cataractous lens epithelium. Methods. Human lens membranes were prepared by a protocol utilizing homogenization and centrifugation. Ca²⁺-ATPase activity was measured biochemically using Gamma-³²P labeled ATP. Lipid order was measured using infrared and Raman spectroscopy. Results. Ca²⁺-ATPase activity was similar in membranes prepared from cataractous lenses that were classified as nuclear subcapsular, nuclear and brunescent cataracts. Ca²⁺-ATPase activity was approximately 50% less in membranes prepared from cataractous lenses in comparison to clear lenses. Because clear lenses from Indian donors was unavailable, clear human lenses were used as a qualitative control for the measurement for Ca²⁺-ATPase activity. Lipid order was measured in lens fibers from cataractous and clear lenses from the United States donors. Lipid order increased from 55% in the hydrocarbon chains from clear lens fibers to 84% in cataractous lens fibers. Conclusions. These findings support the hypothesis that membranes are deranged in cataractous tissue, which should lead to altered levels of calcium.