Ca(2+)-ATPase activity and lens lipid composition in reconstituted systems.

Lens lipid composition and lipid hydrocarbon chain structure change with age, region and cataract. Since the lens Ca(2+)-ATPase pump is important to the maintenance of calcium homeostasis and lens clarity, muscle sarcoplasmic reticulum Ca(2+)-ATPase was reconstituted with bovine lens lipids and dihydrosphingomyelin, the rare and major phospholipid of the human lens. Ca(2+)-ATPase activity was found to be about 5 times lower when the pump was reconstituted into dihydrosphingomyelin or lens lipids compared to native sarcoplasmic reticulum lipids. The addition of cholesterol to levels ranging from 13-53 mole%, had no affect on reconstituted Ca(2+)-ATPase activity. Ca(2+)-ATPase activity correlated with the degree of hydrocarbon chain saturation. The greater levels of saturation are a consequence of the high sphingolipid content in the reconstituted systems. These data support the hypothesis that changes in lens lipid composition or structure could affect Ca(2+)-ATPase activity in human lenses. Because the mechanisms governing Ca(2+)-ATPase activity in vivo are much more complex than in these simple reconstituted systems, this study represents an initial step in the elucidation of the relationships of endogenous membrane lipid composition-structure and function.