Ca2+-ATPase activity and lens lipid composition in reconstituted systems

Lens lipid composition and lipid hydrocarbon chain structure change with ag e, region and cataract. Since the lens Ca2+-ATPase pump is important to the maintenance of calcium homeostasis and lens clarity, muscle sarcoplasmic r eticulum Ca2+-ATPase was reconstituted with bovine lens lipids and dihydro sphingomyelin, the rare and major phospholipid of the human lens. Ca2+-ATPa se activity was found to be about 5 times lower when the pump was reconstit uted into dihydrosphingomyelin or lens lipids compared to native sarcoplasm ic reticulum lipids. The addition of cholesterol to levels ranging from 13- 53 mole%, had no affect on reconstituted Ca2+-ATPase activity. Ca2+-ATPase activity correlated with the degree of hydrocarbon chain saturation. The gr eater levels of saturation are a consequence of the high sphingolipid conte nt in the reconstituted systems. These data support the hypothesis that cha nges in lens lipid composition or structure could affect Ca2+-ATPase activi ty in human lenses. Because the mechanisms governing Ca2+-ATPase activity i n vivo are much more complex than in these simple reconstituted systems, th is study represents an initial step in the elucidation of the relationships of endogenous membrane lipid composition-structure and function. (C) 1999 Academic Press.