Membrane lipid α-crystallin interaction and membrane Ca2+-ATPase activities

Purpose. To determine the effect of a-crystallin binding on lens membrane lipid characteristics and the stability of Ca2+-ATPase activity when challenged with H2O2 or elevated temperatures. Methods. alpha-Crystallin binding to muscle sarcoplasmic reticulum membranes was quantified using a centrifugation protocol. alpha-Crystallin binding to lens epithelial lipids was measured by a fluorescence energy transfer technique. Lipid phase transition temperature and lipid order was measured using fluorescence spectroscopy. Ca2+-ATPase activity was measured using classical biochemical assays. Results. The main phase transition temperatures of multilamellar vesicles composed of sphingomyelin or lipids extracted from bovine lens were 40 degrees C and 20 degrees C, respectively. In the presence of saturating amounts of alpha-crystallin, the phase transition temperature and lipid order of both sphingomyelin and lens lipid membranes remained almost the same as that without alpha-crystallin. The interaction of alpha-crystallin and lipid is likely to be restricted to the membrane surface. The binding of alpha-crystallin did not influence the oxidative or thermal inactivation of the Ca2+-ATPase pump. Conclusion. alpha-Crystallin-lens membrane binding does not protect the Ca2+-ATPase pump from thermal derangement or oxidation by H2O2.