Strength of Ca2+ binding to retinal lipid membranes:: Consequences for lipid organization

There is evidence that membranes of rod outer segment (ROS) disks are a high-affinity Ca2+ binding site. We were interested to see if the high occurrence of sixfold unsaturated docosahexaenoic acid in ROS lipids influences Ca2+-membrane interaction. Ca2+ binding to polyunsaturated model membranes that mimic the lipid composition of ROS was studied by microelectrophoresis and H-2 NMR. Ca2+ association constants of polyunsaturated membranes were found to be a factor of similar to 2 smaller than constants of monounsaturated membranes. Furthermore, strength of Ca2+ binding to monounsaturated membranes increased with the addition of cholesterol, while binding to polyunsaturated lipids was unaffected. The data suggest that the lipid phosphate groups of phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylserine (PS) in PC/PE/PS (4:4:1, mol/mol) are primary targets for Ca2+. Negatively charged serine in PS controls Ca2+ binding by lowering the electric surface potential and elevating cation concentration at the membrane/water interface. The influence of hydrocarbon chain unsaturation on Ca2+ binding is secondary compared to membrane PS content. Order parameter analysis of individual lipids in the mixture revealed that Ca2+ ions did not trigger lateral phase separation of lipid species as long as all lipids remained liquid-crystalline. However, depending on temperature and hydrocarbon chain unsaturation, the lipid with the highest chain melting temperature converted to the gel state, as observed for the monounsaturated phosphatidylethanolamine (PE) in PC/PE/PS (4.4.1, mol/mol) at 25 degrees C.