Biophys J, June 2000, p. 3011-3018, Vol. 78, No. 6

Strength of Ca²⁺ Binding to Retinal Lipid Membranes: Consequences for Lipid Organization

Daniel Huster,^* Klaus Arnold, and Klaus Gawrisch^*

 ^*Laboratory of Membrane Biochemistry and Biophysics, NIAAA, National Institutes of Health, Rockville, Maryland 20852 USA, and  Institute of Medical Physics and Biophysics, University of Leipzig, 04103 Leipzig, Germany

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 ²H NMR. Ca²⁺ association constants of polyunsaturated membranes were found to be a factor of ~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 Ca ²⁺ 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°C.

Biophys J, June 2000, p. 3011-3018, Vol. 78, No. 6
© 2000 by the Biophysical Society   0006-3495/00/06/3011/08  $2.00

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