Phosphatidylcholine and cholesterol inhibit phosphatidate-mediated calcium traversal of liposomal bilayers

Rates of phosphatidic acid- (PA-) mediated Ca2+-traversal are maximal in ‘passive bilayers’ void of lipid CO and OH groups: dietherphosphatidylcholine (diether-PC) or OH-blocked cholesterol liposomes. Phosphatidylcholine (PC) as bilayer matrix causes 99% inhibition, while 45 mol% cholesterol in passive bilayers inhibits by about 70%. Possibly, the absence of CO and OH groups causes a dehydration of the ‘hydrogen belts’, i.e., the membrane strata occupied by hydrogen bond acceptors (CO of phospholipids) and donors (OH of cholesterol, sphingosine) and thereby facilitates the formation of dehydrated Ca(PA)2, the ionophoric vehicle; or (our preferred explanation) PC engages in a (non-ionophoric) Ca(PA - PC) complex and thus reduces the concentration of the ionophore, while cholesterol competes with Ca2+ for the CO groups of phosphatidic acid by hydrogen-bonding. The Ca2+-traversal rates realized in bilayers with modified hydrogen belts lend support to the speculation that a Ca(PA)2 ferry may be of physiological importance, e.g., in membranes (such as myelin) containing much ether phospholipid (plasmalogen); and that Ca2+-membrane association and traversal may be controlled by the composition of the hydrogen belts.