pH-Dependent Ca²⁺ interaction with phospholipids related to phosphatidylethanolamine N-methylation

The interactions of PE and its N-methylated derivatives (PME, PDE and PC) with Ca²⁺ were examined. PE and the intermediate phospholipids of PE N-methylation (PME and PDE) interacted with Ca²⁺ in a pH-dependent and reversible manner. When these phospholipids were present in the heptane phase, Ca²⁺ in the aqueous phase was translocated into the heptane phase at alkaline pH but not at acidic pH. PDE was also effective for the translocation even at around neutral pH, while PC hardly translocated Ca²⁺ at pH 6.0-9.2. The amounts of Ca²⁺ interacting with these phospholipids were in the following order; PDE > PME > PE > PC. PI, phosphatidic acid and PS interacted with Ca²⁺ in the whole pH range examined. The Ca²⁺ interactions with PI and phosphatidic acid were independent of pH, while PS interacted with more Ca²⁺ at alkaline pH. These phospholipids interacted with Ca²⁺ most strongly among the cations studied. Liposomes containing PDE also bound the highest amounts of Ca²⁺ among PE and its N-methylated derivatives. Furthermore, mammalian cultured cell membranes, which contain increased amounts of PDE by in vivo modification with N,N'-dimethylethanolamine, bound more Ca²⁺ than those prepared from choline-treated control cells.