Synthesis and characterization of N-acylated, pH-sensitive 'caged' aminophospholipids.

A series of acid-labile 'caged' phosphatidylserine (PS) and phosphatidylethanolamine (PE) molecules have been synthesized by N-acylation of the aminophospholipid with maleic, citraconic, dimethylmaleic, phthalic, or 3,4,5,6-tetrahydrophthalic anhydride. N-citraconyl-dioleoylphosphatidylethanolamine (C-DOPE) and N-citraconyl-dioleoyl-phosphatidylserine (C-DOPS) exhibited the highest degree of sensitivity to acidic pH; incubation at pH 5.5 and 6.5, respectively, resulted in 50% cleavage to the native aminophospholipid within 60 min. Significant cleavage of the phthalyl- and maleyl-PE derivatives did not occur at physiologically relevant pH values (pH 5.5-8), while tetrahydrophthalyl-PE and dimethylmaleyl-PE could not be isolated, reflecting their inherent instability. At pH 5.5, the half time for cleavage of C-DOPE and C-DOPS was 110 min and 85 min, respectively. The utility of these 'caged' lipids was demonstrated in human erythrocytes. When mixed with cells, C-DOPS, or the short chain analog, N-citraconyl-dilauroylphosphatidylserine (C-DLPS), transferred from liposome membranes into erythrocytes and remained in the cell outer monolayer. Low pH treatment released the citraconyl group and the free PS was transported to the inner monolayer by the aminophospholipid transporter. These novel 'caged' phospholipids may be useful for the study of transmembrane aminophospholipid transport, protein-lipid interactions and membrane fusion.