Abstract
Because an amphiphile with a positive or negative charge, shch as dialkylphosphate or stearylamine, is often added to liposomal phosphatidylcholine (PC) dispersions to prevent aggregation of the liposomes, we investigated the long-term stability of liposomes prepared from saturated PC in the presence of various amphiphiles. On storage of these liposomes at 40°C, PC was gradually hydrolyzed by dialkylphosphate, a negatively charged lipid, while neither stearylamine, a positively charged lipid, nor a non-charged lipid hydrolyzed PC at all. This hydrolysis of PC was examined using dialkylphosphates of various alkyl chain lengths (C10, C12, C14, C16, C18 and C20) and PC with different fatty acyl chain lengths (C14, C16 and C18). The rate of hydrolysis was maximum when the alkyl chain length of dialkylphosphate was almost equal to the fatty acyl chain length of PC. That is, the hydrolysis of dimyristoyl (C14) and dipalmitoyl (C16) acyl chains of PC was accelerated most by the incorporation of dimyristylphosphate (C14) and dipalmitylphosphate (C16), respectively. Distearoyl (C18) acyl chains of PC were hydrolyzed effectively by the incorporation of distearylphosphate (C18) as well as dipalmitylphosphate (C16). The hydrolysis did not occur when methyl dipalmitylphosphate was added instead of dipalmitylphosphate, or when the liposomal structure was decomposed by adding ethanol. These results suggest that dialkylphosphate and PC are aligned head to tail in liposomes and that the phosphate functional group causes the hydrolysis of the esters of PC.The incorporation of cholesterol into PC bilayers suppressed the hydrolysis of PC above the phase transition temperature (Tc) of PC, but increased it below the Tc. The hydrolysis of PC by dialkylphosphate appears to depend on membrane fluidity and to be accelerated with increased membrane fluidity, because cholesterol reduces the fluidity of the liposomal membrane above the Tc and enhances it below the Tc.
