Chemical hydrolysis of phospholipids

Hydrolysis kinetics of phospholipids in liposomes composed of dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidylcholine/cholesterol (DPPC/CHOL) 10/4 (molar ratio) and egg phosphatidylcholine (EPC) at pH 4.0 and different temperatures could be described by Arrhenius curves without breaks. However, the Arrhenius curves for the hydrolysis of liposomal DPPC and distearoylphosphatidylcholine (DSPC) under the same conditions were biphasic. A break was observed in the curves extending over a broad range before and after the known Tm of each of these phospholipids in liposomes (42 and 56 degrees C, respectively). The activation energy (Ea) for the hydrolysis of liposomal DPPC and DSPC below the Tm was substantially larger than the Ea for liposomal DMPC, DPPC/CHOL 10/4, and EPC and decreased when DPPC was mixed with CHOL in a 10/4 molar ratio. Hardly any influence of the presence of alpha-tocopherol, cryoprotectants (glucose, trehalose, sucrose, and propylene glycol), and the major hydrolysis products lysophospholipids and fatty acids or of the absence of sodium chloride on the hydrolysis kinetics of DPPC at pH 4.0 and 30 degrees C was observed. Changes in fatty acid chains and size did not influence the hydrolysis rate constant (kobs) of liposomal phospholipids at pH 4.0 and 30 degrees C either. The only effects of uncharged compounds on the kobs of liposomal DPPC at pH 4.0 and 30 degrees C were found upon mixing with a high concentration of the detergent Triton X-100 or palmitic acid.(ABSTRACT TRUNCATED AT 250 WORDS)