Decreased stability in liposomal suspensions: accelerated loss of p-nitrophenyl acetate.

The goal of this investigation was to determine the reason for the previously reported increase in the rate of hydrolysis of p-nitrophenyl acetate to p-nitrophenol in the presence of positively charged liposomes. When this charge was due to incorporation of stearylamine, the rate of loss increased 5- to 10-fold relative to the control buffers. This rate enhancement was accompanied by formation of N-stearylacetamide, an event which was not previously considered. Similar results were obtained with either L-alpha- or dimyristoyl phosphatidylcholine. When the positive charge on the liposomes was conferred by the cetrimonium ion, however, the acceleration was replaced by a reduction in rate together with the absence of amide formation. Separation of the continuous phases from the liposomes provided media which were kinetically equivalent to the control buffers, indicating that rate enhancement and reduction were both due to the liposomal phases. Increasing the pH produced an increase in ester clearance values due to the stearylamine-containing liposomal phase, which is consistent with the formation of free amine, providing increased aminolysis. Although amide formation was also observed in stearylamine suspensions, the rate of p-nitrophenyl acetate loss was much greater in liposomal suspensions. Accelerated loss in the presence of positively charged liposomes is due to the formation of N-stearylacetamide by reaction with stearylamine and not to the positive charge, a hypothesis disproved by use of cetrimonium ion containing liposomes.