Effect of membrane fluidity upon binding of Electrophorus acetylcholinesterase to lipid vesicles

The multiple biological roles of the cholinesterases

It is tacitly assumed that the biological role of acetylcholinesterase is termination of synaptic transmission at cholinergic synapses. However, together with its structural homolog, butyrylcholinesterase, it is widely distributed both within and outside the nervous system, and, in many cases, the role of both enzymes remains obscure. The transient appearance of the cholinesterases in embryonic tissues is especially enigmatic. The two enzymes' extra-synaptic roles, which are known as 'non-classical' roles, are the topic of this review. Strong evidence has been presented that AChE and BChE play morphogenetic roles in a variety of eukaryotic systems, and they do so either by acting as adhesion proteins, or as trophic factors. As trophic factors, one mode of action is to directly regulate morphogenesis, such as neurite outgrowth, by poorly understood mechanisms. The other mode is by regulating levels of acetylcholine, which acts as the direct trophic factor. Alternate substrates have been sought for the cholinesterases. Quite recently, it was shown that levels of the aggression hormone, ghrelin, which also controls appetite, are regulated by butyrylcholinesterase. The rapid hydrolysis of acetylcholine by acetylcholinesterase generates high local proton concentrations. The possible biophysical and biological consequences of this effect are discussed. The biological significance of the acetylcholinesterases secreted by parasitic nematodes is reviewed, and, finally, the involvement of acetylcholinesterase in apoptosis is considered.

The topography of acetylcholinesterase in dimyristoylphosphatidylcholine liposomes

Unilamellar liposomes prepared from sn-3-(dimyristoyl)phosphatidylcholine (DMPC) in the presence and absence of acetylcholinesterase were examined by ESR for lipid/protein interactions. Using 5-, 12-, and 16-doxyl stearic acid probes incorporated into the phospholipid bilayers, no measurable differences in the gel to liquid-crystalline phase transition temperature of DMPC (as determined by ESR spectroscopy) were observed when the enzyme was present. These results have established that no significant incorporation of acetylcholinesterase into the hydrophobic region of the phospholipid bilayer is detectable at the protein: lipid ratios used in these experiments. Confirmation of these results was also obtained by differential scanning calorimetry. Interaction of the enzyme with the outermost region of the bilayer was established by trypsin digestion which indicated that as much as 25% of liposome-associated enzyme was removable and was, therefore, exposed to the outer surface. The results of this study have established that acetylcholinesterase associated with unilamellar DMPC liposomes was primarily entrapped within the aqueous compartment of the vesicles and was not present in the phospholipid bilayer.

Liposomes are effective carriers for the ocular delivery of prophylactics

Liposomes containing acetylcholinesterase were prepared by the freeze-drying method. The multilamellar morphology of the vesicles was revealed by freeze-fracture electron microscopy and their size distribution was determined by quasi-elastic light scattering. The vesicle diameters were in the range of about 0.2-4.0 micron. The liposome preparations were tested for their ocular delivery of an entrapped cholinesterase enzyme in counteracting the miotic effect of diisopropylfluorophosphate (DFP), a prototype of a family of organophosphate poisons. The topical application of the enzyme-containing liposomes to the rabbit eye was found to confer a significant level of protection against DFP-induced miosis. In comparing the prophylactic effectiveness of different enzyme-bearing liposomes, positively charged vesicles were found to be more effective than either neutral or negatively charged vesicles. Although the precise protective mechanism is not clear, our in vitro studies indicate that DFP molecules freely associate with liposomes and tear fluid promotes the release of liposome-entrapped enzymes. Thus, it is conceivable that the enzyme-liposome complex may act somewhat like a sponge by sequestering DFP molecules which diffuse into the vesicle, and also by releasing the entrapped enzyme to combine with DFP, thereby neutralizing its in vivo toxic effect.