Butyrylcholinesterase formulated in liposomes.

Exogenous cholinesterases have the potential to take part in defence against organophosphate toxins, by acting as scavenger systems. Postulating that formulation in liposomes could enhance the toxin-scavenging potential of these enzymes, we have initiated studies of such formulations and are reporting here our first steps, exploring butyrylcholinesterase (BChE) in multilamellar liposomes composed of phosphatidylcholine. We started by developing an essential research tool: a multisample, sensitive and rapid enzyme-activity assay, based on the Ellman reaction, that could be performed directly on liposome-containing samples. Using an ELISA reader equipped to follow time-dependant absorbency changes, 10 min sufficed to assay 96 samples simultaneously. Next, several key properties of liposome-formulated BChE were explored and the major findings were: (i) the encapsulated enzyme was found to retain its activity. (ii) Enzyme activity was found to increase (at constant enzyme concentration) in the presence of the lipid, in a lipid-concentration-dependant manner. Through data analysis it was possible to attribute this effect to changes in k(cat). (iii) Good, reproducible, encapsulation efficiencies (for macromolecules) in the range of 30% were obtained at liposome concentrations of 100 mM lipid. (iv) Free BChE was completely susceptible to proteolysis under conditions mimicking enzymically-hostile biological environments, whereas > or = 60% of the liposome-formulated BChE was protected, found to be inaccessible to the proteolytic enzymes. (v) Short-term exposures of free and liposome-encapsulated BChE to the inhibitor paraoxon, generated significant losses in enzyme activity. Residual activities of both BChE formulations dropped considerably over the paraoxon concentration range of 0.02-0.11 microM, down to 3 and 11% for free and liposome-encapsulated enzyme respectively. These data are a clear indication that the encapsulated BChE was accessible to the inhibitor, indicating that such liposomal formulations have the potential to perform as the desired scavenger systems.