Reactivation by imidazo-pyridinium oximes of acetylcholinesterase inhibited by organophosphates. A study with an immobilized enzyme method.

A new procedure is described for studying inhibition and reactivation of acetylcholinesterase (AChE). The enzyme, electric eel AChE, is immobilized on fiberglass and the enzymatic activity is continuously monitored in an open reactor by an assay adapted from the Ellman's method. The use of immobilized AChE permits independent inhibition and reactivation of the enzyme. Side-reactions between substrate, inhibitor and reactivator are avoided. This method is used to determine the reactivating efficiency of a new series of imidazo-pyridinium oximes for the enzyme inhibited by different organophosphorous compounds. Kinetic parameters of reactivation were determined after AChE inhibition by sarin, VX and paraoxon. The more efficient reactivators have a short methylene bridge (C3 to C6) between imidazolium and pyridinium rings. Against soman inhibition, the pyrimidoxime or 1-(1-methyl-imidazolinium) 3-(4-carbaldoxime-pyridinium) propane dibromide, introduced immediately after the inhibitor, gives the same result as TMB-4 (37% reactivation). 1-benzyl 2-carbaldoxime pyridinium bromide was found to be more potent in reactivating tabun inhibited AChE than pyrimidoxime. Imidazo-pyridinium oximes with a long methylene bridge (C8 to C10) are good reversible inhibitors of free AChE (Ki less than 1 microM).