Role of edrophonium in prevention of the re-inhibition of acetylcholinesterase by phosphorylated oxime

Abstract

We examined the role of edrophonium in the acceleration phenomenon using mouse wild-type and mutant D74N AChE inhibited with 7-(O,O-diethyl-phosphinyloxy)-1-methylquinolinium methylsulfate (DEPQ). With DEPQ-inhibited wild-type mouse acetylcholinesterase (AChE), the reactivation kinetic profile demonstrated one-phase exponential association only when 2-[hydroxyimino methyl]-1-methylpyridinium chloride (2-PAM) and 1-(2-hydroxy-iminomethyl-1-pyridinium)-1-(4-carboxy-aminopyridi nium)-dimethyl ether hydrochloride (HI-6) were used as reactivators. When 1,1[oxybis-methylene)bis[4-(hydroxyimino)methyl] pyridinium dichloride (LüH6) and 1,1-trimethylene bis(4-hydroxyimino methyl) pyridinium dichloride (TMB4) were used, the reactivation kinetic profile was biphasic in nature. Edrophonium had no effect on reactivation by 2-PAM and HI-6, but significantly accelerated LüH6- and TMB4-induced reactivation of DEPQ-inhibited wild-type mouse AChE. Comparison of the initial and overall reactivation rate constants with five oximes indicated that acceleration by edrophonium may be due to the prevention of re-inhibition of the reactivated enzyme by the phosphorylated oxime (POX) produced during the reactivation. With LüH6 and TMB4, about 2.5-fold increase in the reactivation rate constants was observed in the presence of edrophonium, but little or no effect was observed with the other three oximes. The initial reactivation rate constants were 5.4- and 4.2-fold of the overall rate constants with LüH6 and TMB4 as reactivators respectively, however, very little change was found between the initial and overall rate constants with the other three oximes. In experiments with D74N AChE, for which the inhibition potency of charged organophosphate (OP) was two to three orders less than wild-type enzyme, edrophonium had no effect on the reactivation by LüH6 and TMB4 and the time courses of reactivation were monophasic. The data from mutant enzyme substantiate the involvement of edrophonium in protecting POX re-inhibition of reactivated enzyme formed during the reactivation of OP-inhibited AChE.