Acceleration of oxime-induced reactivation of organophosphate-inhibited fetal bovine serum acetylcholinesterase by monoquaternary and bisquaternary ligands

Reactivation of organophosphate (OP)-inhibited acetylcholinesterase (AChE) by oximes is the primary reason for their effectiveness in the treatment of OP poisoning. Reactivation is reported to accelerate by quaternary ligands such as decamethonium, which is devoid of nucleophilicity. The mechanism of this enhancement is not known. To better understand the acceleration phenomenon, we examined ligand modulations of oxime-induced reactivation of methylphosphorylated AChE using 7-(methylethoxyphosphinyloxy)-1-methylquinolinium iodide and fetal bovine serum AChE. Edrophonium, decamethonium, and propidium, three quaternary AChE ligands of different types, were tested as potential accelerators. Experiments were carried out with both soluble enzyme preparation and AChE conjugated to polyurethane. Kinetic measurements with oximes 2-[hydroxyiminomethyl]-1-methylpyridinium chloride, 1,1'-trimethylene bis-(4-hydroxyimino methyl)-pyridinium dibromide, and 1,1'-[oxybis-methylene)bis[4-(hydroxyimino)-methyl]pyridiniuum dichloride showed that in the presence of 50 mu M edrophonium, the reactivation rate constants increased 3.3-12.0-fold; 200 mu M decamethonium produced a 1.6-3.0-fold enhancement of reactivation rate constants by the same oximes. Reactivation of the inhibited enzyme by 1-(2-hydroxyiminomethyl-1-pyridinium)-1-(4-carboxy-aminopyridinium)-dimethyl ether hydrochloride, 1-(2-hydroxyiminomethyl-1-pyridinium)-1-(3-carboxy-aminopyridinium)-dimethyl ether hydrochloride, and 1-[[[4-(aminocarbonyl)pyridino]methoxy]methyl]-2,4,-bis(hydroxyimino)methyl pyridinium dichloride was not affected by either ligand. Propidium slowed the reactivation of 7-(methylethoxyphosphinyloxy)-1-methylquinolinium iodide-inhibited AChE by all oximes. Results suggest that the accelerator site may reside inside the catalytic gorge rather than at its entrance and acceleration may be due to the prevention of reinhibition of the regenerated enzyme by the putative product, the phosphorylated oxime. In addition to the nucleophilic property of the oximate anion, some of the reactivators may carry an accelerating determinant, as characterized with respect to edrophonium and decamethonium. Results offer possible explanations for the superiority of 1-(2-hydroxyiminomethyl-1-pyridinium)-1-(4-carboxy-aminopyridinium)-dimethyl ether hydrochloride over other oximes in the reactivation of specific AChE-OP conjugates.