ACCELERATION OF OXIME-INDUCED REACTIVATION OF ORGANOPHOSPHATE-INHIBITED FETAL BOVINE SERUM ACETYLCHOLINESTERASE BY MONOQUATERNARY AND BISQUATERNARY LIGANDS

Reactivation of organophosphate (OP)-inhibited acetylcholinesterase (A ChE) by oximes is the primary reason for their effectiveness in the tr eatment of OP poisoning. Reactivation is reported to accelerate by qua ternary ligands such as decamethonium, which is devoid of nucleophilic ity. The mechanism of this enhancement is not known. To better underst and the acceleration phenomenon, we examined ligand modulations of oxi me-induced reactivation of methylphosphorylated AChE using 7-(methylet hoxyphosphinyloxy)-1-methylquinolinium iodide and fetal bovine serum A ChE. Edrophonium, decamethonium, and propidium, three quaternary AChE ligands of different types, were tested as potential accelerators. Exp eriments were carried out with both soluble enzyme preparation and ACh E conjugated to polyurethane. Kinetic measurements with oximes 2-[hydr oxyiminomethyl]-1-methylpyridinium chloride, 1,1'-trimethylene bis-(4- hydroxyimino methyl)-pyridinium dibromide, and -methylene)bis[4-(hydro xyimino)-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 in hibited enzyme by pyridinium)-1-(4-carboxy-aminopyridinium)-dimethyl e ther hydrochloride, pyridinium)-1-(3-carboxy-aminopyridinium)-dimethyl ether hydrochloride, and idino]methoxy]methyl]-2,4,-bis(hydroxyimino) methyl pyridinium dichloride was not affected by either ligand. Propid ium slowed the reactivation of 7-(methylethoxyphosphinyloxy)-1-methylq uinolinium iodide-inhibited AChE by all oximes. Results suggest that t he accelerator site may reside inside the catalytic gorge rather than at its entrance and acceleration may be due to the prevention of reinh ibition of the regenerated enzyme by the putative product, the phospho rylated 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. Resu lts offer possible explanations for the superiority of pyridinium)-1-( 4-carboxy-aminopyridinium)-dimethyl ether hydrochloride over other oxi mes in the reactivation of specific AChE-OP conjugates.