Effects of some mono- and bisquaternary ammonium compounds on the reactivatability of soman-inhibited human acetylcholinesterase in vitro

Abstract

Acetylcholinesterase (AChE) inhibited by the organophosphate soman (1,2,2-trimethyl-propylmethylphosphonofluoridate) rapidly becomes resistant to reactivation by oximes due to dealkylation of the soman-enzyme complex. This reaction is called aging. The effect of the four mono- and bisquaternary ammonium compounds tetramethylammonium (TMA), hexamethonium, decamethonium and suxamethonium on the reactivatability of soman-inhibited, solubilized AChE from human erythrocytes was investigated in vitro. All compounds were reversible inhibitors of AChE; the respective dissociation constants and the type of inhibition exhibited considerable differences. The affinities to both the active and the allosteric site were considerably higher for suxamethonium ( K ii 81.3 μM; K i 15.9 μM) and decamethonium ( K ii 15.4 μM; K i 4.4 μM) than for TMA ( K ii 1 mM; K i 289.6 μM) and hexamethonium ( K ii 4.5 mM; K i 331.8 μM). The reactivation experiments were performed in a four-step procedure (soman-inhibition at 0° and pH 10, aging at 37° and pH 7.3, reactivation by the oxime HI 6 at 37° and pH 7.3 followed by AChE assay). After these four steps (total duration 55 min), AChE was inhibited by soman to 95–100%. HI 6 could reactivate about 20% of the inhibited enzyme. All effectors increased the AChE reactivatability by HI 6 when added before aging was started. The maximal increase in reactivatability was higher in the presence of 1.6 mM suxamethonium (+35.8%) and 150 μM decamethonium (+40%) than of 22 mM TMA (+22.5%) and 8.3 mM hexamethonium (+19.2%). If the effectors were added after 5 min of aging they increased the activity of soman-inhibited AChE, but to a considerably smaller extent than HI 6. A good correlation of the respective K ii values and the effective concentrations of these drugs was observed, indicating that an allosteric binding site of AChE might be involved in the protective effect of these drugs.