Methantheline improves the reactivation by HI 6 of human erythrocyte acetylcholinesterase inhibited by soman in vitro

Abstract

The effect of methantheline, a quaternary ammonium compound, on the reactivation by HI 6 of soman-inhibted human erythrocyte acetylcholinesterase (AChE) was investigated in vitro using purified human erythrocyte AChE or washed human erythrocytes. Methantheline itself was found to be a mixed competitive/non-competitive inhibitor of AChE (K_ii 360±70 μmol/l; K_i 240±10 μmol/l). In all experiments the enzyme was first inhibited by soman for 30 min under conditions preventing ageing (pH 10,0°C) and then ageing was allowed by changing the pH to 7.3 and the temperature to 37°C. Methantheline addition (0.36 or 3.6 mmol/l) at the start of ageing increased the portion of AChE which could be reactivated by HI 6 (0.32 mmol/l) added 5 min later, from 24.6±1.0% (mean±SEM) of the original activity to 42.1±1.8% or 45±2.9%, respectively. With HI 6 alone, the portion of AChE activity which could be reactivated 25 min after the start of ageing decreased rapidly with the delay of the oxime addition (100% of the original activity at immediate addition), the half-life being approximately 2.5 min. With methantheline alone (0.36 mmol/l) the AChE activity was lower after immediate addition (37% of the original value), but the loss of activity due to the increasing delay of methantheline addition exhibited a similar half-life as with HI 6. Finally, when methantheline (0.36 mmol/l) was added at the start of ageing and HI 6 at various intervals thereafter the half-life of AChE activity loss due to the delay of HI 6 addition at least doubled, compared to incubations without methantheline.