Spontaneous reactivation of mouse plasma cholinesterase after inhibition by various organophosphorus compounds

We investigated the spontaneous reactivation of mouse plasma cholinesterase (ChE) after inhibition by various organophosphorus compounds. The remarkable spontaneous reactivations during storage at 24 degrees C were observed in plasma ChE prepared 30 min after oral administration of three O,O-dimethyl organophosphorus compounds, i.e. malathion, methylparathion and cyanox; while the spontaneous reactivation did not occur after inhibition by tolclofos-methyl, one of O,O-dimethyl organophosphorus compounds. The plasma ChEs inhibited by surecide, salithion and leptophos, which contain no O,O-dimethyl moiety, were not reactivated or only slightly so. These results suggest that a more sufficient attention should be paid in the determination of activity of plasma ChE inhibited by O,O-dimethyl organophosphorus compounds than that of plasma ChE inhibited by organophosphorus compounds without O,O-dimethyl moiety, since plasma ChE inhibited by the organophosphorus compounds with O,O-dimethyl moiety, except for tolclofos-methyl, was more easily reactivated, and the correct activity of inhibited plasma ChE can not be obtained without attention to the spontaneous reactivation. Furthermore, these spontaneous reactivations were examined by using butyrylthiocholine as well as acetylthiocholine as a substrate, and results showed that there was little difference between the spontaneous reactivations observed in using acetylthiocholine and butyrylthiocholine. So, it is concluded that these spontaneous reactivations take place only in pseudo ChE.