Spontaneous and oxime-induced reactivation of acetylcholinesterase inhibited by phosphoramidates.

Methamidophos (CH3O(NH2)P(O)SCH3) and phosphoramidates, with the general structure RO(NH2)P(O)OC6H4-p-NO2, in which R = C2H5, C1CH2CH2, FCH2CH2 and F3CCH2, as well as (NH2)2P(O)OC6H4-p-NO2 were synthesized to investigate the relationship between the rates of inhibition and of spontaneous reactivation of AChE inhibited by these organophosphates and their potential as prophylactics against nerve agent poisoning. The phosphoramidates inhibit electric eel acetylcholinesterase (EEAChE), the bimolecular inhibition rate constants ranging from 5 x 10(4) to 3 x 10(6) M-1.min-1 at pH 7.5, 25 degrees C. The inhibited enzymes reactivate spontaneously, with half-lives ranging from 1.3 to 15 h at pH 7.5, 25 degrees C. These half-lives increase 2-4 fold when the temperature is raised to 37 degrees C. Reactivation is accelerated by micromolar concentrations of oximes such as obidoxime and HI-6. Aging of the inhibited enzymes was not observed. Nevertheless, reactivation appears to be incomplete for some of the inhibited enzymes. The title compounds seem promising as prophylactic agents against nerve agent intoxication.