Interaction of some unsubstituted phosphoramidate analogs of methamidophos ( O,S-dimethyl phosphorothioamidate) with acetylcholinesterase and neuropathy target esterase of hen brain

Abstract

At 37°C and pH 7.4–8.0, five higher O-alkyl analogs of methamidophos and four O-alkyl O-2,5-dichlorophenyl phosphoramidates all were more potent progressive inhibitors of hen brain AChE and neuropathy target esterase (NTE) than was methamidophos itself. For AChE, k a increased from 7.2 × 10 2 to 1.0 × 10 5 M −1 min −1 between methyl and n-hexyl S-methyl esters and from 9.3 × 10 3 to 8.9 × 10 5 M −1 min −1 between ethyl and n-hexyl dichlorophenyl analogs. For NTE, the ranges were from 16 to 7.9 × 10 4 for S-methyl esters, and were 9.7 × 10 4 to 7.8 × 10 6 M −1 min −1 for dichlorophenyl. S-methyl esters were more active against AChE than against NTE and all the dichlorophenyl esters were more active against NTE than against AChE. Spontaneous reactivation of 75–100% activity without aging of AChE was found after 19 hr incubation at 37°C after inhibition by all nine straight-chain alkyl analogs. After inhibition by O-isopropyl S-methyl phosphorothioamidate, some spontaneous reactivation with complete aging of all remaining inhibited AChE occurred during 19 hr. No spontaneous reactivation or aging of inhibited NTE was detected. It was concluded that the molecular structures of the inhibited enzymes obtained from equivalent compounds in the two series of inhibitors were identical and that the leaving groups were, therefore, S-methyl and O-2,5-dichlorophenyl, respectively. Although hen brain NTE inhibited by methamidophos in vitro did not age, cases of delayed neuropathy in man have been reported and, presumably, require aging as well as inhibition of NTE. Possible explanations of this apparent discrepancy include (i) the fact that methamidophos consists of two chiral forms and that the form seen to be active in vitro may be disposed of preferentially in vivo, (ii) the possibility of activation in vivo to a different inhibitor, (iii) differences between conformation and ease of aging of inhibited NTE in vitro and in vivo, and (iv) species differences.