INTERACTIONS INVITRO OF SOME ORGANOPHOSPHORAMIDATES WITH NEUROPATHY TARGET ESTERASE AND ACETYLCHOLINESTERASE OF HEN BRAIN

For organophosphates or phosphonates to initiate delayed neuropathy two steps are necessary: (1) progressive covalent reaction with neuropathy target esterase (NTE) to produce a form of inhibited NTE which can be reactivated by incubation with aqueous potassium fluoride (KF) and (2) progressive ''aging'' of inhibited NTE to a form which can no longer be reactivated by KF. However, it has been shown recently that certain N-unsubstituted organophosphoro-monoamidates (analogues of methamidophos) cause delayed neuropathy even though the inhibited NTE appeared not to have aged (Johnson et al. (1991). Arch. Toxicol., 65, 618-624). In order to study the generality of this phenomenon, we have examined some N-substituted compounds. We report in vitro studies of inhibition and reactivation and aging of both NTE and acetylcholinesterase (AChE) prior to toxicological tests. All the compounds studied were less inhibitory to both NTE and AChE in concentrated rather than in dilute suspensions of EDTA-washed brain particles without added cofactors. There was an apparent disposal of up to 100 mumoles of test compound by particles from 95 mg hen brain, which is far greater than can be explained by covalent binding. The activity is distinct from calcium-dependent ''A'' esterase. Several N-alkyl phosphoromonoamidates were found to be potent and selective inhibitors of NTE: second-order rate constant for O-n-pentyl N-benzylphosphoramidofluoridate (Cmpd 6) = 5.6 x 10(7) M-1 min-1 at 37-degrees, which is about 100x higher than for acetylcholinesterase (AChE). Inhibited NTE and AChE from several chiral phosphoromono-amidates did not reactivate spontaneously (21 hours at 37-degrees). Virtually 100% reactivation by KF of AChE inhibited by phosphoromonoamidates was achieved at all times tested. Acetylcholinesterase inhibited by 2,5-dichlorophenyl N,N'-di-n-butylphosphorodiamidate was 42-56% reactivated by incubation with KF (192 mM in pH 5.2 buffer for 30 minutes at 37-degrees). We believe this is the first report of reactivation of any enzyme after inhibition by a phosphorodiamidate. For NTE inhibited by tabun (O-ethyl N-dimethylphosphoroamidocyanidate), virtually complete and rapid aging (t1/2 = 5.5-8.4 minutes) was observed. Consistent but only partial reactivation by KF was achieved 2 or more hours after inhibition of NTE by Cmpd 6 or by its 2,6-difluoro-analogue (Cmpd 7). However, a small but significant aging (approximately 15-20% loss of reactivatability) was measured soon after a 1 minute inhibition by Cmpd 7, but no further change occurred in 21 hours. We conclude that, of the two chiral forms of NTE obtainable from chiral N-substituted phosphoromonoamidates, one form can and does age rapidly while the enantiomeric form does not. The toxicological significance of these results is discussed.