Sensitivity and selectivity of compounds interacting with neuropathy target esterase. Further structure-activity studies.
Assay of neuropathy target esterase (NTE) which accounts for about 70% of paraoxon-resistant phenyl valerate (PV) esterase activity of hen brain depends on the fact that it is selectively inhibited by mipafox. A previous study of structure/activity relationships (Biochem. Pharmac. 24, 797, 1975) has been extended. Among 14 potential substrates NTE hydrolysed phenyl phenoxyacetate and phenyl thiophenoxyacetate faster (1.5-1.7X) than PV, but selectivity of these substrates for NTE among the paraoxon-resistant esterases was only 35-52%. Seventy-seven other potential inhibitors (organophosphates, phosphonates, phosphoramidates, phosphinates and carbamates) were examined to determine I50NTE and effects on both NTE and "non-NTE" at 3-4 x I50NTE (I 85-95) and, where possible, at 6-20 X I50NTE. Hydrophophic inhibitors with small/flexible leaving groups were generally very inhibitory: several 2,2-dichlorovinyl phosphates and fluorides were active at low nanomolar concentrations. In the dichlorovinyl phosphate series increasing dialkyl chain length beyond n-pentyl decreased inhibitory power, presumably due to steric hindrance since the methyl/n-decyl ester was 15X more active than di-n-decyl. Chloro-substitution of both ortho-positions of a phenyl leaving group for benzylcarbamates reduced inhibitory power more than 20X but had little effect in a phenyl leaving group of methyl phenylphosphonates where the acyl-leaving group bond is longer and less subject to steric hindrance. N-phenylbenzohydroxamyl benzylcarbamate is 10X more potent than any previously described carbamate against NTE. Among stereo-isomers differences of activity ranged from less than 2- to 15-fold. Only diphenylphosphinyl fluoride appeared to be virtually specific for NTE: at 0.5-1 microM it inhibited ca.92% of NTE and 10-13% of "non-NTE" which is similar to the specificity found for 2,6-dichlorophenyl methyl phenylphosphonate which has been claimed to be specific. Diphenylphosphinyl fluoride has an advantage in that it is easily synthesized and should be protective rather than neuropathic, but it is not stable in store. We cannot repeat experiments purporting to show a substantial proportion of a second isozyme of NTE. However, according to first-order kinetics, concentrations of inhibitor greater than 6 X I50 should inhibit NTE greater than 98% and for 19 out of 26 compounds a residue greater than 3% (limit of precision) was found under these conditions: in nearly every case the quantity was 3-5%. This quantity may not be "true NTE" but it cannot be the target for organophosphate-induced delayed neuropathy since it is resistant to various neuropathic and protective compounds.(ABSTRACT TRUNCATED AT 400 WORDS)
Johnson
M K
MK
Toxicology Unit, Medical Research Council Laboratories, Carshalton, Surrey, U.K.
D000818:Animals; D002265:Carboxylic Ester Hydrolases / Q000037:antagonists & inhibitors*; D002645:Chickens; D007531:Isoflurophate / Q000031:analogs & derivatives / Q000494:pharmacology; D010261:Paraoxon / Q000494:pharmacology; D013329:Structure-Activity Relationship; D013379:Substrate Specificity
D007531:Isoflurophate; C005238:mipafox; D002265:Carboxylic Ester Hydrolases; C021859:neurotoxic esterase; D010261:Paraoxon