METRIFONATE INDUCES CHOLINESTERASE INHIBITION EXCLUSIVELY VIA SLOW-RELEASE OF DICHLORVOS

Metrifonate, a long-acting cholinesterase (ChE) inhibitor with very lo w toxicity in warm-blooded animals, inhibits rat brain and serum choli nesterase (ChE) in vitro through its hydrolytic degradation product, d ichlorvos. This conclusion is based on the finding that metrifonate-in duced ChE inhibition showed the same pH dependence as its reported deh ydrochlorination to dichlorvos. The ChE inhibition induced by dichlorv os was not pH dependent. It was mediated by a competitive drug interac tion with the catalytic site of the enzyme, which led to irreversible inhibition within several minutes of incubation. After this time, addi tion of further substrate to the inhibited enzyme was not able to prom ote drug dissociation and hence enzyme reactivation. Similar character istics of inhibition, i.e. interaction with the substrate binding site and time-dependent switch to noncompetitive inhibition were observed with the reference compound, physostigmine. However, the physostigmine -induced inhibition of ChE could be readily reversed by further substr ate addition. Another reference compound, tetrahydroaminoacridine (THA ), also induced a reversible inhibition of rat brain and serum choline sterase, but with a mechanism of action different from that of both di chlorvos and physostigmine in that enzyme inhibition occurred rapidly upon drug addition at an allosteric site on the enzyme surface. It is suggested that the unique slow release plus the slow inhibition of ChE by dichlorvos is responsible for the lower toxicity of metrifonate co mpared to that of directly acting ChE inhibitors.