Organophosphorus and other inhibitors of brain 'neurotoxic esterase' and the development of delayed neurotoxicity in hens.

1. The delayed neurotoxic effects of some organophosphorus compounds are associated with phosphorylation of the active site of a nervous-tissue enzyme capable of hydrolysing phenyl phenylacetate. 2. Neurotoxic organophosphorus compounds and some carbamates and sulphonyl fluorides progressively inhibit the enzyme, attaching a substituent covalently at the active site. 3. Prolonged inhibition of the enzyme by phenyl N-benzyl-N-methylcarbamate or phenylmethane-sulphonyl fluoride does not lead to neurotoxic effects. 4. Prior inhibition of the enzyme by carbamates or sulphonyl fluorides in vivo prevents the neurotoxic effects of several organophosphorus compounds. 5. After dosage of hens with protective compounds, protection lasts until about 70% of the enzyme site again becomes available for phosphorylation. 6. Reaction of all the inhibitors at the active site of the enzyme leads to the same inhibitory effect with respect to hydrolysis of phenyl phenylacetate but does not in all cases lead to delayed neurotoxicity. It is concluded that the nature of the group substituted at the enzyme active site determines the toxic response.