Characterization of [3H]di-isopropyl phosphorofluoridate-binding proteins in hen brain. Rates of phosphorylation and sensitivity to neurotoxic and non-neurotoxic organophosphorus compounds

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Abstract
[3H]DFP binding proteins were isolated by sodium dodecyl sulfate/polyacrylamide-gel electrophoresis to characterize and identify potential initiation sites for organophosphorus-compound-induced delayed neurotoxicity. The major Paraoxon-insensitive Mipafox-sensitive binding protein (MW 160,000) was identical with 1 previously identified as neurotoxic esterase an enzyme that has been proposed to be the target site for organophosphorus-compound-induced delayed neurotoxicity. However, 2 other binding proteins with suitable binding characteristics were also found in smaller amounts, one of which has not been detected previously. DFP phosphorylates all 3 of these proteins at rates similar to the rate at which neurotoxic esterase is inhibited by DFP. Varying the concentration of DFP or the time of incubation produced similar increases in binding to each of the labeled proteins. The reaction rates of DFP with proteins may be described by 1st-order kinetics, and the concentration of the Michaelis complex formed during binding is minimal for all the phosphorylated proteins. The recovery of the binding activity in the 160,000-MW band was similar to the recovery of neurotoxic esterase activity, lending further support to the contention that this band is identical with neurotoxic esterase.