Effect of histidine modification on the aging of organophosphate-inhibited acetylcholinesterase

Abstract

Organophosphate-inhibited bovine erythrocyte acetylcholinesterase undergoes aging, a process whereby the phosphoenzyme is transformed to a form which is no longer reactivated by oximes. Aging was estimated indirectly by reactivation of trichlorfon-inhibited acetylcholinesterase by pyridine-2-aldoxime methiodide or directly by loss of tritium from [1,3- 3H]diisopropylfluorophosphate inhibited enzyme. The aging of diisopropylfluorophosphate(DFP)-inhibited acetylcholinesterase was due primarily to dealkylation, rather than to the alternative β-elimination mechanism which was also examined. Aging of trichlorfon-inhibited acetylcholinesterase was not decreased by modification of sulfhydryl, carboxyl or amino groups outside the active site of the enzyme. Modification of histidine by 2 mM diethylpyrocarbonate irreversibly inhibited acetylcholinesterase activity. Dealkylation of [1,3- 3H]DFP-inhibited acetylcholinesterase was slowed 2-to 3-fold by reaction of active site histidine with 2 mM diethylpyrocarbonate, and it was abolished at higher concentrations. It was concluded that an active site histidine catalyzes both aging and acetylcholine hydrolysis in erythrocyte acetylcholinesterase.