A chemical compound that deactivates the enzyme cholinesterase, thus preventing or retarding hydrolytic breakdown of the highly toxic acetylcholine formed in the body by the nervous system. Nerve gases act in this way, as do a number of insecticides, usually organic esters of phosphoric acid derivatives. Serious poisoning and death may occur on ingestion or prolonged inhalation of such compounds. Cholinesterase can be reactivated by administration of atropine sulfate or pralidoxime iodide.See Parathion; Nerve Gas.