Echothiophate iodide is a long-lasting cholinesterase inhibitor of the irreversible type, as is isofluorphate. Unlike the latter, however, it is a quaternary salt, and when applied locally, its distribution in tissues is limited, which can be very desirable. It is used as a long-acting anticholinesterase agent in the treatment of glaucoma.
Hexaethyltetraphosphate (HETP) and Tetraethyl pyrophosphate (TEPP). HETP and TEPP are compounds that also show anticholinesterase activity. HETP was developed by the Germans during World War II and is used as an insecticide against aphids. When used as insecticides, these compounds have the advantage of being hydrolyzed rapidly to the relatively nontoxic, water-soluble compounds phosphoric acid and ethyl alcohol. Fruit trees or vegetables sprayed with this type of compound retain no harmful residue after a period of a few days or weeks, depending on the weather conditions. Workers spraying with these agents should use extreme caution so that the vapors are not breathed and none of the vapor or liquid comes in contact with the eyes or skin.
Malathion. Malathion, 2-[(dimethoxyphosphinothioyl) thio]butanedioic acid diethyl ester, is a water-insoluble phosphodithioate ester that has been used as an agricultural insecticide. Malathion is a poor inhibitor of cholinesterases. Its effectiveness as a safe insecticide is a result of the different rates at which humans and insects metabolize the chemical. Microsomal oxidation, which causes desulfuration, occurs slowly to form the phosphothioate (malaoxon), which is 10,000 times more active than the phosphodithioate (malathion) as a cholinesterase inhibitor. Insects detoxify the phosphothioate by a phosphatase, forming dimethyl phosphorothioate, which is inactive as an inhibitor. Humans, however, can rapidly hydrolyze malathion by a carboxyesterase enzyme, yielding malathion acid, a still poorer inhibitor of AChE. Phosphatases and car-boxyesterases further metabolize malathion acid to di-methylphosphothioate. The metabolic reactions are shown in Figure 17.18.
Parathion. Parathion, O,O-diethyl O-p-nitrophenyl phosphorothioate (Thiophos), is a yellow liquid that is freely soluble in aromatic hydrocarbons, ethers, ketones, esters, and alcohols but practically insoluble in water, petroleum ether, kerosene, and the usual spray oils. It is decomposed at a pH above 7.5. Parathion is used as an agricultural insecticide. It is a relatively weak inhibitor of cholinesterase;
s ch3o ch3o
Malathion (poor ChE inhibitor)
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