The Role of Esterases in Parathion Poisoning of Eggs of the Large Milkweed Bug1

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Abstract
The milkweed bug egg is strikingly insensitive to the ovi-cidal action of parathion, although nymphs hatching from treated eggs die shortly after hatching. The role of esterases in toxicity was studied, using manometric techniques to determine gross inhibition and histo-chemical technique to determine the location of esterases involved and levels of inhibition. Inhibition levels determined by the 2 techniques were not in close agreement suggesting that identical esterases were not being measured by the 2 assay methods. Aliesterase and cholinesterase (ChE) activity was measured manometrically and in both cases inhibition was found to be low prior to hatch of treated eggs. Following hatching, inhibition rose sharply and coincided with the onset of toxic symptoms. The parallel between inhibition and toxic symptoms did not clearly implicate one enzyme over the other. By contrast, histochemical assay showed close agreement between toxic symptoms and cholinesterase inhibition; inhibition of aliesterase occurred only in advanced stages of poisoning, and no inhibition of aromatic esterase occurred in any stage of poisoning. The weight of evidence, particularly that obtained by histochemical technique, appears to implicate ChE as the enzyme involved in toxic action. The results suggest that cross reference by histochemical and manometric techniques, an approach which holds promise in elucidating the role of esterases, cannot be employed effectively until more definitive characterization of esterases is established.