Parathion, an organophosphate insecticide, undergoes several reactions, one of which (desulfuration) activates it to a toxic enzyme inhibitor, paraoxon. Another biotrans-
formation reaction, ester hydrolysis, converts parathion or its active intermediate to inactive products (see structures, Figure 6.13). This insecticide has some species specificity because the ester hydrolysis reaction of inactivation occurs more readily in mammals than in insects. There are four possible outcomes in biotransformation of a xenobiotic, all of which are illustrated by parathion. The xenobiotic can be metabolized to a nontoxic compound, to a toxic compound, or it may be activated and then inactivated, and, last, it may bind to some biological receptor and exert a pharmacological effect.
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