PRINCIPLES OF TOXICOLOGY - Biology East Borneo

3.2 BIOTRANSFORMATION REACTIONS 71both of which are suitable for conjugation, if the metabolites are not first excreted as the phase I products(Figure 3.10).Epoxide hydrolase activity is predominantly microsomal, but an enzyme is also present in thecytosol.Most hydrolyses occur to a significant extent in tissues other than liver. Their quantitativeimportance is variable, depending on the chemical challenge. One significant extrahepatic location ofesterases is in the blood (plasma and erythrocytes), and of great concern is the enzyme normallyresponsible for the hydrolysis of acetylcholine. Blockade of this enzyme is the mode of action of manyinsecticides and “ nerve gases.”Figure 3.10 Hydrolytic and reductive phase I reactions.