Xenobiotic metabolism, survey

In this chapter we survey recent findings that describe the metabolic fate of the xenobiotic moieties of mercapturic acid pathway (MAP) metabolites that are excreted from the liver with the bile. We show that the products of the MAP undergo an enterohepatic circulation that is mediated by intestinal enzymes and/or intestinal microflora. 

Dietary exposure to pesticides (or to xenobiotics in general) is determined by calculating the product of the amount of chemical in or on the food and the total quantity of food consumed. The quantity of chemical potentially consumed in foods can be estimated from data obtained from residue field trials, metabolism studies, and/or monitoring data. Information from these sources is then analyzed with one of several available models containing food consumption factors from surveys conducted by the United States Department of Agriculture (USDA). For calculation of... 

It is important from a biochemical and practical viewpoint to note that criterion A is essential to define conjugation reactions of xenobiotics. In contrast, criteria B to E are neither sufficient nor necessary. They are not sufficient, since in hydrogenation reactions (i.e. typical reactions of functionalization) the hydride is also transferred from a cofactor (NADPH or NADH). And they are not necessary, since all the above criteria suffer from some important exceptions mentioned below. For example, some GSH conjugations are non-enzymatic. A survey of transferases involved in drug metabolism is offered in Table 32.5. 

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