Case Study 11: Considerations for Enzyme Mapping Experiments—Interaction Between the Aldehyde Oxidase Inhibitor Hydralazine and Glutathione

Often it may be convenient and efficient to address multiple research questions with a single experiment. In many instances, however, the best approach is to design the experiment to address one question at a time. The design of enzyme mapping experiments is discussed in this chapter, focusing on considerations pertinent to the study of aldehyde oxidase (AO) vs. cytochrome P450 metabolism. Specifically, a case is presented in which reduced glutathione (GSH) was included in an experiment with human liver S9 fraction to trap reactive metabolites generated from cytochrome P450-mediated metabolism of lapatinib and its O-dealkylated metabolite, M1 (question 1). The AO inhibitor hydralazine was included in this experiment to investigate the involvement of AO-mediated metabolism of M1 (question 2). The presence of GSH was found to interfere with the inhibitory activity of hydralazine. Consideration of the time-dependent nature of hydralazine inhibitory activity toward AO when designing this experiment could have predicted the potential for GSH to interfere with hydralazine. This case underscores the importance of clearly identifying the research question, tailoring the experimental protocol to answer that question, and then meticulously considering how the experimental conditions could influence the results, particularly if attempting to address multiple questions with a single experiment.