ABSTRACT:In the present study, the diversity of a library of drug-metabolizing bacterial cytochrome P450 (P450) BM3 mutants was evaluated by a liquid chromatography-mass spectrometry (LC-MS)-based screening method. A strategy was designed to identify a minimal set of BM3 mutants that displays differences in regio-and stereoselectivities and is suitable to metabolize a large fraction of drug chemistry space. We first screened the activities of six structurally diverse BM3 mutants toward a library of 43 marketed drugs (encompassing a wide range of human P450 phenotypes, cLogP values, charges, and molecular weights) using a rapid LC-MS method with an automated method development and data-processing system. Significant differences in metabolic activity were found for the mutants tested and based on this drug library screen; nine structurally diverse probe drugs were selected that were subsequently used to study the metabolism of a library of 14 BM3 mutants in more detail. Using this alternative screening strategy, we were able to select a minimal set of BM3 mutants with high metabolic activities and diversity with respect to substrate specificity and regiospecificity that could produce both human relevant and BM3 unique drug metabolites. This panel of four mutants (M02, MT35, MT38, and MT43) was capable of producing P450-mediated metabolites for 41 of the 43 drugs tested while metabolizing 77% of the drugs by more than 20%. We observed this as the first step in our approach to use of bacterial P450 enzymes as general reagents for lead diversification in the drug development process and the biosynthesis of drug(-like) metabolites.

A fluorescence-based continuous-flow enzyme affinity detection (EAD) setup was used to screen cytochrome P450 BM3 mutants on-line for diversity. The flow-injection screening assay is based on the BM3-mediated O-dealkylation of alkoxyresorufins forming the highly fluorescent product resorufin, and can be used in different configurations, namely injection of ligands, enzymes and substrates. Screening conditions were optimized and the activity of a library of 32 BM3 mutants towards the recently synthesized new probe substrate allyloxyresorufin was measured in flow-injection analysis (FIA) mode and it was shown that large activity differences between the mutants existed. Next, six BM3 mutants containing mutations at different positions in the active site were selected for which on-line enzyme kinetics were determined. Subsequently, for these six BM3 mutants affinity towards a set of 30 xenobiotics was determined in FIA EAD mode. It was demonstrated that significant differences existed for the affinity profiles of the mutants tested and that these differences correlated to alterations in the BM3 mutant-generated metabolic profiles of the drug buspirone. In conclusion, the developed FIA EAD approach is suitable to screen for diversity within BM3 mutants and this alternative screening technology offers new perspectives for rapid and sensitive screening of compound libraries towards BM3 mutants.

The estrogenic mycotoxin zearalenone (ZEN) can undergo hepatic reductive metabolism to form the estrogenic α and β isomers of zearalenol. ZEN also undergoes cytochrome P450 monooxygenase (P450)-mediated oxidative metabolism to form monohydroxylated products, but until now nothing is known about the estrogenic potency of these metabolites. This study aimed at investigating the metabolism of ZEN by different P450 isoforms and to determine the estrogen receptor α (ERα) affinities of the in vitro P450-generated ZEN metabolites in an online high-resolution screening (HRS) setup. Human liver microsomes (HLM), recombinant P450s, and mutants of the bacterial P450 BM3 were used to investigate the oxidative metabolism of ZEN. It was shown that mutants of the bacterial P450 BM3 could be used to produce the human relevant 13- and 15-OH-ZEN catechol metabolites at such levels that their ERα affinity could be determined in an HRS setup, which was not possible with HLM. It was demonstrated that P450-mediated hydroxylation at the 13 and 15 positions of ZEN resulted in a loss of ERα affinity. The approach presented here can be used for the elucidation of the metabolism of other endocrine disrupting compounds and xenobiotics to get clear pictures of the total effects of these compounds and their metabolites.

The endocrine system is a major communication system in the body and is involved in maintenance of the reproductive system, fetal development, growth, maturation, energy production, and metabolism,. The endocrine system responds to the needs of an organism by secreting a wide variety of hormones that enable the body to maintain homeostasis, to respond to external stimuli, and to follow various developmental programs. This occurs through complex signalling cascades,with multiple sites at which the signals can be regulated. Endocrine disrupting compounds (EDCs) affect the endocrine system by simulating the action of the naturally produced hormones, by inhibiting the action of natural hormones, by changing the function and synthesis of hormone receptors, or by altering the synthesis, transport, metabolism, and elimination of hormones. It has been established that exposure to environmental EDCs is a risk factor for disruption of reproductive development and oncogenesis in both humans and wildlife. For accurate risk assessment of EDCs, the possibility of bioactivation through biotransformation processes needs to be included since neglecting these mechanisms may lead to undervaluation of adverse effects on human health caused by EDCs and/or their metabolites. This accurate risk assessment should include: (1) possibility of EDCs to be bioactivated into metabolites with enhanced endocrine disruption (ED) effects, and (2) possibility of EDCs to be biotransformed into reactive metabolites that may cause DNA damage. Here, we present an overview of different metabolic enzymes that are involved in the biotransformation of EDCs. In addition, we describe how biotransformation by Cytochromes P450 (CYPs), human estrogen sulfotransferase 1E1 (SULT1E1) and selected other phase II enzymes, can lead to the formation of bioactive metabolites. This review mainly focuses on CYP- and SULT-mediated bioactivation of estrogenic EDCs and summarizes our views on this topic while also showing the importance of including bioactivation and biotransformation processes for improved risk assessment strategies.

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