Discovery of a widespread metabolic pathway within and among phenolic xenobiotics

Metabolism is an organism's primary defense against xenobiotics, yet it also increases the production of toxic metabolites. It is generally recognized that phenolic xenobiotics, a group of ubiquitous endocrine disruptors, undergo rapid phase II metabolism to generate more water-soluble glucuronide and sulfate conjugates as a detoxification pathway. However, the toxicological effects of the compounds invariably point to the phase I metabolic cytochrome P450 enzymes. Here we show that phenolic xenobiotics undergo an unknown metabolic pathway to form more lipophilic and bioactive products. In a nontargeted screening of the metabolites of a widely used antibacterial ingredient: triclosan (TCS), we identified a metabolic pathway via in vitro incubation with weever, quail, and human microsomes and in vivo exposure in mice, which generated a group of products: TCS-O-TCS. The lipophilic metabolite of TCS was frequently detected in urine samples from the general population, and TCS-O-TCS activated the constitutive androstane receptor with the binding activity about 7.2 times higher than that of the parent compound. The metabolic pathway was mediated mainly by phase I enzymes localized on the microsomes and widely observed in chlorinated phenols, phenols, and hydroxylated aromatics. The pathway was also present in different phenolic xenobiotics and formed groups of unknown pollutants in organisms (e.g., TCS-O-bisphenol A and TCS-O-benzo(a)pyrene), thus providing a cross-talk reaction between different phenolic pollutants during metabolic processes in organisms.