AHR Activation, Metabolic Fate, and Endocrine Disruption of 1,3,6,8-Tetrachloro-9H-Carbazole: A Complete Toxicity Pathway of Drinking Water Disinfection Byproduct.

Polyhalogenated carbazoles (PHCZs) are disinfection by-products formed during drinking water treatment and represent an important class of emerging contaminants. PHCZs can enter the human body through multiple exposure routes and pose potential health risks. However, their biotransformation pathways and toxic effects in humans remain insufficiently understood, and the underlying molecular mechanisms of toxicity are still unclear. This study focused on 1,3,6,8-tetrachloro-9H-carbazole (1,3,6,8-CCZ) and constructed an activation-metabolism-toxicity pathway using multiple computational strategies. The results indicated that 1,3,6,8-CCZ was a potent activator of the aryl hydrocarbon receptor (AHR). AHR underwent a distinct conformational change upon activation, leading to a stable non-covalent interaction between TYR322 and the ligand. During AHR translocation from the cytoplasm to the nucleus, significant conformational rearrangements occurred in the Dα and Cα helices and the C-terminal region. Furthermore, P450-mediated electrophilic addition to 1,3,6,8-CCZ exhibited strong regioselectivity, which was driven by the C-O distance and the C-O-Fe angle. Epoxidation and NIH shift were the main metabolic pathways. Endocrine disruption assessment showed that 1,3,6,8-CCZ and its hydroxylated metabolites bound to five endocrine receptors with high affinity. This study provides mechanistic insights into the metabolic fate and toxicological effects of exogenous compounds in humans.