Metabolism exploration of disinfection byproducts halonitromethanes (HNMs) by cytochrome P450 enzymes and toxicity evaluation.

The nitrogen-contained disinfection by-products, halonitromethanes (HNMs), are known for their high cytotoxicity and genotoxicity. Although HNMs can be metabolized by cytochrome P450 enzymes (P450s), the specific mechanism has remained unclear. To shed light on this, density functional theory (DFT) calculations were performed to elucidate the potential oxidative P450-catalytic activation of the nine HNMs. Our findings reveal that active species of P450s (Cpd I) predominantly react with halogen-substituted nitromethanes via hydrogen abstraction and bromine atom abstraction, rather than chlorosylation. As a result of these reactions, oxidized HNMs are produced and can undergo further hydrolysis, leading to nitro-formaldehyde, nitro formyl halogen, halogen hydride, hypobromous acid, and nitroformic acid. To experimentally validate the computational predictions, in vitro experiments were conducted on five typical nitromethanes using human liver microsomes and the results reveal that DCNM, BCNM and DBCNM form nitroformyl chlorine (NO2CClO), while BCNM, DBNM and TBNM are transferred into nitroformyl bromide (NO2CBrO). Nitroformic acid is also identified as a metabolite in the TBNM metabolism reaction. Toxicity assessment reveals that metabolic transformation leads to an overall reduction in the ecological toxicity. However, metabolites showed similar toxicity to Fathead minnow and even higher acute toxicity to rat, as well as larger probability of hERG inhibition effects than HNMs, underscoring the need for caution in health risk assessment. By integrating in silico and in vitro approaches, this work has provided a comprehensive understanding of the metabolism of HNMs and offered potential toxicity data basis of these compounds.