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Precision Biotransformation of Emerging Pollutants by Human Cytochrome P450 Using Computational–Experimental Synergy: A Case Study of Tris(1,3-dichloro-2-propyl) Phosphate

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journal contribution
posted on 16.08.2021, 16:34 by Lihong Chai, Huanni Zhang, Runqian Song, Haohan Yang, Haiying Yu, Piotr Paneth, Kasper P. Kepp, Miki Akamatsu, Li Ji
Precision biotransformation is an envisioned strategy offering detailed insights into biotransformation pathways in real environmental settings using experimentally guided high-accuracy quantum chemistry. Emerging pollutants, whose metabolites are easily overlooked but may cause idiosyncratic toxicity, are important targets of such a strategy. We demonstrate here that complex metabolic reactions of tris­(1,3-dichloro-2-propyl) phosphate (TDCIPP) catalyzed by human CYP450 enzymes can be mapped via a three-step synergy strategy: (i) screening the possible metabolites via high-throughout (moderate-accuracy) computations; (ii) analyzing the proposed metabolites in vitro by human liver microsomes and recombinant human CYP450 enzymes; and (iii) rationalizing the experimental data via precise mechanisms using high-level targeted computations. Through the bilateral dialogues from qualitative to semi-quantitative to quantitative levels, we show how TDCIPP metabolism especially by CYP3A4 generates bis­(1,3-dichloro-2-propyl) phosphate (BDCIPP) as an O-dealkylation metabolite and bis­(1,3-dichloro-2-propyl) 3-chloro-1-hydroxy-2-propyl phosphate (alcoholβ‑dehalogen) as a dehalogenation/reduction metabolite via the initial rate-determining H-abstraction from αC- and βC-positions. The relative yield ratio [dehalogenation/reduction]/[O-dealkylation] is derived from the relative barriers of H-abstraction at the βC- and αC-positions by CYP3A4, estimated as 0.002 to 0.23, viz., an in vitro measured ratio of 0.04. Importantly, alcoholβ‑dehalogen formation points to a new mechanism involving successive oxidation and reduction functions of CYP450, with its precursor aldehydeβ‑dehalogen being a key intermediate detected by trapping assays and rationalized by computations. We conclude that the proposed three-step synergy strategy may meet the increasing challenge of elucidating biotransformation mechanisms of substantial synthesized organic compounds in the future.