College of Environment , Zhejiang University of Technology , Hangzhou 310032 , China.
College of Environmental and Resource Sciences , Zhejiang University , Hangzhou 310058 , China.
Environ Sci Technol. 2018 Dec 18;52(24):14411-14421. doi: 10.1021/acs.est.8b03965. Epub 2018 Nov 28.
Understanding metabolic mechanisms is critical and remains a difficult task in the risk assessment of emerging pollutants. Triphenyl phosphate (TPHP), a widely used aryl phosphorus flame retardant (aryl-PFR), has been frequently detected in the environment, and its major metabolite was considered as diphenyl phosphate (DPHP). However, knowledge of the mechanism for TPHP leading to DPHP and other metabolites is lacking. Our in vitro study shows that TPHP is metabolized into its diester metabolite DPHP and mono- and dihydroxylated metabolites by cytochromes P450 (CYP) in human liver microsomes, while CYP1A2 and CYP2E1 isoforms are mainly involved in such processes. Molecular docking gives the conformation for TPHP binding with the active species Compound I (an iron IV-oxo heme cation radical) in specific CYP isoforms, showing that the aromatic ring of TPHP is likely to undergo metabolism. Quantum chemical calculations have shown that the dominant reaction channel is the O-addition of Compound I onto the aromatic ring of TPHP, followed by a hydrogen-shuttle mechanism leading to ortho-hydroxy-TPHP as the main monohydroxylated metabolite; the subsequent H-abstraction-OH-rebound reaction acting on ortho-hydroxy-TPHP yields the meta- and ipso-position quinol intermediates, while the former of which can be metabolized into dihydroxy-TPHP by fast protonation, and the latter species needs to go through type-I ipso-substitution and fast protonation to be evolved into DPHP. We envision that the identified mechanisms may give inspiration for studying the metabolism of several other aryl-PFRs by CYP.
理解代谢机制对于新兴污染物的风险评估至关重要，但仍然是一项艰巨的任务。磷酸三苯酯（TPHP）是一种广泛使用的芳基磷阻燃剂（aryl-PFR），已在环境中频繁检出，其主要代谢物被认为是磷酸二苯酯（DPHP）。然而，对于 TPHP 导致 DPHP 和其他代谢物的机制知之甚少。我们的体外研究表明，TPHP 在人肝微粒体中被细胞色素 P450（CYP）代谢为其二酯代谢物 DPHP 和单羟基化和二羟基化代谢物，而 CYP1A2 和 CYP2E1 同工型主要参与这些过程。分子对接给出了 TPHP 与特定 CYP 同工型中活性物质复合物 I（铁 IV-氧血红素阳离子自由基）结合的构象，表明 TPHP 的芳环可能发生代谢。量子化学计算表明，主要反应通道是复合物 I 对 TPHP 芳环的 O-加成，随后是氢转移机制，导致邻位羟基-TPHP 作为主要的单羟基化代谢物；随后对邻位羟基-TPHP 进行 H 抽提-OH 回跳反应，生成间位和对位醌醇中间体，其中前者可通过快速质子化代谢为二羟基-TPHP，后者需要经历 I 型对位取代和快速质子化才能演变为 DPHP。我们设想，所确定的机制可能为研究 CYP 代谢几种其他芳基-PFR 的机制提供启示。
