Ploemen J P, Wormhoudt L W, Haenen G R, Oudshoorn M J, Commandeur J N, Vermeulen N P, de Waziers I, Beaune P H, Watabe T, van Bladeren P J
TNO Toxicology, Zeist, The Netherlands.
Toxicol Appl Pharmacol. 1997 Mar;143(1):56-69. doi: 10.1006/taap.1996.8004.
Ethylene dibromide (1,2-dibromoethane, EDB) is metabolized by two routes: a conjugative route catalyzed by glutathione S-transferases (GST) and an oxidative route catalyzed by cytochrome P450 (P450). The GST route is associated with carcinogenicity. An approach is presented to use human purified GST and P450 enzymes to explore the importance of these metabolic pathways for man in vivo. This strategy basically consists of four steps: (i) identification of the most important isoenzymes in vitro, (ii) scaling to rate per milligram cytosolic and microsomal protein, (iii) scaling to rate per gram liver, and (iv) incorporation of data in a physiologically based pharmacokinetic (PBPK) model. In the first step, several GST isoenzymes were shown to be active toward EDB and displayed pseudo-first-order kinetics, while the EDB oxidation was catalyzed by CYP2E1, 2A6, and 2B6, which all displayed saturable kinetics. In the second step, the predictions were in agreement with the measured activity in a batch of 21 human liver samples. In the third step, rat liver P450 and GST metabolism of EDB was predicted to be in the same range as human metabolism (expressed per gram). Interindividual differences in GST activity were modeled to determine "extreme cases." For the most active person, an approximately 1.5-fold increase of the amount of conjugative metabolites was predicted. Lastly, it was shown that the GST route, even at low concentrations, will always contribute significantly to total metabolism. In the fourth step, a PBPK model describing liver metabolism after inhalatory exposure to EDB was used. The saturation of the P450 route was predicted to occur faster in the rat than in man. The rat was predicted to have a higher turnover of EDB from both routes. Nevertheless, when all data are combined, it is crucial to recognize that the GST remains significantly active even at low EDB concentrations. The limitations and advantages of the presented strategy are discussed.
二溴乙烷（1,2 - 二溴乙烷，EDB）通过两条途径进行代谢：一条是由谷胱甘肽S - 转移酶（GST）催化的结合途径，另一条是由细胞色素P450（P450）催化的氧化途径。GST途径与致癌性有关。本文提出了一种利用人纯化的GST和P450酶来探究这些代谢途径对人体体内重要性的方法。该策略主要包括四个步骤：（i）体外鉴定最重要的同工酶；（ii）按每毫克胞质和微粒体蛋白的速率进行换算；（iii）按每克肝脏的速率进行换算；（iv）将数据纳入基于生理学的药代动力学（PBPK）模型。在第一步中，几种GST同工酶对EDB表现出活性并呈现伪一级动力学，而EDB氧化由CYP2E1、2A6和2B6催化，它们均呈现饱和动力学。在第二步中，预测结果与一批21份人肝脏样本中的实测活性一致。在第三步中，预测大鼠肝脏中EDB的P450和GST代谢与人类代谢处于相同范围（以每克表示）。对GST活性的个体间差异进行建模以确定“极端情况”。对于最活跃的个体，预测结合代谢物的量将增加约1.5倍。最后，结果表明即使在低浓度下，GST途径对总代谢的贡献也始终显著。在第四步中，使用了一个描述吸入EDB后肝脏代谢的PBPK模型。预测P450途径的饱和在大鼠中比在人类中发生得更快。预计大鼠从两条途径对EDB的周转率都更高。然而，当所有数据综合起来时，至关重要的是要认识到即使在低EDB浓度下，GST仍然具有显著活性。本文讨论了所提出策略的局限性和优点。
