Hong J Y, Yang C S, Lee M, Wang Y Y, Huang W Q, Tan Y, Patten C J, Bondoc F Y
Department of Chemical Biology, College of Pharmacy, Rutgers University, Piscataway, NJ 08854, USA.
Arch Toxicol. 1997;71(4):266-9. doi: 10.1007/s002040050386.
Methyl tert-butyl ether (MTBE) is widely used as a gasoline oxygenate for more complete combustion in order to reduce the air pollution caused by motor vehicle exhaust. The possible adverse effects of MTBE on human health is a major public concern. However, information on the metabolism of MTBE in human tissues is lacking. The present study demonstrates that human liver is active in metabolizing MTBE to tert-butyl alcohol (TBA), a major circulating metabolite and a marker for exposure to MTBE. The activity is localized in the microsomal fraction (125 +/- 11 pmol TBA/ min per mg protein, n = 8) but not in the cytosol. This activity level in human liver microsomes is approximately one-half of the value in rat and mouse liver microsomes. Formation of TBA in human liver microsomes is NADPH-dependent, and is significantly inhibited by carbon monoxide (CO), an inhibitor of cytochrome P450 (CYP) enzymes, suggesting that CYP enzymes play a critical role in the metabolism of MTBE in human livers. Both CYP2A6 and 2E1 are known to be constitutively expressed in human livers. To examine their involvement in MTBE metabolism, human CYP2A6 and 2E1 cDNAs were individually co-expressed with human cytochrome P450 reductase by a baculovirus expression system and the expressed enzymes were used for MTBE metabolism. The turnover number for CYP2A6 and 2E1 was 6.1 and 0.7 nmol TBA/min per nmol P450, respectively. The heterologously expressed human CYP2A6 was also more active than 2E1 in the metabolism of two other gasoline ethers, ethyl tert-butyl ether (ETBE) and tert-amyl methyl ether (TAME). Although the contributions of other human CYP forms to MTBE metabolism remain to be determined, these results strongly suggest that CYP enzymes play an important role in the metabolism of MTBE in human livers.
甲基叔丁基醚（MTBE）作为一种汽油含氧化合物被广泛使用，目的是实现更完全的燃烧，以减少机动车尾气造成的空气污染。MTBE对人体健康可能产生的不利影响是公众主要关注的问题。然而，目前缺乏关于MTBE在人体组织中代谢的信息。本研究表明，人类肝脏在将MTBE代谢为叔丁醇（TBA）方面具有活性，叔丁醇是一种主要的循环代谢物，也是接触MTBE的标志物。该活性定位于微粒体部分（每毫克蛋白质每分钟125±11皮摩尔TBA，n = 8），而不是胞质溶胶中。人类肝脏微粒体中的这种活性水平约为大鼠和小鼠肝脏微粒体中活性水平的一半。人类肝脏微粒体中TBA的形成依赖于NADPH，并且受到一氧化碳（CO）的显著抑制，一氧化碳是细胞色素P450（CYP）酶的抑制剂，这表明CYP酶在人类肝脏中MTBE的代谢中起关键作用。已知CYP2A6和2E1在人类肝脏中组成性表达。为了研究它们在MTBE代谢中的作用，通过杆状病毒表达系统将人类CYP2A6和2E1 cDNA分别与人细胞色素P450还原酶共表达，并将表达的酶用于MTBE代谢。CYP2A6和2E1的转换数分别为每纳摩尔P450每分钟6.1和0.7纳摩尔TBA。在另外两种汽油醚乙基叔丁基醚（ETBE）和叔戊基甲基醚（TAME）的代谢中，异源表达的人类CYP2A6也比2E1更具活性。尽管其他人类CYP形式对MTBE代谢的贡献仍有待确定，但这些结果强烈表明CYP酶在人类肝脏中MTBE的代谢中起重要作用。
