Metabolism of methyl tert-butyl ether and other gasoline ethers by human liver microsomes and heterologously expressed human cytochromes P450: Identification of CYP2A6 as a major catalyst

To reduce the production of carbon monoxide and other pollutants in motor v ehicle exhaust, methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETB E), and tert-amyl methyl ether (TAME) are added to gasoline as oxygenates f or more complete combustion. Previously, we demonstrated that human liver i s active in metabolizing MTBE to tert-butyl alcohol (TBA) and that cytochro me P450 (CYP) enzymes play a critical role in the metabolism of MTBE. The p resent study demonstrates that human liver is also active in the oxidative metabolism of ETBE and TAME. A large interindividual variation in metaboliz ing these gasoline ethers was observed in 15 human liver microsomal samples . The microsomal activities in metabolizing MTBE, ETBE, and TAME were highl y correlated among each other (r, 0.91-0.96), suggesting that these ethers are metabolized by the same enzyme(s). Correlation analysis of the ether-me tabolizing activities with individual CYP enzyme activities in the liver mi crosomes showed that the highest degree of correlation was with human CYP2A 6 (r, 0.90-0.95), which is constitutively expressed in human Livers and kno wn to be polymorphic. CYP2A6 displayed the highest turnover number in metab olizing gasoline ethers among a battery of human CYP enzymes expressed in h uman B-lymphoblastoid cells. Kinetic studies on MTBE metabolism with three human liver microsomes exhibited apparent K-m values that ranged from 28 to 89 mu M and the V-max values from 215 to 783 pmol/min/mg, with similar cat alytic efficiency values (7.7 to 8.8 mu l/min/mg protein). Metabolism of MT BE, ETBE, and TAME by human liver microsomes was inhibited by coumarin, a k nown substrate of human CYP2A6, in a concentration-dependent manner. Monocl onal antibody against human CYP2A6 caused a significant inhibition (75% to 95%) of the metabolism of MTBE, ETBE, and TAME in human liver microsomes. T aken together, these results clearly indicate that in human liver, CYP2A6 i s the major enzyme responsible for the metabolism of MTBE, ETBE, and TAME. (C) 1999 Academic Press.