ROLE OF CYTOCHROMES P450 IN THE METABOLISM OF METHYL TERT-BUTYL ETHERIN HUMAN LIVERS

Methyl tert-butyl ether (MTBE) is widely used as a gasoline oxygenate for more complete combustion in order to reduce the air pollution caus ed by motor vehicle exhaust. The possible adverse effects of MTBE on h uman health is a major public concern. However, information on the met abolism of MTBE in human tissues is lacking. The present study demonst rates that human liver is active in metabolizing MTBE to tert-butyl al cohol (TEA), 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, It = 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 TEA in human liver microsomes is NADPH-dependent, and is significantly inhibited b y carbon monoxide (GO), 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 constitutiv ely expressed in human livers. To examine their involvement in MTBE me tabolism, human CYP2A6 and 2E1 cDNAs were individually coexpressed wit h human cytochrome P450 reductase by a baculovirus expression system a nd the expressed enzymes were used for MTBE metabolism. The turnover n umber for CYP2A6 and 2E1 was 6.1 and 0.7 nmol TBA/min per nmol P450, r espectively. The heterologously expressed human CYP2A6 was also more a ctive than 2E1 in the metabolism of two other gasoline ethers, ethyl t ert-butyl ether (ETBE) and tert-amyl methyl ether (TAME). Although the contributions of other human CYP forms to MTBE metabolism remain to b e determined, these results strongly suggest that CYP enzymes play an important role in the metabolism of MTBE in human livers.