BIOTRANSFORMATION OF C-12-LABELED AND 2-C-13-LABELED METHYL TERT-BUTYL ETHER, ETHYL TERT-BUTYL ETHER, AND TERT-BUTYL ALCOHOL IN RATS - IDENTIFICATION OF METABOLITES IN URINE BY C-13 NUCLEAR-MAGNETIC-RESONANCE AND GAS-CHROMATOGRAPHY MASS-SPECTROMETRY

The biotransformation of the fuel oxygenates methyl tert-butyl ether ( MTBE) and ethyl tert-butyl ether (ETBE) was studied in rats after inha lation exposure; the biotransformation of the initial metabolite of th ese ethers, tert-butyl alcohol, was studied after oral gavage. To stud y ether metabolism, rats were exposed for 6 h to initial concentration s of 2000 ppm of MTBE or ETBE, respectively [2-C-13]MTBE and [2-C-13]E TBE. Urine was collected for 48 h after the end of the exposure, and u rinary metabolites were identified by C-13 NMR (C-13-labeled ethers) a nd gas chromatography/mass spectrometry (GC/MS) (C-12- and C-13-labele d ethers). To study tert-butyl alcohol metabolism, rats were dosed eit her with tert-butyl alcohol at natural carbon isotope ratio or with C- 13-enriched tert-butyl alcohol (250 mg/kg of body weight), urine was c ollected, and metabolites were identified by NMR and GC/MS. tert-Butyl alcohol was identified as a minor product of the biotransformation of MTBE and ETBE. In addition, small amounts of a tert-butyl alcohol con jugate, likely a glucuronide, were present in the urine of the treated animals. Moreover, the mass spectra obtained indicate the presence of small amounts of [C-13]acetone in the urine of [C-13]MTBE and [C-13]E TBE-treated rats. 2-Methyl-1,2-propanediol, 2-hydroxyisobutyrate, and another unidentified conjugate of tert-butyl alcohol, most probably a sulfate, were major urinary metabolites of MTBE and ETBE as judged by the intensities of the NMR signals. In [C-13]-tert-butyl alcohol-dosed rats, [C-13]acetone, tert-butyl alcohol, and its glucuronide represen ted minor metabolites; as with the ethers, 2-methyl-1,2-propanediol, 2 -hydroxyisobutyrate, and the presumed tert-butyl alcohol sulfate were the major metabolites present. In one human individual given 5 mg/kg [ C-13]-tert-butyl alcohol orally, 2-methyl-1,2-propanediol and 2-hydrox yisobutyrate were major metabolites in urine detected by C-13 NMR anal ysis. Unconjugated tert-butyl alcohol and tert-butyl alcohol glucuroni de were present as minor metabolites, and traces of the presumed tert- butyl alcohol sulfate were also present. Our results suggest that tert -butyl alcohol formed from MTBE and ETBE is intensively metabolized by further oxidation reactions. Studies to elucidate mechanisms of toxic ity for these ethers to the kidney need to consider potential toxiciti es induced by these metabolites.