Lack of evidence for the involvement of formaldehyde in the hepatocarcinogenicity of methyl tertiary-butyl ether in CD-1 mice.

The oxygenated fuel additive methyl tertiary-butyl ether (MTBE) induced hepatocellular adenomas in female but not male CD-1 mice exposed to 8000 ppm; liver cancer was not induced in female or male mice exposed to 3000 or 400 ppm. Since MTBE is metabolized by cytochrome P450 to formaldehyde (HCHO), a potentially mutagenic intermediate capable of forming DNA-protein cross-links (DPX), the formation of DPX and of another HCHO derivative, RNA-formaldehyde adducts (RFA), from MTBE was investigated using freshly isolated hepatocytes from female CD-1 mice incubated with MTBE-(O-methyl-14C). DPX and RFA were detected, but the adduct yields were very small and were independent of the concentration of MTBE in the hepatocyte suspension over a wide concentration range (0.33-6.75 mM). Similar results were obtained using hepatocytes from male B6C3F1 mice and male F344 rats. Induction of cytochrome P450 by pretreatment of mice with MTBE prior to isolation of hepatocytes did not result in a measurable increase in the yields of either DPX or RFA. In contrast to the absence of concentration-dependent DPX and RFA formation from MTBE, there was a marked, concentration-dependent increase in the yields of both DPX and RFA when [14C]formaldehyde was added directly to the medium. These results suggest that the metabolism of MTBE to HCHO approaches saturation at concentrations below 0.33 mM, and that the rate of HCHO production from metabolism of MTBE is slow relative to the rate of HCHO metabolism. The lack of concentration dependence and the absence of species or sex differences in the formation of DPX and RFA from MTBE indicate that metabolism of MTBE to HCHO is not a critical component of its carcinogenic mechanism in mice.