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) induce d 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 fo rmaldehyde (HCHO), a potentially mutagenic intermediate capable of for ming DNA-protein cross-links (DPX), the formation of DPX and of anothe r HCHO derivative, RNA-formaldehyde adducts (RFA), from MTBE was inves tigated using freshly isolated hepatocytes from female CD-1 mice incub ated with MTBE-(O-methyl-C-14). DPX and RFA were detected, but the add uct yields were very small and were independent of the concentration o f MTBE in the hepatocyte suspension over a wide concentration range (0 .33-6.75 mM). Similar results were obtained using hepatocytes from mal e B6C3F(1) mice and male F344 rats. Induction of cytochrome P450 by pr etreatment of mice with MTBE prior to isolation of hepatocytes did not result in a measurable increase in the yields of either DPX or RFA. I n contrast to the absence of concentration-dependent DPX and RFA forma tion from MTBE, there was a marked, concentration-dependent increase i n the yields of both DPX and RFA when [C-14]formaldehyde was added dir ectly to the medium. These results suggest that the metabolism of MTBE to HCHO approaches saturation at concentrations below 0.33 mM, and th at the rate of HCHO production from metabolism of MTBE is slow relativ e 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 c ritical component of its carcinogenic mechanism in mice. (C) 1997 Else vier Science Ireland Ltd.