DNA adducts formed by ethene and butadiene : quantitative analysis in rats and humans

Abstract

The present study has focused on DNA adduct formation by two important industrial and environmental chemicals, ethene and butadiene, in rats and humans. Ethene and butadiene are among the 50 chemicals produced in greatest volume in the United States. Ethene is the most used petrochemical, while butadiene is widely used for the production of synthetic rubbers. They also occur as environmental contaminants present in automobile exhaust and cigarette smoke. In vivo these two compounds are metabolized by cytochrome P450-dependent monooxygenases to simple epoxides, which can alkylate various sites in DNA. DNA adduct formation is generally considered to be the basis of genotoxicity and related to cancer development. The detection of the DNA adducts is of great importance in risk assessment of chemicals as well as in determination of human exposures to carcinogens.

Quantification of DNA adduct formation in rats indicated that 7-(2-hydroxyethyl)guanine (7-HEG) accumulated to a similar extent in liver and lymphocytes after repeated exposures to ethene. The half-lives of removal from DNA for 7-HEG in various tissues from ethene-exposed rats suggested that the removal of this adduct in vivo is primarily due to spontaneous depurination. 7-HEG adducts were quantified by the 32P-postlabelling method combined with TLC and HPLC separation. When this method was compared with the high-resolution GC/MS method in the quantification of 7-HEG in DNA obtained from the same tissue samples of rats, a good agreement between these two methods was observed. To demonstrate the effect of tobacco smoking on the formation of adducts, 7-alkylguanine (including 7-HEG and 7-methylguanine) adducts were determined in white blood cell and lung DNA of smokers and non-smokers. Significantly higher levels of 7-alkylguanine were found in smokers than in non-smokers. The background level of 7-HEG adducts in human DNA may result from endogenous ethylene oxide, formed by the metabolism of endogenously produced ethene.

Epoxybutanediol is one of the epoxide metabolites of butadiene. N-1-adenine adducts of epoxybutanediol were identified in liver DNA samples from rats exposed to butadiene. Because of the chemical stability of N-1-adenine adducts and the high sensitivity of the developed 32P-postlabelling assay, this adduct was considered to be a potential biomarker of butadiene exposure. This was investigated in occupationally exposed workers. The adduct level in lymphocyte DNA in butadiene-exposed workers was significantly higher than in control subjects. Modulation of adduct levels by genetic polymorphism of metabolizing enzymes was also studied in these subjects. A significant correlation was found between the adduct level and GSTM1 genotype for the exposed workers (higher adduct level in those lacking the gene). These results suggest that N-1-adenine adducts of epoxybutanediol may be used for biomonitoring of human exposure to butadiene and that DNA adducts may serve as a sensitive and specific biomarker integrating exposure and host metabolic capacity.

Taken together, these results have contributed to the knowledge on the carcinogenic mechanisms of ethene and butadiene and human risk assessment from exposures to ethene and butadiene.