The human carcinogen vinyl chloride is metabolized in the liver to reactive intermediates which form N2,3-ethenoguanine in DNA. N2,3-Ethenoguanine is known to cause G --A transitions during DNA replication inEscherichia coli, and its formation may be a carcinogenic event in higher organisms. To investigate the repair ofN2,3-ethenoguanine, we have prepared an N2,3-etheno[14Clguanine-containing DNA substrate by nick-translating DNA with [14CldGTP and modifying the product with chloroacetaldehyde. E. coli 3-methyladenine DNA glycosylase II, purified from cells which carry the plasmid pYN1000, releases N2,3-ethenoguanine from chloroacetaldehyde-modified DNA in a protein-and time-dependent manner. This finding widens the known substrate specificity of glycosylase II to include a modified base which may be associated with the carcinogenic process. Similar enzymatic activity in eukaryotic cells might protect them from exposure to metabolites of vinyl chloride. gated the activity of this enzyme towards an EG-containing DNA substrate.Both glycosylase II and 06-alkylguanine-DNA alkyltransferase are synthesized by E. coli in increased amounts as part of the adaptive response to methylating carcinogens (17). 06-alkylguanine-DNA alkyltransferase removes alkyl groups from the Q6 position of guanine and from phosphotriesters, thereby restoring the original structure. Glycosylase II acts in a different way, releasing modified bases from the DNA and leaving apurinic sites which are subject to further repair.Glycosylase II releases 3-and 7-alkylated purines as well as 02-methylcytosine and 02-methylthymine from DNA (17). The recent finding that N2,3-ethanoguanine is also released extends the specificity of this enzyme to DNA bases that bear exocyclic groups. The studies reported here show that the enzyme has activity towards a base that has been associated with vinyl chloride carcinogenicity.The industrial chemical vinyl chloride is carcinogenic in animal tests and is associated with an increased incidence of hepatic angiosarcomas in exposed workers (1). Vinyl chloride is metabolized by the cytochrome P450 system in the liver to the unstable electrophile chloroethylene oxide, which rearranges spontaneously to the more stable alkylating agent chloroacetaldehyde (CAA) (2, 3). Chloroethylene oxide and CAA react with DNA to form a variety of adducts, many of which have been identified in vivo (4-10). The question has arisen, therefore, as to which DNA modification or modifications may be responsible for initiating the carcinogenic process.The modified base that is found in the greatest quantity in livers of animals exposed to vinyl chloride is 7-(2-oxoethyl)-guanine (oeG) (4, 6, 7). This base does not cause misincorporation when it is present in a DNA template strand (11), but it does form apurinic sites, which could be mutagenic or carcinogenic. On the other hand, N2,3-ethenoguanine (EG) has been shown to mispair in in vitro transcription systems (12, 13) and causes G --A transitions in Escherichia coli (14).These results...