DNA Guanine Adducts from 3-Methyl-1,2,3-oxadiazolinium Ions

Abstract

The reaction of 3-methyl-1,2,3-oxadiazolinium tosylate 10, a close model for a putative reactive intermediate in the carcinogenic activation of ethanol nitrosamines such as (2-hydroxyethyl)methylnitrosamine 1, with various guanine derivatives, including acycloguanosine 12, deoxyguanosine, deoxyguanosine monophosphate, and cyclic guanosine monophosphate, various DNA oligomers, and calf-thymus DNA has been examined to determine whether this compound methylates and hydroxyethylates guanine residues as proposed. In all of the transformations, 7-(2-(methylnitrosamino)ethyl)guanine (14) is the major product, following acidic hydrolysis, and exceeds the formation of 7-methylguanine by ratios ranging from 4:1 to 48:1, depending upon the guanine bearing substrate. O6-(2-(Methylnitrosamino)ethyl)deoxyguanosine (20) was prepared from the Mitsunobu coupling of 1 and a protected deoxyguanosine derivative. 20 is not produced in the reaction of 10 and deoxyguanosine and decomposes to 1 and guanine upon mild acid treatment, suggesting possible neighboring group participation in its facile hydrolytic cleavage. All of the major products from the reaction of 10 and 12 have been characterized, including the direct alkylation product, 7-(2-(methylnitrosamino)ethyl)acycloguanosine (13), and N2-(2-(methylnitrosamino)ethyl)guanine, which was independently synthesized. Elucidation of the reactions of DNA with 10 and other electrophiles was facilitated by the development of both partial and total enzymatic hydrolysis assays utilizing 32P-5‘-labeled DNA oligotetramers containing one of each base type and HPLC with radiometric detection. The partial hydrolysis assay gives information as to the type of base being modified, and the total hydrolysis assay permits a determination of the number of adducts produced for a given base. The assays permit a comparison between reactions where the same type of base adduct could be expected. Comparisons of the reactions of ethylene oxide and 10 using this methodology showed that 10 does not hydroxyethylate guanine in DNA