Adenine adducts with diepoxybutane: Isolation and analysis in exposed calf thymus DNA

1,3-Butadiene (BD) is a high-volume industrial chemical and a common environmental pollutant. Although BD is classified as a 'probable human carcinogen', only limited evidence is available for its tumorigenic effects in occupationally exposed populations. Animal studies show a surprisingly high sensitivity of mice to the carcinogenic effects of BD compared to rats (≃10³-fold), making interspecies extrapolations difficult. Identification and quantitation of specific BD-induced DNA adducts are important for improving our understanding of the mechanisms of BD biological effects and for explaining the observed species differences. Covalent binding of BD to DNA is probably due to its two epoxy metabolites: 3,4-epoxy-1-butene (EB) and 1,2:3,4-diepoxybutane (DEB). Both EB and DEB are direct mutagens producing frameshift and point mutations at both A:T and G:C base pairs. DEB is 100 times more mutagenic than EB and is found in quantity only in tissues of the most sensitive species (mouse). This has led to the suggestion that the higher sensitivity of mice to BD could be due to greater exposure to DEB. The present work was initiated in order to isolate and structurally characterize DEB-induced adenine adducts. The adducts were formed by reacting DEB with free adenine (Ade), 2'-deoxyadenosine (2'-dAdo), and calf thymus DNA followed by HPLC separation and analysis of the products by UV spectrophotometry, electrospray ionization mass spectrometry, and nuclear magnetic resonance. The adenine reaction resulted in three products which were identified as N-3-, N-7-, and N-9-(2'-hydroxy-3',4'-epoxybut-1'- yl)adenine. These adducts underwent acid-catalyzed hydrolysis to their corresponding (2',3',4'-trihydroxybut-1'-yl)adenines upon heating or storage. The 2'-dAdo reaction with DEB followed by acid hydrolysis yielded a single adduct, N⁶-(2',3',4'-trihydroxybut-1'-yl)adenine (N⁶-DEB-Ade). N- 3-DEB-Ade and N⁶-DEB-Ade were also found in hydrolysates of calf thymus DNA exposed to DEB. The amounts of N-3-DEB-Ade (13/10³ normal Ade) and N⁶- DEB-Ade (5/10³ normal Ade) were slightly lower than those of the corresponding EB-induced adducts in similar experiments, suggesting comparable reactivity of the two epoxy metabolites of BD toward adenine in DNA. The findings of this study provide a basis for future analyses of BD- induced adenyl DNA adducts in vitro and in vivo.