Chemical Modification of Deoxycytidine at Different Sites Yields Adducts of Different Stabilities: Characterization ofN3- andO2-Deoxycytidine andN3-Deoxyuridine Adducts of Butadiene Monoxide

Eight adducts were characterized from the reaction of deoxycytidine with the chemical carcinogen, butadiene monoxide (BM). They were identified as diastereomeric pairs ofN3-(2-hydroxy-3-buten-1-yl)deoxycytidine,N3-(2-hydroxy-3-buten-1-yl)deoxyuridine,N3-(1-hydroxy-3-buten-2-yl)deoxyuridine, andO2-(2-hydroxy-3-buten-1-yl)deoxycytidine based on UV spectra,1H NMR, FAB/MS, and stability studies. TheN3-(2-hydroxy-3-buten-1-yl)deoxycytidine adducts were unstable at pH 7.4, 37°C, and deaminated to the correspondingN3-deoxyuridine adducts with half-lives of 2.3 and 2.5 h. TheN3-(1-hydroxy-3-buten-2-yl)deoxycytidine diastereomers were not detected, apparently because of faster rates of deamination compared to theN3-(2-hydroxy-3-buten-1-yl)deoxycytidine adducts. The corresponding fourN3-deoxyuridine adducts were stable for up to 168 h. TheO2-deoxycytidine adducts were unstable and decomposed with a half-life of 11 h. TheN3-(2-hydroxy-3-buten-1-yl)deoxycytidine adducts were initially the major adducts formed upon reaction of deoxycytidine with BM at 37°C in phosphate buffer (pH 7.4), but the correspondingN3-deoxyuridine adducts showed a lag in formation due to the time needed for deamination. TheN3-(1-hydroxy-3-buten-2-yl)deoxyuridine andO2-deoxycytidine adducts had linear formation rates, but were formed to a lesser extent. Heating the reaction mixture at 80°C for 1 h converted allN3-deoxycytidine adducts to the stableN3-deoxyuridine adducts. Incubation of deoxycytidine with an excess of BM at pH 7.4, 37°C, followed by the extraction and heating steps allowed calculation of the pseudo-first-order kinetic rate constants for the four uridine adducts. If the heating step was eliminated, then the pseudo-first-order kinetic rate constants could be calculated for theN3-(2-hydroxy-3-buten-1-yl)deoxycytidine andO2-(2-hydroxy-3-buten-1-yl)deoxycytidine adducts. The rate constants forN3-(2-hydroxy-3-buten1-yl)deoxycytidine and the correspondingN3-(2-hydroxy-3-buten-1-yl)deoxyuridine were five- to sixfold the rate constants for theN3-(1-hydroxy-3-buten-2-yl)deoxyuridine andO2-(2-hydroxy-3-buten-1-yl)deoxycytidine adducts. Thus, the results show that the reaction of deoxycytidine with BM yields adducts at different sites with different rates of formation and stabilities. Understanding the chemical interactions of deoxycytidine with BM and the stability of the various adducts may contribute to a better understanding of the molecular mechanisms of mutagenesis and carcinogenesis of BM and the development of useful biomarkers of exposure.