Chemical modification of deoxycytidine at different sites yields adducts of different stabilities: characterization of N3- and O2-deoxycytidine and N3-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 of N3-(2-hydroxy-3-buten-1-yl)deoxycytidine, N3-(2-hydroxy-3-buten-1-yl)deoxyuridine, N3-(1-hydroxy-3-buten-2-yl)deoxyuridine, and O2-(2-hydroxy-3-buten-1-yl)deoxycytidine based on UV spectra, 1H NMR, FAB/MS, and stability studies. The N3-(2-hydroxy-3-buten-1-yl)deoxycytidine adducts were unstable at pH 7.4, 37 degrees C, and deaminated to the corresponding N3-deoxyuridine adducts with half-lives of 2.3 and 2.5 h. The N3-(1-hydroxy-3-buten-2-yl)deoxycytidine diastereomers were not detected, apparently because of faster rates of deamination compared to the N3-(2-hydroxy-3-buten-1-yl)deoxycytidine adducts. The corresponding four N3-deoxyuridine adducts were stable for up to 168 h. The O2-deoxycytidine adducts were unstable and decomposed with a half-life of 11 h. The N3-(2-hydroxy-3-buten-1-yl)deoxycytidine adducts were initially the major adducts formed upon reaction of deoxycytidine with BM at 37 degrees C in phosphate buffer (pH 7.4), but the corresponding N3-deoxyuridine adducts showed a lag in formation due to the time needed for deamination. The N3-(1-hydroxy-3-buten-2-yl)deoxyuridine and O2-deoxycytidine adducts had linear formation rates, but were formed to a lesser extent. Heating the reaction mixture at 80 degrees C for 1 h converted all N3-deoxycytidine adducts to the stable N3-deoxyuridine adducts. Incubation of deoxycytidine with an excess of BM at pH 7.4, 37 degrees 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 the N3-(2-hydroxy-3-buten-1-yl)deoxycytidine and O2-(2-hydroxy-3-buten-1-yl)deoxycytidine adducts. The rate constants for N3-(2-hydroxy-3-buten1-yl)deoxycytidine and the corresponding N3-(2-hydroxy-3-buten-1-yl)deoxyuridine were five- to sixfold the rate constants for the N3-(1-hydroxy-3-buten-2-yl)deoxyuridine and O2-(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.