Deuterium exchange studies in the identification of alkylated DNA bases found in urine, by tandem mass spectrometry

Humans are exposed to a large number of carcinogens which may react at various sites throughout the body, including the N-7-, N2-, and O6-positions of guanine. The effects of this are various but may result in depurination and eventual excretion of the modified base in the urine. Various alkylguanine derivatives with substituents at the N-7-, N2- and O6-positions were synthesized and daughter-ion spectra obtained. Apart from the methyl and dialkylguanines all other spectra exhibited an ion at m/z 151 using electron ionization (EI) and m/z 152 using fast-atom bombardment (FAB). The daughter-ion spectra of dialkylguanines contained an ion at m/z 150 (EI). Hence, scans of m/z 150 and 151 using EI, to detect all parent-ions from which they are formed (parent-ion scans) should indicate the presence of alkyl and dialkylguanine bases in a complex biological matrix such as human urine. Parent-ion scans of m/z 150 and 151 (EI) of a partially purified human urine sample exhibited numerous ions, including a prominent ion at m/z 179. A daughter-ion spectrum of m/z 179 revealed fragment ions that suggested the presence of N2-dimethylguanine and an ethylated guanine. Any confusion due to the presence of daughter ions from different alkylguanines in the same spectrum can be resolved by the fact that an ethylated guanine has four exchangeable protons, whereas N2-dimethylguanine has only three. By performing hydrogen/deuterium exchange it is possible to distinguish the N2-dimethylguanine from ethylated guanine isomers.(ABSTRACT TRUNCATED AT 250 WORDS)