Tandem mass spectrometric approaches for the analysis of alkylguanines in human urine

Human exposure to carcinogenic alkylating agents can lead to the formation of covalently bound adducts in DNA, some of which are excreted in urine as alkylated purines following DNA degradation and repair. Tandem mass spectrometric methods have been developed for the qualitative and quantitative determination of such alkylpurines in human urine. Short-chain alkyl- and hydroxyalkylguanines have been synthesized with the substituents at the N-7-, O⁶- and N2-positions of guanine. Examination of the product ion scans of their molecular ions (electron impact (EI) ionization) revealed that the ion at m/z 151, guanine]⁺, was common to all of the alkylguanines studied, with the exception of the methylated analogues. Precursor ion scans of this ion on partially purified human urine extracts showed the presence of several ions (e.g. m/z 179, 195) which were consistent with molecular ions for alkylguanines. The presence of these and other constituents was confirmed by product ion spectra of molecular ions (EI and fast atom bombardment), and by high-performance liquid chromatographic separation prior to tandem mass spectrometry (MS/MS). Evidence was obtained for the presence of N-7-methyl-, N²-dimethyl-, N²-dimethyl-, N²-ethyl- and N-7-(2-hydroxyethyl)guanine. Quantitative methods were established for these five alkyl guanines using gas chromatography mass spectrometry (GC/MS) and GC/MS/MS. Deuterated internal standards were synthesized and added to the urine prior to extraction of alkylpurines by Sep-Pak cartridge chromatography. The products were converted into their tert-butyldimethylsilyl derivatives and analysed by selected ion monitoring (SIM) of M – 57]⁺ or by multiple reaction monitoring (MRM) of the fragmentation M^+˙ → M – 57]⁺. The MRM method yielded values for N-7-methylguanine of 2.57 ± S.D. 1.32 mg day^?1 (n = 6), N2-methylguanine of 0.31 ± 0.10 mg day^?1 (n = 10) and N²-dimethylguanine of 0.21 ± 0.23 mg day^?1 (n = 10). N²-Ethyl- and N-7-(2-hydroxyethyl)guanine could only be detected by SIM at levels of ～0.5 and 2 μg day^?1, respectively. The MRM analyses, although inherently less sensitive than the SIM analyses, exhibit greater selectivity and consequently fewer contaminant ions.