Quantification of Azaserine-induced Carboxymethylated and Methylated DNA Lesions in Cells by Nanoflow Liquid Chromatography-Nanoelectrospray Ionization Tandem Mass Spectrometry Coupled with the Stable Isotope-Dilution Method

摘要

Humans are exposed to N-nitroso compounds through environmental exposure and endogenous metabolism. Some N-nitroso compounds can be metabolically activated to yield diazoacetate, which is known to induce DNA carboxymethylation. DNA lesion measurement remains one of the core tasks in toxicology and in evaluating human health risks associated with carcinogen exposure. In this study, we developed a highly sensitive nano-flow liquid chromatography-nanoelectrospray ionization-multi-stage tandem mass spectrometry (nLC-nESI-MS³) method for the simultaneous quantification of O⁶-carboxymethyl-2′-deoxyguanosine (O⁶-CMdG), O⁶-methyl-2′-deoxyguanosine (O⁶-MedG), and N⁶-carboxymethyl-2′-deoxyadenosine (N⁶-CMdA). We were able to measure the levels of these three lesions with the use of low-microgram quantities of DNA from cultured human skin fibroblasts and human colorectal carcinoma cells treated with azaserine, a DNA carboxymethylating agent. Our results revealed that the levels of O⁶-CMdG and O⁶-MedG increased when the dose of azaserine was increased from 0 to 450 µM. We, however, did not observe an apparent dose-dependent induction of N⁶-CMdA, suggesting the presence of repair mechanism(s) for the rapid clearance of this lesion in cells. This is the first report about the application of nLC-nESI-MS³ technique for the simultaneous quantification of O⁶-CMdG, O⁶-MedG and N⁶-CMdA. The method reported here will be useful for the future investigations about the repair of the carboxymethylated DNA lesions and about the implications of these lesions in carcinogenesis.