Measurement of O6-alkylguanine-DNA alkyltransferase activity in tumour cells using stable isotope dilution HPLC-ESIâ¿¿MS/MS

The repair of DNA mediated by O6-alkylguanine-DNA alkyltransferase (AGT) provides protection against DNA damage from endogenous or exogenous alkylation of the O6 position of guanine. However, this repair acts as a double-edged sword in cancer treatment, as it not only protects normal cells from chemotherapy-associated toxicities, but also results in cancer cell resistance to guanine O6-alkylating antitumour agents. Thus, AGT plays an important role in predicting the individual susceptibility to guanine O6-alkylating carcinogens and chemotherapies. Accordingly, it is necessary to establish a quantitative method for determining AGT activity with high accuracy, sensitivity and practicality. Here, we describe a novel nonradioactive method for measuring AGT activity using stable isotope dilution high-performance liquid chromatography electrospray ionization tandem mass spectrometry (HPLC-ESIâ¿¿MS/MS). This method is based on the irreversibility of the removal of the O6-alkyl group from guanine by AGT and on the high affinity of O6-benzylguanine (O6-BG) as an AGT substrate. HPLC-ESIâ¿¿MS/MS was used to measure the AGT activities in cell protein extracts from eight tumour lines, demonstrating that AGT activity was quite variable among different cell lines, ranging from nondetectable to 1021 fmol/mg protein. The experiments performed in intact tumour cells yielded similar results but exhibited slightly higher activities than those observed in cell protein extracts. The accuracy of this method was confirmed by an examination of AGT expression levels using western blotting analysis. To our knowledge, this method is the first mass spectrometry-based AGT activity assay, and will likely provide assistance in the screening of cancer risk or the application of chemotherapies.