title: Dealkylation rates of O6-alkyldeoxyguanosine, O4-alkylthymidine and related compounds in an alkyl-transfer system.
abstract: Bacterial O6-alkylguanine-DNA alkyltransferase (AGT) removes alkyl group from O6-alkylguanine and O4-alkylthymine residues in DNA, both of which are considered to be DNA damages most related to the induction of cancer and/or mutation. The repair process involves alkyl-transfer of an O-alkyl group to the active site of the enzyme, where an SH-group of cysteine residue plays the role of alkyl acceptor. In order to elucidate the chemical characteristics of substrates for this enzyme, dealkylation rates of O6-alkyldeoxyguanosine, O4-alkylthymidine and related compounds were measured using an alkyl-transfer system. Thiophenol-triethylamine system was employed as an alkyl acceptor and twenty-one O-alkyl compounds were tested. Dealkylation proceeded with pseudo first order kinetics. The half-life of O6-methyldeoxyguanosine (MedG) was 122 h and no remarkable dependence on N-9 substituents (H, CH3 and deoxyribose) was observed. A compound lacking 2-NH2 group underwent demethylation about three times faster than O6-methylguanines did, while, a compound lacking imidazole moiety underwent demethylation about 2.5 times more slowly. The half-life of O4-methylthymidine (MedT) was 38 h and no remarkable dependence on N-1 (H, CH3 and deoxyribose) and C-5 (H and CH3) substituents was observed. Deethylation proceeded much more slowly than demethylation. Substitution of selenophenol for thiophenol resulted in a 4.5 times faster MedG demethylation rate. Demethylation rates were moderately correlated with values for NMR chemical shift of CH3 group, an indicator of electron density, although the correlation curves of a series of MedG and MedT derivatives were quite different. This result suggests that some different rate-determining factors other than electron density are playing a role. These findings may be of help in resolving the details of the mechanisms of enzymic repair by bacterial and mammalian AGT.
authors.affiliation: Faculty of Pharmaceutical Sciences, Nagoya City University, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Kohda
authors.forename: M
authors.initials: M
authors.lastname: Yasuda
authors.forename: N
authors.initials: N
authors.lastname: Sawada
authors.forename: K
authors.initials: K
authors.lastname: Itano
authors.forename: Y
authors.initials: Y
authors.lastname: Kawazoe
mesh_terms: D000478:Alkylation; D001555:Benzene Derivatives / Q000378:metabolism; D002247:Carbon Isotopes; D003849:Deoxyguanosine / Q000031:analogs & derivatives* / Q000378:metabolism; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008745:Methylation; D008780:Methyltransferases / Q000378:metabolism; D019853:O(6)-Methylguanine-DNA Methyltransferase; D016566:Organoselenium Compounds; D012643:Selenium / Q000378:metabolism; D013936:Thymidine / Q000031:analogs & derivatives* / Q000378:metabolism
chemical_list: D001555:Benzene Derivatives; D002247:Carbon Isotopes; D016566:Organoselenium Compounds; C008505:O-methylthymidine; C018846:benzeneselenol; C015558:O(6)-methyl-2'-deoxyguanosine; D008780:Methyltransferases; D019853:O(6)-Methylguanine-DNA Methyltransferase; D003849:Deoxyguanosine; D012643:Selenium; D013936:Thymidine