title: Cross-linking of the human DNA repair protein O6-alkylguanine DNA alkyltransferase to DNA in the presence of 1,2,3,4-diepoxybutane.
abstract: 1,2,3,4-Diepoxybutane (DEB) is a key carcinogenic metabolite of the important industrial chemical 1,3-butadiene. DEB is a bifunctional alkylating agent capable of reacting with DNA and proteins. Initial DNA alkylation by DEB produces N7-(2'-hydroxy-3',4'-epoxybut-1'-yl)-guanine monoadducts, which can react with another nucleophilic site to form cross-linked adducts. A recent report revealed a strong correlation between cellular expression of the DNA repair protein O6-alkylguanine DNA alkyltransferase (AGT) and the cytotoxic and mutagenic activity of DEB, suggesting that DEB induces AGT-DNA cross-links (Valadez, J. G., et al. (2004) Activation of bis-electrophiles to mutagenic conjugates by human O6-alkylguanine-DNA alkyltransferase. Chem. Res. Toxicol. 17, 972-982). The purpose of our study was to analyze the formation and structures of DEB-induced AGT-DNA conjugates and to identify specific amino acid residues within the protein involved in cross-linking. DNA-protein cross-link formation was detected by SDS-PAGE when 32P-labeled double-stranded oligodeoxynucleotides were exposed to DEB in the presence of either wild-type hAGT or a C145A hAGT mutant. Capillary HPLC-electrospray ionization mass spectrometry (ESI-MS) analysis of hAGT that had been treated with N7-(2'-hydroxy-3',4'-epoxybut-1'-yl)-deoxyguanosine (dG monoepoxide) revealed the ability of the protein to form either one or two butanediol-dG cross-links, corresponding to mass shifts of +353 and +706 Da, respectively. HPLC-ESI+ -MS/MS sequencing of the tryptic peptides obtained from dG monoepoxide-treated protein indicated that the two cross-linking sites were the alkyl acceptor site, Cys145, and a neighboring active site residue, Cys150. The same two amino acid residues of hAGT became covalently cross-linked to DNA following DEB treatment. Modification of Cys145 was further confirmed by HPLC-ESI+ -MS/MS analysis of dG monoepoxide-treated synthetic peptide GNPVPILIPCHR which represents the active site tryptic fragment of hAGT (C = Cys145). The replacement of the catalytic cysteine residue with alanine in the C145A hAGT mutant abolished DEB-induced cross-linking at this site, while the formation of conjugates via neighboring Cys150 was retained. The exact chemical structure of the cross-linked lesion was established as 1-(S-cysteinyl)-4-(guan-7-yl)-2,3-butanediol by HPLC-ESI+ -MS/MS analysis of the amino acids resulting from the total digestion of modified proteins analyzed in parallel with an authentic standard. AGT-DNA cross-linking is a likely mechanism of DEB-mediated cytotoxicity in cells expressing this important repair protein.
authors.affiliation: University of Minnesota Cancer Center and Department of Medicinal Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, USA.
authors.forename: Rachel
authors.initials: R
authors.lastname: Loeber
authors.forename: Mathur
authors.initials: M
authors.lastname: Rajesh
authors.forename: Qingming
authors.initials: Q
authors.lastname: Fang
authors.forename: Anthony E
authors.initials: AE
authors.lastname: Pegg
authors.forename: Natalia
authors.initials: N
authors.lastname: Tretyakova
mesh_terms: D000477:Alkylating Agents / Q000737:chemistry; D000595:Amino Acid Sequence; D002273:Carcinogens / Q000737:chemistry; D003432:Cross-Linking Reagents / Q000737:chemistry*; D004247:DNA / Q000737:chemistry*; D004260:DNA Repair; D003849:Deoxyguanosine / Q000737:chemistry; D004852:Epoxy Compounds / Q000737:chemistry*; D006801:Humans; D008969:Molecular Sequence Data; D019853:O(6)-Methylguanine-DNA Methyltransferase / Q000737:chemistry*; D010455:Peptides / Q000032:analysis / Q000737:chemistry
chemical_list: D000477:Alkylating Agents; D002273:Carcinogens; D003432:Cross-Linking Reagents; D004852:Epoxy Compounds; D010455:Peptides; C007366:diepoxybutane; D004247:DNA; D019853:O(6)-Methylguanine-DNA Methyltransferase; D003849:Deoxyguanosine