CROSS-LINKING OF THE HUMAN DNA REPAIR PROTEIN O6-ALKYLGUANINE DNA ALKYLTRANSFERASE TO DNA IN THE PRESENCE OF 1234-DIEPOXYBUTANE

摘要

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 (J. G. Valadez et al., Activation of bis-electrophiles to mutagenic conjugates by human O⁶-alkylguanine-DNA alkyltransferase. Chem. Res. Toxicol. 17 (2004) 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 AGT (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.