The environmental contaminant 1,2-dibromoethane and diepoxybutane, an oxidation product of the important industrial chemical butadiene, are bis-functional electrophiles and known to be mutagenic and carcinogenic. One mechanism by which bis-electrophiles can exert their toxic effects is through the induction of genotoxic and mutagenic DNA-peptide crosslinks. This mechanism has been shown in systems overexpressing the DNA repair protein O 6 -alkylguanine DNA-alkyltransferase (AGT) or glutathione S-transferase and involves reactions with nucleophilic cysteine residues. The hypothesis that DNA-protein crosslink formation is a more general mechanism for genotoxicity by bis-electrophiles was investigated by screening nuclear proteins for reactivity with model monofunctional electrophiles. Glyceraldehyde 3-phosphate dehydrogenase (GAPDH) was identified as a candidate due to the nucleophilicity of two cysteine residues (Cys 152 and Cys 246 ) in reaction screens with model electrophiles (Dennehy, M. K. et al. (2006) Chem. Res. Toxicol. 19,[20][21][22][23][24][25][26][27][28][29]. Incubation of GAPDH with bis-electrophiles resulted in inhibition of its catalytic activity, but only at high concentrations of diepoxybutane. In vitro assays indicated DNA-GAPDH crosslink formation in the presence of diepoxybutane, and biselectrophile reactivity at Cys 246 was confirmed using mass spectral analysis. In contrast to AGT, overexpression of human GAPDH in Escherichia coli did not enhance mutagenesis by diepoxybutane. We propose that the lack of mutational enhancement is in part due to the inherently lower reactivity of GAPDH toward bis-electrophiles as well as the reduced DNA binding ability relative to AGT, preventing the in vivo formation of DNA-protein crosslinks and enhanced mutagenesis.

Abstract1,2-Dibromoethane and 1,3-butadiene are cancer suspects present in the environment and have been used widely in industry. The mutagenic properties of 1,2-dibromoethane and the 1,3-butadiene oxidation product diepoxybutane are thought to be related to the bis-electrophilic character of these chemicals. The discovery that overexpression of O 6 -alkylguanine alkyltransferase (AGT) enhances bis-electrophile-induced mutagenesis prompted a search for other proteins that may act by a similar mechanism. A human liver screen for nuclear proteins that cross-link with DNA in the presence of 1,2-dibromoethane identified histones H2b and H3 as candidate proteins. Treatment of isolated histones H2b and H3 with diepoxybutane resulted in DNA-protein cross-links and produced protein adducts, and DNA-histone H2b cross-links were identified (immunochemically) in Escherichia coli cells expressing histone H2b. However, heterologous expression of histone H2b in E. coli failed to enhance bis-electrophile-induced mutagenesis. These results are similar to those found with the cross-link candidate glyceraldehyde 3-phosphate dehydrogenase (GAPDH) (Loecken, E. M. and Guengerich, F. P. (2008) Chem. Res. Toxicol. 21, 453−458) but, in contrast to GAPDH, histone H2b bound DNA with even higher affinity than AGT. The extent of DNA cross-linking of isolated histone H2b was similar to that of AGT, suggesting that differences in post-cross-linking events explain the difference in mutagenesis.

The mutagenicity and carcinogenicity of the important commodity chemical 1,3-butadiene are attributed to the epoxide products. We confirmed our previous work showing that expression of rat glutathione (GSH) transferase 5-5 enhances the mutagenicity of butadiene diepoxide in Salmonella typhimurium TA1535. A GSH-butadiene diepoxide was isolated and fully characterized by mass spectrometry and NMR as S-(2-hydroxy-3,4-epoxybutyl)GSH. The conjugate had a t1/2 of 2.6 h (pH 7.4, 37 °C) and was considerably more mutagenic than butadiene diepoxide or monoepoxide in S. typhimurium. We propose that the GSH conjugate may be a major species involved in butadiene genotoxicity, not a detoxication product.