Sangaiah R, Yen T Y, Swenberg J A
Department of Environmental Sciences and Engineering, University of North Carolina, Chapel Hill 27599-7400, USA.
Chem Res Toxicol. 1997 Jul;10(7):779-85. doi: 10.1021/tx970004q.
Diepoxybutane (DEB) is an important metabolite of 1,3-butadiene (BD), a high-volume industrial chemical classified as a probable human carcinogen. Rodent inhalation studies show strikingly high sensitivity of mice to carcinogenic effects of butadiene compared to rats, which has been linked to differences in metabolism. Both species convert BD to 3,4-epoxy-1-butene (EB), but mice further oxidize a significantly greater part of EB to DEB. DEB is a potent bifunctional genotoxic agent which is 100-fold more mutagenic than EB and is likely to be involved in BD-induced carcinogenesis. Identification of specific BD-induced DNA adducts is critical to understanding the mechanism of its biological activity. We have previously described reactions of EB with guanine and adenine as nucleobases, nucleosides, and constituents of DNA. In this work, DEB-induced guanine adducts were isolated and structurally characterized by UV spectroscopy, mass spectrometry, and nuclear magnetic resonance. When guanosine was reacted with DEB in glacial acetic acid followed by hydrolysis in hydrochloric acid, three products were isolated: N-7-(2',3',4'-trihydroxybut-1'-yl)guanine (DEB-Gua I, major adduct), N-7-(2',4'-dihydroxy-3'-chlorobut-1'-yl)guanine (DEB-Gua II), and N-7-(2',3'-dihydroxy-4'-acetoxybut-1'-yl)guanine (DEB-Gua III). We suggest initial formation of the N-7-(2'-hydroxy-3',4'-epoxybut-1'-yl)guanine intermediate followed by nucleophilic substitution at the 3',4'-epoxy ring with hydroxide, chloride, or acetate anions to give DEB-Gua I, II, or III, respectively. DEB-Gua I and the epoxy intermediate were also isolated from hydrolysates of DEB-exposed calf thymus DNA (CT DNA). N-7-Guanine adducts are known to undergo spontaneous and enzymatic depurination producing apurinic sites. If not repaired before DNA replication, apurinic sites can give rise to mutations and ultimately cancer. The extent of alkylation at the N-7 of guanine in DEB-exposed DNA (58.7 +/- 1.1 adducts/10(3) normal guanines) was similar to that previously reported for CT DNA exposed to EB at the same molar ratio. Since EB and DEB appear to induce comparable levels of overall DNA alkylation at the conditions applied in this work, other factors, such as formation of DNA cross-links by DEB but not EB or differences in repair of EB and DEB adducts, may be responsible for the differences in mutagenicity.
1,2 - 二环氧丁烷（DEB）是1,3 - 丁二烯（BD）的一种重要代谢产物，BD是一种大量生产的工业化学品，被归类为可能的人类致癌物。啮齿动物吸入研究表明，与大鼠相比，小鼠对丁二烯的致癌作用具有极高的敏感性，这与代谢差异有关。两种物种都能将BD转化为3,4 - 环氧 - 1 - 丁烯（EB），但小鼠会将显著更多部分的EB进一步氧化为DEB。DEB是一种强效的双功能基因毒性剂，其致突变性比EB高100倍，可能参与了BD诱导的致癌过程。鉴定特定的BD诱导的DNA加合物对于理解其生物活性机制至关重要。我们之前已经描述了EB与鸟嘌呤和腺嘌呤作为核碱基、核苷以及DNA成分的反应。在这项工作中，通过紫外光谱、质谱和核磁共振对DEB诱导的鸟嘌呤加合物进行了分离和结构表征。当鸟苷在冰醋酸中与DEB反应，随后在盐酸中水解时，分离出了三种产物：N - 7 - （2',3',4' - 三羟基丁 - 1' - 基）鸟嘌呤（DEB - Gua I，主要加合物）、N - 7 - （2',4' - 二羟基 - 3' - 氯丁 - 1' - 基）鸟嘌呤（DEB - Gua II）和N - 7 - （{2',3' - 二羟基 - 4' - 乙酰氧基丁 - 1' - 基}）鸟嘌呤（DEB - Gua III）。我们认为首先形成N - 7 - （2' - 羟基 - 3',4' - 环氧丁 - 1' - 基）鸟嘌呤中间体，随后3',4' - 环氧环上发生亲核取代反应，分别与氢氧根、氯离子或醋酸根阴离子反应，生成DEB - Gua I、II或III。DEB - Gua I和环氧中间体也从暴露于DEB的小牛胸腺DNA（CT DNA）的水解产物中分离出来。已知N - 7 - 鸟嘌呤加合物会发生自发和酶促脱嘌呤反应，产生无嘌呤位点。如果在DNA复制之前未得到修复，无嘌呤位点会导致突变并最终引发癌症。暴露于DEB的DNA中鸟嘌呤N - 7位的烷基化程度（58.7±1.1个加合物 / 10³个正常鸟嘌呤）与之前报道的以相同摩尔比暴露于EB的CT DNA的烷基化程度相似。由于在本工作所采用的条件下，EB和DEB似乎诱导了相当水平的总体DNA烷基化，其他因素，如DEB而非EB形成DNA交联，或EB和DEB加合物修复的差异，可能是致突变性差异的原因。
