Speina Elzbieta, Kierzek Andrzej M, Tudek Barbara
Institute of Biochemistry and Biophysics, Polish Academy of Sciences, Pawińskiego 5A, 02-106 Warsaw, Poland.
Mutat Res. 2003 Oct 29;531(1-2):205-17. doi: 10.1016/j.mrfmmm.2003.07.007.
1,N(6)-Ethenoadenine (epsilonA) is an exocyclic DNA adduct introduced to DNA by vinyl chloride and related compounds as well as in the consequence of oxidative stress and lipid peroxidation (LPO). This highly genotoxic DNA damage is chemically unstable and either depurinates or converts into pyrimidine ring-opened secondary lesions. We have studied the structures of derivatives formed during epsilonA chemical rearrangement and identified enzymes repairing one of the rearrangement products. Rearrangement involves a water molecule addition to the C(2)-N(3) bond of epsilonA, resulting in formation of pyrimidine ring-closed B1 product, which is in equilibrium with pyrimidine ring-opened B2 compound. B2 further deformylates to yield compound C. N-Glycosidic bond of compound C is unstable and C depurinates, yielding compound D. These secondary lesions are not repaired by alkylpurine DNA N-glycosylase, which excises the parental epsilon A. Compound B, when paired with thymine and cytosine is efficiently excised by Escherichia coli formamidopirymidine DNA N-glycosylase (Fpg), and thymine glycol DNA N-glycosylases from E. coli (Nth) and Saccharomyces cerevisiae (Ntg2). B is eliminated from B:G pair only by Nth and Ntg2 glycosylases, however none of the enzymes studied is excising B from B:A pair. This enables finishing of rearrangement, formation of AP sites and subsequently DNA strand breaks. During in vitro translesion synthesis, C is much easier bypassed by DNA polymerases, than compound B, and also than the parental epsilonA as well as than the AP site. This bypass beyond C proceeds mainly by misinsertion of adenine and guanine, or by insertion of thymine, the latter restoring the parental A:T pair. Alternatively, looping out of adducted nucleotide alone or with adjacent one generates one- or two-nucleotide deletions. This may explain the previously reported 20-fold higher mutagenic potency of product C in comparison to epsilon A in E. coli [Biochemistry 32 (1993) 12793].
1，N（6）-乙烯腺嘌呤（εA）是一种外环DNA加合物，由氯乙烯及相关化合物引入DNA，也是氧化应激和脂质过氧化（LPO）的结果。这种具有高度遗传毒性的DNA损伤在化学上不稳定，要么发生脱嘌呤，要么转化为嘧啶环打开的二级损伤。我们研究了εA化学重排过程中形成的衍生物的结构，并鉴定了修复其中一种重排产物的酶。重排涉及水分子加成到εA的C（2）-N（3）键上，导致嘧啶环闭合的B1产物形成，其与嘧啶环打开的B2化合物处于平衡状态。B2进一步脱甲酰基生成化合物C。化合物C的N-糖苷键不稳定，C发生脱嘌呤，生成化合物D。这些二级损伤不能被切除亲本εA的烷基嘌呤DNA N-糖苷酶修复。当化合物B与胸腺嘧啶和胞嘧啶配对时，可被大肠杆菌甲酰胺嘧啶DNA N-糖苷酶（Fpg）以及大肠杆菌（Nth）和酿酒酵母（Ntg2）的胸腺嘧啶乙二醇DNA N-糖苷酶有效切除。只有Nth和Ntg2糖苷酶能从B:G对中消除B，然而，所研究的酶中没有一种能从B:A对中切除B。这使得重排完成、AP位点形成并随后导致DNA链断裂。在体外跨损伤合成过程中，DNA聚合酶绕过化合物C比绕过化合物B容易得多，也比绕过亲本εA以及AP位点容易得多。这种绕过C的过程主要通过腺嘌呤和鸟嘌呤的错误插入，或者通过胸腺嘧啶的插入来进行，后者恢复亲本A:T对。或者，加合核苷酸单独或与相邻核苷酸一起环出会产生一个或两个核苷酸的缺失。这可能解释了先前报道的产物C在大肠杆菌中的诱变效力比εA高20倍[《生物化学》32（1993）12793]。
