Department of Chemistry, University of Utah, 315 South 1400 East, Salt Lake City, Utah 84112-0850, USA.
J Am Chem Soc. 2012 Sep 12;134(36):15091-102. doi: 10.1021/ja306077b. Epub 2012 Aug 29.
8-Oxo-7,8-dihydroguanine (OG) is the most common base damage found in cells, where it resides in many structural contexts, including the nucleotide pool, single-stranded DNA at transcription forks and replication bubbles, and duplex DNA base-paired with either adenine (A) or cytosine (C). OG is prone to further oxidation to the highly mutagenic hydantoin products spiroiminodihydantoin (Sp) and 5-guanidinohydantoin (Gh) in a sharply pH-dependent fashion within nucleosides. In the present work, studies were conducted to determine how the structural context affects OG oxidation to the hydantoins. These studies revealed a trend in which the Sp yield was greatest in unencumbered contexts, such as nucleosides, while the Gh yield increased in oligodeoxynucleotide (ODN) contexts or at reduced pH. Oxidation of oligomers containing hydrogen-bond modulators (2,6-diaminopurine, N(4)-ethylcytidine) or alteration of the reaction conditions (pH, temperature, and salt) identify base stacking, electrostatics, and base pairing as the drivers of the key intermediate 5-hydroxy-8-oxo-7,8-dihydroguanine (5-HO-OG) partitioning along the two hydantoin pathways, allowing us to propose a mechanism for the observed base-pairing effects. Moreover, these structural effects cause an increase in the effective pK(a) of 5-HO-OG, following an increasing trend from 5.7 in nucleosides to 7.7 in a duplex bearing an OG·C base pair, which supports the context-dependent product yields. The high yield of Gh in ODNs underscores the importance of further study on this lesion. The structural context of OG also determined its relative reactivity toward oxidation, for which the OG·A base pair is ~2.5-fold more reactive than an OG·C base pair, and with the weak one-electron oxidant ferricyanide, the OG nucleoside reactivity is >6000-fold greater than that of OG·C in a duplex, leading to the conclusion that OG in the nucleoside pool should act as a protective agent for OG in the genome.
8-氧代-7,8-二氢鸟嘌呤（OG）是细胞中最常见的碱基损伤，它存在于许多结构环境中，包括核苷酸池、转录叉和复制泡中的单链 DNA 以及与腺嘌呤（A）或胞嘧啶（C）碱基配对的双链 DNA。OG 很容易在核苷中进一步氧化为高度诱变的海因产物螺旋亚胺二氢嘧啶（Sp）和 5-胍基海因（Gh），这一过程强烈依赖于 pH 值。在本工作中，研究了结构环境如何影响 OG 氧化为海因。这些研究表明，在无阻碍的环境中（如核苷），Sp 的产率最高，而在寡脱氧核苷酸（ODN）环境或降低 pH 值时，Gh 的产率增加。含有氢键调节剂（2,6-二氨基嘌呤、N（4）-乙基胞嘧啶）的寡聚物的氧化或改变反应条件（pH 值、温度和盐），确定碱基堆积、静电和碱基配对是关键中间体 5-羟基-8-氧代-7,8-二氢鸟嘌呤（5-HO-OG）沿两条海因途径分配的驱动力，使我们能够提出观察到的碱基配对效应的机制。此外，这些结构效应导致 5-HO-OG 的有效 pK(a)增加，遵循从核苷中的 5.7 到带有 OG·C 碱基对的双链体中的 7.7 的增加趋势，这支持了依赖于上下文的产物产率。Gh 在 ODN 中的高产率突出表明需要进一步研究这种损伤。OG 的结构环境也决定了其相对氧化反应性，其中 OG·A 碱基对的反应性比 OG·C 碱基对高约 2.5 倍，并且使用弱单电子氧化剂铁氰化物，OG 核苷的反应性比双链体中的 OG·C 高 6000 倍以上，这导致结论是核苷池中的 OG 应该作为基因组中 OG 的保护剂。
