Keyer K, Gort A S, Imlay J A
Department of Microbiology, University of Illinois, Urbana 61801, USA.
J Bacteriol. 1995 Dec;177(23):6782-90. doi: 10.1128/jb.177.23.6782-6790.1995.
The conventional model of oxidative DNA damage posits a role for superoxide (O2-) as a reductant for iron, which subsequently generates a hydroxyl radical by transferring the electron to H2O2. The hydroxyl radical then attacks DNA. Indeed, mutants of Escherichia coli that lack superoxide dismutase (SOD) were 10-fold more vulnerable to DNA oxidation by H2O2 than were wild-type cells. Even the pace of DNA damage by endogenous oxidants was great enough that the SOD mutants could not tolerate air if enzymes that repair oxidative DNA lesions were inactive. However, DNA oxidation proceeds in SOD-proficient cells without the involvement of O2-, as evidenced by the failure of SOD overproduction or anaerobiosis to suppress damage by H2O2. Furthermore, the mechanism by which excess O2- causes damage was called into question when the hypersensitivity of SOD mutants to DNA damage persisted for at least 20 min after O2- had been dispelled through the imposition of anaerobiosis. That behavior contradicted the standard model, which requires that O2- be present to rereduce cellular iron during the period of exposure to H2O2. Evidently, DNA oxidation is driven by a reductant other than O2-, which leaves the mechanism of damage promotion by O2- unsettled. One possibility is that, through its well-established ability to leach iron from iron-sulfur clusters, O2- increases the amount of free iron that is available to catalyze hydroxyl radical production. Experiments with iron transport mutants confirmed that increases in free-iron concentration have the effect of accelerating DNA oxidation. Thus, O2- may be genotoxic only in doses that exceed those found in SOD-proficient cells, and in those limited circumstances it may promote DNA damage by increasing the amount of DNA-bound iron.
传统的氧化性DNA损伤模型认为，超氧化物（O2-）作为铁的还原剂发挥作用，随后通过将电子转移到H2O2生成羟基自由基。然后羟基自由基攻击DNA。事实上，缺乏超氧化物歧化酶（SOD）的大肠杆菌突变体比野生型细胞对H2O2介导的DNA氧化更敏感10倍。即使内源性氧化剂造成的DNA损伤速度足够快，以至于如果修复氧化性DNA损伤的酶失活，SOD突变体也无法耐受空气。然而，DNA氧化在SOD功能正常的细胞中也会发生，且不涉及O2-，这一点可通过SOD过量表达或厌氧状态无法抑制H2O2造成的损伤得以证明。此外，当通过厌氧处理消除O2-后，SOD突变体对DNA损伤的超敏性至少持续20分钟，这使得过量O2-导致损伤的机制受到质疑。这种行为与标准模型相矛盾，标准模型要求在暴露于H2O2期间要有O2-存在，以便重新还原细胞内的铁。显然，DNA氧化是由O2-以外的还原剂驱动的，这使得O2-促进损伤的机制仍未明确。一种可能性是，O2-凭借其从铁硫簇中浸出铁的既定能力，增加了可用于催化羟基自由基生成的游离铁的量。对铁转运突变体的实验证实，游离铁浓度的增加具有加速DNA氧化的作用。因此，O2-可能只有在超过SOD功能正常的细胞中所发现的剂量时才具有基因毒性，在这些有限的情况下，它可能通过增加与DNA结合的铁的量来促进DNA损伤。
