Keyer K, Imlay J A
Department of Microbiology, University of Illinois, Urbana 61801, USA.
Proc Natl Acad Sci U S A. 1996 Nov 26;93(24):13635-40. doi: 10.1073/pnas.93.24.13635.
Superoxide promotes hydroxyl-radical formation and consequent DNA damage in cells of all types. The long-standing hypothesis that it primarily does so by delivering electrons to adventitious iron on DNA was refuted by recent studies in Escherichia coli. Alternative proposals have suggested that superoxide may accelerate oxidative DNA damage by leaching iron from storage proteins or enzymic [4Fe-4S] clusters. The released iron might then deposit on the surface of the DNA, where it could catalyze the formation of DNA oxidants using other electron donors. The latter model is affirmed by the experiments described here. Whole-cell electron paramagnetic resonance demonstrated that the level of loose iron in superoxide-stressed cells greatly exceeds that of unstressed cells. Bacterial iron storage proteins were not the major source for free iron, since superoxide also increased iron levels in mutants lacking these iron storage proteins. However, overproduction of an enzyme containing a labile [4Fe-4S] cluster dramatically increased the free iron content of cells when they were growing in air. The rates of spontaneous mutagenesis and DNA damage from exogenous H2O2 increased commensurately. It is striking that both growth defects and DNA damage caused by superoxide ensue from its ability to damage a subset of iron-sulfur clusters.
超氧化物可促进各类细胞中羟基自由基的形成及随之而来的DNA损伤。长期以来的假说认为，超氧化物主要通过向DNA上的偶然铁传递电子来实现这一作用，但近期对大肠杆菌的研究驳斥了这一假说。其他观点认为，超氧化物可能通过从储存蛋白或酶的[4Fe - 4S]簇中浸出铁来加速氧化性DNA损伤。释放出的铁随后可能沉积在DNA表面，在那里它可以利用其他电子供体催化DNA氧化剂的形成。本文所述的实验证实了后一种模型。全细胞电子顺磁共振表明，超氧化物应激细胞中游离铁的水平大大超过未应激细胞。细菌铁储存蛋白不是游离铁的主要来源，因为超氧化物也会增加缺乏这些铁储存蛋白的突变体中的铁水平。然而，当细胞在空气中生长时，含有不稳定[4Fe - 4S]簇的酶的过量表达会显著增加细胞中的游离铁含量。自发突变率和外源性H2O2引起的DNA损伤率也相应增加。引人注目的是，超氧化物引起的生长缺陷和DNA损伤均源于其破坏一部分铁硫簇的能力。
