title: NAD-dependent malate dehydrogenase protects against oxidative damage in Escherichia coli K-12 through the action of oxaloacetate.
abstract: Reactive oxygen species including hydrogen peroxide (H(2)O(2)) and hydroxyl radical (OH) can be generated by ionizing radiation and has the potential to induce diseases. We provide the evidence that NAD-dependent malate dehydrogenase (MDH) is involved in the antioxidant role in preventing H(2)O(2) or γ-radiation-induced damage in Escherichia coli through the action of oxaloacetate. The E. colimdh mutant strain defective in MDH activity was more sensitive to H(2)O(2) or γ-radiation than was the wild type strain, when challenged in the exponential growth phase. The mdh mutant cells pretreated with oxaloacetate (2.5 mM), a product of NAD-dependent MDH activity, prior to H(2)O(2) treatment or γ-irradiation are resistant to H(2)O(2) or γ-radiation-induced damage, so cell survivability is restored to similar levels with the wild type. The SOS induction of umu'-'lacZ fusion gene by H(2)O(2) is significantly repressed by pretreatment of oxaloacetate in a dose-dependent way. These results indicate that oxaloacetate effectively protects E. coli cells against damage caused by oxidative stress. Oxaloacetate strongly prevented the DNA strand breaks by OH in a metal-catalyzed oxidation (MCO) system that generated H(2)O(2) as a mediator. By contrast, the prevention of DNA damage by oxaloacetate in an γ-irradiation system that directly generates OH from H(2)O in vitro was far less than that in an MCO system. Our results demonstrated that oxaloacetate, metabolite of NAD-dependent MDH action, plays a role as an antioxidant, possibly by scavenging H(2)O(2).
authors.affiliation: Department of Radiation Biology, Environmental Radiation Research Group, Korea Atomic Energy Research Institute, P.O. Box 105, Yusong, Taejon 305-600, South Korea.
authors.forename: Tae Jeong
authors.initials: TJ
authors.lastname: Oh
authors.forename: In Gyu
authors.initials: IG
authors.lastname: Kim
authors.forename: Seon Young
authors.initials: SY
authors.lastname: Park
authors.forename: Kug Chan
authors.initials: KC
authors.lastname: Kim
authors.forename: Hye Won
authors.initials: HW
authors.lastname: Shim