NAD-dependent malate dehydrogenase protects against oxidative damage in Escherichia coli K-12 through the action of oxaloacetate

Reactive oxygen species including hydrogen peroxide (H2O2) and hydroxyl radical ((OH)-O-.) 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 H2O2 or gamma -radiation-induced damage in Escherichia coli through the action of oxaloacetate. The E. coli mdh mutant strain defective in MDH activity was more sensitive to H2O2 or gamma -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 H2O2 treatment or gamma -irradiation are resistant to H2O2 or gamma -radiation-induced damage, so cell survivability is restored to similar levels with the wild type. The SOS induction of umu'-'lacZ fusion gene by H2O2 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)-O-. in a metal-catalyzed oxidation (MCO) system that generated H2O2 as a mediator. By contrast, the prevention of DNA damage by oxaloacetate in an gamma -irradiation system that directly generates (OH)-O-. from H2O 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 H2O2. (C) 2002 Elsevier Science B.V. All rights reserved.